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Lee Drbohlav, Hae-Kyung, and V. Krishnamurthy. "Spatial Structure, Forecast Errors, and Predictability of the South Asian Monsoon in CFS Monthly Retrospective Forecasts." Journal of Climate 23, no. 18 (September 15, 2010): 4750–69. http://dx.doi.org/10.1175/2010jcli2356.1.
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Abstract The spatial structure of the boreal summer South Asian monsoon in the ensemble mean of monthly retrospective forecasts by the Climate Forecast System of the National Centers for Environmental Prediction is examined. The forecast errors and predictability of the model are assessed. Systematic errors in the forecasts consist of deficient rainfall over India, excess rainfall over the Arabian Sea, and a dipole structure over the equatorial Indian Ocean. On interannual time scale during 1981–2003, two different characteristics of the monsoon are recognized—both in observation and forecasts. One feature seems to indicate that the monsoon is regionally controlled, while the other shows a strong relation with El Niño–Southern Oscillation (ENSO). The spatial structure of the regional monsoon can be characterized by the dominant rainfall between the latitudes of 15°N and 5°S in the western Indian Ocean. The maximum precipitation anomalies in the northern Arabian Sea are associated with the cyclonic circulation, while the precipitation anomalies in the equatorial western Indian Ocean accompany the easterlies over the equatorial Indian Ocean. In the ENSO-related monsoon, strong positive precipitation anomalies prevail from the equatorial eastern Indian Ocean to the western Pacific, inducing westerlies over the equatorial Indian Ocean. The spatial structure of the forecast error shows that the model is inclined to predict the ENSO-related feature more accurately than the regional feature. The predictability is found to be lower over certain areas in the northern and equatorial eastern Indian Ocean. The predictability errors in the northern Indian Ocean diminish for longer forecast leads, presumably because the impact of different initial conditions dissipates with time. On the other hand, predictability errors over the equatorial eastern Indian Ocean grow as the forecast lead increases.
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JOSEPH, P. V. "Monsoon variability in relation to equatorial trough activity over Indian and West Pacific Oceans." MAUSAM 41, no. 2 (February 22, 2022): 150–55. http://dx.doi.org/10.54302/mausam.v41i2.2560.
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Variability of Indian monsoon rainfall has been examined in relation to the convective activity of the equatorial trough over the Indian Ocean a~d the Pacific Qcean west of the International Date Line. It is found that the cyclogenesis (tropical cyclones) near the West Pacific equatorial trough is closely related to this variability through a see-saw in. convection between this ocean basin and north Indian Ocean, with period in the range 30-50 days. SST anomalies over north Indian Ocean and West Pacific Ocean can cause variability of the date of onset of monsoon and also the quantum of monsoon rainfall over India through the 30-50 day mode.
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Zhou, Zhen-Qiang, Renhe Zhang, and Shang-Ping Xie. "Interannual Variability of Summer Surface Air Temperature over Central India: Implications for Monsoon Onset." Journal of Climate 32, no. 6 (February 18, 2019): 1693–706. http://dx.doi.org/10.1175/jcli-d-18-0675.1.
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Abstract Year-to-year variability of surface air temperature (SAT) over central India is most pronounced in June. Climatologically over central India, SAT peaks in May, and the transition from the hot premonsoon to the cooler monsoon period takes place around 9 June, associated with the northeastward propagation of intraseasonal convective anomalies from the western equatorial Indian Ocean. Positive (negative) SAT anomalies during June correspond to a delayed (early) Indian summer monsoon onset and tend to occur during post–El Niño summers. On the interannual time scale, positive SAT anomalies of June over central India are associated with positive SST anomalies over both the equatorial eastern–central Pacific and Indian Oceans, representing El Niño effects in developing and decay years, respectively. Although El Niño peaks in winter, the correlations between winter El Niño and Indian SAT peak in the subsequent June, representing a post–El Niño summer capacitor effect associated with positive SST anomalies over the north Indian Ocean. These results have important implications for the prediction of Indian summer climate including both SAT and summer monsoon onset over central India.
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Ogata, Tomomichi, and Shang-Ping Xie. "Semiannual Cycle in Zonal Wind over the Equatorial Indian Ocean." Journal of Climate 24, no. 24 (December 15, 2011): 6471–85. http://dx.doi.org/10.1175/2011jcli4243.1.
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Abstract The semiannual cycle in zonal wind over the equatorial Indian Ocean is investigated by use of ocean–atmospheric reanalyses, and linear ocean–atmospheric models. In observations, the semiannual cycle in zonal wind is dominant on the equator and confined in the planetary boundary layer (PBL). Results from a momentum budget analysis show that momentum advection generated by the cross-equatorial monsoon circulation is important for the semiannual zonal-wind cycle in the equatorial Indian Ocean. In experiments with a linearized primitive model of the atmosphere, semiannual momentum forcing due to the meridional advection over the central equatorial Indian Ocean is important to simulate the observed maxima of the semiannual cycle in equatorial zonal wind. Off Somalia, diabatic heating and surface friction over land weaken the semiannual response to large momentum forcing there. Results from a linear ocean model suggest that the semiannual zonal wind stress over the central equatorial Indian Ocean generates large semiannual variability in zonal current through a basin-mode resonance.
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Rao, Suryachandra A., Sebastien Masson, Jing-Jia Luo, Swadhin K. Behera, and Toshio Yamagata. "Termination of Indian Ocean Dipole Events in a Coupled General Circulation Model." Journal of Climate 20, no. 13 (July 1, 2007): 3018–35. http://dx.doi.org/10.1175/jcli4164.1.
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Abstract Using 200 yr of coupled general circulation model (CGCM) results, causes for the termination of Indian Ocean dipole (IOD) events are investigated. The CGCM used here is the Scale Interaction Experiment-Frontier Research Center for Global Change (SINTEX-F1) model, which consists of a version of the European Community–Hamburg (ECHAM4.6) atmospheric model and a version of the Ocean Parallelise (OPA8.2) ocean general circulation model. This model reproduces reasonably well the present-day climatology and interannual signals of the Indian and Pacific Oceans. The main characteristics of the intraseasonal disturbances (ISDs)/oscillations are also fairly well captured by this model. However, the eastward propagation of ISDs in the model is relatively fast in the Indian Ocean and stationary in the Pacific compared to observations. A sudden reversal of equatorial zonal winds is observed, as a result of significant intraseasonal disturbances in the equatorial Indian Ocean in November–December of IOD events, which evolve independently of ENSO. A majority of these IOD events (15 out of 18) are terminated mainly because of the 20–40-day ISD activity in the equatorial zonal winds. Ocean heat budget analysis in the upper 50 m clearly shows that the initial warming after the peak of the IOD phenomenon is triggered by increased solar radiation owing to clear-sky conditions in the eastern Indian Ocean. Subsequently, the equatorial jets excited by the ISD deepen the thermocline in the southeastern equatorial Indian Ocean. This deepening of the thermocline inhibits the vertical entrainment of cool waters and therefore the IOD is terminated. IOD events that co-occur with ENSO are terminated owing to anomalous incoming solar radiation as a result of prevailing cloud-free skies. Further warming occurs seasonally through the vertical convergence of heat due to a monsoonal wind reversal along Sumatra–Java. On occasion, strong ISD activities in July–August terminated short-lived IOD events by triggering downwelling intraseasonal equatorial Kelvin waves.
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Brown, J., C. A. Clayson, L. Kantha, and T. Rojsiraphisal. "North Indian Ocean variability during the Indian Ocean dipole." Ocean Science Discussions 5, no. 2 (June 9, 2008): 213–53. http://dx.doi.org/10.5194/osd-5-213-2008.
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Abstract. The circulation in the North Indian Ocean (NIO henceforth) is highly seasonally variable. Periodically reversing monsoon winds (southwesterly during summer and northeasterly during winter) give rise to seasonally reversing current systems off the coast of Somalia and India. In addition to this annual monsoon cycle, the NIO circulation varies semiannually because of equatorial currents reversing four times each year. These descriptions are typical, but how does the NIO circulation behave during anomalous years, during an Indian Ocean dipole (IOD) for instance? Unfortunately, in situ observational data are rather sparse and reliance has to be placed on numerical models to understand this variability. In this paper, we estimate the surface current variability from a 12-year hindcast of the NIO for 1993–2004 using a 1/2° resolution circulation model that assimilates both altimetric sea surface height anomalies and sea surface temperature. Presented in this paper is an examination of surface currents in the NIO basin during the IOD. During the non-IOD period of 2000–2004, the typical equatorial circulation of the NIO reverses four times each year and transports water across the basin preventing a large sea surface temperature difference between the western and eastern NIO. Conversely, IOD years are noted for strong easterly and westerly wind outbursts along the equator. The impact of these outbursts on the NIO circulation is to reverse the direction of the currents – when compared to non-IOD years – during the summer for negative IOD events (1996 and 1998) and during the fall for positive IOD events (1994 and 1997). This reversal of current direction leads to large temperature differences between the western and eastern NIO.
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Ihara, Chie, Yochanan Kushnir, Mark A. Cane, and Victor H. de la Peña. "Climate Change over the Equatorial Indo-Pacific in Global Warming*." Journal of Climate 22, no. 10 (May 15, 2009): 2678–93. http://dx.doi.org/10.1175/2008jcli2581.1.
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Abstract The response of the equatorial Indian Ocean climate to global warming is investigated using model outputs submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report. In all of the analyzed climate models, the SSTs in the western equatorial Indian Ocean warm more than the SSTs in the eastern equatorial Indian Ocean under global warming; the mean SST gradient across the equatorial Indian Ocean is anomalously positive to the west in a warmer twenty-first-century climate compared to the twentieth-century climate, and it is dynamically consistent with the anomalous westward zonal wind stress and anomalous positive zonal sea level pressure (SLP) gradient to the east at the equator. This change in the zonal SST gradient in the equatorial Indian Ocean is detected even in the lowest-emission scenario, and the size of the change is not necessarily larger in the higher-emission scenario. With respect to the change over the equatorial Pacific in climate projections, the subsurface central Pacific displays the strongest cooling or weakest warming around the thermocline depth compared to that above and below in all of the climate models, whereas changes in the zonal SST gradient and zonal wind stress around the equator are model dependent and not straightforward.
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Yuan, Dongliang, Jing Wang, Tengfei Xu, Peng Xu, Zhou Hui, Xia Zhao, Yihua Luan, Weipeng Zheng, and Yongqiang Yu. "Forcing of the Indian Ocean Dipole on the Interannual Variations of the Tropical Pacific Ocean: Roles of the Indonesian Throughflow." Journal of Climate 24, no. 14 (July 15, 2011): 3593–608. http://dx.doi.org/10.1175/2011jcli3649.1.
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Abstract Controlled numerical experiments using ocean-only and ocean–atmosphere coupled general circulation models show that interannual sea level depression in the eastern Indian Ocean during the Indian Ocean dipole (IOD) events forces enhanced Indonesian Throughflow (ITF) to transport warm water from the upper-equatorial Pacific Ocean to the Indian Ocean. The enhanced transport produces elevation of the thermocline and cold subsurface temperature anomalies in the western equatorial Pacific Ocean, which propagate to the eastern equatorial Pacific to induce significant coupled evolution of the tropical Pacific oceanic and atmospheric circulation. Analyses suggest that the IOD-forced ITF transport anomalies are about the same amplitudes as those induced by the Pacific ENSO. Results of the coupled model experiments suggest that the anomalies induced by the IOD persist in the equatorial Pacific until the year following the IOD event, suggesting the importance of the oceanic channel in modulating the interannual climate variations of the tropical Pacific Ocean at the time lag beyond one year.
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Hastenrath, Stefan, and Dierk Polzin. "Circulation mechanisms of climate anomalies in the equatorial Indian Ocean." Meteorologische Zeitschrift 12, no. 2 (April 25, 2003): 81–93. http://dx.doi.org/10.1127/0941-2948/2003/0012-0081.
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Kido, Shoichiro, and Tomoki Tozuka. "Salinity Variability Associated with the Positive Indian Ocean Dipole and Its Impact on the Upper Ocean Temperature." Journal of Climate 30, no. 19 (September 1, 2017): 7885–907. http://dx.doi.org/10.1175/jcli-d-17-0133.1.
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Abstract Both surface and subsurface salinity variability associated with positive Indian Ocean dipole (pIOD) events and its impacts on the sea surface temperature (SST) evolution are investigated through analysis of observational/reanalysis data and sensitivity experiments with a one-dimensional mixed layer model. During the pIOD, negative (positive) sea surface salinity (SSS) anomalies appear in the central-eastern equatorial Indian Ocean (southeastern tropical Indian Ocean). In addition to these SSS anomalies, positive (negative) salinity anomalies are found near the pycnocline in the eastern equatorial Indian Ocean (southern tropical Indian Ocean). A salinity balance analysis shows that these subsurface salinity anomalies are mainly generated by zonal and vertical salt advection anomalies induced by anomalous currents associated with the pIOD. These salinity anomalies stabilize (destabilize) the upper ocean stratification in the central-eastern equatorial (southeastern tropical) Indian Ocean. By decomposing observed densities into contribution from temperature and salinity anomalies, it is shown that the contribution from anomalous salinity stratification is comparable to that from anomalous thermal stratification. Furthermore, impacts of these salinity anomalies on the SST evolution are quantified for the first time using a one-dimensional mixed layer model. Since enhanced salinity stratification in the central-eastern equatorial Indian Ocean suppresses vertical mixing, significant warming of about 0.3°–0.5°C occurs. On the other hand, stronger vertical mixing associated with reduced salinity stratification results in significant SST cooling of about 0.2°–0.5°C in the southeastern tropical Indian Ocean. These results suggest that variations in salinity may potentially play a crucial role in the evolution of the pIOD.
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Huang, Bohua, and J. Shukla. "Mechanisms for the Interannual Variability in the Tropical Indian Ocean. Part I: The Role of Remote Forcing from the Tropical Pacific." Journal of Climate 20, no. 13 (July 1, 2007): 2917–36. http://dx.doi.org/10.1175/jcli4151.1.
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Abstract A series of experiments are conducted using a coupled ocean–atmosphere general circulation model in regional coupled mode, which permits active air–sea interaction only within the Indian Ocean to the north of 30°S, with sea surface temperatures (SSTs) prescribed over the rest of the world oceans. In this paper, an ensemble of nine simulations has been analyzed with the observed SST anomalies for 1950–98 prescribed over the uncoupled region. The purpose of this study is to determine the major patterns of interannual variability in the tropical Indian Ocean that could be related to the global low-frequency fluctuations and to understand the physical links between the remote forcing and the regional coupled variations. The ensemble coupled simulations with prescribed SST outside the Indian Ocean are able to reproduce a considerable amount of observed variability in the tropical Indian Ocean during 1950–98. The first EOF modes of the simulated upper-ocean heat content and SST anomalies show structures that are quite consistent with those from the historical upper oceanic temperature and SST analyses. The dominant pattern of response is associated with an oceanic dynamical adjustment of the thermocline depth in the southwestern Indian Ocean. In general, a deepening of the thermocline in the southwest is usually accompanied by the enhanced upwelling and thermocline shoaling centered near the Sumatra coast. Further analysis shows that the leading external forcing is from the El Niño–Southern Oscillation (ENSO), which induces an anomalous fluctuation of the atmospheric anticyclones on both sides of the equator over the Indian Ocean, starting from the evolving stage of an El Niño event in boreal summer. Apart from weakening the Indian monsoon, the surface equatorial easterly anomalies associated with this circulation pattern first induce equatorial and coastal upwelling anomalies near the Sumatra coast from summer to fall, which enhance the equatorial zonal SST gradient and stimulate intense air–sea feedback in the equatorial ocean. Moreover, the persistent anticyclonic wind curl over the southern tropical Indian Ocean, reinforced by the equatorial air–sea coupling, forces substantial thermocline change centered at the thermocline ridge in the southwestern Indian Ocean for seasons. The significant thermocline change has profound and long-lasting influences on the SST fluctuations in the Indian Ocean. It should be noted that the ENSO forcing is not the only way that this kind of basinwide Indian Ocean fluctuations can be generated. As will be shown in the second part of this study, similar low-frequency fluctuations can also be generated by processes within the Indian and western Pacific region without ENSO influence. The unique feature of the ENSO influence is that, because of the high persistence of the atmospheric remote forcing from boreal summer to winter, the life span of the thermocline anomalies in the southwestern Indian Ocean is generally longer than that generated by regional coupled processes.
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Rao, Suryachandra A., Hemantkumar S. Chaudhari, Samir Pokhrel, and B. N. Goswami. "Unusual Central Indian Drought of Summer Monsoon 2008: Role of Southern Tropical Indian Ocean Warming." Journal of Climate 23, no. 19 (October 1, 2010): 5163–74. http://dx.doi.org/10.1175/2010jcli3257.1.
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Abstract While many of the previous positive Indian Ocean dipole (IOD) years were associated with above (below)-normal monsoon rainfall over central (southern) India during summer monsoon months [June–September (JJAS)], the IOD event in 2008 is associated with below (above)-normal rainfall in many parts of central (southern peninsular) India. Because understanding such regional organization is a key for success in regional prediction, using different datasets and atmospheric model simulations, the reasons for this abnormal behavior of the monsoon in 2008 are explored. Compared to normal positive IOD events, sea surface temperature (SST) and rainfall in the southern tropical Indian Ocean (STIO) in JJAS 2008 were abnormally high. Downwelling Rossby waves and oceanic heat advection played an important role in warming SST abnormally in the STIO. It was also found that the combined influence of a linear warming trend in the tropical Indian Ocean and warming associated with the IOD have resulted in abnormal warming of the STIO. This abnormal SST warming resulted in enhancement of convection in the southwest tropical Indian Ocean and forced anticyclonic circulation anomalies over the Bay of Bengal and central India, leading to suppressed rainfall over this region in JJAS 2008. The above mechanism is tested by conducting several model sensitivity experiments with an atmospheric general circulation model (AGCM). These experiments confirmed that the subsidence over central India and the Bay of Bengal was forced mainly by the anomalous warming in the STIO region driven by coupled ocean–atmosphere processes. This study provides the first evidence of combined Indian Ocean warming, associated with global warming, and IOD-related warming influence on Indian summer monsoon rainfall. The combined influence may force below-normal rainfall over central India by inducing strong convection in the STIO region. The conventional seesaw in convection between the Indian subcontinent and the eastern equatorial Indian Ocean may shift to the central equatorial Indian Ocean and the Bay of Bengal if the central Indian Ocean consistently warms in the global warming scenario.
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Han, Weiqing, Julian P. McCreary, Yukio Masumoto, Jérôme Vialard, and Benét Duncan. "Basin Resonances in the Equatorial Indian Ocean." Journal of Physical Oceanography 41, no. 6 (June 1, 2011): 1252–70. http://dx.doi.org/10.1175/2011jpo4591.1.
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Abstract Previous studies have investigated how second-baroclinic-mode (n = 2) Kelvin and Rossby waves in the equatorial Indian Ocean (IO) interact to form basin resonances at the semiannual (180 day) and 90-day periods. This paper examines unresolved issues about these resonances, including the reason the 90-day resonance is concentrated in the eastern ocean, the time scale for their establishment, and the impact of complex basin geometry. A hierarchy of ocean models is used: an idealized one-dimensional (1D) model, a linear continuously stratified ocean model (LCSM), and an ocean general circulation model (OGCM) forced by Quick Scatterometer (QuikSCAT) wind during 2000–08. Results indicate that the eastern-basin concentration of the 90-day resonance happens because the westward-propagating Rossby wave is slower, and thus is damped more than the eastward-propagating Kelvin wave. Results also indicate that superposition with other baroclinic modes further enhances the eastern maximum and weakens sea level variability near the western boundary. Without resonance, although there is still significant power at 90 and 180 days, solutions have no spectral peaks at these periods. The key time scale for the establishment of all resonances is the time it takes a Kelvin wave to cross the basin and a first-meridional-mode (ℓ = 1) Rossby wave to return; thus, even though the amplitude of the 90-day winds vary significantly, the 90-day resonance can be frequently excited in the real IO, as evidenced by satellite-observed and OGCM-simulated sea level. The presence of the Indian subcontinent enhances the influence of equatorial variability in the north IO, especially along the west coast of India. The Maldives Islands weaken the 180-day resonance amplitude but have little effect on the 90-day resonance, because they fall in its “node” region. Additionally, resonance at the 120-day period for the n = 1 mode is noted.
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Hastenrath, Stefan. "Zonal Circulations over the Equatorial Indian Ocean." Journal of Climate 13, no. 15 (August 2000): 2746–56. http://dx.doi.org/10.1175/1520-0442(2000)013<2746:zcotei>2.0.co;2.
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Wang, Jing, and Dongliang Yuan. "Roles of Western and Eastern Boundary Reflections in the Interannual Sea Level Variations during Negative Indian Ocean Dipole Events." Journal of Physical Oceanography 45, no. 7 (July 2015): 1804–21. http://dx.doi.org/10.1175/jpo-d-14-0124.1.
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AbstractThe equatorial wave dynamics of sea level variations during negative Indian Ocean dipole (nIOD) events are investigated using the LICOM ocean general circulation model forced with the European Centre for Medium-Range Weather Forecast reanalysis wind stress and heat flux from 1990 to 2001. The work is a continuation of the study by Yuan and Liu, in which the equatorial wave dynamics during positive IOD events are investigated. The model has reproduced the sea level anomalies of satellite altimeter data well. Long equatorial waves extracted from the model output suggest two kinds of negative feedback during nIOD events: the western boundary reflection and the easterly wind bursts. During the strong 1998–99 nIOD event, the downwelling anomalies in the eastern Indian Ocean are terminated by persistent and strong upwelling Kelvin waves from the western boundary, which are reflected from the wind-forced equatorial Rossby waves over the southern central Indian Ocean. During the 1996–97 nIOD, however, the reflection of upwelling anomalies at the western boundary is terminated by the arrival of downwelling equatorial Rossby waves from the eastern boundary reflection in early 1997. Therefore, the negative feedback of this nIOD event is not provided by the western boundary reflection. The downwelling anomalies in the eastern basin during the 1996–97 nIOD event are terminated by easterly wind anomalies over the equatorial Indian Ocean in early 1997. The disclosed equatorial wave dynamics are important to the simulation and prediction of IOD evolution.
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Pérez-Hernández, M. Dolores, Alonso Hernández-Guerra, Terrence M. Joyce, and Pedro Vélez-Belchí. "Wind-Driven Cross-Equatorial Flow in the Indian Ocean." Journal of Physical Oceanography 42, no. 12 (December 1, 2012): 2234–53. http://dx.doi.org/10.1175/jpo-d-12-033.1.
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Abstract Meridional velocity, mass, and heat transport in the equatorial oceans are difficult to estimate because of the nonapplicability of the geostrophic balance. For this purpose a steady-state model is utilized in the equatorial Indian Ocean using NCEP wind stress and temperature and salinity data from the World Ocean Atlas 2005 (WOA05) and Argo. The results show a Somali Current flowing to the south during the winter monsoon carrying −11.5 ± 1.3 Sv (1 Sv ≡ 106 m3 s−1) and −12.3 ± 0.3 Sv from WOA05 and Argo, respectively. In the summer monsoon the Somali Current reverses to the north transporting 16.8 ± 1.2 Sv and 19.8 ± 0.6 Sv in the WOA05 and Argo results. Transitional periods are considered together and in consequence, there is not a clear Somali Current present in this period. Model results fit with in situ measurements made around the region, although Argo data results are quite more realistic than WOA05 data results.
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Terray, Pascal, and Sébastien Dominiak. "Indian Ocean Sea Surface Temperature and El Niño–Southern Oscillation: A New Perspective." Journal of Climate 18, no. 9 (May 1, 2005): 1351–68. http://dx.doi.org/10.1175/jcli3338.1.
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Abstract Here the 1976–77 climate regime shift that was accompanied by a remarkable change in the lead–lag relationships between Indian Ocean sea surface temperature (SST) and El Niño evolution is shown. After the 1976–77 regime shift, a correlation analysis suggests that southern Indian Ocean SSTs observed during late boreal winter are a key precursor in predicting El Niño evolution as the traditional oceanic heat content anomalies in the equatorial Pacific or zonal wind anomalies over the equatorial western Pacific. The possible physical mechanisms underlying this highly significant statistical relationship are discussed. After the 1976–77 regime shift, southern Indian Ocean SST anomalies produced by Mascarene high pulses during boreal winter trigger coupled air–sea processes in the tropical eastern Indian Ocean during the following seasons. This produces a persistent remote forcing on the Pacific climate system, promoting wind anomalies over the western equatorial Pacific and modulating the regional Hadley cell in the southwest Pacific. These modulations, in turn, excite Rossby waves, which produce quasi-stationary circulation anomalies in the extratropical South Pacific, responsible for the development of the southern branch of the “horseshoe” El Niño pattern. The change of the background SST state that occurred in the late 1970s over the Indian Ocean may also explain why ENSO evolution is different before and after the 1976–77 regime shift. These results shed some light on the possible influence of global warming or decadal fluctuations on El Niño evolution through changes in teleconnection patterns between the Indian and Pacific Oceans.
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Schott, Friedrich A. "Shallow overturning circulation of the Western Indian Ocean." Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 363, no. 1826 (January 15, 2005): 143–49. http://dx.doi.org/10.1098/rsta.2004.1483.
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The Indian Ocean differs from the other two oceans in not possessing an eastern equatorial upwelling regime. Instead, the upwelling occurs dominantly in the northwestern Arabian Sea and, to a lesser degree, around the Indian subcontinent. Subduction, on the other hand, occurs dominantly in the Southern Hemisphere. The result is a shallow Cross–Equatorial Cell connecting both regimes. The northward flow at thermocline levels occurs as part of the Somali Current and the southward upper–layer return flow is carried by the Ekman transports that are directed southward in both hemispheres. The main forcing is by the Southwest Monsoon that overwhelms the effects of the Northeast Monsoon and is the cause for the annual mean Northern Hemisphere upwelling and southward Ekman transports. In the Southern Hemisphere, the annual mean upwelling at the northern rim of the Southeast Trades causes a zonally extended open–ocean upwelling regime that is apparent in isopycnal doming in the 3–12○ S band; it drives a shallow Subtropical Cell.
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Yang, Yiya, Renguang Wu, and Chenghai Wang. "Individual and Combined Impacts of Tropical Indo-Pacific SST Anomalies on Interannual Variation of the Indochina Peninsular Precipitation." Journal of Climate 33, no. 3 (February 1, 2020): 1069–88. http://dx.doi.org/10.1175/jcli-d-19-0262.1.
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AbstractThis study documents interannual rainfall variations over the Indochina Peninsula (ICP) during the rainy season and individual and combined influences of tropical Indo-Pacific sea surface temperature (SST) anomalies. The rainfall variability is large along the west coast in May–June, along the west coast and over the eastern mountains in July–August, and along the central Vietnam coast in September–November. More rainfall in May–June, July–August, and October–November occurs in the La Niña decaying years, La Niña decaying years and/or El Niño developing years, and La Niña developing years, respectively. The May–June rainfall variation along the west coast is associated with equatorial central-eastern Pacific (EP), south Indian Ocean, and western North Pacific SST anomalies. The July–August rainfall variation along the west coast and over the eastern mountains is related to equatorial central Pacific and tropical southeastern Indian Ocean SST anomalies. The October–November rainfall variation along the central Vietnam coast is affected by EP and tropical western Indian Ocean SST anomalies. The EP and tropical western Indian Ocean SST influence is through anomalous Walker circulation. The south Indian Ocean SST influence is via cross-equatorial flows. The tropical southeastern Indian Ocean SST influence is via an anomalous cross-equatorial overturning circulation. The equatorial central Pacific and western North Pacific SST influence is via a Rossby wave–type response. The analysis illustrates the importance of combined effects of regional SST anomalies on the ICP precipitation variation in different stages of the rainy season. Numerical experiments with SST anomalies imposed in different regions confirm the combined effects of the Indo-Pacific SST anomalies on the ICP rainfall variation.
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Joseph, Sudheer, M. Ravichandran, B. Praveen Kumar, Raju V. Jampana, and Weiqing Han. "Ocean atmosphere thermal decoupling in the eastern equatorial Indian ocean." Climate Dynamics 49, no. 1-2 (September 22, 2016): 575–94. http://dx.doi.org/10.1007/s00382-016-3359-1.
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SINGH, M. S., and B. Lakshmanaswamy. "Evolution of two troughs in the tropical Indian Ocean and their characteristic features." MAUSAM 43, no. 4 (December 31, 2021): 395–98. http://dx.doi.org/10.54302/mausam.v43i4.3507.
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Evolution and characteristic features of double trough systems in the tropical Indian Ocean have been studied with the help of Climatological Atlas (Part I andIl) ~f the Tropical Indian Oc.ean (Hastenrath and Lamb 1979). It is confirmed that there are two troughs (Northern Hemisphere EquatorIal Trough and Southern Hemisphere Equatorial Trough) in this region (including south Asian landmass) all the year round, one in northern hemisphere and the other in southern. Both are migratory in nature and, perhaps, thermal in origin. In the convergent zones of the two troughs, there is extensive cloudiness. The migration of these trough systems during their respective summer seasons appear to be related to the extensive heating of the south Asian/ African land masses surrounding the Indian Ocean in north and west.
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Okello, Ochieng, Guirong Tan, Victor Ongoma, and Isaiah Nyandega. "Influence of convectively coupled equatorial Kelvin waves on March-May precipitation over East Africa." Geographica Pannonica 25, no. 1 (2021): 24–34. http://dx.doi.org/10.5937/gp25-31132.
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Convectively coupled equatorial Kelvin waves (CCEKWs) are those types of equatorially trapped disturbances that propagate eastward and are among the most common intra-seasonal oscillations in the tropics. There exists two-way feedback between the inter-tropical convergence zone (ITCZ) and these equatorially trapped disturbances. Outgoing Longwave Radiation (OLR) was utilized as a proxy for deep convection. For CCEKWs, the modes are located over the West Atlantic, equatorial West Africa, and the Indian Ocean. The influence of other circulations and climate dynamics is studied for finding other drivers of climate within East Africa. The results show a positive relationship between Indian and Atlantic Oceans Sea Surface Temperatures and March-May rainfall over equatorial East Africa over the period of 1980 to 2010. This influence is driven by the Walker circulation and anomalous moisture influx enhanced by winds. Composite analysis reveals strong lower-tropospheric westerlies during the active phase of the CCKWs activities over Equatorial East Africa. The winds are in the opposite direction with the upper-tropospheric winds, which are easterlies. Singular Value Decomposition shows a strong coupling interaction between rainfall over equatorial East Africa and CCKWs. This study concludes that Kelvin waves are not the main factors that influence rainfall during the rainy season. Previous studies show that the main influencing factors are ITCZ, El-Nino Southern Oscillation (ENSO), and tropical anticyclones that borders the African continent. However, CCKWs are a significant factor during the dry seasons.
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Watanabe, Masahiro. "Two Regimes of the Equatorial Warm Pool. Part I: A Simple Tropical Climate Model." Journal of Climate 21, no. 14 (July 15, 2008): 3533–44. http://dx.doi.org/10.1175/2007jcli2151.1.
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Abstract Atmosphere–ocean coupled processes responsible for generating and maintaining the equatorial warm pool were investigated using models of different complexities. The primary focus was to answer the following question: why is the observed warm pool concentrated around the maritime continent? In this first of a two-part series, the solutions of a simple conceptual model that represents the tropical Pacific and Indian Oceans interacting via the Walker circulation are examined. When the interbasin coupling is sufficiently strong, surface wind convergence over the Maritime Continent associated with easterly trades over the Pacific acts to generate the equatorial westerly over the Indian Ocean, leading to a warm pool spontaneously emerging between the two ocean basins. The conceptual model shows that tropical climate has two equilibria, depending upon the ocean basin widths—a single warm pool regime corresponding to the current climate and a split warm pool regime that accompanies warm pools created in the western parts of each ocean basin. The latter is found to be unstable and hence exhibits large-amplitude vacillations between the ocean basins being further amplified by the Bjerknes feedback. The above two regimes of the equatorial warm pool are identified in the model incorporating the interactive Atlantic Ocean as well, wherein the mean state and variability in the three ocean basins qualitatively agree with the observations.
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Chen, Gengxin, Weiqing Han, Yuanlong Li, and Dongxiao Wang. "Interannual Variability of Equatorial Eastern Indian Ocean Upwelling: Local versus Remote Forcing." Journal of Physical Oceanography 46, no. 3 (March 2016): 789–807. http://dx.doi.org/10.1175/jpo-d-15-0117.1.
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AbstractThe equatorial eastern Indian Ocean (EIO) upwelling occurs in the Indian Ocean warm pool, differing from the equatorial Pacific and Atlantic upwelling that occurs in the cold tongue. By analyzing observations and performing ocean model experiments, this paper quantifies the remote versus local forcing in causing interannual variability of the equatorial EIO upwelling from 2001 to 2011 and elucidates the associated processes. For all seasons, interannual variability of thermocline depth in the EIO, as an indicator of upwelling, is dominated by remote forcing from equatorial Indian Ocean winds, which drive Kelvin waves that propagate along the equator and subsequently along the Sumatra–Java coasts. Upwelling has prominent signatures in sea surface temperature (SST) and chlorophyll-a concentration but only in boreal summer–fall (May–October). Local forcing plays a larger role than remote forcing in producing interannual SST anomaly (SSTA). During boreal summer–fall, when the mean thermocline is relatively shallow, SSTA is primarily driven by the upwelling process, with comparable contributions from remote and local forcing effects. In contrast, during boreal winter–spring (November–April), when the mean thermocline is relatively deep, SSTA is controlled by surface heat flux and decoupled from thermocline variability. Advection affects interannual SSTA in all cases. The remote and local winds that drive the interannual variability of the equatorial EIO upwelling are closely associated with Indian Ocean dipole events and to a lesser degree with El Niño–Southern Oscillation.
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Chen, Gengxin, Weiqing Han, Yuanlong Li, Jinglong Yao, and Dongxiao Wang. "Intraseasonal Variability of the Equatorial Undercurrent in the Indian Ocean." Journal of Physical Oceanography 49, no. 1 (January 2019): 85–101. http://dx.doi.org/10.1175/jpo-d-18-0151.1.
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AbstractBy analyzing in situ observations and conducting a series of ocean general circulation model experiments, this study investigates the physical processes controlling intraseasonal variability (ISV) of the Equatorial Undercurrent (EUC) of the Indian Ocean. ISV of the EUC leads to time-varying water exchanges between the western and eastern equatorial Indian Ocean. For the 2001–14 period, standard deviations of the EUC transport variability are 1.92 and 1.77 Sv (1 Sv ≡ 106 m3 s−1) in the eastern and western basins, respectively. The ISV of the EUC is predominantly caused by the wind forcing effect of atmospheric intraseasonal oscillations (ISOs) but through dramatically different ocean dynamical processes in the eastern and western basins. The stronger ISV in the eastern basin is dominated by the reflected Rossby waves associated with intraseasonal equatorial zonal wind forcing. It takes 20–30 days to set up an intraseasonal EUC anomaly through the Kelvin and Rossby waves associated with the first and second baroclinic modes. In the western basin, the peak intraseasonal EUC anomaly is generated by the zonal pressure gradient force, which is set up by radiating equatorial Kelvin and Rossby waves induced by the equatorial wind stress. Directly forced and reflected Rossby waves from the eastern basin propagate westward, contributing to intraseasonal zonal current near the surface but having weak impact on the peak ISV of the EUC.
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Krishnan, R., and P. Swapna. "Significant Influence of the Boreal Summer Monsoon Flow on the Indian Ocean Response during Dipole Events." Journal of Climate 22, no. 21 (November 1, 2009): 5611–34. http://dx.doi.org/10.1175/2009jcli2176.1.
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Abstract A majority of positive Indian Ocean dipole (IOD) events in the last 50 years were accompanied by enhanced summer monsoon circulation and above-normal precipitation over central-north India. Given that IODs peak during boreal autumn following the summer monsoon season, this study examines the role of the summer monsoon flow on the Indian Ocean (IO) response using a suite of ocean model experiments and supplementary data diagnostics. The present results indicate that, if the summer monsoon Hadley-type circulation strengthens during positive IOD events, then the strong off-equatorial southeasterly winds over the northern flanks of the intensified Australian high can effectively promote upwelling in the southeastern tropical Indian Ocean and amplify the zonal gradient of the IO heat content response. While it is noted that a strong monsoon cross-equatorial flow by itself may not generate a dipolelike response, a strengthening (weakening) of monsoon easterlies to the south of the equator during positive IOD events tends to reinforce (hinder) the zonal gradient of the upper-ocean heat content response. The findings show that an intensification of monsoonal winds during positive IOD periods produces nonlinear amplification of easterly wind stress anomalies to the south of the equator because of the nonlinear dependence of wind stress on wind speed. It is noted that such an off-equatorial intensification of easterlies over the SH enhances upwelling in the eastern IO off Sumatra–Java, and the thermocline shoaling provides a zonal pressure gradient, which drives anomalous eastward equatorial undercurrents (EUC) in the subsurface. Furthermore, the combination of positive IOD and stronger-than-normal monsoonal flow favors intensification of shallow transient meridional overturning circulation in the eastern IO and enhances the feed of cold subsurface off-equatorial waters to the EUC.
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Youngs, Madeleine K., and Gregory C. Johnson. "Basin-Wavelength Equatorial Deep Jet Signals across Three Oceans." Journal of Physical Oceanography 45, no. 8 (August 2015): 2134–48. http://dx.doi.org/10.1175/jpo-d-14-0181.1.
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AbstractEquatorial deep jets (EDJs) are equatorially trapped, stacked, zonal currents that reverse direction every few hundred meters in depth throughout much of the water column. This study evaluates their structure observationally in all three oceans using new high-vertical-resolution Argo float conductivity–temperature–depth (CTD) instrument profiles from 2010 to 2014 augmented with historical shipboard CTD data from 1972 to 2014 and lower-vertical-resolution Argo float profiles from 2007 to 2014. The vertical strain of density is calculated from the profiles and analyzed in a stretched vertical coordinate system determined from the mean vertical density structure. The power spectra of vertical strain in each basin are analyzed using wavelet decomposition. In the Indian and Pacific Oceans, there are two distinct peaks in the power spectra, one Kelvin wave–like and the other entirely consistent with the dispersion relation of a linear, first meridional mode, equatorial Rossby wave. In the Atlantic Ocean, the first meridional mode Rossby wave signature is very strong and dominates. In all three ocean basins, Rossby wave–like signatures are coherent across the basin width and appear to have wavelengths the scale of the basin width, with periods of about 5 yr in the Indian and Atlantic Oceans and about 12 yr in the Pacific Ocean. Their observed meridional scales are about 1.5 times the linear theoretical values. Their phase propagation is downward with time, implying upward energy propagation if linear wave dynamics hold.
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Yuhong, Zhang, Du Yan, Zheng Shaojun, Yang Yali, and Cheng Xuhua. "Impact of Indian Ocean Dipole on the salinity budget in the equatorial Indian Ocean." Journal of Geophysical Research: Oceans 118, no. 10 (October 2013): 4911–23. http://dx.doi.org/10.1002/jgrc.20392.
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Meehl, Gerald A., Julie M. Arblaster, and Johannes Loschnigg. "Coupled Ocean–Atmosphere Dynamical Processes in the Tropical Indian and Pacific Oceans and the TBO." Journal of Climate 16, no. 13 (July 1, 2003): 2138–58. http://dx.doi.org/10.1175/2767.1.
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Abstract The transitions (from relatively strong to relatively weak monsoon) in the tropospheric biennial oscillation (TBO) occur in northern spring for the south Asian or Indian monsoon and northern fall for the Australian monsoon involving coupled land–atmosphere–ocean processes over a large area of the Indo-Pacific region. Transitions from March–May (MAM) to June–September (JJAS) tend to set the system for the next year, with a transition to the opposite sign the following year. Previous analyses of observed data and GCM sensitivity experiments have demonstrated that the TBO (with roughly a 2–3-yr period) encompasses most ENSO years (with their well-known biennial tendency). In addition, there are other years, including many Indian Ocean dipole (or zonal mode) events, that contribute to biennial transitions. Results presented here from observations for composites of TBO evolution confirm earlier results that the Indian and Pacific SST forcings are more dominant in the TBO than circulation and meridional temperature gradient anomalies over Asia. A fundamental element of the TBO is the large-scale east–west atmospheric circulation (the Walker circulation) that links anomalous convection and precipitation, winds, and ocean dynamics across the Indian and Pacific sectors. This circulation connects convection over the Asian–Australian monsoon regions both to the central and eastern Pacific (the eastern Walker cell), and to the central and western Indian Ocean (the western Walker cell). Analyses of upper-ocean data confirm previous results and show that ENSO El Niño and La Niña events as well as Indian Ocean SST dipole (or zonal mode) events are often large-amplitude excursions of the TBO in the tropical Pacific and Indian Oceans, respectively, associated with anomalous eastern and western Walker cell circulations, coupled ocean dynamics, and upper-ocean temperature and heat content anomalies. Other years with similar but lower-amplitude signals in the tropical Pacific and Indian Oceans also contribute to the TBO. Observed upper-ocean data for the Indian Ocean show that slowly eastward-propagating equatorial ocean heat content anomalies, westward-propagating ocean Rossby waves south of the equator, and anomalous cross-equatorial ocean heat transports contribute to the heat content anomalies in the Indian Ocean and thus to the ocean memory and consequent SST anomalies, which are an essential part of the TBO.
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Jansen, Malte F., Dietmar Dommenget, and Noel Keenlyside. "Tropical Atmosphere–Ocean Interactions in a Conceptual Framework." Journal of Climate 22, no. 3 (February 1, 2009): 550–67. http://dx.doi.org/10.1175/2008jcli2243.1.
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Abstract Statistical analysis of observations (including atmospheric reanalysis and forced ocean model simulations) is used to address two questions: First, does an analogous mechanism to that of El Niño–Southern Oscillation (ENSO) exist in the equatorial Atlantic or Indian Ocean? Second, does the intrinsic variability in these basins matter for ENSO predictability? These questions are addressed by assessing the existence and strength of the Bjerknes and delayed negative feedbacks in each tropical basin, and by fitting conceptual recharge oscillator models, both with and without interactions among the basins. In the equatorial Atlantic the Bjerknes and delayed negative feedbacks exist, although weaker than in the Pacific. Equatorial Atlantic variability is well described by the recharge oscillator model, with an oscillatory mixed ocean dynamics–sea surface temperature (SST) mode present in boreal spring and summer. The dynamics of the tropical Indian Ocean, however, appear to be quite different: no recharge–discharge mechanism is found. Although a positive Bjerknes-like feedback from July to September is found, the role of heat content seems secondary. Results also show that Indian Ocean interaction with ENSO tends to damp the ENSO oscillation and is responsible for a frequency shift to shorter periods. However, the retrospective forecast skill of the conceptual model is hardly improved by explicitly including Indian Ocean SST. The interaction between ENSO and the equatorial Atlantic variability is weaker. However, a feedback from the Atlantic on ENSO appears to exist, which slightly improves the retrospective forecast skill of the conceptual model.
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Yoneyama, Kunio, Yukio Masumoto, Yoshifumi Kuroda, Masaki Katsumata, Keisuke Mizuno, Yukari N. Takayabu, Masanori Yoshizaki, et al. "MISMO FIELD EXPERIMENT IN THE EQUATORIAL INDIAN OCEAN." Bulletin of the American Meteorological Society 89, no. 12 (December 2008): 1889–904. http://dx.doi.org/10.1175/2008bams2519.1.
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Sengupta, Debasis, Retish Senan, B. N. Goswami, and Jérôme Vialard. "Intraseasonal Variability of Equatorial Indian Ocean Zonal Currents." Journal of Climate 20, no. 13 (July 1, 2007): 3036–55. http://dx.doi.org/10.1175/jcli4166.1.
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Abstract New satellite and in situ observations show large intraseasonal (10–60 day) variability of surface winds and upper-ocean current in the equatorial Indian Ocean, particularly in the east. An ocean model forced by the Quick Scatterometer (QuikSCAT) wind stress is used to study the dynamics of the intraseasonal zonal current. The model has realistic upper-ocean currents and thermocline depth variabilities on intraseasonal to interannual scales. The quality of the simulation is directly attributed to the accuracy of the wind forcing. At the equator, moderate westerly winds are punctuated by strong 10–40-day westerly wind bursts. The wind bursts force swift, intraseasonal (20–50 day) eastward equatorial jets in spring, summer, and fall. The zonal momentum balance is between local acceleration, stress, and pressure, while nonlinearity deepens and strengthens the eastward current. The westward pressure force associated with the thermocline deepening toward the east rapidly arrests eastward jets and, subsequently, generates (weak) westward flow. Thus, in accord with direct observations in the east, the spring jet is a single intraseasonal event, there are intraseasonal jets in summer, and the fall jet is long lived but strongly modulated on an intraseasonal scale. The zonal pressure force is almost always westward in the upper 120 m, and changes sign twice a year in the 120–200-m layer. Transient eastward equatorial undercurrents in early spring and late summer are associated with semiannual Rossby waves generated at the eastern boundary following thermocline deepening by the spring and fall jets. An easterly wind stress is not necessary to generate the undercurrents. Experiments with a single westerly wind burst forcing show that apart from the intraseasonal response, the zonal pressure force and current in the east have an intrinsic 90-day time scale that arises purely from equatorial adjustment.
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Reverdin, Gilles, and James Luyten. "Near-Surface Meanders in the Equatorial Indian Ocean." Journal of Physical Oceanography 16, no. 6 (June 1986): 1088–100. http://dx.doi.org/10.1175/1520-0485(1986)016<1088:nsmite>2.0.co;2.
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Chatterjee, A., D. Shankar, J. P. McCreary, and P. N. Vinayachandran. "Yanai waves in the western equatorial Indian Ocean." Journal of Geophysical Research: Oceans 118, no. 3 (March 2013): 1556–70. http://dx.doi.org/10.1002/jgrc.20121.
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Stanger, Gordon, and Abdullahi Majeed. "Extreme rainfall events in the equatorial Indian Ocean." Weather 49, no. 10 (October 1994): 330–37. http://dx.doi.org/10.1002/j.1477-8696.1994.tb05944.x.
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Zhong, Aihong, Harry H. Hendon, and Oscar Alves. "Indian Ocean Variability and Its Association with ENSO in a Global Coupled Model." Journal of Climate 18, no. 17 (September 1, 2005): 3634–49. http://dx.doi.org/10.1175/jcli3493.1.
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Abstract The evolution of the Indian Ocean during El Niño–Southern Oscillation is investigated in a 100-yr integration of an Australian Bureau of Meteorology coupled seasonal forecast model. During El Niño, easterly anomalies are induced across the eastern equatorial Indian Ocean. These act to suppress the equatorial thermocline to the west and elevate it to the east and initially cool (warm) the sea surface temperature (SST) in the east (west). Subsequently, the entire Indian Ocean basin warms, mainly in response to the reduced latent heat flux and enhanced shortwave radiation that is associated with suppressed rainfall. This evolution can be partially explained by the excitation of an intrinsic coupled mode that involves a feedback between anomalous equatorial easterlies and zonal gradients in SST and rainfall. This positive feedback develops in the boreal summer and autumn seasons when the mean thermocline is shallow in the eastern equatorial Indian Ocean in response to trade southeasterlies. This positive feedback diminishes once the climatological surface winds become westerly at the onset of the Australian summer monsoon. ENSO is the leading mechanism that excites this coupled mode, but not all ENSO events are efficient at exciting it. During the typical El Niño (La Niña) event, easterly (westerly) anomalies are not induced until after boreal autumn, which is too late in the annual cycle to instigate strong dynamical coupling. Only those ENSO events that develop early (i.e., before boreal summer) instigate a strong coupled response in the Indian Ocean. The coupled mode can also be initiated in early boreal summer by an equatorward shift of the subtropical ridge in the southern Indian Ocean, which stems from uncoupled extratropical variability.
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Ihara, Chie, Yochanan Kushnir, Mark A. Cane, and Alexey Kaplan. "Timing of El Niño–Related Warming and Indian Summer Monsoon Rainfall." Journal of Climate 21, no. 11 (June 1, 2008): 2711–19. http://dx.doi.org/10.1175/2007jcli1979.1.
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Abstract The relationship between all-India summer monsoon rainfall (ISMR) and the timing of (El Niño–Southern Oscillation) ENSO-related warming/cooling is investigated, using observational data during the period from 1881 to 1998. The analysis of the evolutions of Indo-Pacific sea surface temperature (SST) anomalies suggests that when ISMR is not below normal despite the co-occurrence of an El Niño event, warming over the eastern equatorial Pacific starts from boreal winter and evolves early so that the western-central Pacific and Indian Ocean are warmer than normal during the summer monsoon season. In contrast, when the more usual El Niño–dry ISMR relationship holds, the eastern equatorial Pacific starts warming rapidly only about a season before the reference summer so that the western-central Pacific and Indian Oceans remain cold during the monsoon season.
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Rydbeck, Adam V., and Tommy G. Jensen. "Oceanic Impetus for Convective Onset of the Madden–Julian Oscillation in the Western Indian Ocean." Journal of Climate 30, no. 11 (May 9, 2017): 4299–316. http://dx.doi.org/10.1175/jcli-d-16-0595.1.
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Abstract A theory for intraseasonal atmosphere–ocean–atmosphere feedback is supported whereby oceanic equatorial Rossby waves are partly forced in the eastern Indian Ocean by the Madden–Julian oscillation (MJO), reemerge in the western Indian Ocean ~70 days later, and force large-scale convergence in the atmospheric boundary layer that precedes MJO deep convection. Downwelling equatorial Rossby waves permit high sea surface temperature (SST) and enhance meridional and zonal SST gradients that generate convergent circulations in the atmospheric boundary layer. The magnitude of the SST and SST gradient increases are 0.25°C and 1.5°C Mm−1 (1 megameter is equal to 1000 km), respectively. The atmospheric circulations driven by the SST gradient are estimated to be responsible for up to 45% of the intraseasonal boundary layer convergence observed in the western Indian Ocean. The SST-induced boundary layer convergence maximizes 3–4 days prior to the convective maximum and is hypothesized to serve as a trigger for MJO deep convection. Boundary layer convergence is shown to further augment deep convection by locally increasing boundary layer moisture. Warm SST anomalies facilitated by downwelling equatorial Rossby waves are also associated with increased surface latent heat fluxes that occur after MJO convective onset. Finally, generation of the most robust downwelling equatorial Rossby waves in the western Indian Ocean is shown to have a distinct seasonal distribution.
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Chacko, Neethu, Meer M. Ali, and Mark A. Bourassa. "Impact of Ocean Currents on Wind Stress in the Tropical Indian Ocean." Remote Sensing 14, no. 7 (March 23, 2022): 1547. http://dx.doi.org/10.3390/rs14071547.
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This study examines the effect of surface currents on the bulk algorithm calculation ofwind stress estimated using the scatterometer data during 2007–2020 in the Indian Ocean. In the study region as a whole, the wind stress decreased by 5.4% by including currents in the wind stress equation. The most significant reduction in the wind stress is found along the most energetic regions with strong currents such as Somali Current, Equatorial Jets, and Agulhas retroflection. The highest reduction of 11.5% is observed along the equator where the Equatorial Jets prevail. A sensitivity analysis has been carried out for the study region and for different seasons to assess the relative impact of winds and currents in the estimation of wind stress by changing the winds while keeping the currents constants and vice versa. The inclusion of currents decreased the wind stress (consistent with scatterometer winds) and this decrease is prominent when the currents are stronger. This study showed that the equatorial Indian Ocean is the most sensitive region where the current can impact wind stress estimation. The results showed that uncertainties in the wind stress estimations are quite large at regional levels and hence better representation of wind stress incorporating ocean currents should be considered in the ocean/climatic models for accurate air-sea interaction studies that are not based on remotely sensed winds.
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Hastenrath, Stefan, Dierk Polzin, and Charles Mutai. "Circulation Mechanisms of Kenya Rainfall Anomalies." Journal of Climate 24, no. 2 (January 15, 2011): 404–12. http://dx.doi.org/10.1175/2010jcli3599.1.
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Abstract Expanding earlier studies on the boreal spring and autumn rainy seasons in equatorial East Africa, pending challenges on the mechanisms of rainfall variability, are investigated. Eastward pressure gradient and slack south Indian Ocean trade winds allow surface equatorial westerlies in spring and autumn. Complementing that, upper-tropospheric easterlies are required for the development of a zonal vertical circulation cell along the Indian Ocean equator. Because of the summer warming and high stand of upper-tropospheric topography over South Asia, strong upper-tropospheric easterlies over the tropical northern and equatorial Indian Ocean persist from summer into autumn, thus allowing the development of a zonal vertical circulation cell. By contrast, the winter cooling entails low stand of upper-tropospheric topography in the north, thus hindering easterlies over the equator. Consequently, an equatorial zonal circulation cell does not develop in boreal spring. The equatorial zonal circulation cell, with subsidence over East Africa, strongly controls the boreal autumn rains, as evidenced in their tight correlation with the equatorial westerlies. In a related vein, rain gauge stations show much shared variance in boreal autumn as compared to spring. Plausibly consistent with this, boreal autumn rather than spring has brought the extreme flood and drought disasters in the course of the past half-century.
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Ce Seow, Marvin Xiang, Yushi Morioka, and Tomoki Tozuka. "Roles of Tropical Remote Forcings on the South China Sea Winter Atmospheric and Cold Tongue Variabilities." Journal of Climate 34, no. 10 (May 2021): 4103–18. http://dx.doi.org/10.1175/jcli-d-20-0657.1.
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AbstractInfluences from the tropical Pacific and Indian Oceans and atmospheric internal variability on the South China Sea (SCS) atmospheric circulation and cold tongue (CT) variabilities in boreal winter and the relative roles of remote forcings at interannual time scales are studied using observational data, reanalysis products, and coupled model experiments. In the observation, strong CT years are accompanied by local cyclonic wind anomalies, which are an equatorial Rossby wave response to enhanced convection over the warmer-than-normal western equatorial Pacific associated with La Niña. Also, the cyclonic wind anomalies are an atmospheric Kelvin wave response to diabatic cooling anomalies linked to both the decaying late fall negative Indian Ocean dipole (IOD) and winter atmospheric internal variability. Partially coupled experiments reveal that both the tropical Pacific air–sea coupling and atmospheric internal variability positively contribute to the coupled variability of the SCS CT, while the air–sea coupling over the tropical Indian Ocean weakens such variabilities. The northwest Pacific anticyclonic wind anomalies that usually precede El Niño–Southern Oscillation–independent negative IOD generated under the tropical Indian Ocean air–sea coupling undermine such variabilities.
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LI, Chongyin. "Indian Ocean temperature dipole and SSTA in the equatorial Pacific Ocean." Chinese Science Bulletin 47, no. 3 (2002): 236. http://dx.doi.org/10.1360/02tb9056.
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MURALI, BODDEPALLI, M. S. GIRISH KUMAR, M. RAVICHANDRAN, and G. BHARATHI. "Role of equatorial Indian Ocean convection on the Indian summer monsoon." MAUSAM 72, no. 2 (October 28, 2021): 457–62. http://dx.doi.org/10.54302/mausam.v72i2.610.
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Annamalai, H., Shinichiro Kida, and Jan Hafner. "Potential Impact of the Tropical Indian Ocean–Indonesian Seas on El Niño Characteristics*." Journal of Climate 23, no. 14 (July 15, 2010): 3933–52. http://dx.doi.org/10.1175/2010jcli3396.1.
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Abstract Diagnostics performed with twentieth-century (1861–2000) ensemble integrations of the Geophysical Fluid Dynamics Laboratory Climate Model, version 2.1 (CM2.1) suggest that, during the developing phase, El Niño events that co-occur with the Indian Ocean Dipole Zonal Mode (IODZM; class 1) are stronger than those without (class 2). Also, during class 1 events coherent sea surface temperature (SST) anomalies develop in the Indonesian seas that closely follow the life cycle of IODZM. This study investigates the effect of these regional SST anomalies (equatorial Indian Ocean and Indonesian seas) on the amplitude of the developing El Niño. An examination of class 1 minus class 2 composites suggests two conditions that could lead to a strong El Niño in class 1 events: (i) during January, ocean–atmosphere conditions internal to the equatorial Pacific are favorable for the development of a stronger El Niño and (ii) during May–June, coinciding with the development of regional SST anomalies, an abrupt increase in westerly wind anomalies is noticeable over the equatorial western Pacific with a subsequent increase in thermocline and SST anomalies over the eastern equatorial Pacific. This paper posits the hypothesis that, under favorable conditions in the equatorial Pacific, regional SST anomalies may enable the development of a stronger El Niño. Owing to a wealth of feedbacks in CM2.1, solutions from a linear atmosphere model forced with May–June anomalous precipitation and anomalous SST from selected areas over the equatorial Indo-Pacific are examined. Consistent with our earlier study, the net Kelvin wave response to contrasting tropical Indian Ocean heating anomalies cancels over the equatorial western Pacific. In contrast, Indonesian seas SST anomalies account for about 60%–80% of the westerly wind anomalies over the equatorial western Pacific and also induce anomalous precipitation over the equatorial central Pacific. It is argued that the feedback between the precipitation and circulation anomalies results in an abrupt increase in zonal wind anomalies over the equatorial western Pacific. Encouraged by these results, the authors further examined the processes that cause cold SST anomalies over the Indonesian seas using an ocean model. Sensitivity experiments suggest that local wind anomalies, through stronger surface heat loss and evaporation, and subsurface upwelling are the primary causes. The present results imply that in coupled models, a proper representation of regional air–sea interactions over the equatorial Indo-Pacific warm pool may be important to understand and predict the amplitude of El Niño.
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Prasanna Kumar, S., T. Divya David, P. Byju, J. Narvekar, Kunio Yoneyama, Naoki Nakatani, Akio Ishida, Takanori Horii, Yukio Masumoto, and Keisuke Mizuno. "Bio-physical coupling and ocean dynamics in the central equatorial Indian Ocean during 2006 Indian Ocean Dipole." Geophysical Research Letters 39, no. 14 (July 24, 2012): n/a. http://dx.doi.org/10.1029/2012gl052609.
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Du, Yan, and Yuhong Zhang. "Satellite and Argo Observed Surface Salinity Variations in the Tropical Indian Ocean and Their Association with the Indian Ocean Dipole Mode." Journal of Climate 28, no. 2 (January 15, 2015): 695–713. http://dx.doi.org/10.1175/jcli-d-14-00435.1.
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Abstract This study investigates sea surface salinity (SSS) variations in the tropical Indian Ocean (IO) using the Aquarius/Satelite de Aplicaciones Cientificas-D (SAC-D) and the Soil Moisture and Ocean Salinity (SMOS) satellite data and the Argo observations during July 2010–July 2014. Compared to the Argo observations, the satellite datasets generally provide SSS maps with higher space–time resolution, particularly in the regions where Argo floats are sparse. Both Aquarius and SMOS well captured the SSS variations associated with the Indian Ocean dipole (IOD) mode. Significant SSS changes occurred in the central equatorial IO, along the Java–Sumatra coast, and south of the equatorial IO, due to ocean circulation variations. During the negative IOD events in 2010, 2013, and 2014, westerly wind anomalies strengthened along the equator, weakening coastal upwelling off Java and Sumatra and decreasing SSS. South of the equatorial IO, an anomalous cyclonic gyre changed the tropical circulation, which favored the eastward high-salinity tongue along the equator and the westward low-saline tongue in the south. An upwelling Rossby wave favored the increase of SSS farther to the south. During the positive IOD events in 2011 and 2012, the above-mentioned processes reversed, although the decrease of SSS was weaker in magnitude.
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Watanabe, Masahiro. "Two Regimes of the Equatorial Warm Pool. Part II: Hybrid Coupled GCM Experiments." Journal of Climate 21, no. 14 (July 15, 2008): 3545–60. http://dx.doi.org/10.1175/2007jcli2152.1.
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Abstract In this second of a two-part study, the two regimes in a simple tropical climate model identified in Part I are verified using a hybrid coupled general circulation model (HCM) that can reproduce the observed climatology and the interannual variability reasonably well. Defining a ratio of basin width between the Pacific and Indian Oceans, a series of parameter sweep experiments was conducted with idealized tropical land geometry. Consistent with the simple model, the HCM simulates two distinct states: the split warm pool regime with large vacillation between the two ocean basins and the single warm pool regime representing current climate. The former is suddenly switched to the latter as the Pacific becomes wider than the Indian Ocean. Furthermore, the vacillation in the split regime reveals a preferred transition route that the warm phase in the Pacific follows that in the Indian Ocean. This route occurs due to convectively coupled Kelvin waves that accompany precipitation anomalies over land. Additional experiments show that the inclusion of the idealized Eurasian continent stabilizes the split regime by reducing the Bjerknes feedback in the Indian Ocean, suggesting the atmosphere–ocean–land interaction at work in maintaining the observed warm pool. No difference in cloud feedback was found between two regimes; this feature may, however, be model dependent. Both the simple model and the HCM results suggest that the tropical atmosphere–ocean system inherently involves multiple solutions, which may have an implication on climate modeling as well as on the understanding of the observed mean climate.
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Annamalai, H., and K. R. Sperber. "Regional Heat Sources and the Active and Break Phases of Boreal Summer Intraseasonal (30–50 Day) Variability*." Journal of the Atmospheric Sciences 62, no. 8 (August 1, 2005): 2726–48. http://dx.doi.org/10.1175/jas3504.1.
Full textAbstract:
Abstract The boreal summer intraseasonal variability (BSISV) associated with the 30–50-day mode is represented by the coexistence of three components: poleward propagation of convection over the Indian and tropical west Pacific longitudes and eastward propagation along the equator. The hypothesis that the three components influence each other has been investigated using observed outgoing longwave radiation (OLR), NCEP–NCAR reanalysis, and solutions from an idealized linear model. The null hypothesis is that the three components are mutually independent. Cyclostationary EOF (CsEOF) analysis is applied on filtered OLR to extract the life cycle of the BSISV. The dominant CsEOF mode is significantly tied to the observed spatial rainfall pattern associated with the active/break phases over the Indian subcontinent. The components of the heating patterns from CsEOF analysis serve as prescribed forcings for the dry version of the linear model. This allows one to investigate the possible roles that the regional heat sources and sinks play in driving the large-scale monsoon circulation at various stages of the BSISV life cycle. To understand the interactive nature between convection and circulation, the moist version of the model is forced with intraseasonal SST anomalies. The linear models reproduce the major features of the BSISV seen in the reanalysis. The linear model suggests three new findings: (i) The circulation anomalies that develop as a Rossby wave response to suppressed convection over the equatorial Indian Ocean associated with the previous break phase of the BSISV results in low-level convergence and tropospheric moisture enhancement over the equatorial western Indian Ocean and helps trigger the next active phase of the BSISV. (ii) The development of convection over the tropical west Pacific forces descent anomalies to the west. This, in conjunction with the weakened cross-equatorial flow due to suppressed convective anomalies over the equatorial Indian Ocean, reduces the tropospheric moisture over the Arabian Sea and promotes westerly wind anomalies that do not recurve over India. As a result the low-level cyclonic vorticity shifts from India to Southeast Asia and break conditions are initiated over India. (iii) The circulation anomalies forced by equatorial Indian Ocean convective anomalies significantly influence the active/break phases over the tropical west Pacific. The model solutions support the hypothesis that the three components of the BSISV influence each other but do not imply that such an influence is responsible for the space–time evolution of the BSISV. Further, the applicability of the model results to the observed system is constrained by the assumption that linear interactions are sufficient to address the BSISV and that air–sea interaction and transient forcing are excluded.
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Yuan, Dongliang, and Hailong Liu. "Long-Wave Dynamics of Sea Level Variations during Indian Ocean Dipole Events." Journal of Physical Oceanography 39, no. 5 (May 1, 2009): 1115–32. http://dx.doi.org/10.1175/2008jpo3900.1.
Full textAbstract:
Abstract Long-wave dynamics of the interannual variations of the equatorial Indian Ocean circulation are studied using an ocean general circulation model forced by the assimilated surface winds and heat flux of the European Centre for Medium-Range Weather Forecasts. The simulation has reproduced the sea level anomalies of the Ocean Topography Experiment (TOPEX)/Poseidon altimeter observations well. The equatorial Kelvin and Rossby waves decomposed from the model simulation show that western boundary reflections provide important negative feedbacks to the evolution of the upwelling currents off the Java coast during Indian Ocean dipole (IOD) events. Two downwelling Kelvin wave pulses are generated at the western boundary during IOD events: the first is reflected from the equatorial Rossby waves and the second from the off-equatorial Rossby waves in the southern Indian Ocean. The upwelling in the eastern basin during the 1997–98 IOD event is weakened by the first Kelvin wave pulse and terminated by the second. In comparison, the upwelling during the 1994 IOD event is terminated by the first Kelvin wave pulse because the southeasterly winds off the Java coast are weak at the end of 1994. The atmospheric intraseasonal forcing, which plays an important role in inducing Java upwelling during the early stage of an IOD event, is found to play a minor role in terminating the upwelling off the Java coast because the intraseasonal winds are either weak or absent during the IOD mature phase. The equatorial wave analyses suggest that the upwelling off the Java coast during IOD events is terminated primarily by western boundary reflections.
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Yuan, Dongliang, Hui Zhou, and Xia Zhao. "Interannual Climate Variability over the Tropical Pacific Ocean Induced by the Indian Ocean Dipole through the Indonesian Throughflow." Journal of Climate 26, no. 9 (April 26, 2013): 2845–61. http://dx.doi.org/10.1175/jcli-d-12-00117.1.
Full textAbstract:
Abstract The authors’ previous dynamical study has suggested a link between the Indian and Pacific Ocean interannual climate variations through the transport variations of the Indonesian Throughflow. In this study, the consistency of this oceanic channel link with observations is investigated using correlation analyses of observed ocean temperature, sea surface height, and surface wind data. The analyses show significant lag correlations between the sea surface temperature anomalies (SSTA) in the southeastern tropical Indian Ocean in fall and those in the eastern Pacific cold tongue in the following summer through fall seasons, suggesting potential predictability of ENSO events beyond the period of 1 yr. The dynamics of this teleconnection seem not through the atmospheric bridge, because the wind anomalies in the far western equatorial Pacific in fall have insignificant correlations with the cold tongue anomalies at time lags beyond one season. Correlation analyses between the sea surface height anomalies (SSHA) in the southeastern tropical Indian Ocean and those over the Indo-Pacific basin suggest eastward propagation of the upwelling anomalies from the Indian Ocean into the equatorial Pacific Ocean through the Indonesian Seas. Correlations in the subsurface temperature in the equatorial vertical section of the Pacific Ocean confirm the propagation. In spite of the limitation of the short time series of observations available, the study seems to suggest that the ocean channel connection between the two basins is important for the evolution and predictability of ENSO.