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The multi‐model ensemble mean sea surface temperature (SST) of CMIP5 models shows an El Niño‐like Pacific warming (PW) trend, contradictory with the observational result which manifests a La Niña‐like PW pattern. Here, we demonstrate that these two SST PWs coexist in the CMIP5 models and they are largely determined by the model's tropical Indo‐Pacific SST gradient. When the Pacific warms faster than the Indian Ocean (IO), the model tends to project an El Niño‐like PW pattern. In contrast, a La Niña‐like warming trend prevails if a more rapid IO warming is simulated. We suggest that the PW pattern in an individual model is nonstationary and may transform from a La Niña‐like to an El Niño‐like when its interbasin SST gradient changes with more robust warming in the Pacific, and vice versa. Our conclusions shed great light on the future SST PW pattern change projected by the coupled models. Plain Language Summary: Previous studies demonstrate that an El Niño‐like SST warming pattern prevails in a majority of CMIP5 models. But a La Niña‐like warming pattern is detected based on historical SST records. This inconsistency between the observations and model simulation triggers an intense debate in the climate community. In this study, we find that both El Niño‐like and La Niña‐like SST warming patterns can be identified in the CMIP5 model simulations. The warming pattern is largely determined by the SST gradient between the Indian and Pacific oceans. Distinct from previous studies, an El Niño‐like pattern in an individual model can shift to a La Niña‐like pattern when the Indian warming is stronger than the Pacific warming trend, and vice versa. Key Points: El Niño‐like and La Niña‐like Pacific sea surface temperature warming patterns coexist and are interchangeable in climate model projectionsThe tropical Pacific warming pattern is primarily determined by a zonal gradient in the Indo‐Pacific sea surface warmingThis Pacific warming pattern diversity sheds light on future sea surface temperature change under greenhouse gases concentrations [ABSTRACT FROM AUTHOR]

Using the ensemble hindcasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) coupled model for the period of 1980–2005, spatiotemporal evolution in the covariability of sea surface temperature (SST) and low-level winds in the ensemble mean and spread over the tropical Atlantic is investigated with the month-reliant singular value decomposition (SVD) method, which treats the variables in a given monthly sequence as one time step. The leading mode of the ensemble mean represents a coevolution of SST and winds over the tropical Atlantic associated with a phase transition of El Niño from the peak to decay phase, while the second mode is related to a phase transition from El Niño to La Niña, indicating a precursory role of the north tropical Atlantic (NTA) SST warming in La Niña development. The leading mode of ensemble spread in SST and winds further illustrates that an NTA SST anomaly acts as a precursor for El Niño–Southern Oscillation (ENSO). A north-tropical pathway for the delayed effect of the NTA SST anomaly on the subsequent ENSO event is identified; the NTA SST warming induces the subtropical northeast Pacific SST cooling through the modulation of a zonal–vertical circulation, setting off a North Pacific meridional mode (NPMM). The coupled SST–wind anomalies migrate southwestward to the central equatorial Pacific and eventually amplify into a La Niña event in the following months due to the equatorial Bjerknes feedback. Ensemble spread greatly increases the sample size and affords insights into the interbasin interactions between the tropical Atlantic and Pacific, as demonstrated here in the NTA SST impact on ENSO. [ABSTRACT FROM AUTHOR]

The net heat flux and meridional temperature advection in the ocean are two factors in the North Pacific subtropical sea surface temperature front (NPSTF) frontogenesis occurring from October to the following February. However, the relative importance of these two factors has been rarely explored. In this study, frontogenesis of the NPSTF is examined quantitatively based on the mixed-layer heat budget equation to clarify the relative importance of net heat flux and meridional temperature advection and to further explore its connection with the atmosphere above. Diagnosis results show that the net heat flux dominates the frontogenesis from October to December, while the meridional temperature advection in the ocean contributes equally as or even more than the net heat flux in January and February. The atmosphere is critical to the frontogenesis of the NPSTF, including the direct effect of the net heat flux and the indirect effect through the Aleutian low. Further analyses demonstrate that the latent heat flux (the shortwave radiation) dominates the net heat flux in October (from November to February). The meridional temperature advection in the ocean is mostly due to the meridional Ekman convergence, which is related to the Aleutian low. Climatologically, the strengthening and southward migration of the Aleutian low from October to the following February are characterized by the acceleration and southward shift of the westerly wind to the south, respectively, which can drive southward ocean currents. Correspondingly, the southward ocean currents provide for colder meridional advection to the north of the NPSTF in January and February, favoring frontogenesis. In addition, the Aleutian low plays a role in transforming the dominant effect of the net heat flux into the joint effect of meridional temperature advection and net heat flux in January. [ABSTRACT FROM AUTHOR]

ABSTRACT: The North Pacific subtropical front (NPSTF) of sea surface temperature (SST) is an important subtropical feature in the North Pacific. In this study, we reveal that the inter‐annual variability of the spring NPSTF has a robust correlation with the El Niño‐Southern Oscillation (ENSO) in the following winter over the period of 1961–2016. When there is a strong spring NPSTF, anomalous cyclonic and anticyclonic circulations in the low troposphere occur to its north and south, respectively. The northeasterly wind anomaly associated with the anticyclonic circulation increases the climatological northeasterly wind over the subtropical Northeast Pacific, resulting in SST decrease via the wind–evaporation–SST feedback. The SST and wind perturbations propagate southwards from the subtropical to the eastern central equatorial Pacific in summer and grow via the Bjerknes feedback from summer to winter, eventually leading to a La Niña event in winter. A strong spring NPSTF induces an anomalous anticyclonic circulation to its south through both transient eddy activity and atmospheric heat source anomaly. The enhanced air temperature gradient due to a strong NPSTF increases the atmospheric baroclinicity, favouring an enhanced transient eddy activity over the NPSTF and to the north. The enhanced transient eddy activity triggers a significant negative height anomaly to the north via high‐frequency transient eddy feedback forcing, which is conductive to an anticyclonic circulation over the subtropical North Pacific through the downstream effect of Rossby waves. On the other hand, the convergence and increased atmospheric baroclinicity related to the strong NPSTF act to enhance precipitation over the NPSTF and to the north, corresponding to the increased atmospheric heat source, which can also excite an anomalous anticyclonic circulation over the subtropical North Pacific. [ABSTRACT FROM AUTHOR]

Using observational data covering 1948–2020, the environmental factors controlling the winter precipitation in California were investigated. Empirical orthogonal function (EOF) analysis was applied to identify the dominant climate regimes contributing to the precipitation. The first EOF mode described a consistent change, with 70.1% variance contribution, and the second mode exhibited a south–east dipole change, with 11.7% contribution. For EOF1, the relationship was positive between PC1(principal component) and SST (sea surface temperature) in the central Pacific Ocean, while it was negative with SST in the southeast Indian Ocean. The Pacific–North America mode, induced by the positive SST and precipitation in the central Pacific Ocean, leads to California being occupied by southwesterlies, which would transport warm and wet flow from the ocean, beneficial for precipitation. As for the negative relationship, California is controlled by biotrophically high pressure, representing part of the Rossby wave train induced by the positive SST in the Indian ocean, which is unfavorable for the precipitation. For EOF2, California is controlled by positive vorticity at the upper level, whereas at the lower level, there is positive vorticity to the south and negative vorticity to the north, the combination of which leads to the dipole mode change in the precipitation. [ABSTRACT FROM AUTHOR]

Previous studies have documented the significant influences of the North Pacific subtropical sea surface temperature (SST) front (NPSTF) on the overlying atmospheric circulation. However, whether and how the NPSTF could affect remote circulations has not been well addressed. Here, we demonstrate that the enhanced NPSTF can induce a southward shift of the downstream North Atlantic westerly jet stream (NAWJS) in winter. A strong NPSTF with an increased meridional SST gradient induces a positive (negative) air temperature anomaly in its south (north) flank through the heat flux, leading to a direct thermal wind response and an indirect transient eddy feedback. The direct thermal wind response benefits a negative height anomaly south of Gulf of Alaska below 250 hPa and a positive one above 200 hPa, while the transient eddy activity could locally enhance and tilt northward with height. The kinetic energy significantly grows from the baroclinic energy conversion below 250 hPa over the NPSTF and from the barotropic energy conversion north of the NPSTF at the upper levels. This enhanced transient eddy activity gives rise to a negative height anomaly south of Gulf of Alaska through high-frequency transient eddy feedback forcing at all levels dominated by transient eddy vorticity flux. The anomalous low south of Gulf of Alaska favors an anomalous high over Canada and an anomalous low over the United States through Rossby energy propagation, causing the southward shift of the NAWJS. The numerical model further verifies the downstream influences of the NPSTF on the NAWJS in winter. • In addition to local effects, NPSTF can deliver its effect further downstream and modulate the meridional shift in NAWJS. • The height response south of Alaska Bay fosters downstream wind and height anomalies via Rossby energy propagation. • The transient eddy activity jointly induced by BTEC and BCEC dominates the local atmospheric response of NPSTF. [ABSTRACT FROM AUTHOR]

In this study, we use the long-term satellite data to investigate seasonal and interannual variation of low-cloud fraction (LCF) and the associated controlling factors over the eastern and western North Pacific. On the seasonal time scale, the enhanced LCF over the eastern North Pacific in summer is actively coupled with strong estimated inversion strength (EIS) and 700-hPa relative humidity, and the LCF over the western North Pacific in winter is large and mainly caused by increased sensible heat flux and tropospheric low-level cold advection. On the interannual time scale, the increased LCF over the eastern North Pacific in summer is associated with increased EIS and decreased sea surface temperatures, in which the El Niño plays an important role; the enhanced LCF over the western North Pacific in spring and winter has a positive correlation with enhanced sensible heat flux (SHF) and tropospheric low-level cold advection, which can be partly explained by the subpolar frontal zone (SPFZ) intensity. [ABSTRACT FROM AUTHOR]

It has been well documented that storm track activity are closely related to the weather and short-term climate variability in the extratropics, which is affected by sea surface temperature anomalies over the tropical eastern Pacific Ocean. Interannual relationship between the El Niño-Southern Oscillation (ENSO) and the Atlantic storm track (AST) in spring modulated by the Atlantic multidecadal oscillation (AMO) was investigated using reanalysis data and model simulations in this study. The meridional displacement of the AST is significantly correlated with ENSO during negative AMO phase, while no significant relationship is found during positive AMO phase. This may be due to the difference of 500-hPa geopotential height anomalies induced by ENSO in different AMO phases. For an El Niño event during the negative AMO phase, an anomalous 500-hPa wave train propagates eastward across the North American continent, with positive height anomalies at the high latitudes, extending from South Canada to Newfoundland. Thus, easterly wind anomalies appear over central North America, upstream of the negative AST anomaly. Accordingly, the local eddy growth rate (EGR) and baroclinic energy conversion (BC) are obviously reduced, which weaken (strengthen) the southern (northern) part of the climatological AST. As a result, the AST is shifted northward significantly. During the positive AMO phase, the ENSO-related anomalous wave train at 500 hPa only propagates northeastward and is largely suppressed over Northwest Canada, with positive height anomalies confined to the northwest of North America. Therefore, no significant changes of the westerly jet, EGR and BC are found in the upstream region of the AST, and the meridional location of the AST generally remains unchanged. Most previous studies investigate AST variabilities in winter, and few focus on AST in spring. This work may be helpful in understanding more about the interannual and interdecadal variations of springtime AST and in further studying the weather and short-term climate changes caused by AST. [ABSTRACT FROM AUTHOR]