Strong Meridional Temperature Gradients Research Articles

Antarctic Warm Extremes Across Seasons and Their Response to Advection

AbstractAntarctic warm extremes impact the cryosphere, with very warm extremes driving surface melt on ice shelves. Here, we analyze temperatures exceeding the 90th percentile and the associated circulation patterns and radiation anomalies. ERA5 reanalysis data show positive geopotential height anomalies related to the occurrence of warm extremes. The highest temperature during warm extremes appears on the western periphery of high‐pressure systems, consistent with anticyclonic advection. Temperature anomalies during warm extremes are strongest in winter due to the transport of warm and moist air and a strong meridional temperature gradient. In summer, the weak meridional gradients of top‐of‐atmosphere downward solar radiation flux and surface air temperature contribute to weak temperature anomalies. Warm extremes are associated with positive longwave radiation anomalies in all seasons, but with negative shortwave radiation anomalies at the surface except during polar night. These relationships are verified by station observations. Our results confirm that Antarctic warm extremes are mostly driven by meridional advection of warm air, and suggest that these warm air masses are predominantly moist and cloudy.

Climatology of Clear‐Air Turbulence in Upper Troposphere and Lower Stratosphere in the Northern Hemisphere Using ERA5 Reanalysis Data

AbstractSpatial and temporal distributions of Clear‐Air Turbulence (CAT) in the Northern Hemisphere were investigated using 41 years (1979–2019) of the European Centre for Medium‐range Weather Forecast Reanalysis version 5 (ERA5) data. We used two groups of CAT diagnostics to determine occurrence frequencies: (a) commonly used empirical turbulence indices (TI1, TI2, and TI3) and their components [vertical wind shear (VWS), deformation, ‐divergence, and divergence tendency], and (b) theoretical instability indicators [Richardson number (Ri), potential vorticity (PV), and Brunt‐Vӓisӓlӓ frequency]. The empirical indices showed high frequencies of moderate‐or‐greater (MOG)‐level CAT potential over the East Asian, Eastern Pacific, and Northwestern Atlantic regions in winter. Over East Asia, the entrance region of strong upper‐level jets showed the highest frequencies in TI1, TI2, and TI3 due mainly to strong VWS. The Eastern Pacific and Northwestern Atlantic areas near the exit region of jets had relatively high frequencies of these indices and also Ri. PV frequency was high on the southern side of jet primarily due to negative relative vorticity. Long‐term increasing trends of MOG‐level CAT potential also appeared in those three regions mainly due to warming in lower latitudes. The most significant increasing trend was found over East Asia, due to the strengthening of the East Asian jet and increased VWS due to the strong meridional temperature gradients in the mid‐troposphere induced by warming in the tropics and cooling in eastern Eurasia. These trends over East Asia, if continued, are expected to be of importance to efficient aviation operations across the northwestern Pacific Ocean.

Assessing the Vertical Velocity of the East Pacific ITCZ

AbstractReanalysis and satellite data sets disagree on the vertical structure of the meridional overturning circulation over the East Pacific (EP) intertropical convergence zone (ITCZ). Recent observations from the Organization of Tropical East Pacific Convection (OTREC) 2019 field campaign show a meridional slope of vertical motion with a shallow mode from 3° to 7°N, over a strong meridional sea surface temperature (SST) gradient, and a deeper mode from 7° to 10.5°N, where the SST is predominantly warm. However, reanalysis fields from the OTREC period show a predominant shallow mode from 3° to 9°N unless OTREC dropsondes are assimilated; then a weak deep mode is found at 8°N. Vertical motion over the OTREC region derived from Global Precipitation Measurement (GPM) precipitation radar latent heating retrievals has the opposite problem in that the deep mode is predominant from 3° to 10°N. A shallow mode is present with the addition of cloud radar data.

Diagnostic Analysis of the Generative Mechanism of Extratropical Cyclones in the Northwest Pacific and Northwest Atlantic

We investigated the early-stage development of cyclones occurring in the strong baroclinic regions in the Northwest Pacific and the Northwest Atlantic based on European Center for Medium-range Weather Forecasts Re-Analysis-Interim (ERA-Interim) data. The composite background conditions corresponding to the cyclones on the onset day are characterized by upper troposphere divergence of westerly jet ahead of a trough, low troposphere convergence of westerly jet behind a trough, and strong meridional air temperature gradient (baroclinicity) both in the Northwest Pacific and the Northwest Atlantic, but with stronger baroclinicity in the Northwest Pacific. The composite velocity and temperature fields of the cyclone on the onset day show a clear horizontal front and a westward and northward vertical tilting of cyclonic circulation to the cold zone. The composite Northwest Pacific cyclone filed on the onset day has a warm core, whereas the composite Northwest Atlantic cyclone field has a cold core in the low troposphere. The leading adiabatic processes that contribute to the developing of the cold core cyclone in the Northwest Atlantic on the onset day is the temperature advection, while stronger vertical motion induces stronger adiabatic warming in the Northwest Pacific cyclones, which has a significant contribution to the development of warm core cyclones on the onset day.

On the role of aerosol radiative effect in the wet season onset timing over the Congo rainforest during boreal autumn

Abstract. The boreal summer dry season length is reported to have been increasing in the last 3 decades over the Congo rainforest, which is the second-largest rainforest in the world. In some years, the wet season in boreal autumn starts early, while in others it arrives late. The mechanism behind such a change in the wet season onset date has not been investigated yet. Using multi-satellite data sets, we discover that the variation in aerosols in the dry season plays a major role in determining the subsequent wet season onset. Dry season aerosol optical depth (AOD) influences the strength of the southern African easterly jet (AEJ-S) and, thus, the onset of the wet season. Higher AOD associated with a higher dust mass flux reduces the net downward shortwave radiation and decreases the surface temperature over the Congo rainforest region, leading to a stronger meridional temperature gradient between the rainforest and the Kalahari Desert as early as in June. The latter, in turn, strengthens the AEJ-S, sets in an early and a stronger easterly flow, and leads to a stronger equatorward convergence and an early onset of the wet season in late August to early September. The mean AOD in the dry season over the region is strongly correlated (r=0.7) with the timing of the subsequent wet season onset. Conversely, in low AOD years, the onset of the wet season over the Congo basin is delayed to mid-October.

Improved clouds over Southern Ocean amplify Antarctic precipitation response to ozone depletion in an earth system model

Increasing precipitation on the Antarctic Ice Sheet (AIS) in a warming climate has the potential to partially mitigate Antarctica’s contribution to sea level rise. We show that a simple, physically motivated change to the shallow convective cloud phase in the Community Earth System Model (CESM)—improving a long-standing bias in shortwave cloud forcing over the Southern Ocean—leads to an enhanced response of precipitation when the model is forced with realistic stratospheric ozone depletion, with other radiative forcing remaining constant. We analyze two ozone-forced ensemble experiments with the CESM version 1.1: one using the standard version of the model and the other using the cloud-modified version. The standard version exhibits a precipitation increase on the AIS of 34 gigatons year−1; the cloud-modified version shows an increase of 109 Gt year−1. The cloud-modified version shows a more robust, year-round poleward shift in the westerly jet and storm tracks, which brings more precipitation to the AIS, compared to the standard version. Greater surface warming and larger-amplitude stationary waves further increase the Antarctic precipitation response. The enhanced warming in the cloud-modified version is explained by larger positive shortwave cloud feedbacks, while the enhanced poleward jet shift is associated with a stronger meridional temperature gradient in the upper troposphere—lower stratosphere. These results illustrate (1) the sensitivity of forced changes in Antarctic precipitation to the mean state of a climate model and (2) the strong role of atmospheric dynamics in driving that forced precipitation response.

Synoptic and Large-Scale Determinants of Extreme Austral Frost Events

AbstractWe define and examine extreme frost events at three station locations across southern Australia. A synoptic assessment of the events shows that they are generally characterized by passage of a front or trough followed by a developing blocking high. Frost typically occurs at the leading edge of the block. The very cold air pool leading to the frost event is the result of descent of cold, dry midtropospheric air parcels from regions poleward of the station. The air is exceptionally cold because it is advected across the strong meridional temperature gradients in the storm track. The air is dry because this equatorward meridional pathway requires descent and so must have origins well above the surface in the dryer midtroposphere. The position of the block and location of the dry descent are dynamically determined by large-scale waveguide modes in the polar jet waveguide. The role of the waveguide modes is deduced from composites of midtropospheric flow anomalies over the days preceding and after the frost events. These show organized wavenumber 3 or 4 wave trains, with the block associated with the frost formed as a node of the wave train. The wave trains resemble known waveguide modes such as the Pacific–South America mode, and the frost event projects clearly onto these modes during their life cycle. The strong interannual and decadal variability of extreme frost events at a location can be understood in light of event dependence on organized waveguide modes.

Improving the simulation of the climatology of the East Asian summer monsoon by coupling the Stochastic Multicloud Model to the ECHAM6.3 atmosphere model

The East Asian summer monsoon (EASM) has significant impacts on local and global hydrological and climatic systems. However, most current atmospheric and coupled models have difficulty in capturing the climatological mean state of the EASM. The present study investigated the possible improvement of EASM simulation via coupling the Stochastic Multicloud Model (SMCM) to the state-of-the-art ECHAM6.3 atmosphere model. Evaluated by the pattern correlation coefficient and root-mean-square error, the modified ECHAM6.3, i.e., with SMCM coupling, showed better performance in simulating the EASM in terms of the precipitation pattern and intensity, the pattern of the western North Pacific subtropical high, the monsoon onset, and its seasonal evolution. Analyses also revealed that the modified ECHAM6.3 outperformed the default ECHAM6.3 in terms of large-scale circulation and vertically integrated moisture transport. Finally, it was that the intensified land–sea thermal contrast in both the meridional and zonal directions, as well as the stronger meridional temperature gradient in the upper troposphere and enhanced diabatic heating over the Tibetan Plateau, which are associated with the stochastic cloud, were the main reasons for the better performance of the modified ECHAM6.3 in simulating the EASM.

Magneto–Thermal Evolution of Neutron Stars with Emphasis to Radio Pulsars

The magnetic and thermal evolution of neutron stars is a very complex process with many nonlinear interactions. For a decent understanding of neutron star physics, these evolutions cannot be considered isolated. A brief overview is presented, which describes the main magnetothermal interactions that determine the fate of both isolated neutron stars and accreting ones. Special attention is devoted to the interplay of thermal and magnetic evolution at the polar cap of radio pulsars. There, a strong meridional temperature gradient is maintained over the lifetime of radio pulsars. It may be strong enough to drive thermoelectric magnetic field creation which perpetuate a toroidal magnetic field around the polar cap rim. Such a local field component may amplify and curve the poloidal surface field at the cap, forming a strong and small scale magnetic field as required for the radio emission of pulsars

Equatorial heat accumulation as a long-term trigger of permanent Antarctic ice sheets during the Cenozoic

Growth of the first permanent Antarctic ice sheets at the Eocene-Oligocene Transition (EOT), ∼33.7 million years ago, indicates a major climate shift within long-term Cenozoic cooling. The driving mechanisms that set the stage for this glaciation event are not well constrained, however, owing to large uncertainties in temperature reconstructions during the Eocene, especially at lower latitudes. To address this deficiency, we used recent developments in coccolith biogeochemistry to reconstruct equatorial Atlantic sea surface temperature (SST) and atmospheric pCO2 values from pelagic sequences preceding and spanning the EOT. We found significantly more variability in equatorial SSTs than previously reported, with pronounced cooling from the Early to Middle Eocene and subsequent warming during the Late Eocene. Thus, we show that the Antarctic glaciation at the Eocene-Oligocene boundary was preceded by a period of heat accumulation in the low latitudes, likely focused in a progressively contracting South Atlantic gyre, which contributed to cooling high-latitude austral regions. This prominent redistribution of heat corresponds to the emplacement of a strong meridional temperature gradient that typifies icehouse climate conditions. Our equatorial coccolith-derived geochemical record thus highlights an important period of global climatic and oceanic upheaval, which began 4 million years before the EOT and, superimposed on a long-term pCO2 decline, drove the Earth system toward a glacial tipping point in the Cenozoic.

Role of inertial instability in the West African monsoon jump

AbstractThe West African monsoon jump is a sudden shift in the latitude of the West African precipitation maximum from the Guinean coast near 4°N into Sahel near 12°N in late June or early July. An examination of reanalyses and observations indicates that the Sahel rainy season develops smoothly and the monsoon jump occurs because of an abrupt decrease in Guinean coast rainfall. We show that this abrupt end of the coastal rainy season occurs when inertial instability develops over the region, 1 month later than it develops in the vicinity of the marine Atlantic Intertropical Convergence Zone. The reason for this delay is the presence of the African easterly jet, which places strong negative meridional zonal wind gradients over the coast to preserve the inertially stable environment. When the African easterly jet moves farther north due to the seasonal solar forcing, these gradients weaken and then reverse to satisfy the threshold condition for inertial instability; the rapid end of the Guinean coast rainy season follows. The northward movement and intensity of the African easterly jet are controlled by the seasonal development of strong meridional land surface temperature gradients and are independent of the formation of the Atlantic cold tongue. This explanation for the West African monsoon jump relates the phenomenon to the shape and location of the African continent, including the low‐latitude position of the Guinean coast and the large expanse of the continent to the north.

Systematic winter sea-surface temperature biases in the northern Arabian Sea in HiGEM and the CMIP3 models

Analysis of 20th century simulations of the High resolution Global Environment Model (HiGEM) and the Third Coupled Model Intercomparison Project (CMIP3) models shows that most have a cold sea-surface temperature (SST) bias in the northern Arabian Sea during boreal winter. The association between Arabian Sea SST and the South Asian monsoon has been widely studied in observations and models, with winter cold biases known to be detrimental to rainfall simulation during the subsequent monsoon in coupled general circulation models. However, the causes of these SST biases are not well understood. Indeed this is one of the first papers to address causes of the cold biases. The models show anomalously strong north-easterly winter monsoon winds and cold air temperatures in north-west India, Pakistan and beyond. This leads to the anomalous advection of cold, dry air over the Arabian Sea. The cold land region is also associated with an anomalously strong meridional surface temperature gradient during winter, contributing to the enhanced low-level convergence and excessive precipitation over the western equatorial Indian Ocean seen in many models.

Interaction between Antarctic sea ice and synoptic activity in the circumpolar trough: implications for ice-core interpretation

AbstractInteractions between Antarctic sea ice and synoptic activity in the circumpolar trough have been investigated using meteorological data from European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-analysis and sea-ice data from passive-microwave measurements. Total Antarctic sea-ice extent does not show large interannual variations. However, large differences are observed on a regional/monthly scale, depending on prevailing winds and currents, and thus on the prevailing synoptic situations. the sea-ice edge is also a preferred region for cyclogenesis due to the strong meridional temperature gradient (high baroclinicity) in that area. the motivation for this study was to gain a better understanding of the interaction between sea-ice extent and the general atmospheric flow, particularly the frequency of warm-air intrusions into the interior of the Antarctic continent, since this influences precipitation seasonality and must be taken into account for a correct climatic interpretation of ice cores. Two case studies of extraordinary sea-ice concentration anomalies in relation to the prevailing atmospheric conditions are presented. However, both strong positive and negative anomalies can be related to warm biases in ice cores (indicated by stable-isotope ratios), especially in connection with the negative phase of the Southern Annular Mode.

A Numerical Study of Clear-Air Turbulence (CAT) Encounters over South Korea on 2 April 2007

Abstract On 2 April 2007, nine cases of moderate-or-greater-level clear-air turbulence (CAT) were observed from pilot reports over South Korea during the 6.5 h from 0200 to 0830 UTC. Those CAT events occurred in three different regions of South Korea: the west coast, Jeju Island, and the eastern mountain areas. The characteristics and possible mechanisms of the CAT events in the different regions are investigated using the Weather Research and Forecasting model. The simulation consists of six nested domains focused on the Korean Peninsula, with the finest horizontal grid spacing of 0.37 km. The simulated wind and temperature fields in a 30-km coarse domain are in good agreement with those of the Regional Data Assimilation and Prediction System (RDAPS) analysis data of the Korean Meteorological Administration and observed soundings of operational radiosondes over South Korea. In synoptic features, an upper-level front associated with strong meridional temperature gradients is intensified, and the jet stream passing through the central part of the Korean Peninsula exceeds 70 m s−1. Location and timing of the observed CAT events are reproduced in the finest domains of the simulated results in three different regions. Generation mechanisms of the CAT events revealed in the model results are somewhat different in the three regions. In the west coast area, the tropopause is deeply folded down to about z = 4 km because of the strengthening of an upper-level front, and the maximized vertical wind shear below the jet core produces localized turbulence. In the Jeju Island area, localized mixing and turbulence are generated on the anticyclonic shear side of the enhanced jet, where inertial instability and ageostrophic flow are intensified in the lee side of the convective system. In the eastern mountain area, large-amplitude gravity waves induced by complex terrain propagate vertically and subsequently break down over the lee side of topography, causing localized turbulence. For most of the CAT processes considered, except for the mountain-wave breaking, standard NWP resolutions of tens of kilometers are adequate to capture the CAT events.

Climatology of summer midtropospheric perturbations in the US northern plains. Part II: large-scale effects of the Rocky Mountains on genesis

Propagating convective storms across the US northern plains are often coupled with preexisting midtropospheric perturbations (MPs) initiated over the Rocky Mountains. A companion study (Part I) notes that such MPs occur most commonly at 12 UTC (early morning) and 00 UTC (late afternoon). Using a regional reanalysis and a general circulation model (GCM), this study investigates how such a bimodal distribution of the MP frequency is formed. The results point to two possible mechanisms working together while each has a different timing in terms of maximum effect. The diurnal evolutions between the midtropospheric flows over the Rockies and over the Great Plains are nearly out-of-phase due to inertial oscillation. During the nighttime, the westerly flows at 700–500 mb over the Rockies intensify while flows at the same level over the Great Plains turn easterly. These two flows converge over the eastern Rockies and induce cyclonic vorticity through vortex stretching. After sunrise, the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection, moving it downstream. This process creates a climatological zonally propagating vorticity signal which, in turn, facilitates the early-morning MP genesis at 12 UTC. The analysis also reveals marked dynamic instability conducive to subsynoptic-scale disturbances in the midtroposphere over the Rockies. Strong meridional temperature gradients appear over the north-facing slopes of the Rockies due to terrain heating to the south and the presence of cooler air to the north. This feature, along with persistent vertical shear, creates a Charney–Stern type of instability (i.e. sign changes of the meridional potential vorticity gradient). Meanwhile, the development of terrain boundary layer reduces the Rossby deformation radius which, subsequently, enhances the likelihood for baroclinic short waves. Such effects are most pronounced in the late afternoon and therefore are supportive to the MP genesis around 00 UTC. Examination of GCM experiments with and without orography further supports the critical role of the Rocky Mountains and its associated boundary layer impacts on the formation of MPs.

South Australian rainfall variability and climate extremes

Rainfall extremes over South Australia are connected with broad-scale atmospheric rearrangements associated with strong meridional sea surface temperature (SST) gradients in the eastern Indian Ocean. Thirty-seven years of winter radiosonde data is used to calculate a time series of precipitable water (PW) and convective available potential energy (CAPE) in the atmosphere. Principle component analysis on the parameters of CAPE and PW identify key modes of variability that are spatially and seasonally consistent with tropospheric processes over Australia. The correlation of the leading principle component of winter PW to winter rainfall anomalies reveal the spatial structure of the northwest cloudband and fronts that cross the southern half of the continent during winter. Similarly the second and third principle components, respectively, reveal the structures of the less frequent northern and continental cloudbands with remarkable consistency. 850 hPa-level wind analysis shows that during dry seasons, anomalous offshore flow over the northwest of Australia inhibits advection of moisture into the northwest, while enhanced subsidence from stronger anticyclonic circulation over the southern half of the continent reduces CAPE. This coincides with a southward shift of the subtropical ridge resulting in frontal systems passing well to the south of the continent, thus producing less frequent interaction with moist air advected from the tropics. Wet winters are the reverse, where a weaker meridional pressure gradient to the south of the continent allows rain-bearing fronts to reach lower latitudes. The analysis of SSTs in the Indian Ocean indicate that anomalous warm (cool) waters in the southeast Indian Ocean coincide with a southward (northward) shift in the subtropical ridge during dry (wet) seasons.

Dynamics of the Intertropical Convergence Zone of the East Pacific

Abstract The dynamical factors controlling the mean state and variability of the east Pacific intertropical convergence zone (ITCZ) and the associated cross-equatorial boundary layer flow are investigated using observations from the East Pacific Investigation of Climate (EPIC2001) project. The tropical east Pacific exhibits a southerly boundary layer flow that terminates in the ITCZ. This flow is induced by the strong meridional sea surface temperature (SST) gradient in the region. Away from the equator and from deep convection, it is reasonably well described on a day-to-day basis by an extended Ekman balance model. Variability in the strength and northward extent of this flow is caused by variations in free-tropospheric pressure gradients that either reinforce or oppose the pressure gradient associated with the SST gradient. These free-tropospheric gradients are caused by easterly waves, tropical cyclones, and the Madden–Julian oscillation. Convergence in the boundary layer flow is often assumed to be responsible for destabilizing the atmosphere to deep convection. An alternative hypothesis is that enhanced total surface heat fluxes associated with high SSTs and strong winds act to produce the necessary destabilization. Analysis of the moist entropy budget of the planetary boundary layer shows that, on average, surface fluxes generate over twice the destabilization produced by boundary layer convergence in the east Pacific ITCZ.

Effects of changing baroclinicity on the southern hemisphere extratropical circulation

AbstractExtratropical cyclones of the southern hemisphere provide the primary mechanism of poleward horizontal heat and moisture transport. Their behaviour is influenced by the strength of the meridional temperature gradient in the mid troposphere. As the semi‐annual oscillation (SAO) in the annual cycle of this gradient has become less dominant in recent years, we investigate the impact of modifying the gradient in two simulations with the Arpège general‐circulation model. Model temperatures at 50°S near the 500 hPa level are increased (strengthening the gradient) or decreased (weakening the gradient) in two different simulations T50+ and T50−, respectively.The temperature nudging had a greater impact on the temperature gradient in T50+, but both simulations showed significant surface warming on the Antarctic coast in winter. In T50+, 500 hPa temperature and surface pressure gradients had weaker SAOs, as the circumpolar trough contracted and intensified and cyclone numbers increased along the Antarctic coast. The SAOs strengthened in T50−, and the trough became more disperse, but cyclone numbers increased north of 50°S.Baroclinicity increased (decreased) with a stronger (weaker) meridional temperature gradient, but the static stability of the atmosphere also changed with the opposite impact on baroclinicity. Thus, in T50+ the increase in cyclone numbers was greatest near the Antarctic coast but this was counteracted somewhat by the static stability increase north of 60°S. In T50− cyclone numbers increased north of 50°S because baroclinicity was increased by a stronger meridional temperature gradient and reduced static stability. Hence, the effect on the Antarctic surface climate was nonlinear, with a warmer coast in both cases in winter, and a windier East Antarctica in T50+ but calmer katabatic flow in T50−. Copyright © 2002 Royal Meteorological Society

Traveling waves in the Northern Hemisphere of Mars

Analysis of temperature retrievals from Mars Global Surveyor Thermal Emission Spectrometer data has revealed the presence of regular, eastward propagating waves in the Northern Hemisphere. A large amplitude, zonal wave 1 with a long (∼20 sol) period is particularly prominent during early winter (Ls = 220–270°). After Ls = 270°, a weaker and more rapidly propagating (6.5 sol period) zonal wave 1 is dominant. These waves have a deep vertical structure (>40 km) correlated with the axis of the winter hemisphere westerly jet. Simulations with a Mars general circulation model suggest that the fast wave is associated with baroclinic instability due to the strong meridional temperature gradient at the surface and is consistent with surface pressure oscillations seen in Viking Lander data. By contrast, the slow wave has the appearance of a large‐amplitude Rossby wave that is coupled with an inertially unstable region in the subtropics.

Relative Roles of Elevated Heating and Surface Temperature Gradients in Driving Anomalous Surface Winds over Tropical Oceans

Elevated heating by cumulus convection and sea surface temperature gradients are both thought to contribute to surface winds over tropical oceans. The relative strength and role of each mechanism is examined by imposing forcing derived from data on a linear primitive equation model with idealized parameterizations for the two forcings, and comparing the response with observed surface winds. Two test cases are studied: one related to the El Niño–Southern Oscillation, and the other related to the “dipole” mode in the tropical Atlantic. It is found that in both cases, elevated heating dominates the surface zonal wind response, and contributes significantly to the meridional wind response, especially in the subtropics and the South Pacific and South Atlantic convergence zone regions. Surface temperature gradients dominate the meridional wind forcing in regions near the equator with strong meridional temperature gradients.