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Artykuły w czasopismach na temat "Equatorial waves"
1
Grise, Kevin M., and David W. J. Thompson. "Equatorial Planetary Waves and Their Signature in Atmospheric Variability." Journal of the Atmospheric Sciences 69, no. 3 (2012): 857–74. http://dx.doi.org/10.1175/jas-d-11-0123.1.
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Abstract Equatorial planetary waves are a fundamental component of the tropical climate system. Previous studies have examined their structure in the climatological-mean circulation, their role in the climatological-mean momentum balance of the tropics, and their contribution to the climatological-mean upwelling across the tropical tropopause. In this study, the authors focus on the contribution of the equatorial planetary waves to variability in the tropical circulation about its climatological-mean state. The equatorial planetary waves that dominate the climatological mean exhibit considerable variability on intraseasonal and interannual time scales. Variability in the amplitude of the equatorial planetary waves is associated with a distinct pattern of equatorially symmetric climate variability that also emerges from empirical orthogonal function analysis of various tropical dynamical fields. Variability in the equatorial planetary waves is characterized by variations in 1) convection in the deep tropics, 2) eddy momentum flux convergence and zonal-mean zonal wind in the tropical upper troposphere, 3) the mean meridional circulation of the tropical and subtropical troposphere, 4) temperatures in the tropical lower stratosphere and subtropical troposphere of both hemispheres, and 5) the amplitude of the upper tropospheric anticyclones over the western tropical Pacific Ocean. It is argued that pulsation of the equatorial planetary waves provides an alternative framework for interpreting the response of the tropical circulation to a range of climate phenomena. Pulsation of the equatorial planetary waves is apparent in association with opposing phases of El Niño–Southern Oscillation and select phases of the Madden–Julian oscillation. Pulsation of the equatorial planetary waves also contributes to variability in measures of the width of the tropical belt.
2
Zhang, K. "On equatorially trapped boundary inertial waves." Journal of Fluid Mechanics 248 (March 1993): 203–17. http://dx.doi.org/10.1017/s0022112093000746.
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Solutions of the Poincaré equation describing equatorially trapped three-dimensional boundary travelling waves in rotating spherical systems are discussed. It is shown that the combined effects of Coriolis forces and spherical curvature enable the equatorial region to form an equatorial waveguide tube with characteristic latitudinal radius (2/m)1/2 and radial radius (1/m), where m is azimuthal wavenumber. Inertial waves with sufficiently simple structure along the axis of rotation and sufficiently small azimuthal wavelength must be trapped in the equatorial waveguide tube. The structure and frequency of the inertial waves are thus hardly affected by the presence of an inner sphere or by the condition of higher latitudes. Further calculations on rotating spherical fluid shells of finite internal viscosity and stressfree boundaries are also discussed.
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Durland, Theodore S., Roger M. Samelson, Dudley B. Chelton, and Roland A. de Szoeke. "Modification of Long Equatorial Rossby Wave Phase Speeds by Zonal Currents." Journal of Physical Oceanography 41, no. 6 (2011): 1077–101. http://dx.doi.org/10.1175/2011jpo4503.1.
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Abstract Previously unaddressed aspects of how equatorial currents affect long Rossby wave phase speeds are investigated using solutions of the shallow-water equations linearized about quasi-realistic currents. Modification of the background potential vorticity (PV) gradient by curvature in the narrow equatorial currents is shown to play a role comparable to the Doppler shift emphasized by previous authors. The important variables are the meridional projections of mean-current features onto relevant aspects of the wave field. As previously shown, Doppler shifting of long Rossby waves is determined by the projection of the mean currents onto the wave’s squared zonal-velocity and pressure fields. PV-gradient modification matters only to the extent that it projects onto the wave field’s squared meridional velocity. Because the zeros of an equatorial wave’s meridional velocity are staggered relative to those of the zonal velocity and pressure, and because the meridional scales of the equatorial currents are similar to those of the low-mode Rossby waves, different parts of the current system dominate the advective and PV-gradient modification effects on a single mode. Since the equatorial symmetry of classical equatorial waves alternates between symmetric and antisymmetric with increasing meridional mode number, the currents produce opposite effects on adjacent modes. Meridional mode 1 is slowed primarily by a combination of eastward advection by the Equatorial Undercurrent (EUC) and the PV-gradient decrease at the peaks of the South Equatorial Current (SEC). The mode-2 phase speed, in contrast, is increased primarily by a combination of westward advection by the SEC and the PV-gradient increase at the core of the EUC. Perturbation solutions are carried to second order in ε, the Rossby number of the mean current, and it is shown that this is necessary to capture the full effect of quasi-realistic current systems, which are asymmetric about the equator. Equatorially symmetric components of the current system affect the phase speed at O(ε), but antisymmetric components of the currents and distortions of the wave structures by the currents do not influence the phase speed until O(ε2).
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McGregor, Shayne, Neil J. Holbrook, and Scott B. Power. "Interdecadal Sea Surface Temperature Variability in the Equatorial Pacific Ocean. Part II: The Role of Equatorial/Off-Equatorial Wind Stresses in a Hybrid Coupled Model." Journal of Climate 21, no. 17 (2008): 4242–56. http://dx.doi.org/10.1175/2008jcli2057.1.
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Abstract Many modeling studies have been carried out to investigate the role of oceanic Rossby waves linking the off-equatorial and equatorial Pacific Ocean. Although the equatorial ocean response to off-equatorial wind stress forcing alone tends to be relatively small, it is clear that off-equatorial oceanic Rossby waves affect equatorial Pacific Ocean variability on interannual through to interdecadal time scales. In the present study, a hybrid coupled model (HCM) of the equatorial Pacific (between 12.5°S and 12.5°N) was developed and is used to estimate the magnitude of equatorial region variability arising from off-equatorial (poleward of 12.5° latitude) wind stress forcing. The HCM utilizes a reduced-gravity ocean shallow-water model and a statistical atmosphere derived from monthly output from a 100-yr Australian Bureau of Meteorology Research Centre (now the Centre for Australian Weather and Climate Research) coupled general circulation model integration. The equatorial region wind stress forcing is found to dominate both the interannual and interdecadal SST variability. The equatorial response to off-equatorial wind stress forcing alone is insufficient to initiate an atmospheric feedback that significantly amplifies the original equatorial region variability. Consequently, the predictability of equatorial region SST anomalies (SSTAs) could be limited to ∼1 yr (the maximum time it takes an oceanic Rossby wave to cross the Pacific Ocean basin in the equatorial region). However, the results also suggest that the addition of off-equatorial wind stress forcing to the HCM leads to variations in equatorial Pacific background SSTA of up to almost one standard deviation. This off-equatorially forced portion of the equatorial SSTA could prove critical for thresholds of El Niño–Southern Oscillation (ENSO) because they can constructively interfere with equatorially forced SSTA of the same sign to produce significant equatorial region ENSO anomalies.
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Kinoshita, Takenari, and Kaoru Sato. "A Formulation of Three-Dimensional Residual Mean Flow and Wave Activity Flux Applicable to Equatorial Waves." Journal of the Atmospheric Sciences 71, no. 9 (2014): 3427–38. http://dx.doi.org/10.1175/jas-d-13-0161.1.
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Abstract The large-scale waves that are known to be trapped around the equator are called equatorial waves. The equatorial waves cause mean zonal wind acceleration related to quasi-biennial and semiannual oscillations. The interaction between equatorial waves and the mean wind has been studied by using the transformed Eulerian mean (TEM) equations in the meridional cross section. However, to examine the three-dimensional (3D) structure of the interaction, the 3D residual mean flow and wave activity flux for the equatorial waves are needed. The 3D residual mean flow is expressed as the sum of the Eulerian mean flow and Stokes drift. The present study derives a formula that is approximately equal to the 3D Stokes drift for equatorial waves on the equatorial beta plane (EQSD). The 3D wave activity flux for equatorial waves whose divergence corresponds to the wave forcing is also derived using the EQSD. It is shown that the meridionally integrated 3D wave activity flux for equatorial waves is proportional to the group velocity of equatorial waves.
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Jouanno, Julien, Frédéric Marin, Yves du Penhoat, and Jean-Marc Molines. "Intraseasonal Modulation of the Surface Cooling in the Gulf of Guinea." Journal of Physical Oceanography 43, no. 2 (2013): 382–401. http://dx.doi.org/10.1175/jpo-d-12-053.1.
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Abstract A regional numerical model of the tropical Atlantic Ocean and observations are analyzed to investigate the intraseasonal fluctuations of the sea surface temperature at the equator in the Gulf of Guinea. Results indicate that the seasonal cooling in this region is significantly shaped by short-duration cooling events caused by wind-forced equatorial waves: mixed Rossby–gravity waves within the 12–20-day period band, inertia–gravity waves with periods below 11 days, and equatorially trapped Kelvin waves with periods between 25 and 40 days. In these different ranges of frequencies, it is shown that the wave-induced horizontal oscillations of the northern front of the mean cold tongue dominate the variations of mixed layer temperature near the equator. But the model mixed layer heat budget also shows that the equatorial waves make a significant contribution to the mixed layer heat budget through modulation of the turbulent cooling, especially above the core of the Equatorial Undercurrent (EUC). The turbulent cooling variability is found to be mainly controlled by the intraseasonal modulation of the vertical shear in the upper ocean. This mechanism is maximum during periods of seasonal cooling, especially in boreal summer, when the surface South Equatorial Current is strongest and between 2°S and the equator, where the presence of the EUC provides a background vertical shear in the upper ocean. It applies for the three types of intraseasonal waves. Inertia–gravity waves also modulate the turbulent heat flux at the equator through vertical displacement of the core of the EUC in response to equatorial divergence and convergence.
7
Back, Amanda, and Joseph A. Biello. "Effect of Overturning Circulation on Long Equatorial Waves: A Low-Frequency Cutoff." Journal of the Atmospheric Sciences 75, no. 5 (2018): 1721–39. http://dx.doi.org/10.1175/jas-d-17-0173.1.
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Zonally long tropical waves in the presence of a large-scale meridional and vertical overturning circulation are studied in an idealized model based on the intraseasonal multiscale moist dynamics (IMMD) theory. The model consists of a system of shallow-water equations describing barotropic and first baroclinic vertical modes coupled to one another by the zonally symmetric, time-independent background circulation. To isolate the effects of the meridional circulation alone, an idealized background flow is chosen to mimic the meridional and vertical components of the flow of the Hadley cell; the background flow meridionally converges and rises at the equator. The resulting linear eigenvalue problem is a generalization of the long-wave-scaled version of Matsuno’s equatorial wave problem with the addition of meridional and vertical advection. The results demonstrate that the meridional circulation couples equatorially trapped baroclinic Rossby waves to planetary, barotropic free Rossby waves. The meridional circulation also causes the Kelvin wave to develop an equatorially trapped barotropic component, imparting a westward-tilted vertical structure to the wave. The total energy of the linear system is positive definite, so all waves are shown to be neutrally stable. A critical layer exists at latitudes where the meridional background flow vanishes, resulting in a minimum frequency cutoff for physically feasible waves. Therefore, linear Matsuno waves with periods longer than the vertical transport time of the meridional circulation do not exist in the equatorial waveguide. This implies a low-frequency cutoff for long equatorial waves.
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CHAO, Winston C. "Chimeric Equatorial Waves as a Better Descriptor for “Convectively-Coupled Equatorial Waves”." Journal of the Meteorological Society of Japan 85, no. 4 (2007): 521–24. http://dx.doi.org/10.2151/jmsj.85.521.
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Zhou, Cheng, and John P. Boyd. "Cross-equatorial structures of equatorially trapped nonlinear Rossby waves." Dynamics of Atmospheres and Oceans 64 (November 2013): 53–61. http://dx.doi.org/10.1016/j.dynatmoce.2013.08.001.
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Grise, Kevin M., and David W. J. Thompson. "On the Signatures of Equatorial and Extratropical Wave Forcing in Tropical Tropopause Layer Temperatures." Journal of the Atmospheric Sciences 70, no. 4 (2013): 1084–102. http://dx.doi.org/10.1175/jas-d-12-0163.1.
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Abstract Temperatures in the tropical tropopause layer (TTL) play an important role in stratosphere–troposphere exchange and in the formation and maintenance of thin cirrus clouds. Many previous studies have examined the contributions of extratropical and equatorial waves to the TTL using coarse-vertical-resolution satellite and reanalysis data. In this study, the authors provide new insight into the role of extratropical and equatorial waves in the TTL using high-vertical-resolution GPS radio occultation data. The results examine the influence of four different wave forcings on the TTL: extratropical waves that propagate vertically into the stratosphere, extratropical waves that propagate meridionally into the subtropical stratosphere, extratropical waves that propagate meridionally into the subtropical troposphere, and the equatorial planetary waves. The vertically and meridionally propagating extratropical stratospheric waves are associated with deep, zonally symmetric temperature anomalies that extend and amplify with height throughout the lower-to-middle tropical stratosphere. In contrast, the extratropical tropospheric waves and the equatorial planetary waves are associated with tropical temperature anomalies that are confined below 20-km altitude. The equatorial planetary waves dominate the zonally asymmetric component of the TTL temperature field, and both the equatorial planetary waves and the extratropical tropospheric waves are linked to large temperature variability in a 1–2-km-deep layer near the tropical tropopause. The fine vertical scale of the TTL temperature features associated with the equatorial planetary waves and the extratropical tropospheric waves is only readily apparent in high-vertical-resolution data.