Sea Surface Temperature Feedback Research Articles

Quantifying the impact of SST feedback frequency on Madden–Julian oscillation simulations

Abstract. This study uses the Community Atmosphere Model 5.3 coupled to a 1-D ocean model to investigate the effects of intraseasonal sea surface temperature (SST) feedback frequency on Madden–Julian oscillation (MJO) simulations with intervals at 30 min and 1, 3, 6, 12, 18, 24, and 30 d. The large-scale nature of the MJO in simulations remains intact with decreasing feedback frequency, although it becomes increasingly unrealistic in both structure and amplitude, until 1 per 30 d when the intraseasonal fluctuations are overwhelmingly dominated by unorganized small-scale perturbations in both atmosphere and ocean, as well as at the atmosphere–ocean interface where heat and energy are rigorously exchanged. The main conclusion is that the less frequent the SST feedback, the more unrealistic the simulations. Our results suggest that more spontaneous atmosphere–ocean interaction (e.g., ocean response once every time step to every 3 d in this study) with high vertical resolution in the ocean model is a key to the realistic simulation of the MJO and should be properly implemented in climate models.

Propagation and Maintenance of the Quasi-Biweekly Oscillation over the Western North Pacific in Boreal Winter

Abstract This study investigates the propagation and maintenance mechanisms of the dominant intraseasonal oscillation over the western North Pacific in boreal winter, the quasi-biweekly oscillation (QBWO). The wintertime QBWO over the western North Pacific is characterized by the westward-northwestward movement from the tropical western Pacific to the western North Pacific and resembles the n = 1 equatorial Rossby wave. Its westward migration is primarily driven by the seasonal-mean zonal winds that advect vorticity anomalies in the lower-middle troposphere and moisture anomalies in the lower troposphere. Its northward movement is preconditioned by the vorticity dynamics of the beta effect, the low-level vertical moisture variation, and the local air-sea interaction. The latter involves the atmospheric forcing on the underlying ocean by changing the surface heat flux fluctuations and the sea surface temperature feedback on the low-level atmospheric instability. Its maintenance is primarily through atmospheric external energy sources from diabatic heating, which first generates eddy available potential energy and then converts it to eddy kinetic energy.

Uncertainty in the Past and Future Changes of Tropical Pacific SST Zonal Gradient: Internal Variability versus Model Spread

Abstract The zonal sea surface temperature (SST) gradient across the tropical Pacific is a pacemaker of the variable rates of global warming. In both historical simulations and future projections, the current state-of-the-art climate models show evident spread in the changes of zonal SST gradient, but the reasons remain unknown. Here, we quantify the contributions of internal variability and model spread to the uncertainty of zonal SST gradient changes by analyzing 342 realizations from CMIP5 and CMIP6 models and several sets of large-ensemble simulations. We found that the internal variability dominates the total uncertainty at multidecadal time scales (∼31-yr trends). Although the ratio of internal uncertainty to the total uncertainty declines along with higher emission of greenhouse gases under global warming, it is still over 80% at the multidecadal time scales in the future. The Pacific decadal oscillation is identified as the key internal mode responsible for the multidecadal uncertainty. For the future projections at centurial time scales, the uncertainty of zonal SST gradient changes is mainly from the intermodel spread in response to external forcing, accounting for about 70% of the uncertainty based on the difference between 2070–99 and 1970–99. The model spread in the cloud–shortwave radiation–SST feedback over the tropical Pacific is important in the uncertainty of zonal SST gradient changes. In particular, the intensity of negative convective cloud feedback in the western Pacific dominates the spread in CMIP5 models, while the intensity of stratocumulus cloud feedback over the southeastern Pacific is the primary process influencing the uncertainty in CMIP6 models.

On the Role of Wind–Evaporation–SST Feedbacks in the Subseasonal Variability of the East Pacific ITCZ

Abstract The intertropical convergence zone (ITCZ) is a zonally elongated band of near-surface convergence and precipitation near the equator. During boreal spring, the eastern Pacific ITCZ migrates latitudinally on daily to subseasonal time scales, and climate models exhibit the greatest ITCZ biases during this time of the year. In this work, we investigate the air–sea interactions associated with the variability in the eastern Pacific ITCZ’s latitudinal location for consecutive days when the ITCZ is only located north of the equator (nITCZ events) compared to when the ITCZ is on both sides of the equator or south of the equator (dsITCZ events) during February–April. The distribution of sea surface temperature (SST) anomalies and surface latent heat flux (SLHF) anomalies during the nITCZ and dsITCZ events follow the classic wind–evaporation–SST (WES) positive feedback mechanism. However, an alternative mechanism, embracing the effect of SST anomalies on vertical stratification and momentum mixing, gives rise to a negative WES feedback. Our results show that in the surface layer, there is a general progression of positive WES feedbacks happening in the weeks leading to the events followed by negative WES feedbacks occurring after the ITCZ events, with an alternate mechanism involving air–sea humidity differences limiting evaporation occurring in between. Additionally, the spatial structures of the components of the feedbacks are nearly mirror images for these opposite ITCZ events over the east Pacific during boreal spring. In closing, we find that understanding the air–sea interactions during daily to weekly varying ITCZ events (nITCZ and dsITCZ) helps to pinpoint how fundamental processes differ for ITCZs in different hemispheres.

The Gulf Stream Front Amplifies Large-Scale SST Feedback to the Atmosphere in North Atlantic Winter

The Gulf Stream (GS) ocean front releases intense moisture and heat to the atmosphere and regulates storm tracks and zonal jets in winter. The large-scale sea surface temperature (SST) anomaly in the central North Atlantic provides important feedback to the atmosphere in winter, but the role played in this feedback by the GS front inside the SST anomaly has not been extensively studied. In this study, two sets of ensemble experiments were conducted using a global community atmosphere model forced by SST in boreal winters from 2000 to 2013. The regional averaged SST and its variation in the experiments were identical, with the only difference being the strength of the SST front in the GS region. The large-scale SST anomaly in the central North Atlantic in our model provides feedback to the atmosphere and excites a wave train that extends across Eurasia. With the inclusion of the strong GS front, the first center of the wave train in the North Atlantic is strengthened by approximately 40%, and the wave activity flux toward downstream is highly intensified. When the large-scale SST anomaly is combined with a strong GS front, greatly increased water vapor is released from the GS region, resulting in a 50% increase in moisture transport toward Western Europe. In this scenario, precipitation and diabatic heating both increase greatly on the western Scandinavian Peninsula. With the release of deep diabatic heating, a strong upward wave activity flux is triggered, and the wave train excited by the large-scale SST variation is significantly intensified. These findings suggest that the strong SST front in the large-scale SST anomaly in the central North Atlantic significantly amplifies its feedback to the atmosphere in winter.

Seasonal Predictability of North American Surface Temperature Arises from SST Propagation over the Western Pacific

Abstract The predictability of temperatures in North America is of great importance for local agriculture and human health. In autumn (SON), the temperature in North America is correlated with ENSO, but its predictive skill is limited. Here, we show that spring (MAM) sea surface temperature (SST) anomalies in the southwest Pacific (SWP) exhibit a higher correlation with autumn temperature in North America (r = 0.73) than ENSO. This cross-seasonal and cross-hemispheric relationship is established via the western tropical Pacific (WTP) region. The spring SWP SST anomalies show a cross-hemispheric propagation embedded in the southerly monsoonal flow through the wind–evaporation–SST feedback, which sustains the progression of SST anomalies toward the WTP in autumn. The AGCM simulations from multimodels show that active convection in the WTP caused by SST warming stimulate a Rossby wave train propagating downward to North America, where it is governed by an anomalous high with increased atmospheric thickness and tropospheric temperature. Consequently, the longwave radiative heating over North America is enhanced, raising the surface air temperature. These results indicate that the SWP SST is a useful predictor of North American temperature 6 months in advance through the cross-hemispheric influence. A model based on the spring SWP SST and preceding winter ENSO (Niño-3.4) shows a high predictive skill for the autumn temperature anomalies in North America. Significance Statement Seasonal predictions of surface temperature are important for agricultural decision-making, disaster prevention, and mitigation. Over the western Pacific, associated with the seasonal evolution of ITCZ, there is a strong northward progression of SST anomaly signals generated in the southwest Pacific. This study finds that this cross-hemispheric SST propagation can significantly impact the autumn surface temperature over central North America through the atmospheric bridge. A statistical model incorporating southwest Pacific SST for spring is constructed to predict the North American autumn surface temperature and exhibits high consistency with observations. This improves our understanding of the interhemispheric interactions at seasonal time scales and the seasonal predictability of the North American climate.

Asymmetric influence of the Pacific meridional mode on tropical cyclone formation over the western North Pacific

AbstractThis study investigates the asymmetric response of western North Pacific (WNP) tropical cyclone (TC) formation during August–November to the Pacific meridional mode (PMM) from 1961 to 2021. Basinwide WNP TC frequency significantly increases (slightly decreases) during positive (negative) PMM years, implying a nonlinear PMM–TC frequency relationship. Only small spatial changes in TC formation occur during negative PMM years, while there is nearly a basinwide enhancement of TC formation during positive PMM years, particularly over the region of 5°–30°N and 135°–155°E. This region is characterized by significantly enhanced low‐level vorticity, mid‐level updrafts and upper‐level divergence during positive PMM years, all favouring TC development. By contrast, environmental anomalies are of a smaller magnitude and mostly insignificant over the WNP during negative PMM years. These distinct environmental changes during different PMM phases can be explained by differences in PMM strength. Positive PMM events are of a greater strength and are associated with a stronger wind–evaporation–sea surface temperature feedback than negative PMM events, leading to a notable anomalous large‐scale low‐level cyclonic circulation over the WNP in positive PMM events.

Western Mediterranean Precipitation Extremes, the Result of Quasi-Resonant Sea–Atmosphere Feedbacks

The Mediterranean region has been identified as a climate change hotspot, and 13 case studies of extreme rainfall events (EREs) make it possible to categorize convective systems according to whether they are tropical-like or extratropical cyclones. This study, which focuses on the western Mediterranean basin from 2000 to 2021, is based on the cross-wavelet analysis in the period range of 11.4 to 45.7 days of (1) the height of precipitation at a particular place representative of the deep convective system used as the temporal reference and (2) the amount of precipitation in the western Mediterranean basin, as well as the sea surface temperature (SST) in the Mediterranean, the Adriatic, the Aegean Sea, the Black Sea, the Baltic, the North Sea and the Atlantic Ocean. Extratropical cyclones result from quasi-resonant atmospheric water and SST feedbacks, reflecting the co-evolution of the clustering of lows and the harmonization of thermocline depths and a relative stability of the atmospheric blocking circulation. When the SST anomaly in the western Mediterranean is greater than 0.5 °C, in its paroxysmal phase, the deep convective system is centered both over the southeast of France and the Mediterranean off the French coast. However, when the SST anomaly is weaker, deep convective systems can develop in different patterns, depending on SST anomalies in the peripheral seas. They can produce a low-pressure system extending from the Pyrenees to southern Italy or Sicily when the SST anomaly in the western Mediterranean is in phase opposition with EREs. In some cases, partial clustering of Atlantic and Mediterranean low-pressure systems occurs, producing a large cyclonic system. Tropical-like cyclones develop in the absence of any significant SST anomalies. Like extratropical cyclones, they occur in autumn or even winter, when the thermal gradient between the sea surface and the upper atmosphere is greatest but, this way, non-resonantly. Their return period is around 2 to 3 years. However, due to the gradual increase in the SST of the western Mediterranean in summer resulting from global warming, they can now lead to an ERE as happened on 21 January 2020.

Optimal Precursors for Central Pacific El Niño Events in GFDL CM2p1

Abstract Compared to the canonical eastern Pacific El Niño, the understanding and ability to predict the central Pacific (CP)-type event still need further improvement. In this study, the principal component analysis–based particle swarm optimization algorithm (PPSO) is applied in Geophysical Fluid Dynamics Laboratory Climate Model version 2p1 (GFDL CM2p1) to obtain the optimal precursors (OPRs) for CP El Niño events, based on the conditional nonlinear optimal perturbation (CNOP) method. For this, three normal years with neither El Niño nor La Niña events, i.e., three cases, are chosen as the reference states. The obtained OPRs for these cases exhibit a consistent positive sea surface temperature (SST) perturbation distribution in the subtropical North Pacific (20°–40°N, 175°E–140°W), which is further proven to be crucial for the evolution of CP El Niño based on the Northern and Southern Hemisphere significance test results. Mechanically, these positive SST perturbations are enhanced and reach the equatorial Pacific via wind–evaporation–SST (WES) feedback to evolve into a CP El Niño at the end of the year. The nonlinear approach is adopted to investigate the predictability of CP El Niño events and can shed some light on future studies.

Rainfall–Mixed Layer–SST Feedback Contributing to Atlantic Meridional Mode Development

Abstract A simple air–sea coupled model for wind–evaporation–sea surface temperature (SST), wind-induced turbulence–mixed layer (ML)–SST, and wind–evaporation–ML–SST feedback is extended to unitedly represent the precipitation anomaly associated with moisture convergence and the ML depth (MLD) anomaly due to freshwater-induced buoyancy flux. An eigenanalysis reveals the presence of yet another feedback accompanying a cross-equatorial SST gradient. The feedback operates as follows: A cross-equatorial SST gradient anomaly forces surface wind anomalies to blow toward the warmer hemisphere, causing low-level convergence (divergence) and hence a positive (negative) precipitation anomaly in the warmer (cooler) hemisphere. The positive (negative) precipitation anomaly stratifies (destabilizes) the near-surface ocean and results in a shallower (deeper) ML, which enhances (reduces) the warming by climatological shortwave radiation, and thus provides positive feedback to the initial SST gradient anomaly. The strength of this feedback is similar to the three known feedbacks in terms of stability. Sensitivity experiments with the coupled general circulation model MIROC6 reveal that the precipitation-induced buoyancy flux anomaly accounts for up to ∼14% of the Atlantic meridional mode (AMM) amplitude in boreal spring through affecting the MLD anomaly in the deep tropics, which is consistent with the simple model results, supporting the existence of the feedback. In contrast, the evaporation-induced buoyancy anomaly contributes only marginally to the MLD and thus the SST anomalies. The ML temperature budget from MIROC6 confirms that sensitivity changes of the ML to the warming by climatological shortwave radiation due to the MLD anomaly are important in generating the SST anomalies associated with the AMM, which is consistent with previous observational studies. Significance Statement It is known that year-to-year variations in the sea surface north–south temperature gradient in the tropical Atlantic can affect the climate in both surrounding and remote regions. In this study, we used theoretical and state-of-the-art climate models to investigate a tropical air–sea coupled process and establish its contribution to the Atlantic climate variability. As a result, we identified a previously unknown process contributing to north–south gradient variations in which the atmosphere and ocean interact to enhance the initial variation, thus forming a positive feedback loop. In particular, precipitation and near-surface ocean state changes were determined to be essential. This feedback process accounts for up to 14% of the north–south temperature gradient variations in the tropical Atlantic during the boreal spring.

Low Cloud–SST Feedback over the Subtropical Northeast Pacific and the Remote Effect on ENSO Variability

Abstract Low clouds frequent the subtropical northeastern Pacific Ocean (NEP) and interact with the local sea surface temperature (SST) to form positive feedback. Wind fluctuations drive SST variability through wind–evaporation–SST (WES) feedback, and surface evaporation also acts to damp SST. This study investigates the relative contributions of these feedbacks to NEP SST variability. Over the summer NEP, the low cloud–SST feedback is so large that it exceeds the evaporative damping and amplifies summertime SST variations. The WES feedback causes the locally enhanced SST variability to propagate southwestward from the NEP low cloud deck, modulating El Niño–Southern Oscillation (ENSO) occurrence upon reaching the equator. As a result, a second-year El Niño tends to occur when there are significant warm SST anomalies over the subtropical NEP in summer following an antecedent El Niño event and a second-year La Niña tends to occur when there are significant cold SST anomalies over the subtropical NEP in summer following an antecedent La Niña event The mediating role of the NEP low cloud–SST feedback is confirmed in a cloud-locking experiment with the Community Earth System Model, version 1 (CESM1). When the cloud–ocean coupling is disabled, SST variability over the NEP weakens and the modulating effect on ENSO vanishes. The nonlocal effect of the NEP low cloud–SST feedback on ENSO has important implications for climate prediction.

A Continuing Increase of the Impact of the Spring North Pacific Meridional Mode on the Following Winter El Niño and Southern Oscillation

Abstract This study reveals that the impact of the spring North Pacific meridional mode (PMM) on the following-winter El Niño–Southern Oscillation (ENSO) shows a continuing increase in the past. A comparative analysis is conducted for the high- and low-correlation periods to understand the factors for the strengthened impact of the PMM. The spring PMM-related sea surface temperature (SST) and atmospheric anomalies over the subtropical northeastern Pacific propagate southwestward to the tropical central Pacific via wind–evaporation–SST feedback in the high-correlation period. The tropical SST and atmospheric anomalies further develop to an ENSO-like pattern via positive air–sea interaction. In the low-correlation period, SST and atmospheric anomalies over the subtropical northeastern Pacific related to the PMM cannot extend to the deep tropics. Therefore, the spring PMM has a weak impact on ENSO. The extent to which the PMM-related SST and atmospheric anomalies extend toward the tropics is related to the background flow. The stronger mean trade winds in the high-correlation period lead to an increase in the air–sea coupling strength over the subtropical northeastern Pacific. As such, the spring PMM-related SST and atmospheric anomalies can more efficiently propagate southwestward to the tropical Pacific and exert stronger impacts on the succeeding ENSO. In addition, the southward shifted intertropical convergence zone in the high-correlation period also favors the southward extension of the PMM-related SST anomalies to the tropics and contributes to a stronger PMM–ENSO relation. The variation and its formation mechanism of the spring PMM–winter ENSO relationship appear in both the observations and the long historical simulation of Earth system models. Significance Statement The North Pacific meridional mode (PMM) is the leading atmosphere–ocean coupling pattern over the subtropical northeastern Pacific after removing the ENSO variability, with maximum variance during boreal spring. Previous studies indicated that the PMM plays an important role in relaying the impact of the atmosphere–ocean forcings over the extratropics on the tropical ENSO. This study reveals that the impact of the spring PMM on the following winter ENSO shows a continuing increase in the past 70 years. The physical mechanisms for this strengthened impact are further examined. Results obtained in this study have important implications for improving the prediction of the tropical ENSO variability.

How Well Can Current Climate Models Simulate the Connection of the Early Spring Aleutian Low to the Following Winter ENSO?

Abstract A recent study revealed an impact of the intensity of early spring Aleutian low (AL) on the succeeding winter ENSO. This study examines the ability of 41 climate models that participated in CMIP6 in simulating the early spring AL–winter ENSO connection. It is shown that there exists a large diversity among the models in simulating this AL–ENSO linkage. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. Diversity of the AL–ENSO connection is related to the spread in the spatial structure of AL-related atmospheric anomalies over the North Pacific. In the models that capture the observed AL–ENSO connection, weakened AL induces an anomalous anticyclone over the northern middle and high latitudes and an anomalous cyclone over the subtropical North Pacific. The resultant westerly wind anomalies over the tropical western-central Pacific (TWCP) induce an El Niño sea surface temperature (SST) anomaly pattern in the following winter. By contrast, in the models with the AL–ENSO relation opposite to the observations, the AL-associated anomalous anticyclone over the North Pacific extends too southward. As such, the subtropical North Pacific is dominated by northeasterly wind anomalies and SST cooling. The subtropical North Pacific SST cooling induces easterly wind anomalies over the TWCP via wind–evaporation–SST feedback, and leads to a La Niña anomaly pattern in the following winter. The spread in the spatial structure of the AL-associated atmospheric anomalies over the North Pacific is partly due to the diversity in the amplitude of the climatological mean flow. Significance Statement A recent study suggested that variation of the AL intensity in early spring could exert a significant impact on the following winter ENSO. It indicated that inclusion of the early spring AL signal could improve the prediction of ENSO and to some extent help reduce the spring predictability barrier of ENSO. To employ the AL as a predictor in the ENSO prediction and forecast, the current climate model should have the ability to simulate realistically the early spring AL variation as well as the physical process linking the early spring AL with the subsequent winter ENSO. Hence, this study examines the performance of the current coupled climate models that participated in the phase 6 of the Coupled Model Intercomparison Project (CMIP6) in simulating the linkage between the early spring AL and the following winter ENSO. We show that there exists a large diversity among the CMIP6 models in simulating the early spring AL–winter ENSO connection. A number of models capture well the observed AL–ENSO connection and the associated physical processes. However, the AL–ENSO relation in several models is opposite to the observed. The factors leading to the spread are further examined. Results of this study would have implications in improving our understanding of the impact of extratropical atmospheric forcing on the ENSO and improving the seasonal forecasting of the ENSO.

A Positive Low Cloud–Sea Surface Temperature Feedback in the East Asian Marginal Seas during El Niño Mature Winters and Their Following Spring

Abstract During the winter and subsequent spring of an El Niño year, the East Asian marginal sea (EAMS) exhibits positive sea surface temperature anomalies (SSTAs) and fewer low clouds, while the western North Pacific experiences negative SSTAs. In this study, we suggest that the positive SSTAs in EAMS are maintained by a positive low cloud–SST feedback. In neutral winters and springs, the EAMS is covered by low clouds, which have a cooling effect on surface temperatures. During an El Niño year, a western North Pacific anomalous anticyclone is established, and along its northwestern flank, there are favorable conditions for convergence of moisture and weaker surface latent heat flux over the EAMS. Once a positive SSTA has been established, a further reduction of turbulent mixing results in less low cloud and enhanced solar heating of the ocean mixed layer; this reinforces and maintains both the positive SSTA and the lack of low cloud via a positive feedback mechanism. The concurrent increase of low cloud–SST feedback and anticyclone circulation strengths is evident in the coupled-model simulations from phase 6 of the Coupled Model Intercomparison Project. Furthermore, sensitivity experiments, performed with the atmospheric components of Community Earth System Model (CESM2), reveal that a positive SSTA helps to maintain the western North Pacific anomalous anticyclone. Four pacemaker-coupled experiments by CESM2, with sea surface temperature in the equatorial Pacific restored to the observational anomalies plus the model climatology and altered low cloud feedback over EAMS, suggest that the low cloud–SST feedback results in more than the maintenance of a positive SSTA over the EAMS: the positive feedback is also a previously overlooked mechanism for the maintenance of the western North Pacific anomalous anticyclone. Significance Statement The East Asian marginal sea (EAMS) and western North Pacific are important areas that bridge El Niño and the climate of East Asia. Unlike the cold sea surface temperature anomaly (SSTA) over the western North Pacific during El Niño, the positive SSTA over EAMS, which is covered by winter low cloud, has received less attention. We suggest that a “low cloud–SST” feedback—namely, one in which decreasing low-level clouds allows more sunlight to strike the ocean surface and favors higher SST—maintains the positive SSTA over EAMS. We also configure a widely used atmospheric model with a set of preset SSTA patterns that mimic different climate patterns. Our experiments with different climate patterns and CMIP6 historical runs show that the low cloud–SST feedback (through the positive SSTA) is a possible supplementary mechanism for reinforcing the WNP anomalous anticyclone.

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.

A possible remote tropical forcing for the interannual variability of peak summer muggy hot days in Northeast China

The peak summer (July–August; JA) muggy hot weather over Northeast China (NEC) negatively impacts local socioeconomic development and human health. This study investigates the physical connection between sea surface temperature (SST) and year-to-year variations in the number of peak summer muggy hot days (MHDs) in NEC (PSMHDNEC) for the period 1979–2018. We found that on the interannual timescale, SST anomalies (SSTAs) over the tropical North Australia (TNA) sector have a stable and significant negative correlation with PSMHDNEC since the early summer of June; however, the strongest negative correlation occurs in the JA. Our further analyses indicate that the SST cooling over the TNA sector could form a large-scale atmospheric teleconnection emanating northwest of the TNA through the profound in situ diabatic cooling anomalies tied to the SST cooling, which propagates poleward from the northeastern Pacific Ocean. This teleconnection might remotely strengthen the local-scale anticyclonic anomaly centered near NEC, a critical system responsible for a higher PSMHDNEC. Under such circumstances, the NEC region is dominated by high-pressure anomalies, facilitating the establishment of localized MHD-favorable environmental conditions (e.g., increased surface air temperature and enhanced downward solar radiation flux with suppressed convection activities). During years of negative SSTAs over TNA, local SSTAs can persist from early summer until JA via the wind–evaporation–SST feedback. Therefore, it appears that SSTAs over the TNA sector may be a significant remote tropical forcing factor for the interannual variability of PSMHDNEC, and the corresponding June SST cooling may act as a potential predictability source physically contributing to a higher PSMHDNEC.

Impact of the South China Sea Summer Monsoon on the Indian Ocean Dipole in CMIP5 Models

AbstractThe impact of the South China Sea summer monsoon (SCSSM) on the Indian Ocean dipole (IOD) has been systematically investigated in observations. This study focuses on the ability of climate models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5) to reproduce the observed relationship between the SCSSM and IOD and the relevant physical mechanisms. All 23 models reproduce significant correlations between the SCSSM and IOD during boreal summer [June–August (JJA)], whereas the influence of the SCSSM on the IOD varies considerably across the CMIP5 models. To explore the causes, all models are divided into two groups. Models that successfully simulated both the correlations between the SCSSM and JJA IOD and of the SCSSM and JJA IOD with precipitation over the western North Pacific and Maritime Continent are classified as Type I, and these produce stronger low-level wind anomalies over the tropical southeastern Indian Ocean. The stronger low-level wind anomalies enhance local sea surface temperature (SST) anomalies via positive wind–evaporation–SST (WES) and wind–thermocline–SST (Bjerknes) feedbacks. This corresponds to a strengthening of IOD events due to the increased zonal gradient of SST anomalies over the tropical Indian Ocean. In contrast, Type II models perform poorly in representing the relationship between the SCSSM and JJA IOD or relevant physical processes, corresponding to weaker WES and Bjerknes feedbacks, and produce weaker IOD events. These results demonstrate that the better the model simulation of the critical physical processes, the larger contribution of the SCSSM to the IOD.

Influence of the Pacific Meridional Mode on ENSO Evolution and Predictability: Asymmetric Modulation and Ocean Preconditioning

ABSTRACTThis study investigates the mechanisms behind the Pacific meridional mode (PMM) in influencing the development of El Niño–Southern Oscillation (ENSO) events and their seasonal predictability. To examine the relative importance of various factors that may modulate the efficiency of the PMM influence, a series of experiments is conducted for selected ENSO events with different intensity using the Community Earth System Model, in which ensemble predictions are made from slightly different ocean initial states but under a common prescribed PMM surface heat flux forcing. Overall, a PMM forcing matched to ENSO—that is, a positive or negative PMM prior to an El Niño or a La Niña, respectively—plays an enhancing role, whereas a mismatched PMM forcing plays a damping role. For the matched cases, a positive PMM event enhances an El Niño more strongly than a negative PMM event enhances a La Niña. This asymmetry in influencing ENSO largely originates from the asymmetry in intensity between the positive and negative PMM events in the tropics, which can be explained by the nonlinearity in the growth and equatorward propagation of the PMM-related anomalies of sea surface temperature (SST) and surface zonal wind through both wind–evaporation–SST feedback and summer deep convection response. Our model results also indicate that the PMM acts as a modulator rather than a trigger for the occurrence of ENSO event. Furthermore, the response of ENSO to an imposed PMM forcing is modulated by the preconditioning of the upper-ocean heat content, which provides the memory for the coupled low-frequency evolution in the tropical Pacific Ocean.

The Interdecadal Reverse of the Relationship and Feedback Mechanism between Sea Surface Temperature and Evaporation over the Indian Ocean during Boreal Autumn

AbstractThe linkage between sea surface temperature (SST) and evaporation (EVP) plays an important role in air–sea interactions. In this study, the interaction mechanism of SST and EVP during boreal autumn was studied using correlation analysis, composite analysis, the EVP decomposition method, and singular value decomposition. The results showed that the correlation between SST and EVP in the Indian Ocean was reversed from positive to negative in the late 1990s. The significant positive SST–EVP relationship was attributed to the Indian Ocean basin mode forcing upon EVP during 1980–90. The decrease in wind speed–induced EVP and SST warming led to a significant negative SST–EVP relationship during 2005–15. Moreover, the negative SST–EVP correlation occurred when the Indian Ocean dipole (IOD) and subtropical Indian Ocean dipole (SIOD) exhibited inverse phases. The low-level moisture–EVP–SST feedback dominated the negative SST–EVP correlation in the negative IOD and positive SIOD (nIOD–pSIOD) pattern, whereas the wind–EVP–SST feedback played the leading role in the positive IOD and negative SIOD (pIOD–nSIOD) pattern. The EVP anomalies induced by the low-level anomalous anticyclone and cyclone were the main causes of the SST anomalies with inverse phases of the IOD and SIOD. The correlation between the SST and EVP reversal from positive to negative implies that the effect of the atmosphere on the ocean is as important as the external forcing of the ocean on the atmosphere.