Meridional Sea Surface Temperature Research Articles

Decoupled terrestrial temperature and hydroclimate during the Plio-Pleistocene in the East Asian monsoonal region

The independent reconstructions of temperature and hydroclimate records are crucial to understanding past changes in Earth's climate. However, the temperature and hydroclimate history of East Asia during the late Cenozoic remains largely unknown. Here, we reconstructed terrestrial temperature and hydroclimate patterns in the East Asian monsoonal regions using GDGTs (glycerol dialkyl glycerol tetraethers) in a sediment core from North China spanning 5.5 million years (Ma). Our data show a decoupled terrestrial temperature and hydroclimate evolution over the past 5.5 Ma. The methylation index of branched GDGTs (brGDGTs), i.e. MBT'5ME-inferred temperature, significantly decreases at ∼2.6 Ma and likely reflects a response to Northern Hemisphere glaciation (NHG), in agreement with multiple marine temperature records. The MBT'5ME temperature record shows a decline of approximately 6.5 °C at around 2.6 Ma, which is larger than the decline recorded in marine sequences. Interestingly, this temperature decline occurs much later than the large GDGT-inferred (i.e., Ri/b and GDGT-0/Cren) increase in rainfall at ∼4.0 Ma, which aligns with the strengthening of Pacific zonal and meridional sea surface temperature (SST) gradients as well as the increased marine-land temperature gradients. Our study thus suggests differing mechanisms driving temperature and hydroclimate evolution in East Asia during the Plio-Pleistocene.

On the association of Pacific Decadal Oscillation with the interdecadal variability in Asian-Pacific Oscillation

The Asian-Pacific Oscillation (APO), featuring a seesaw pattern in the upper-tropospheric temperatures between Asia and the North Pacific, plays profound roles in the Northern Hemispheric climate anomalies. So, its variability and associated physical mechanisms are of great interest. Although progress has been achieved for the APO variability at the interannual time scale, the understanding of its interdecadal variability remains relatively poor. Based on the twentieth-century reanalysis and the simulations of the preindustrial control, Pacific pacemaker, and large ensemble experiments, this study reports a salient contribution of the Pacific Decadal Oscillation (PDO) to the interdecadal variability of the APO during summer, featured as a warm (cold) PDO phase accompanying a negative (positive) APO polarity. During the warm PDO phase, the sea surface temperature (SST) warming over the eastern Pacific is conducive to local ascending anomalies, which teleconnect with descending anomalies over the Tibetan Plateau through zonal vertical circulations. The anomalous ascent (descent) over the eastern Pacific (Tibetan Plateau) tends to strengthen (weaken) the atmospheric heating, beneficial for a warming (cooling) of the troposphere in situ. In addition, the PDO-related meridional SST gradient anomalies favor a southward shift of the subtropical jet stream, weakening the South Asian high and the North Pacific trough in the upper troposphere, which corresponds to the atmospheric situation of a negative APO phase. Quantitative analysis from the large ensemble simulations indicates that the PDO may contribute to approximately 48% of recent APO interdecadal variation from the negative phase to the positive phase in the late 1990s.

Empirical orthogonal function analysis of lightning flashes over India

Lightning studies are highly focused on spatial and temporal variability in various scales but very limited studies are focused on dominant spatial modes of variability. This study intends to identify the possible spatial modes of climate variability of lightning over India during different seasons and relate them to regional and large-scale climate modes. Empirical orthogonal function analysis of lightning has been carried out and the first three orthogonally independent modes are considered in order to retrieve the maximum variance explained by each mode. To understand the role of remote and local teleconnections on the lightning flash rate (LFR) variability, we have analyzed two Pacific Ocean modes (El Niño Southern Oscillation; ENSO, Pacific Decadal Oscillation; PDO) and two Indian Ocean modes (Indian Ocean Dipole; IOD and Bay of Bengal (BOB) meridional Sea Surface Temperature (SST) gradient). First mode is positively correlated with the warm phase of ENSO and PDO whereas second and third modes are negatively correlated with the warm phase of ENSO and PDO during pre-monsoon, post-monsoon and winter. Reverse is true for the monsoon season due to the shift in walker cell caused by the changes in the location of the heat sources and sinks. A strong positive correlation of IOD and BOB meridional SST gradient with first mode, suggests the vital role of nearby Indian Ocean in explaining the typical lightning flashes over India due to the enhanced zonal and meridional circulation, thereby moisture supply to the Indian subcontinent. The impact of Nino-3.4, IOD and BOB meridional SST gradient on lightning over India further suggest the role of SST in local and remote influence on lightning variability through the distribution and transport of heat and moisture.

Orbital‐Scale Global Ocean Sea Surface Temperatures Coupling With Cryosphere‐Carbon Cycle Changes Over the Past 4 Million Years

AbstractChanges in the thermal conditions of the ocean surface, the interface for air‐sea exchange, are critical for understanding global climate and environmental change. Here we explore the evolution of sea surface temperature (SST) and the meridional SST gradient (STG) at orbital timescales since 4 million years ago (Ma), along with interactions between SSTs, the cryosphere, and the global carbon cycle. We observe orbital eccentricity and obliquity influences on SST evolution and infer that SST changes may have played a key role in atmospheric CO2 and cryosphere changes through key climate transitions in the past 4 Ma. We find a major equator‐to‐pole STG increase in the Northern Hemisphere (NH) close to the initiation of major NH glaciation (at ∼2.7 Ma). In addition, we find substantial increases in the obliquity sensitivity (Sobl) of NH STG at ∼2.7 Ma and in Southern Hemisphere (SH) STG at ∼1 Ma, which may be responses to important expansions of NH and SH ice sheets, respectively. Phase analysis shows that SST changes typically lead global ice volume changes throughout the last 4 Ma. SST changes also lead atmospheric CO2 changes since ∼1.5 Ma, which indicates that SST changes either drove, or directly reflect, processes that changed ocean‐atmosphere carbon exchange and, thus, atmospheric CO2 concentrations. Overall, our study emphasizes that SST changes were a critical component of climate change throughout the last 4 Ma.

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.

Constraining Plio‐Pleistocene Shifts in Northwest African Hydroclimate, Ecosystem Distributions, and Marine Productivity: New Paleo‐Records Across the Mid‐Pleistocene Transition

AbstractNorthwest Africa transitioned from a wet/vegetated landscape toward drier/sparser conditions sometime between the late‐Pliocene and the late‐Pleistocene. However, our understanding of the precise timing and nature of this transition is hampered by a paucity of paleo‐records which bridge these two intervals. Here we report new plant‐wax isotope as well as dust and opal flux records from the relatively brief interval ∼1.1–1.0 million years ago (Ma) to evaluate the astronomical timescale controls of Northwest African hydroclimate and vegetation during the Mid‐Pleistocene Transition (MPT) and, in context with published records, the drivers of long‐term climate and ecological trends over the Plio‐Pleistocene. The tempo and amplitude of the Northwest African monsoon rainfall swings closely track low latitude insolation forcings over the last 5 Ma. However, we demonstrate that a pronounced mean state decline in monsoon strength likely occurred following the MPT most likely instigated by increasing Atlantic meridional sea surface temperature gradients or declines in the strength of the meridional overturning circulation. The northward extent of vegetation does not track changes in monsoon strength over the Plio‐Pleistocene and thus may be more strongly influenced by changes in monsoon rainfall extent or ecosystem disturbances. Progressively diminished dust fluxes following a decline in monsoon strength after 1.0 Ma is consistent with reduced production and subsequent depletion of fine‐grained sediments in the Sahara. Synchroneity between dust and opal fluxes across timescales suggests nutrient delivery to the surface ocean via dust plays a key role in marine primary productivity off the coast of Northwest Africa.

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.

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.

Mechanism of the Record-Breaking Heatwave Event Dynamics in South America in January 2022

Heatwaves in the Southern Hemisphere (SH) occur frequently but have received little attention over the years. This study presents a comprehensive analysis of a long-duration, wide-ranging, and high-intensity heatwave event in South America spanning from 9 to 16 January 2022. Before the heatwave occurred, the meridional sea surface temperature (SST) in the SH intensified due to the warming of the South Pacific, while the Southern Annular Mode (SAM) exhibited a positive phase. As a result, the intensified wave activities in the westerlies led to high-pressure anomalies in South America, which played a dominant role in the generation of the heatwave. The diagnostic analysis of thermodynamic equations in South America indicates that the temperature increase during the heatwave was primarily caused by the vertical advection term. In contrast, horizontal advection had a negative impact on surface warming. Additionally, the diabatic heating term associated with surface land types serves as a significant factor that cannot be disregarded. This study aims to deepen our understanding of the mechanisms underlying heatwave generation in South America, enabling the improved prediction of heatwaves and enhanced assessment of potential risks in the future.

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.

Interannual relationship between South Pacific meridional sea surface temperature dipole and rainfall anomalies over South China in late-spring to early-summer without ENSO impact

Interannual relationship between South Pacific meridional sea surface temperature dipole and rainfall anomalies over South China in late-spring to early-summer without ENSO impact

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.

Influence of tropical Atlantic meridional dipole of sea surface temperature anomalies on Antarctic autumn sea ice

Antarctic sea ice plays an important role in polar ecosystems and global climate, while its variability is affected by many factors. Teleconnections between the tropical and high latitudes have profound impacts on Antarctic climate changes through the stationary Rossby wave mechanism. Recent studies have connected long-term Antarctic sea ice changes to multidecadal variabilities of the tropical ocean, including the Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. On interannual timescales, whether an impact exists from teleconnection of the tropical Atlantic is not clear. Here we find an impact of sea surface temperature (SST) variability of the tropical Atlantic meridional dipole mode on Antarctic sea ice that is most prominent in austral autumn. The meridional dipole SST anomalies in the tropical Atlantic force deep convection anomalies locally and over the tropical Pacific, generating stationary Rossby wave trains propagating eastward and poleward, which induce atmospheric circulation anomalies affecting sea ice. Specifically, convective anomalies over the equatorial Atlantic and Pacific are opposite-signed, accompanied by anomalous wave sources over the subtropical Southern Hemisphere. The planetary-scale atmospheric response has significant impacts on sea ice concentration anomalies in the Ross Sea, near the Antarctic Peninsula, and east of the Weddell Sea.