Meridional Sea Surface Temperature Gradient Research Articles

The Role of the Aleutian Low in the Relationship between Spring Pacific Meridional Mode and Following ENSO

Abstract The spring Pacific meridional mode (PMM) is an important precursor of El Niño–Southern Oscillation (ENSO). However, recent studies reported that only about half of the spring PMM events were followed by ENSO events. This study examines the role of internal climate variability in modulating the impact of PMM on ENSO using 100-member ensemble simulations of the Max Planck Institute Earth System Model (MPI-ESM). The relationship between spring PMM and following winter ENSO shows a large spread among the 100 members. The variation of spring Aleutian low (AL) intensity is identified to be an important factor modulating the PMM–ENSO relation. The spring AL affects the PMM–ENSO relationship by modifying PMM-generated low-level zonal wind anomalies over the tropical western Pacific. The strengthening of the spring AL is accompanied by westerly wind anomalies over the midlatitude northwestern Pacific, leading to sea surface temperature (SST) cooling there via an enhancement of upward surface heat flux. This results in increased meridional SST gradient and leads to northerly wind anomalies over the subtropical northwestern Pacific, which turn to surface westerly wind anomalies after reaching the equatorial western Pacific due to the conservation of potential vorticity. Thus, the low-level westerly (easterly) wind anomalies over the tropical western Pacific associated with the positive (negative) spring PMM were strengthened (weakened), which further contributes to an enhanced (a weakened) PMM–ENSO relation. The mechanism for the modulation of the AL on the spring PMM–ENSO relationship is verified by a set of AGCM simulations. This study suggests that the condition of the spring AL should be considered when predicting ENSO on the basis of the PMM. Significance Statement Spring Pacific meridional mode (PMM) is a predictor of ENSO, but not all spring PMM events are accompanied by the occurrence of ENSO events. This study aims to explore the influence of internal climate variability on the relationship between spring PMM and following ENSO. It is revealed that the Aleutian low exerts a crucial modulation on the spring PMM–ENSO relationship. The underlying physical mechanisms for the impact of the Aleutian low on the relationship between spring PMM and ENSO are further examined. The results of this study have important implications for improving the prediction of ENSO.

Synoptic Moisture Intrusion Provided Heavy Isotope Precipitations in Inland Antarctica During the Last Glacial Maximum

AbstractStable water isotopes in inland Antarctic ice cores are powerful paleoclimate proxies; however, their relationship with dynamical atmospheric circulations remains controversial. Using a water isotope climate model (MIROC5‐iso), we assessed the influence of the Last Glacial Maximum (LGM; ∼21,000 years ago) sea surface temperatures (SST) and sea ice (SIC) on Antarctic precipitation isotopes (δ18Op) through atmospheric circulation. The results revealed that the synoptic circulation mostly maintained southward moisture transport, reaching inland Antarctica. The steepened meridional SST gradient in the mid‐latitudes increased δ18Op in inland Antarctica with the enhanced baroclinic instability and synoptic moisture transport. In contrast, expanded SIC distribution decreased δ18Op over Antarctica by enhanced preferential removal of heavy isotopes during vapor transport due to the increased transport distance and enhanced surface cooling. These findings propose to use Antarctic ice cores to describe the southern hemisphere atmospheric circulation, represented by the westerly jets, during the LGM and other past climates.

Multiyear variability of cloud genera in Krakow in the context of changes in the thermal state of the North Atlantic

AbstractThe analysis conducted in this study examines the relationships between changes in cloud cover and the occurrence of cloud genera in Krakow, Poland, and the variations in the thermal state of the North Atlantic (NA) from 1951 to 2020. The existence of areas where annual sea surface temperature (SST) changes exhibit statistically significant correlations with the annual frequency of specific cloud genera was observed. These relationships vary in space: An increase in SST over the NA, particularly in the western and central regions of the subtropical NA, leads to a decrease in the frequency of stratiform clouds at all levels (Cs, As, Ns and St) and an increase in the frequency of convective clouds (Cu, Sc). An attempt to explain this phenomenon demonstrates that there is a correlation between the frequency of specific cloud genera and the variability of meridional SST gradients, as well as changes in the intensity of the thermohaline circulation in the NA (NA THC), which control the variability of mid‐tropospheric circulation (500 hPa). During positive phases of NA THC (the “warm” state of the NA), zonal circulation prevails over Europe, leading to an increase in the height of h500, an increase in the frequency of anticyclonic weather and a decrease in the frequency of cyclonic weather, including a significant proportion of frontal weather systems. Consequently, there is a reduction in frequency of stratiform clouds and an increase in the occurrence of vertically developed convective clouds, thereby increasing the possibility of observing middle‐level (Ac) and high‐level (Ci, Cc) clouds. In negative phases of NA THC (the “cool” state of the NA), the situation is reversed, with meridional circulation dominating over Europe, h500 lowering, an increase in the frequency of cyclonic systems with fronts, and an increase in the frequency of stratiform clouds. This results in decreased sunshine duration and a reduction in the amount of solar energy reaching the Earth's surface.

Modulation of Equatorial Currents and Tropical Instability Waves During the 2021 Atlantic Niño

AbstractIn the boreal summer of 2021, the equatorial Atlantic experienced the strongest warm event, that is, Atlantic Niño, since the beginning of satellite observations in the 1970s. Such events have far‐reaching impacts on large‐scale wind patterns and rainfall over the surrounding continents. Yet, developing a paradigm of how Atlantic Niño interacts with the upper‐ocean currents and intraseasonal waves remains elusive. Here we show that the equatorial Kelvin wave associated with the onset of the 2021 Atlantic Niño modulated both the background flow and the eddy flux of the equatorial upper‐ocean circulation, causing an extremely weak and delayed tropical instability wave (TIW) season. TIW‐induced variations of sea surface temperature (SST), sea surface salinity, sea surface height, and eddy temperature advection were exceptionally weak during May to July, the climatological peak of TIW activity, but rebounded in August when higher than normal variability was observed. Moored velocity data at 23°W show that during the peak of the 2021 Atlantic Niño from June to August, the Equatorial Undercurrent was deeper and stronger than usual. An anomalously weak eddy momentum flux strongly suppressed barotropic energy conversion north of the equator from May to July, likely contributing to low TIW activity. Reduced baroclinic energy conversion also might have played a role, as the meridional gradient of SST was sharply reduced during the Atlantic Niño. Despite extremely weak TIW velocities, modest intraseasonal variability of chlorophyll‐a (Chl‐a) was observed during the Atlantic Niño, due to pronounced meridional Chl‐a gradients that partly compensated for the weak TIWs.

Southward Shift and Intensification of the Intertropical Convergence Zone in the North Pacific Across the Mid‐Pleistocene Transition

AbstractThe Hadley Circulation and associated westerlies strengthened and moved equatorward across the mid‐Pleistocene transition (MPT). However, the evolution of the intertropical convergence zone (ITCZ) is still elusive due to the scarcity of long‐term hydrological records from regions sensitive to the ITCZ change. Here, high‐resolution sea surface salinity estimates derived from surface‐dwelling planktic foraminiferal δ18O and Mg/Ca in Ocean Drilling Program Site 871 reveal a long‐term freshening trend in the central equatorial Pacific across the MPT. We attribute this secular reorganization of the precipitation‐evaporation balance to the gradual southward migration and intensification of ITCZ in the North Pacific. It is inferred that the long‐term evolution of the ITCZ was modulated by the increased meridional sea surface temperature gradients and the enhancements of trade winds across the MPT.

On the Influence of the Bay of Bengal’s Sea Surface Temperature Gradients on Rainfall of the South Asian Monsoon

Abstract The southwest monsoon delivers over 70% of India’s annual rainfall and is crucial to the success of agriculture across much of South Asia. Monsoon precipitation is known to be sensitive to sea surface temperature (SST) in the Bay of Bengal (BoB). Here, we use a configuration of the Unified Model of the Met Office coupled to an ocean mixed layer model to investigate the role of upper-ocean features in the BoB on southwest monsoon precipitation. We focus on the pronounced zonal and meridional SST gradients characteristic of the BoB; the zonal gradient in particular has an as-yet unknown effect on monsoon rainfall. We find that the zonal SST gradient is responsible for a 50% decrease in rainfall over the southern BoB (approximately 5 mm day−1), and a 50% increase in rainfall over Bangladesh and northern India (approximately 1 mm day−1). This increase is remotely forced by a strengthening of the monsoon Hadley circulation. The meridional SST gradient acts to decrease precipitation over the BoB itself, similarly to the zonal SST gradient, but does not have comparable effects over land. The impacts of barrier layers and high-salinity subsurface water are also investigated, but neither has significant effects on monsoon precipitation in this model; the influence of barrier layers on precipitation is felt in the months after the southwest monsoon. Models should accurately represent oceanic processes that directly influence BoB SST, such as the BoB cold pool, in order to faithfully represent monsoon rainfall.

Effects of paleogeographic changes and CO2 variability on northern mid-latitudinal temperature gradients in the Cretaceous

The Cretaceous ‘greenhouse’ period (~145 to ~66 million years ago, Ma) in Earth’s history is relatively well documented by multiple paleoproxy records, which indicate that the meridional sea surface temperature (SST) gradient increased (non-monotonically) from the Valanginian (~135 Ma) to the Maastrichtian (~68 Ma). Changes in atmospheric CO2 concentration, solar constant, and paleogeography are the primary drivers of variations in the spatiotemporal distribution of SST. However, the particular contribution of each of these drivers (and their underlying mechanisms) to changes in the SST distribution remains poorly understood. Here we use data from a suite of paleoclimate simulations to compare the relative effects of atmospheric CO2 variability and paleogeographic changes on mid-latitudinal SST gradient through the Cretaceous. Further, we use a fundamental model of wind-driven ocean gyres to quantify how changes in the Northern Hemisphere paleogeography weaken the circulation in subtropical ocean gyres, leading to an increase in extratropical SSTs.

Modulation of the Oceanic Mesoscale Activity by the Mesoscale Thermal Feedback to the Atmosphere

Abstract Ocean mesoscale thermal feedback (TFB) is the influence of mesoscale sea surface temperature (SST) anomalies on the overlying atmosphere and its feedback to the ocean. Over the past few decades, TFB has been shown to affect the atmosphere by inducing low-level wind and surface stress anomalies and modulating ocean–atmosphere heat fluxes ubiquitously over the global oceans. These anomalies can alter the climate variability. However, it is not clear yet to what extent heat and momentum flux anomalies modulate the mesoscale ocean activity. Here, using coupled ocean–atmosphere mesoscale simulations over a realistic subtropical channel centered on the equator in which the TFB can be turned off by spatially smoothing the SST as seen by the atmosphere, we show that TFB can damp the mesoscale activity, with a more pronounced effect near the surface. This damping appears to be sensitive to the cutoff filter used: on average, the surface mesoscale activity is attenuated by 9% when smoothing the SST using an ∼1000-km cutoff but by only 2% when using an ∼350-km cutoff. We demonstrate that the mesoscale activity damping is primarily caused by a sink of available eddy potential energy that is controlled by the induced-anomalous heat fluxes, the surface stress anomalies having a negligible role. When TFB is neglected, the absence of sink of potential energy is partly compensated by a more negative eddy wind work. We illustrate that TFB filtering in a coupled model must be done carefully because it can also impact the large-scale meridional SST gradients and subsequently the mean large-scale wind stress curl and ocean dynamics.

Impacts of Sea Surface Temperature Gradient on Monsoon Intraseasonal Oscillation

Abstract The northward-propagating monsoon intraseasonal oscillation (MISO) is the most pronounced variability over the tropical Indian Ocean during the Indian summer monsoon (June–September). MISO is accompanied by significant air–sea interactions; however, the mechanism of the oceanic feedback to MISO is still a great scientific challenge. In this study, the role of the intraseasonal sea surface temperature (SST) gradient in MISO is diagnosed using reanalysis products and model sensitivity experiments. It is found that the positive meridional gradient of intraseasonal SST induces positive wind convergence in the planetary boundary layer (PBL) and leads convection by about 1–2 days. This accounts for approximately half of the total wind convergence in the PBL to the north of convection during MISO. The warm SST anomalies before convection accelerate the intraseasonal northerly wind in the PBL due to the enhanced downward transport of momentum from aloft. By contrast, the cold SST anomalies behind deep convection weaken the downward vertical momentum transport, thereby inducing a deceleration in the intraseasonal northerly. Consequently, changes in the speed of intraseasonal northerly along the meridional direction strengthen the PBL wind convergence ahead of deep convection. It finally results in the intensification of intraseasonal rainfall associated with MISO over the summer monsoon region. In addition, a strong (weak) SST meridional gradient at an intraseasonal time scale amplifies (lessens) the MISO intensity in the model simulation. Thus, this study highlights the role of SST meridional gradient in the feedback to MISO, which differs from the weak contribution of the warm SST itself to the wind convergence mentioned in previous studies. Collectively, these findings indicate that consideration of oceanic feedback is necessary to improve the understanding and simulation of MISO.

Stage-dependent influence of PDO on interdecadal summer precipitation anomalies in eastern China

Pacific decadal oscillation (PDO) has been regarded as one of the most important interdecadal factors to the interdecadal summer precipitation in eastern China, which, however, cannot keep a consistent pattern during the same positive or negative PDO phase. Because sea surface temperature (SST) anomalies vary along with PDO evolvement, the PDO impacts are investigated in terms of developing and decaying stages of its positive and negative phases, respectively. It is found that the SST cooling in mid-latitude North Pacific and SST warming in off-equator central-eastern Pacific are generally intensified from the developing stage to the decaying stage for positive PDO phase, and vice versa. Particularly, during the decaying stage of positive (negative) PDO phase, the enhanced SST warming (cooling) in northeastern tropical Pacific triggers a stronger (weaker) large-scale Walker circulation and suppresses (enhances) the convective activities in western tropical Pacific, which favors a meridional wave train like East Asia–Pacific (EAP) teleconnection over East Asia. Meanwhile, the westward extended SST cooling (warming) in mid-latitude North Pacific together with the SST warming (cooling) to its south significantly increases (decreases) the meridional SST gradient and low-level atmospheric baroclinicity, conductive to the strengthened (weakened) and southward (northward) shifted East Asian subtropical westerly jet as well as an equivalent barotropic “cold trough” in western North Pacific. Because of such tropical and extratropical dynamical processes, a meridional tripole pattern of precipitation anomalies occurs in eastern China which dominates the PDO impact in the same phase and distinctly different from the dipole pattern during the developing stage.

Decadal intensified and slantwise Subpolar Front in the Japan/East Sea

The Subpolar Front in the Japan/East Sea (JES) could far-reaching influence the atmospheric processes over the downstream regions. However its variability on decadal timescale remains less understood. In this study, the decadal trends in the intensity and position of the SPF in the JES during the time period 1985−2020 are analyzed by using four categories of satellite observed high-resolution sea surface temperature products. The results show that there is a significant intensification trend of the SPF at a rate of 0.37°C/100km/decade. The SPF is further divided into three regions based on the meridional sea surface temperature gradient (MSSTG): the eastern (135−138°E), central (130−135°E) and western (128−130°E) regions, respectively. These three regions showed different meridional movements with the eastern SPF moving poleward by 0.08°/decade, the central SPF moving equatorward by −0.11°/decade and the western SPF showing no significant displacements. The reverse meridional movements between the central and eastern SPF increased its skewness. The frontogenesis rate equation is employed to identify the mechanisms of these decadal trends. Results show that the geostrophic advection term, especially its zonal component, had a crucial role in the decadal trends of the intensity and position of the central and eastern SPF. The decadal trend of the central SPF was mainly attributed to the zonal geostrophic advection of the MSSTG associated with the enhancement of the Subpolar Front Current (SFC) in the upstream region, whereas the decadal trend in the eastern SPF was mainly driven by the zonal geostrophic shear advection controlled by the shear of the SFC in the downstream region. Before 2002, the eastern SPF moved poleward at a rate of 0.27°/decade, whereas there was no obvious trend after 2002. Further decomposition showed that this shift was caused by meridional Ekman advection of the MSSTG.

Mitigating Climate Biases in the Midlatitude North Atlantic by Increasing Model Resolution: SST Gradients and Their Relation to Blocking and the Jet

Abstract Starting to resolve the oceanic mesoscale in climate models is a step change in model fidelity. This study examines how certain obstinate biases in the midlatitude North Atlantic respond to increasing resolution (from 1° to 0.25° in the ocean) and how such biases in sea surface temperature (SST) affect the atmosphere. Using a multimodel ensemble of historical climate simulations run at different horizontal resolutions, it is shown that a severe cold SST bias in the central North Atlantic, common to many ocean models, is significantly reduced with increasing resolution. The associated bias in the time-mean meridional SST gradient is shown to relate to a positive bias in low-level baroclinicity, while the cold SST bias causes biases also in static stability and diabatic heating in the interior of the atmosphere. The changes in baroclinicity and diabatic heating brought by increasing resolution lead to improvements in European blocking and eddy-driven jet variability. Across the multimodel ensemble a clear relationship is found between the climatological meridional SST gradients in the broader Gulf Stream Extension area and two aspects of the atmospheric circulation: the frequency of high-latitude blocking and the southern-jet regime. This relationship is thought to reflect the two-way interaction (with a positive feedback) between the respective oceanic and atmospheric anomalies. These North Atlantic SST anomalies are shown to be important in forcing significant responses in the midlatitude atmospheric circulation, including jet variability and the storm track. Further increases in oceanic and atmospheric resolution are expected to lead to additional improvements in the representation of Euro-Atlantic climate.

Drivers of Late Miocene Tropical Sea Surface Cooling: A New Perspective From the Equatorial Indian Ocean

AbstractDuring the late Miocene, global cooling occurred alongside the establishment of near‐modern terrestrial and marine ecosystems. Significant (3°C–5°C) sea surface cooling from 7.5 to 5.5 Ma is recorded by proxies at midlatitudes to high latitudes, yet the magnitude of tropical cooling and the role of atmospheric carbon dioxide (pCO2) in driving this trend are debated. Here, we present a new orbital‐resolution sea surface temperature (SST) record spanning the late Miocene to earliest Pliocene (9–5 Ma) from the eastern equatorial Indian Ocean (International Ocean Discovery Program Site U1443) based on Mg/Ca ratios measured in tests of the planktic foraminifer Trilobatus trilobus. Our SST record reveals a 3.2°C decrease from 7.4 to 5.8 Ma, significantly increasing previous estimates of late Miocene tropical cooling. Analysis of orbital‐scale variability shows that before the onset of cooling, SST variations were dominated by precession‐band (19–23 kyr) variability, whereas tropical temperature became highly sensitive to obliquity (41 kyr) after 7.5 Ma, suggesting an increase in high‐latitude forcing. We compare a revised global SST database with new paleoclimate model simulations and show that a pCO2 decrease from 560 to 300 ppm, in the range suggested by pCO2 proxy records, could explain most of the late Miocene sea surface cooling observed at Site U1443. Using our new Site U1443 record as representative of tropical SST evolution, estimated meridional SST gradients suggest a much more modest increase over the late Miocene than previously suggested, in agreement with modeled meridional SST gradients.

Investigating Extratropical Influence on the Equatorial Atlantic Zonal Bias with Regional Data Assimilation

Abstract A reversal of zonal sea surface temperature (SST) gradient in the equatorial Atlantic is a common bias in climate models. Studies to investigate the origin of this bias mainly focused on the tropics itself. Applying the regional data assimilation method in the GFDL CM2.1 model, we investigate the combined and respective influences of the northern and southern extratropics on this bias. It is found that the reversed zonal SST gradient bias is caused to a considerable extent by the extratropical atmosphere, especially by the northern extratropics. This extratropical impact on the equator occurs mainly through influencing the Hadley circulation. Therefore, the ITCZ position in boreal spring in this model most likely determines the dominant role of northern extratropics in the spring equatorial westerly bias and additionally the zonal SST gradient bias. Due to the cold bias in the extratropical atmosphere, the northward shift of the ITCZ coupled with the increased meridional SST gradient caused by assimilating the northern extratropics strengthens the cross-equatorial southeasterly wind, thus correcting the spring equatorial westerly bias. The strengthened spring equatorial easterlies further steepen the thermocline slope and enhance the eastern upwelling, thus reproducing the summer cold tongue and finally improving the annual-mean zonal SST gradient bias.

Revisiting the different responses of the following Indian summer monsoon rainfall to the diversity of El Niño events

There is evidence that the interannual relationship between El Niño events and the following Indian summer monsoon rainfall (ISMR) has weakened with the more frequent occurrence of central Pacific (CP) El Niño events. We revisited the following ISMR responses to the two different types of El Niño events using observations and reanalysis datasets. Our results show that the ISMR anomalies associated with eastern Pacific (EP) and CP El Niño events are different, with decreased (increased) rainfall in early summer (June–July) following EP (CP) El Niño events. This is primarily attributed to the different responses to anomalous warming of the sea surface temperature (SST) in the northern Indian Ocean (NIO), which is characterized by double peaks in the warming SST during EP El Niño events, but only one peak during CP El Niño events. For EP El Niño events, the second SST warming peak in early summer contributes to the lower level antisymmetric wind pattern over the tropical Indian Ocean (TIO), which delays the onset of the Indian summer monsoon (ISM) and decreases the supply of moisture to India, implying a decrease in the ISMR. By contrast, for CP El Niño events, the cooling SST over the western TIO directly induces a significantly positive meridional SST gradient and drives the lower level southwesterly wind anomalies, resulting in an eastward shift in the decreased antisymmetric winds over TIO and the early onset of ISM. These circulation features are associated with anomalous upper-level divergence over TIO and sinking over India, jointly leading to the excess ISMR in early summer. These results suggest that, in addition to the key role of the warming of the NIO SST, cooling of the SST over the western TIO during CP El Niño events should be considered carefully in understanding the El Niño–ISMR relationship.

Severe Drought Conditions in Northern East Asia During the Early Pliocene Caused by Weakened Pacific Meridional Temperature Gradient

The evolution of hydroclimate in East Asia during the Pliocene—a potential analog of the future greenhouse climate—remains highly uncertain. Here we reconstruct changes in the hydroclimate of Northern China during this epoch. Our results reveal previously undocumented severe drought conditions that persisted through the early Pliocene with a significantly greater magnitude than suggested by other records. Using a broad hierarchy of climate simulations, we find that reduction in the Pacific meridional sea surface temperature (SST) gradient has particularly strong impacts on precipitation over Northern China, which suggests its key role in maintaining drought conditions during the early Pliocene. The weaker Pacific meridional SST gradient causes the weakening of the East Asian summer monsoon precipitation over this region via a reduction of atmospheric northward moisture transport from the ocean, which enhances aridity inland. Our results highlight the fundamental control of the tropical ocean on the extra-tropical hydrological cycle.

The Role of Clouds in Shaping Tropical Pacific Response Pattern to Extratropical Thermal Forcing

AbstractExtratropical influences on tropical sea surface temperature (SST) have implications for decadal predictability. We implement a cloud‐locking technique to highlight the critical role of clouds in shaping the tropical SST response to extratropical thermal forcing. With heating imposed over either the extratropical Northern Atlantic or Pacific, Hadley Cells respond similarly that the trades strengthen south of the rainband. The wind‐evaporation‐SST (WES) feedback leads to cooling over the southern subtropics, which is enhanced in the southeastern Pacific due to the positive feedback between SST and stratiform clouds. Cloud‐locking experiments show that zonal contrasts in SST and cloud feedbacks in the Pacific enhance the zonal surface winds, leading to increased evaporation and strengthens zonal SST difference. We propose that the meridional and zonal SST gradients are tightly linked via WES effects and the cloud‐radiative‐SST feedbacks, which are largely determined by the climatological rainband position and the spatial distribution of cloud properties.

A Simple Coupled Model of the Wind–Evaporation–SST Feedback with a Role for Stability

Abstract The wind–evaporation–SST (WES) feedback describes a coupled mechanism by which an anomalous meridional sea surface temperature (SST) gradient in the tropics evolves over time. As commonly posed, the (positive) WES feedback depends critically on the atmospheric response to SST anomalies being governed by a process akin to that argued by Lindzen and Nigam in 1987, and omits an alternative process by which SST anomalies modulate surface wind speed through vertical momentum mixing as proposed by Wallace et al. and Hayes et al. in 1989. A simple model is developed that captures the essential coupled dynamics of the WES feedback as commonly posed, while also allowing for momentum entrainment in response to evolving SST anomalies. The evolution of the coupled system depends strongly on which effects are enabled in the model. When both effects are accounted for in idealized cases near the equator, the initial anomalous meridional SST gradient grows over a time scale of a few months but is damped within one year. The sign and magnitude of the WES feedback depend on latitude within the tropics and exhibit hemispheric asymmetry. When constrained by realistic profiles of prevailing zonal wind, the model predicts that the WES feedback near the equator is stronger during boreal winter, while the domain over which it is positive is broader during boreal summer, and that low-frequency climate variability can also modulate the strength and structure of the WES feedback. These insights may aid in the interpretation of coupled climate behavior in observations and more complex models. Significance Statement Regional climate variability on time scales from months to decades, including El Niño, relies heavily on feedbacks between the atmosphere and the ocean in which some initial change in the environment is either amplified or damped over time. Several conceptual models for such feedbacks have been devised over the years to explain the coupled climate behavior seen in observations and computer simulations. A rather ubiquitous one is called the wind–evaporation–SST (WES) feedback, but the typical phrasing of it does not incorporate a potentially important influence of ocean temperature changes on the stability of the atmosphere above it. This study adds that effect to the WES feedback framework and examines climate variability through the lens of the augmented conceptual model.

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.

Meridional Eddy Heat Transport Variability in the Surface Mixed Layer of the Atlantic Ocean

AbstractWe analyze the interannual variability and trends of the eddy heat transport (EHT) in the surface mixed layer of the Atlantic Ocean from covariances of sea surface temperature (SST) and geostrophic velocities from satellite observations between 1993 and 2018. The EHT is largest along the path of the Gulf Stream in the Northern Hemisphere and the vicinity of the Agulhas Retroflection and Argentine basin in the Southern Hemisphere. On average, meridional EHT in the mixed layer leads to a divergence of heat away from the subtropics in both hemispheres toward the equator and higher latitudes. Depending on latitude, the divergence of the EHT accounts for around 1%–5% of the atmospheric net surface heat flux, but reaches as much as 20% for certain regions. The EHT can be linked to eddy kinetic energy (EKE) and meridional SST gradients using mixing length hypothesis, but the variability of EKE and SST gradients are not enough to capture local EHT variability. For different latitudes, the EHT shows different behavior over time, with a strong increase in northward EHT in the Gulf Stream region, and a decadal oscillation of poleward EHT in the tropical and subtropical regions. This oscillation on larger scales is highly correlated with spatially averaged EKE and the large‐scale SST gradients. A comparison with climate indices indicates a relation to Atlantic climate variability, especially the meridional modes of the Tropical Atlantic Variability and the North Atlantic Oscillation (NAO).