Equatorial Sea Surface Temperature Research Articles

Why Does Extreme Precipitation Occur in Borneo During Both El Niño and La Niña Winters?

AbstractExtreme precipitation over the Maritime Continent (MC) is not only vital for local livelihoods, but also influences atmospheric circulation in tropical and extra‐tropical regions. The warm (cold) phase of El Niño‐Southern Oscillation usually leads to dry (wet) condition around the MC in boreal winter. This study unravels that extreme precipitation occurs in Borneo during both El Niño and La Niña winters. Extreme precipitation in Borneo is associated with tripole and dipole anomalous vertical motion pattern, respectively, over the MC‐tropical Pacific in El Niño and La Niña years. A parallel analysis reveals that weak precipitation in Borneo in El Niño and La Niña years occurs with dipole and tripole anomalous vertical motion pattern, respectively, over the MC‐tropical Pacific. The amplitude of the equatorial central‐eastern Pacific sea surface temperature (SST) anomalies may be a factor in the change of anomalous vertical motion pattern over the MC‐tropical Pacific. Further analysis identifies a tendency of extreme precipitation in late December to early−mid January and weak precipitation in February in Borneo. This suggests a critical role of seasonal change in the eastern equatorial Pacific SST in the occurrence of extreme precipitation in Borneo.

Origins of Biweekly Sea Surface Temperature Variability in the Eastern Equatorial Pacific and Atlantic

AbstractBiweekly sea surface temperature (SST) variability significantly contributes to over 50% of the intraseasonal variability in the eastern equatorial Pacific (EEP) and Atlantic (EEA). Our study investigates this biweekly variability, employing a blend of in–situ and reanalysis data sets. The research identifies biweekly signals in SST, meridional wind, and ocean currents, notably in September–November in EEP and June–August in EEA. Biweekly southerly (northerly) winds drive instantaneous northward (southward) ocean currents in EEP, but with a 1–2‐day phase delay in EEA. Consequently, these currents lead to SST anomalies with a 3–4‐day lag in both EEP and EEA due to the presence of the cold tongue. The study reveals the origin of biweekly wind fluctuations in the western Pacific for EEP and the subpolar Pacific for EEA, connected by atmospheric Rossby waves validated through a linearized non‐divergent barotropic model. This research affirms the influence of subtropical and subpolar atmospheric forcing on equatorial SST.

Interhemispheric propagation of seasonal sea surface temperature fluctuations over the western Pacific in CMIP6

AbstractInterannual sea surface temperature (SST) variability in the western Pacific is a critical research topic in climate change, as it can significantly impact regional and global weather patterns. Previous observational studies have shown that SST in the southwestern Pacific (SWP) can effectively serve as a precursor signal for SST in the western tropical Pacific (WTP). In this study, we evaluate and compare the modelling capabilities of 32 CMIP6 models to simulate the relationship between SWP and WTP SST. The results indicate that while some CMIP6 models can simulate the spatial connection and physical processes between the SWP and WTP, there are significant intermodel differences. Specifically, the models that best simulate the cross‐equatorial propagation of SWP SST anomalies are those that accurately simulate the cross‐equatorial wind anomalies induced by SWP SST anomalies, highlighting the key role of wind‐evaporation‐SST feedback in SST propagation. Further analysis reveals that the intermodel spread in cross‐equatorial SST propagation is attributable to model biases in simulating climatological SST and wind speed. Models with warm climatological equatorial SST and strong wind speeds provide favourable conditions for the cross‐equatorial propagation of SWP‐related SST anomalies towards the north. Warm SST and strong inter‐season wind speeds near the equator facilitate the northward propagation of the SWP warm signal, leading to increased warming in the WTP.

Interhemispheric Temperature Gradient and Equatorial Pacific SSTs Drive Sahel Monsoon Uncertainties under Global Warming

Abstract The Sahel is one of the most vulnerable regions to climate change. Robust estimation of future changes in the Sahel monsoon is therefore essential for effective climate change adaptation. Unfortunately, state-of-the-art climate models show large uncertainties in their projections of Sahel rainfall. In this study, we use 32 models from CMIP6 to identify the sources of this large intermodel spread of Sahel rainfall. By using maximum covariance analysis, we first highlight two new key drivers of this spread during boreal summer: the interhemispheric temperature gradient and equatorial Pacific sea surface temperature (SST) changes. This contrasts with previous studies, which have focused mainly on the Northern Hemisphere rather than the global scale, and in which the Pacific Ocean has been neglected in favor of the Atlantic. Next, we unravel the physical mechanisms behind these statistical relationships. First, the modulation of the interhemispheric temperature gradient across the models leads to varying latitudinal positions of the intertropical convergence zone and, consequently, varying Sahel rainfall intensity. Second, models that exhibit less warming than the multimodel mean in the equatorial Pacific, thereby projecting a less “El Niño–like” mean state, simulate enhanced precipitation over the central Sahel in the future through modulations of the Walker circulation, the tropical easterly jet, the meridional tropospheric temperature gradient, and hence regional zonal wind shear. Finally, we show that these two indices collectively explain 62% of Sahel rainfall change uncertainty: 40% due to the interhemispheric temperature gradient and 22% through equatorial Pacific SST.

A distinct and reproducible teleconnection pattern over North America during extreme El Niño events

El Niño-Southern Oscillation (ENSO) teleconnections are an important predictability source for extratropical seasonal climate forecasts. Previous studies suggest that the ENSO teleconnection pattern depends on the ENSO phase (El Niño vs. La Niña) and/or Sea Surface Temperature (SST) pattern (central Pacific vs. eastern Pacific El Niño events). Observations and ensemble simulations with the CNRM-CM6.1 atmospheric general circulation model indicate that only extreme El Niño events (e.g. 1982–1983, 1997–1998, 2015–2016) display a statistically significant eastward shift relative to the well-known Pacific-North American teleconnection pattern that occurs during both central and eastern Pacific moderate El Niño or during La Niña. This specific teleconnection pattern emerges when equatorial SST anomalies are both eastward-shifted and sufficiently large to exceed the deep atmospheric convection threshold over most of the eastern Pacific, resulting in a basin-wide reorganization of tropospheric heat sources. It yields> 0.5 std wet conditions over Western United States (74% likelihood) as well as> 0.5 std warm anomalies over Canada and the Northern United States (71% likelihood), with more consistency across events and ensemble members than for any other El Niño or La Niña type. These findings hold important implications for the seasonal forecasting of El Niño’s impacts on the North American climate.

Local and Remote Causes of the Equatorial Pacific Cold Sea Surface Temperature Bias in the Kiel Climate Model

Abstract We investigate the origin of the equatorial Pacific cold sea surface temperature (SST) bias and its link to wind biases, local and remote, in the Kiel Climate Model (KCM). The cold bias is common in climate models participating in phases 5 and 6 of the Coupled Model Intercomparison Project. In the coupled experiments with the KCM, the interannually varying NCEP/CFSR wind stress is prescribed over four spatial domains: globally, over the equatorial Pacific (EP), the northern Pacific (NP), and the southern Pacific (SP). The corresponding EP SST bias is reduced by 100%, 52%, 12%, and 23%, respectively. Thus, the EP SST bias is mainly attributed to the local wind bias, with small but not negligible contributions from the extratropical regions. Erroneous ocean circulation driven by overly strong winds causes the cold SST bias, while the surface heat flux counteracts it. Extratropical Pacific SST biases contribute to the EP cold bias via the oceanic subtropical gyres, which is further enhanced by dynamical coupling in the equatorial region. The origin of the wind biases is examined by forcing the atmospheric component of the KCM in a stand-alone mode with observed SSTs and simulated SSTs from the coupled experiments. Wind biases over the EP, NP, and SP regions originate in the atmosphere model. The cold EP SST bias substantially enhances the wind biases over all three regions, while the NP and SP SST biases support local amplification of the wind bias. This study suggests that improving surface wind stress, at and off the equator, is a key to improve mean-state equatorial Pacific SST in climate models.

Future Projections of the El Niño—Southern Oscillation and Tropical Pacific Mean State in CMIP6

AbstractThe El Niño—Southern Oscillation (ENSO) is an important mode of tropical Pacific atmosphere‐ocean variability that drives teleconnections with weather and climate globally. However, prior studies using state‐of‐the‐art climate models lack consensus regarding future ENSO projections and are often impacted by tropical Pacific sea‐surface temperature (SST) biases. We used 173 simulations from 29 climate models participating in the Coupled Model Intercomparison Project, version 6 (CMIP6) to analyze model biases and future ENSO projections. We analyzed two ENSO indices, namely the ENSO Longitude Index (ELI), which measures zonal shifts in tropical Pacific deep convection and accounts for changes in background SST, and the Niño 3.4 index, which measures SST anomalies in the central‐eastern equatorial Pacific. We found that the warm eastern tropical‐subtropical Pacific SST bias typical of previous generations of climate models persists into many of the CMIP6 models. Future projections of ENSO shift toward more El Niño‐like conditions based on ELI in 48% of simulations and 55% of models, in association with a future weakening of the zonal equatorial Pacific SST gradient. On the other hand, none of the models project a significant shift toward La Niña‐like conditions. The standard deviation of the Niño 3.4 index indicates a lack of consensus on whether an increase or decrease in ENSO variability is expected in the future. Finally, we found a possible relationship between historical SST and low‐level cloud cover biases in the ENSO region and future changes in ELI; however, this result may be impacted by limitations in data availability.

Dual Wave Energy Sources for the Atlantic Niño Events Identified by Wave Energy Flux in Case Studies

AbstractThe evolution of equatorial sea surface temperature anomalies during Atlantic Niño events has demonstrated its diversity in both intensity and timing. To investigate the mechanism responsible for this diversity, this study focuses on ocean responses to atmospheric forcing, manipulating the wind forcing in both equatorial and off‐equatorial regions to excite linear ocean models for three types of events that occurred in 1999, 2019, and 2021 respectively. The results reveal the dual wave energy sources for the equatorial Kelvin waves (KWs): one is the local wind forcing in the western tropical basin; the other is the reflection due to the off‐equatorial Rossby waves in the western boundary. The reflected KWs can precondition the events when wind‐forced KWs insufficiently displace the thermocline (e.g., the boreal winter of 2019 and 2021). The participation of off‐equatorial wave energy hence leads to the diversity of the Atlantic Niños.

Changes in the West Africa Monsoon Precipitation Extremes during ENSO developing Phases

ABSTRACT A 37-year record of rainfall gridded data covering West Africa and a Global Sea Surface Temperature (SST) dataset are used to investigate the remote influence of SST anomalies in the Equatorial Pacific on the interannual variability of West Africa’s extreme rainfall indices over the period 1981–2018. The top five (5) years with the strongest and weakest peak of Niño3.4 SST monthly anomalies are selected, and May-to-September (MJJAS) composite anomalies of the total and extreme rainfall indices are performed. Results reveal that Equatorial Pacific SST’s impacts on daily rainfall intensity are generally more robust than that on their frequency. The significant changes in the mean zonal atmospheric circulation associated with the SST lead to significant dynamic and thermodynamic changes that affect the West African monsoon system locally. During El Niño (La Niña) years, (i) a weakening (strengthening) of the Tropical Easterly Jet (TEJ), (ii) a strengthening (weakening), and southward (northward) shift position of the African Easterly Jet (AEJ), and (iii) a decrease (increase) of the monsoon flow are noted. These changes in the atmospheric circulation prevent (encourage) a supply of moisture, resulting in a reduction (increase) in extreme precipitation observed across West Africa. Equatorial eastern Pacific warming (cooling) is also shown to lead to stable (unstable) atmospheric conditions over West Africa that block (generate) the development of convective systems.

A Simple Multiscale Intermediate Coupled Stochastic Model for El Niño Diversity and Complexity

AbstractEl Niño‐Southern Oscillation (ENSO) is the most prominent interannual climate variability in the tropics and exhibits diverse features in spatiotemporal patterns. This paper develops a simple multiscale intermediate coupled stochastic model to capture the ENSO diversity and complexity. The model starts with a deterministic and linear coupled interannual atmosphere, ocean, and sea surface temperature (SST) system. It can generate two dominant linear solutions representing the eastern Pacific (EP) and the central Pacific (CP) El Niños, respectively. In addition to adopting a stochastic model for characterizing the intraseasonal wind bursts, another simple stochastic process is developed to describe the decadal variation of the background Walker circulation. The latter links the two dominant modes in a simple nonlinear fashion and advances the modulation of the strength and occurrence frequency of the EP and the CP events. Finally, cubic nonlinear damping is adopted to parameterize the relationship between subsurface temperatures and thermocline depth. The model succeeds in reproducing the spatiotemporal dynamical evolution of different types of ENSO events. It also accurately recovers the strongly non‐Gaussian probability density function, the seasonal phase locking, the power spectrum, and the temporal autocorrelation function of the SST anomalies in all the three Niño regions (3, 3.4, and 4) across the equatorial Pacific. Furthermore, both the composites of the SST anomalies for various ENSO events and the strength‐location bivariate distribution of equatorial Pacific SST maxima for the El Niño events from the model simulation highly resemble those from the observations.

Decadal Variability of the Pacific Shallow Overturning Circulation and the Role of Local Wind Forcing

Abstract The Pacific shallow meridional overturning circulations, known as subtropical cells (STCs), link subduction in the subtropical regions to equatorial upwelling, suggesting the possibility for subtropical winds to influence equatorial sea surface temperatures (SSTs) by altering the STCs’ strength. Indeed, STC variability provides the basis for one of the mechanisms proposed to explain the origin of tropical Pacific decadal variability (TPDV). While the relationship between STC strength, as measured by their subsurface transport convergence, and equatorial SST variations is well documented, the location of the wind forcing most influential on STC variability is still being debated. In this study, we use the output of an ocean reanalysis to examine tropical Pacific Ocean surface and subsurface decadal changes during recent decades and relate them to STC variability and surface wind forcing. Our results indicate that the STC interior transport at each latitude is largely controlled by the wind forcing at that latitude rather than induced by remote subtropical wind variations. We also show that the establishment of the anomalous transport at each latitude is associated with the westward propagation of oceanic wind-forced Rossby waves, as part of the ocean adjustment process that also leads to a zonal redistribution of upper-ocean heat content at both interannual and decadal time scales. These results provide guidance for understanding the origin of TPDV by elucidating the underlying dynamics of STC variability and can have practical implications for monitoring STC variability in the tropical Pacific. Significance Statement Slow variations of the surface ocean temperature in the tropical Pacific Ocean have been shown to affect the global climate. Our study aims at better understanding the origin of these temperature anomalies by taking a closer look at the upper ocean circulation variability and its relationship with surface wind forcing. Unlike previous studies, which have related the upper ocean circulation changes to wind variations outside the tropical Pacific, we show here that the variations in upper-ocean circulation are primarily driven by local winds. This result not only clarifies which winds are most important, but also suggests a practical approach for monitoring circulation changes from surface observations.

A regime shift in North Pacific annual mean sea surface temperature in 2013/14

This study revealed a new regime shift in the North Pacific annual mean sea surface temperature (SST) in 2013/14, robust in multiple SST data sets. This regime shift shows a horseshoe pattern with warming SST along the North American western coast and toward the Central Pacific. It suggests a phase reversal of the Victoria Mode (VM) and it seems unrelated to the shift in the Pacific Decadal Oscillation (PDO) and Interdecadal Pacific Oscillation (IPO). Associated with this regime shift, the annual surface air temperature significantly enhanced over the high-latitude landmass after 2014, whereas it declined over northeastern Canada and southwestern Greenland. The annual precipitation generally increased (decreased) in the Northern (Southern) Hemisphere landmass monsoon regions. There are no regime shifts in Equatorial Central-East Pacific SST and the intensity and locations of the Aleutian Low around 2013/14. These facts suggest the regime shift of North Pacific SST in 2013/14 may not originate from the decadal variations of the midlatitude atmosphere and tropical ocean. The extensive influence of this regime shift in 2013/14 on the atmospheric, oceanic and ecological systems warrants further investigation.

Atlantic impacts on subdecadal warming over the tropical Pacific in the 2000s

IntroductionA subdecadal (i.e. , three-year running mean) variation over the tropical Pacific is very distinctively observed in the 2000s.Results and methodsHere, we have demonstrated that sea surface temperature (SST) anomalies in the tropical Atlantic contribute to forming high ocean-temperature anomalies in the tropical Pacific in the early 2000s by performing partial data assimilation of a global climate model. Low SSTs over the equatorial Atlantic change the Walker circulation, and the associated weakening of the Pacific trade winds raises the equatorial SST on subdecadal timescales. At the same time, a high SST anomaly is also generated in the off-equatorial North Pacific through deepening of the upper ocean thermocline due to an accompanying anticyclonic surface wind anomaly aloft. While the subtropical North Atlantic SSTs may help the subdecadal warming in the equatorial Pacific, the resultant SST anomalies show a one-year delay in the phase transition and are modestly accompanied by ocean thermocline deepening.DiscussionIt roughly follows the IMRaD format.

A 15-million-year surface- and subsurface-integrated TEX86 temperature record from the eastern equatorial Atlantic

Abstract. TEX86 is a paleothermometer based on Thaumarcheotal glycerol dialkyl glycerol tetraether (GDGT) lipids and is one of the most frequently used proxies for sea-surface temperature (SST) in warmer-than-present climates. However, GDGTs are not exclusively produced in and exported from the mixed layer, so sedimentary GDGTs may contain a depth-integrated signal that is also sensitive to local subsurface temperature variability. In addition, the correlation between TEX86 and SST is not significantly stronger than that to depth-integrated mixed-layer to subsurface temperatures. The calibration of TEX86 to SST is therefore controversial. Here we assess the influence of subsurface temperature variability on TEX86 using a downcore approach. We present a 15 Myr TEX86 record from Ocean Drilling Program Site 959 in the Gulf of Guinea and use additional proxies to elucidate the source of the recorded TEX86 variability. Relatively high GDGT[2/3] ratio values from 13.6 Ma indicate that sedimentary GDGTs were partly sourced from deeper (>200 m) waters. Moreover, late Pliocene TEX86 variability is highly sensitive to glacial–interglacial cyclicity, as is also recorded by benthic δ18O, while the variability within dinoflagellate assemblages and surface/thermocline temperature records (U37k′ and Mg/Ca) is not primarily explained by glacial–interglacial cyclicity. Combined, these observations are best explained by TEX86 sensitivity to sub-thermocline temperature variability. We conclude that TEX86 represents a depth-integrated signal that incorporates a SST and a deeper component, which is compatible with the present-day depth distribution of Thaumarchaeota and with the GDGT[2/3] distribution in core tops. The depth-integrated TEX86 record can potentially be used to infer SST variability, because subsurface temperature variability is generally tightly linked to SST variability. Using a subsurface calibration with peak calibration weight between 100 and 350 m, we estimate that east equatorial Atlantic SST cooled by ∼5 ∘C between the Late Miocene and Pleistocene. On shorter timescales, we use the TEX86 record as a proxy for South Atlantic Central Water (SACW), which originates from surface waters in the South Atlantic Gyre and mixes at depth with Antarctic Intermediate Water (AAIW). Leads and lags around the Pliocene M2 glacial (∼3.3 Ma) in our record, combined with published information, suggest that the M2 glacial was marked by SACW cooling during an austral summer insolation minimum and that decreasing CO2 levels were a feedback, not the initiator, of glacial expansion.

Diagnosing Hawaii’s Recent Drought

Abstract Hawaii’s recent drought is among the most severe on record. Wet-season (November–April) rainfall deficits during 2010–19 rank second lowest among consecutive 10-yr periods since 1900. Various lines of empirical and model evidence indicate a principal natural atmospheric cause for the low rainfall, mostly unrelated to either internal oceanic variability or external forcing. Empirical analysis reveals that traditional factors have favored wetness rather than drought in recent decades, including a cold phase of the Pacific decadal oscillation in sea surface temperatures (SSTs) and a weakened Aleutian low in atmospheric circulation. But correlations of Hawaiian rainfall with patterns of Pacific sea level pressure and SSTs that explained a majority of its variability during the twentieth century collapsed in the twenty-first century. Atmospheric model simulations indicate a forced decadal signal (2010–19 vs 1981–2000) of Aleutian low weakening, consistent with recent observed North Pacific circulation. However, model ensemble means do not generate reduced Hawaiian rainfall, indicating that neither oceanic boundary forcing nor a weakened Aleutian low caused recent low Hawaiian rainfall. Additional atmospheric model experiments explored the role of anthropogenic forcing. These reveal a strong sensitivity of Hawaiian rainfall to details of long-term SST change patterns. Under an assumption that anthropogenic forcing drives zonally uniform SST warming, Hawaiian rainfall declines, with a range of 3%–9% among three models. Under an assumption that anthropogenic forcing also increases the equatorial Pacific zonal SST gradient, Hawaiian rainfall increases 2%–6%. Large spread among ensemble members indicates that no forced signals are detectable.

Will Increasing Climate Model Resolution Be Beneficial for ENSO Simulation?

AbstractIncreasing climate model resolution offers multifaceted benefits, such as improving modeled tropical cyclones. However, the extent to which it benefits El Niño‐Southern Oscillation (ENSO) simulation remains unknown. Here, comprehensive information on the sensitivity of ENSO performance to various resolutions is provided, based on a multi‐model and multi‐resolution ensemble of global coupled models. Overall, the reduced model biases of the equatorial sea surface temperature (SST) and precipitation mean state in higher‐resolution models may be attributed to increased oceanic resolution, and thus, better resolved eddy‐driven heat transport. ENSO spatial patterns were reproduced clearer in the eddy‐present models, likely due to the improved mean state and associated surface thermodynamic feedback. However, increasing atmospheric resolution alone deteriorates ENSO asymmetry, which may be due to the degradation of nonlinear atmospheric feedback. It remains challenging to alleviate the SST−shortwave‐flux feedback bias, which is a major source of too‐weak net heat flux feedback, irrespective of model resolution.

The Role of Seasonality and the ENSO Mode in Central and East Pacific ENSO Growth and Evolution

Abstract A cyclostationary linear inverse model (CSLIM) is used to investigate the seasonal growth of tropical Pacific Ocean El Niño–Southern Oscillation (ENSO) events with canonical, central Pacific (CP), or eastern Pacific (EP) sea surface temperature (SST) characteristics. Analysis shows that all types of ENSO events experience maximum growth toward final states occurring in November and December. ENSO events with EP characteristics also experience growth into May and June, but CP events do not. A single dominant “ENSO mode,” growing from an equatorial heat content anomaly into a characteristic ENSO-type SST pattern in about 9 months (consistent with the delayed/recharge oscillator model of ENSO), is essential for the predictable development of all ENSO events. Notably, its seasonality is responsible for the late-calendar-year maximum in ENSO amplification. However, this ENSO mode alone does not capture the observed growth and evolution of diverse ENSO events, which additionally involve the seasonal evolution of other nonorthogonal Floquet modes. EP event growth occurs when the ENSO mode is initially “covered up” in combination with other Floquet modes. The ENSO mode’s slow seasonal evolution allows it to emerge while the other modes rapidly evolve and/or decay, leading to strongly amplifying and more predictable EP events. CP events develop when the initial state has a substantial contribution from Floquet modes with meridional mode–like SST structures. Thus, while nearly all ENSO events involve the seasonally varying ENSO-mode dynamics, the diversity and predictability of ENSO events cannot be understood without identifying contributions from the remaining Floquet modes. Significance Statement The purpose of this study is to identify structures that lead to seasonal growth of diverse types of El Niño–Southern Oscillation (ENSO) events. An important contribution from this study is that it uses an observationally constrained, empirically derived seasonal model. We find that processes affecting the evolution of diverse ENSO events are strongly seasonally dependent. ENSO events with eastern equatorial Pacific sea surface temperature (SST) characteristics are closely related to a single “ENSO mode” that resembles theoretical models of ENSO variability. ENSO events that have central equatorial Pacific SST characteristics include contributions from additional “meridional mode” structures that evolve via different physical processes. These findings are an important step in evaluating the seasonal predictability of ENSO diversity.

Equatorial Origin of the Observed Tropical Pacific Quasi‐Decadal Variability From ENSO Nonlinearity

AbstractQuasi‐decadal (QD) climate variability is detected in the tropical Pacific based on the recent 70 years of observations. This QD variability is identified in equatorial sea surface temperatures (SSTs), the pattern of which resembles the Central Pacific (CP) El Niño‐Southern Oscillation (ENSO) but extends further meridionally to the northeastern subtropical Pacific. Whereas equatorial upper‐ocean heat content and SSTs are in quadrature for ENSO, these two quantities are almost in phase on the QD timescale. Further analysis shows that nonlinear dynamical heating, primarily originating from strong El Niño events, tends to lead QD SSTs by a quarter of its dominant period (approximately 30 months) and shapes the dominant QD periodicity in observations. Our results suggest that the observed QD variability largely originates from ENSO nonlinearity and thus is fundamentally different from ENSO's oscillatory nature.

Linkage between cross-equatorial potential vorticity flux and surface air temperature over the mid–high latitudes of Eurasia during boreal spring

The source of potential vorticity (PV) for the global domain is located at the Earth’s surface. PV in one hemisphere can exchange with the other through cross-equatorial PV flux (CEPVF). This study investigates the features of the climatic mean CEPVF, the connection in interannual CEPVF with the surface thermal characteristics, and the associated mechanism. Results indicate that the process of positive (negative) PV carried by a northerly (southerly) wind leads to the climatologically overwhelming negative CEPVF over almost the entire equatorial cross-section, while the change of the zonal circulation over the equator is predominately responsible for CEPVF variation. By introducing the concept of “PV circulation” (PVC), it is demonstrated that the interannual CEPVF over the equator is closely linked to the notable uniform anomalies of spring cold surface air temperature (SAT) over the mid–high latitudes of Eurasia by virtue of the PVC, the PV-θ mechanism, and the surface positive feedback. Further analysis reveals that equatorial sea surface temperature (SST) forcing, such as the El Niño–Southern Oscillation and tropical South Atlantic uniform SST, can directly drive anomalous CEPVF by changing the zonal circulation over the equator, thereby influencing SAT in the Northern Hemisphere. All results indicate that the equilibrium linkage between CEPVF and extratropical SAT is mainly a manifestation of the response of extratropical SAT to tropical forcing by virtue of PVC, and that the perspective of PVC can provide a reasonably direct and simple connection of the circulation and climate between the tropics and the mid–high latitudes.

What Determine the Performance of the ENSO‐East Asian Winter Monsoon Relationship in CMIP6 Models?

AbstractThis study evaluates the relationship between El Niño‐Southern Oscillation (ENSO) and the East Asian winter monsoon (EAWM) in 26 Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Of particular concern is the plausible factors for the model's capability of simulating the ENSO‐EAWM relationship. Results show that the model's ability of simulating the ENSO‐EAWM relationship is more dependent upon the longitudinal extension of ENSO‐related equatorial Pacific sea surface temperature (SST) anomalies than the amplitude of the equatorial central‐eastern Pacific SST anomalies. The influence of the amplitude of ENSO on the simulation of the ENSO‐EAWM relationship depends on the westward extension of ENSO‐related equatorial Pacific SST anomalies. Another factor for the model's ability of simulating the ENSO‐EAWM relationship is the SST anomalies in the tropical western North Pacific (WNP). A westward extension of the equatorial Pacific SST anomalies shifts the west branch of anomalous Walker circulation too far westward, which causes westward displaced anomalous ascending (descending) motion around the Philippines through modulating regional meridional vertical circulation in El Niño (La Niña) years. The weak SST anomalies in the tropical WNP lead to the failure of inducing anomalous lower‐level anticyclone (cyclone) over the Philippine Sea through a Rossby wave response in El Niño (La Niña) years. The accompanying weak anomalous lower‐level southwesterly (northeasterly) winds along the west flank of the anomalous anticyclone (cyclone) account for the weak ENSO‐EAWM relationship.