Academic literature on the topic 'Quasi-biweekly oscillation'

Abstract Southern China, located in the tropical–subtropical East Asian monsoonal region, presents a unique anticyclonic–cyclonic circulation pattern during extreme heat (EH), obviously different from the typical anticyclone responsible for EH in many other regions. Associated with the evolution of EH in southern China, the anticyclonic–cyclonic anomalies propagate northwestward over the Philippines and southern China. Before the EH onsets, the anticyclonic anomaly dominates southern China, resulting in stronger subsidence over southern China and stronger southerly (southwesterly) flow over the western (northern) margins of southern China. The southerly (southwesterly) flow transports more water vapor to the north of southern China, thus, together with the local stronger subsidence, resulting in drier air condition and accordingly favoring the occurrence of EH. Conversely, after the EH onsets, the cyclonic component approaches southern China and offsets the high temperature. The oscillations of temperature and circulation anomalies over southern China exhibit a periodicity of about 10 days and indicate the influence of a quasi-biweekly oscillation, which originates from the tropical western Pacific and propagates northwestward. Therefore, the 5–25-day-filtered data are extracted to further analyze the quasi-biweekly oscillation. It turns out that the evolution of the filtered circulation remarkably resembles the original anomalies with comparable amplitudes, indicating that the quasi-biweekly oscillation is critical for the occurrence of EH in southern China. The quasi-biweekly oscillation could explain more than 50% of the intraseasonal variance of daily maximum temperature Tmax and vorticity over southern China and 80% of the warming amplitude of EH onsets. The close relationship between the circulation of the quasi-biweekly oscillation and the EH occurrence indicates the possibility of medium-range forecasting for high temperature in southern China.

Abstract The quasi-biweekly oscillation (QBW: here defined as a 12–20-day oscillation) is one of the major systems that affect tropical and subtropical weather and seasonal mean climate. However, knowledge is limited concerning its temporal and spatial structures and dynamics, particularly in a global perspective. To advance understanding of the QBW, its life cycle is documented using a tracking method and extended EOF analysis. Both methods yield consistent results. The analyses reveal a wide variety of QBW activity in terms of initiation, movement, development, and dissipation. The convective anomalies associated with the QBW are predominant in the latitude bands between 10° and 30° in both hemispheres. The QBW modes tend to occur regionally and be associated with monsoons. Three boreal summer modes are identified in the Asia–Pacific, Central America, and subtropical South Pacific regions. Five austral summer modes are identified in the Australia–southwest Pacific, South Africa–Indian Ocean, South America–Atlantic, subtropical North Pacific, and North Atlantic–North Africa regions. The QBW modes are classified into two categories: westward- and eastward-propagating modes. The westward mode is found in the Asia–Pacific and Central America regions during boreal summer; it originates in the tropics and dissipates in the subtropics. The behavior of the westward-propagating mode can be understood in terms of equatorial Rossby waves in the presence of monsoon mean flow and convective coupling. The eastward-propagating mode, on the other hand, connects with upstream extratropical Rossby wave trains and propagates primarily eastward and equatorward. Barotropic Rossby wave trains play an essential role in controlling initiation, development, and propagation of the eastward QBW mode in the subtropics. The results therefore suggest that not only tropical but also extratropical dynamics are required for fully understanding the behavior of the QBW systems worldwide. The new conceptual picture of QBW obtained here based on long-term observation provides valuable information on the behavior of QBW systems in a global perspective, which is important for a thorough understanding of tropical variability on a time scale between day-to-day weather and the Madden–Julian oscillation.

Abstract Persistent heavy rainfall events (PHREs) are the product of the combined effects of multiscale systems. A PHRE that occurred during the 2016 mei-yu season was selected to further the understanding of the scale interactions accounting for the persistence of this type of event. The scale interactions were analyzed quantitatively using a piecewise energy budget based on temporal scale separation. Results show that the strongest interactions between the precipitation-related eddy flow and its background circulation (BC) occur in the mid- to lower troposphere, where a significant downscale kinetic energy (KE) cascade alone dominates eddy flow persistence. An obvious upscale KE cascade (i.e., a feedback effect) appears in the mid- to upper troposphere but has a negligible effect on the BC. Overall, within the precipitation region, the downscale KE cascade is primarily dependent on BC signals with shorter periods, whereas the upscale KE cascade is more dependent on BC signals with longer periods. Thus, the BC has asymmetric effects on the KE cascades. The most significant BC signal as determined via wavelet analysis [i.e., quasi-biweekly (10–18 days) oscillations in this event] does not play the leading role in the downscale KE cascade. Instead, the quasi-weekly oscillations provide the maximum amount of energy for eddy flow maintenance. Semi-idealized simulations of various BC signals show similar results: precipitation and the intensities of lower-level shear lines and transversal troughs (both of which are closely related to the precipitation-related eddy flow) are more sensitive to the quasi-weekly oscillation than to the quasi-biweekly oscillation.

Based on the National Center for Environmental Prediction/National Center for Atmospheric Research reanalysis dataset from 1948 to 2009, this study reveals that global low-frequency oscillation features two major temporal bands. One is a quasi-60-day period known as the intraseasonal oscillation (ISO), and the other is a quasi-15-day period known as the quasi-biweekly oscillation (QBWO). After the mid-1970s, both the ISO and QBWO become intensified and more active, and these changes are equivalently barotropic. The primitive barotropic equations are adopted to study the involved mechanism. It reveals that the e-folding time of the least stable modes of both the ISO and QWBO becomes shorter if the model is solved under the atmospheric basic state after the mid-1970s than if solved under the basic state before the mid-1970s. This result suggests that the atmospheric basic flow after the mid-1970s facilitates a more rapid growth of the ISO and QBWO, and thereby an intensification of the low-frequency oscillations at the two bands.

Persistent pollution often occurs in North China in winter. The study of the sub-seasonal evolution characteristics of fine particles (PM2.5) can provide a theoretical basis for the prediction and prevention of persistent pollution. Based on the high-resolution gridded data of PM2.5 and NCEP/NCAR reanalysis, the sub-seasonal variation in PM2.5 in North China in winter and its dominant circulation patterns from 1960/61 to 2019/20 were analyzed. The results show that, in winter, PM2.5 in North China shows a dominant period of 10–20 days, and persistent heavy pollution occurs at the active phase of oscillation. Based on the PM2.5 quasi-biweekly oscillation (QBWO) events, the 850 hPa wave train can be classified into four categories. It was found that, during the active phase of PM2.5 QBWO, the wind speed is weak and humidity is high in the low-troposphere for all of the four event types, while the quasi-biweekly 850 hPa wave train and the track of geopotential height anomaly are significantly different. Based on the characteristics of circulation evolution, these four types of events can be named as eastward, split southward, southeastward, and merged event. The energy conversion between the basic flow and the quasi-biweekly disturbance, and the mean flow difference are responsible for the circulation diversity for different PM2.5 QBWO events. The above research results can provide a theoretical basis for pollutant prediction.

Abstract This study investigates the characteristics and climate impacts of the quasi-biweekly oscillation (QBWO) over the western North Pacific (WNP) in boreal winter based on observational and reanalysis data and numerical experiments with a simplified model. The wintertime convection over the WNP is dominated by significant biweekly variability with a 10–20-day period, which explains about 66% of the intraseasonal variability. Its leading mode on the biweekly time scale is a northwestward-propagating convection dipole over the WNP, which oscillates over a period of about 12 days. When the convection-active center of this QBWO is located to the east of the Philippines, it can generate an anticyclonic vorticity source to the south of Japan via inducing upper-tropospheric divergence and excite a Rossby wave train propagating toward North America along the Pacific rim. The resultant lower-tropospheric circulation facilitates cold advection and leads to cold anomalies over central North America in the following week. This result highlights a cause–effect relationship between the WNP convection and the North American climate on the quasi-biweekly time scale and may provide some prediction potential for the North American climate. Significance Statement This study establishes a cause–effect relationship between the wintertime western North Pacific convection and the central North American air temperature on the quasi-biweekly time scale. In boreal winter, the convection over the western North Pacific oscillates significantly with a 10–20-day period. When the convection is active, it can disturb the atmosphere to the south of Japan and excite a midlatitude Rossby wave train. The latter propagates along the North Pacific rim and leads to cold spells over central North America within one week. This information connects the climate variability across the Pacific and provides an additional subseasonal-to-seasonal prediction potential for the North American winter climate.

Abstract Low-frequency intraseasonal oscillations in the tropical atmosphere in general circulation models (GCMs) were studied extensively in many previous studies. However, the simulation of the quasi-biweekly oscillation (QBWO), which is an important component of the intraseasonal oscillations, in GCMs has not received much attention. This paper evaluates the QBWO features over the South China Sea in early [May–June (MJ)] and late [August–September (AS)] summer in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, version 5.3 (CAM5), using observations and reanalysis data. Results show that the major features of the spatial distribution of the QBWO in both MJ and AS are simulated reasonably well by the model, although the amplitude of the variation is overestimated. CAM5 captures the local oscillation in MJ and the westward propagation in AS of the QBWO. Although there are important biases in geographical location and intensity in MJ, the model represents the QBWO horizontal and vertical structure qualitatively well in AS. The diagnosis of the eddy vorticity budget is conducted to better understand the QBWO activities in the model. Both horizontal advection of relative vorticity and that of planetary vorticity (Coriolis parameter) are important for the local evolution of the QBWO in MJ in observations as well as model simulation, whereas advection of planetary vorticity contributes to the westward propagation of QBWO vorticity anomalies in AS. Since the Coriolis parameter f only changes with latitude, this suggests that the correct simulation of anomalous meridional wind is a key factor in the realistic simulation of the QBWO in the model.

Using ERA-interim Reanalysis data and observational data, the intraseasonal oscillation of the winter rainfall in southern China is studied. The mean square deviation of daily precipitation is used to express precipitation variability, and winter precipitation variability over southern China is determined to be highly correlated with sea surface temperature (SST) in central and eastern tropical Pacific; the dominant period of the precipitation is 10–30 days, which reflects quasi-biweekly oscillation. Examination of 1000 hPa geopotential height suggests that key low-pressure systems affecting the intraseasonal precipitation come from Lake Baikal, but with different travel paths. In El Niño years, key low-pressure systems converge with other low-pressure systems and move southeastward until reaching South China, while in La Niña years, only one low-pressure system can reach southern China. Meanwhile, the explosive development of the low-pressure system is mainly caused by the joint effects of thermal advection and vorticity advection in El Niño, and only vorticity advection accounted for the dominant status in La Niña. Multiscale analysis shows that the meridional distribution of intraseasonal circulation plays an important role on the thermal transmission and brings strong warm advection from low latitudes to high latitudes in El Niño.

碩士
國立臺灣大學
大氣科學研究所
83
Quasi-biweekly oscillation is the feature of low frequency oscillation in the atmosphere and was found in early study of monsoon systems. Precipitation in India has oscillations with periods of 10 to 20 days. Band-passed filter, one point correlation, and singular value decomposition(SVD) are applied to a 7-summer dataset consisting of OLR, 200-mb height, 500-mb height, 850-mb height with seasonal trend being removed over Asia. Sumatra and Yangtze River are regions where quasi-biweekly oscillation appears more clearly in the OLR field and height field in the western Pacific possesses quasi-biweekly oscilla- tion, too. The couple mode between OLR field and 500-mb height field has the following feature: oscillation in OLR over western Pacific, Sumatra, and middle Asia is in phase; oscillation in Korea, eastern China, Indochina, and southern South China Sea is in phase; while oscillations over these two regions are out of phase. Oscillations in 500-mb height field over equatorial western Pacific, southeast China, South China Sea, and Indo- china are in phase and which is out of phase with higher lati- tudes in the northeast and northwest. 500-mb height over southeast China, Indochina, and South China Sea increases after convection over eastern China, Indo- china, and South China Sea increases for 1 to 5 days. And conv- ection over south China, Indochina, and South China Sea decre- ases at the same time or after 500-mb height over southeast China, Indochina, and South China Sea increases for 1 to 3 days.

The Intraseasonal Oscillation (ISO) plays an important role to modulate deep convective activity in the tropical region. In this thesis, I aim to understand the role of land and warm oceans in ISO, using a general circulation model. For this, I conduct a series of experiments in the Community Atmosphere Model (CAM) with various idealized and realistic surface boundary conditions to study tropical ISO. To investigate the influence of tropical sea surface temperature (SST) on ISO and convectively coupled equatorial waves in the global atmosphere, I conduct experiments with idealized, zonally symmetric SST profiles having different widths of warm ocean centered at the equator. I use the model in its basic “Aquaplanet” configuration, with the sun at the equator, i.e. perpetual spring equinox forcing; with idealized zonally symmetric SST, the aquaplanet model produces a double Intertropical Convergence Zone (ITCZ) on either side of the equator, and an eastward propagating Madden Julian oscillation (MJO)like mode with variance at intraseasonal (30 to 96 day) periods and zonal wavenumber one. In the experiment with the narrowest meridional width of warm SST, the variance of moist convective activity lies predominantly in equatorially trapped Kelvin wave band. As the width of the warm equatorial SST is increased, the eastward propagating speed of the MJO-like signal decreases; for the broadest SST profile (warm SST covering 20 degrees of latitude), the speed of the model MJO is about 5.5 m s−1, close to the observed speed. This is because the latitudinal extent of warm SST is comparable to the equatorial Rossby radius, and the model produces off equatorial Rossby waves of sufficient strength to interact with the Kelvin wave and slow down the MJO-like mode. The model also generates westward propagating waves with intraseasonal periods and zonal wavenumber 1–3; the structure of these signals, which extend well into the mid-latitudes, projects onto equatorially trapped Rossby waves with meridional mode numbers 1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the model generates 30–80 day westward moving signals with zonal wavenumber 4–7, particularly in the experiment with a narrow region of warm SST. Although these waves are seen in the wavenumber-frequency spectra in the equatorial region, they have the largest amplitude in the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting, westward propagating Rossby waves comprise a large portion of tropical intraseasonal variability. In the observations (ERA-Interim daily reanalysis), the MJO signal lies in the range of zonal wavenumbers 1 to 5. The variance of MJO at higher wavenumbers (2–5) is absent in the aquaplanet model. For this, I design model experiments in order to study how model MJO responds to the introduction of continents in the presence of zonally symmetric SST, and a realistic SST distribution with the Indo-Pacific warm pool and cool SST in the eastern Pacific. As before, the model is in the aquaplanet-like configuration, to eliminate the effects of seasonality. Model results are compared with 21 years (1995–2015) ERA-Interim reanalysis data and analyzed in terms of the moist static energy (MSE) budget to study the growth and propagation of MJO. When I introduce continents with realistic orography and interactive surface temperature, soil moisture, and albedo, the variance of model MJO is reduced due to weaker boundary layer moisture convergence. However,MJO variance extends to higher wavenumbers. With prescribed climatological January SST boundary condition in the presence of continents, the variance of model MJO is enhanced by a factor of 2–3, and it is distributed across zonal wavenumbers 1 to 5, in closer agreement with observations. Thus, I find that the presence of land by itself is not enough to produce realistic MJO in CAM, but realistic SST distribution is also necessary to simulate MJO with improved spacetime characteristics. Both in simulations and ERA-Interim data, column-integrated longwave radiation plays a key role in the growth of MSE anomaly associated with MJO; in general, meridional and vertical advection of MSE both acts to promote eastward movement of MJO. In the model experiments, meridional advection of low-level MSE anomaly is most significant in the vicinity of the ITCZ. This indicates that the physical processes which determine the location of (single or double) ITCZ are linked to MJO dynamics. The westward propagating “quasi-biweekly” oscillation (QBWO) with 10–25 day period is an important intraseasonal mode of the Asian summer monsoon, yet very few model studies focus on this mode. I study QBWO in the northern and southern tropics in the model and compare it with ERA-Interim reanalysis data. The pure aquaplanet model produces a double Intertropical Convergence Zone (ITCZ), winds that are predominantly zonal, and weak quasi-biweekly variance. When continents are introduced in the model with zonally symmetric SST, the northern ITCZ, as well as quasi-biweekly variance between 10◦N to 24◦N are strengthened in the Pacific Ocean, bringing model results closer to observations. In the model with continents, the QBWO signal dwells inside the mean envelope of high atmospheric moisture, or total precipitable water (TPW), in agreement with observations. However, in the presence of zonally symmetric SST, the model fails to simulate sufficiently high precipitable water in the region extending from the north Indian Ocean to East Asia, resulting in very weak QBWO variance. When the model includes continents and realistic (January) SST boundary conditions, the spatial structure of both TPW and QBWO variance becomes more realistic. I study the mechanisms of propagation and maintenance of the quasi-biweekly mode using vorticity budget and moist static energy (MSE) budget analysis. Advection due to the effect is responsible for the northwestward propagation of QBWO vorticity, while the propagation of column MSE anomaly is mainly due to horizontal advection. Surface turbulent heat fluxes and vertical MSE advection are the dominant contributors to the growth and maintenance of column MSE anomaly in observations and model respectively. Surface heat flux makes a significant contribution to the growth of quasi-biweekly MSE anomaly in the presence of land, in association with the enhanced meridional wind, and vortical structures that resemble moist Rossby waves with a wavelength of about 4000 kilometers.