ENSO-Dependent and ENSO-Independent Variability over the Mid-Latitude North Pacific: Observation and Air-Sea Coupled Model Simulation

During El Niño events, the warm anomalies in the eastern tropical Pacific are seen to occur in conjunction with prominent warm anomalies in the North Pacific SSTs off the west coast of North America as well as with cold anomalies in the central North Pacific. This kind of North Pacific response to ENSO is examined in observational data and IPSL air-sea coupled model simulations. Analyses based on observational data and the model output data both support the hypothesis of an “atmospheric bridge concept”, i.e., the atmospheric response to ENSO, in turn, forces the extra-tropical SST anomalies associated with the El Ninno event, thereby serving as a bridge between the tropical and extra-tropical Pacific. Regarding the mechanism responsible for this, the ocean dynamical response to the atmospheric forcing is suggested to be active, while the contribution of latent heat flux is also significant. The role of solar radiation, longwave radiation, and sensible heat flux are of minor importance however, as indicated in the model. Further analysis shows that the North Pacific mode, which is linearly independent of ENSO, resembles the El Niño-type SST mode in the northern Pacific, i.e. both take the pattern of a zonally-oriented dipole in the subtropical Pacific, though differ slightly in the location of the anomaly center. The coupling between the North Pacific mode and the atmosphere is found to be mainly via air-sea heat flux exchange in the model. Both solar radiation and longwave radiation play important roles, while the contribution of latent heat flux is nearly negligible.

A Physics-informed Deep-learning Intensity Prediction Scheme for Tropical Cyclones over the Western North Pacific

Accurate prediction of tropical cyclone(TC)intensity is challenging due to the complex physical processes involved.Here,we introduce a new TC intensity prediction scheme for the western North Pacific(WNP)based on a time-dependent theory of TC intensification,termed the energetically based dynamical system(EBDS)model,together with the use of a long short-term memory(LSTM)neural network.In time-dependent theory,TC intensity change is controlled by both the internal dynamics of the TC system and various environmental factors,expressed as environmental dynamical efficiency.The LSTM neural network is used to predict the environmental dynamical efficiency in the EBDS model trained using besttrack TC data and global reanalysis data during 1982–2017.The transfer learning and ensemble methods are used to retrain the scheme using the environmental factors predicted by the Global Forecast System(GFS)of the National Centers for Environmental Prediction during 2017–21.The predicted environmental dynamical efficiency is finally iterated into the EBDS equations to predict TC intensity.The new scheme is evaluated for TC intensity prediction using both reanalysis data and the GFS prediction data.The intensity prediction by the new scheme shows better skill than the official prediction from the China Meteorological Administration(CMA)and those by other state-of-art statistical and dynamical forecast systems,except for the 72-h forecast.Particularly at the longer lead times of 96 h and 120 h,the new scheme has smaller forecast errors,with a more than 30%improvement over the official forecasts.

Track-Pattern-Based Characteristics of Extratropical Transitioning Tropical Cyclones in the Western North Pacific

Based on the Regional Specialized Meteorological Center(RSMC)Tokyo-Typhoon Center best-track data and the NCEP-NCAR reanalysis dataset,extratropical transitioning(ET)tropical cyclones(ETCs)over the western North Pacific(WNP)during 1951–2021 are classified into six clusters using the fuzzy c-means clustering method(FCM)according to their track patterns.The characteristics of the six hard-clustered ETCs with the highest membership coefficient are shown.Most tropical cyclones(TCs)that were assigned to clusters C2,C5,and C6 made landfall over eastern Asian countries,which severely threatened these regions.Among landfalling TCs,93.2%completed their ET after landfall,whereas 39.8%of ETCs completed their transition within one day.The frequency of ETCs over the WNP has decreased in the past four decades,wherein cluster C5 demonstrated a significant decrease on both interannual and interdecadal timescales with the expansion and intensification of the western Pacific subtropical high(WPSH).This large-scale circulation pattern is favorable for C2 and causes it to become the dominant track pattern,owning to it containing the largest number of intensifying ETCs among the six clusters,a number that has increased insignificantly over the past four decades.The surface roughness variation and three-dimensional background circulation led to C5 containing the maximum number of landfalling TCs and a minimum number of intensifying ETCs.Our results will facilitate a better understanding of the spatiotemporal distributions of ET events and associated environment background fields,which will benefit the effective monitoring of these events over the WNP.

CMIP 6 models simulation of the connection between North/South Pacific Meridional Mode and ENSO

The subtropical North and South Pacific Meridional Modes(NPMM and SPMM)are well known precursors of El Niño-Southern Oscillation(ENSO).However,relationship between them is not constant.In the early 1980,the relationship experienced an interdecadal transition.Changes in this connection can be attributed mainly to the phase change of the Pacific decadal oscillation(PDO).During the positive phase of PDO,a shallower thermocline in the central Pacific is responsible for the stronger trade wind charging(TWC)mechanism,which leads to a stronger equatorial subsurface temperature evolution.This dynamic process strengthens the connection between NPMM and ENSO.Associated with the negative phase of PDO,a shallower thermocline over southeastern Pacific allows an enhanced wind-evaporation-SST(WES)feedback,strengthening the connection between SPMM and ENSO.Using 35 Coupled Model Intercomparison Project Phase 6(CMIP6)models,we examined the NPMM/SPMM performance and its connection with ENSO in the historical runs.The great majority of CMIP6 models can reproduce the pattern of NPMM and SPMM well,but they reveal discrepant ENSO and NPMM/SPMM relationship.The intermodal uncertainty for the connection of NPMM-ENSO is due to different TWC mechanism.A stronger TWC mechanism will enhance NPMM forcing.For SPMM,few models can simulate a good relationship with ENSO.The intermodel spread in the relationship of SPMM and ENSO owing to SST bias in the southeastern Pacific,as WES feedback is stronger when the southeastern Pacific is warmer.

A teleconnection pattern of upper-tropospheric circulation anomalies over the Eurasian continent associated with the interannual variability of atmospheric convection over the tropical western North Pacific in July

热带西北太平洋大气对流的年际变化对东亚夏季气候有着显著影响,而且受到前冬ENSO的影响.然而,还有相当多的西北太平洋对流年际变化不能由前冬ENSO解释.本文表明,7月西北太平洋对流受到欧亚大陆对流层上层遥相关型的显著影响,且这一影响与前冬ENSO的影响程度相当.该遥相关型表现为位于欧洲的气旋式环流异常,中亚的反气旋式环流异常和东亚带状延伸的气旋式环流异常,并表现为7月对流层上层环流的第一主模态.基于本文的结果并结合前人的研究,作者认为这一遥相关型能通过亚洲急流出口区附近的罗斯贝波破碎引起西北太平洋对流异常.

Westward extension of summer atmospheric circulation over the North Pacific after the 1990s

本文研究了20世纪90年代前后夏季北太平洋大气环流(NPcirculation)向西扩展,并探讨了其可能的原因.结果表明,在1961-1983年期间NP中心主要位于东北太平洋,而在1994-2016年期间NP中心发生西移,至北太平洋中部进一步分析指出,90年代以后NP活动中心的西移和春季中纬度北太平洋海温(SST_NP)与NP关系的加强有关.春季SST_NP异常通过引起后期夏季北太平洋中部地表热通量(即感热通量和潜热通量)和垂直运动异常,有利于NP活动中心向西移动,此外,20世纪90年代以后,北太平洋中部海平面气压的年际变率增加,这可能是NP向西扩展的另一原因。

Impacts of monsoon break events in the western North Pacific on the cross-equatorial flows over the Maritime Continent

本文利用ERA5逐日再分析资料,探讨了1979-2020年间西北太平洋季风中断事件对海洋性大陆越赤道气流的影响.合成结果表明,西北太平洋季风中断事件会造成高,低空越赤道气流减弱,即高层南风异常,低层北风异常,与此相关的环流异常表现为西北太平洋高层气旋,低层反气旋的斜压结构。特别的是,西北太平洋季风中断对高空越赤道气流的影响更为显著,92%的季风中断事件都导致高空越赤道气流减弱,而只有70%的事件造成低空越赤道气流减弱,这是由于低空越赤道气流同时还受到赤道中东太平洋海温异常的调控.

Recent Enhancement in Co-Variability of the Western North Pacific Summer Monsoon and the Equatorial Zonal Wind

The western North Pacific summer monsoon(WNPSM)is an important subcomponent of the Asian summer monsoon.The equatorial zonal wind(EZW)in the lower troposphere over the western Pacific may play a critical role in the evolution of the El Niño-Southern Oscillation(ENSO).The possible linkage between the EZW over the western Pacific and the offequatorial monsoonal winds associated with the WNPSM and its decadal changes have not yet been fully understood.Here,we find a non-stationary relationship between the WNPSM and the western Pacific EZW,significantly strengthening their correlation around the late 1980s/early 1990s.This observed shift in the WNPSM–EZW relationship could be explained by the changes in the related sea surface temperature(SST)configurations across the tropical oceans.The enhanced influence from the springtime tropical North Atlantic,summertime tropical central Pacific,and maritime continent SST anomalies may be working together in contributing to the recent intensified WNPSM–EZW co-variability.The observed recent strengthening of the WNPSM–EZW relationship may profoundly impact the climate system,including prompting more effective feedback from the WNPSM on subsequent ENSO evolution and bolstering a stronger biennial tendency of the WNPSM–ENSO coupled system.The results obtained herein imply that the WNPSM,EZW,ENSO,and the tropical North Atlantic SST may be closely linked within a unified climate system with a quasi-biennial rhythm occurring during recent decades,accompanied by a reinforcement of the WNPSM–ENSO interplay quite possibly triggered by enhanced tropical Pacific–Atlantic cross-basin interactions.These results highlight the importance of the tropical Atlantic cross-basin influences in shaping the spatial structure of WNPSM-related wind anomalies and the WNPSM–ENSO interaction.

The Influence of Meridional Variation in North Pacific Sea Surface Temperature Anomalies on the Arctic Stratospheric Polar Vortex

This study examines the dependence of Arctic stratospheric polar vortex(SPV)variations on the meridional positions of the sea surface temperature(SST)anomalies associated with the first leading mode of North Pacific SST.The principal component 1(PC1)of the first leading mode is obtained by empirical orthogonal function decomposition.Reanalysis data,numerical experiments,and CMIP5 model outputs all suggest that the PC1 events(positive-minus-negative PC1 events),located relatively northward(i.e.,North PC1 events),more easily weaken the Arctic SPV compared to the PC1 events located relatively southward(i.e.,South PC1 events).The analysis indicates that the North PC1-related Aleutian low anomaly is located over the northern North Pacific and thus enhances the climatological trough,which strengthens the planetary-scale wave 1 at mid-to-high latitudes and thereby weakens the SPV.The weakened stratospheric circulation further extends into the troposphere and favors negative surface temperature anomalies over Eurasia.By contrast,the South PC1-related Aleutian low anomaly is located relatively southward,and its constructive interference with the climatological trough is less efficient at high latitudes.Thus,the South PC1 events could not induce an evident enhancement of the planetary-scale waves at high latitudes and thereby a weakening of the SPV on average.The Eurasian cooling associated with South PC1 events(positive-minus-negative PC1 events)is also not prominent.The results of this study suggest that the meridional positions of the PC1 events may be useful for predicting the Arctic SPV and Eurasian surface temperature variations.

Response of the North Pacific Storm Track Activity in the Cold Season to Multi-scale Oceanic Variations of Kuroshio Extension System: A Statistical Assessment

In this paper,a statistical method called Generalized Equilibrium Feedback Analysis(GEFA)is used to investigate the responses of the North Pacific Storm Track(NPST)in the cold season to the multi-scale oceanic variations of the Kuroshio Extension(KE)system,including its large-scale variation,oceanic front meridional shift,and mesoscale eddy activity.Results show that in the cold season from the lower to the upper troposphere,the KE large-scale variation significantly weakens the storm track activity over the central North Pacific south of 30°N.The northward shift of the KE front significantly strengthens the storm track activity over the western and central North Pacific south of 40°N,resulting in a southward shift of the NPST.In contrast,the NPST response to KE mesoscale eddy activity is not so significant and relatively shallow,which only shows some significant positive signals near the dateline in the lower and middle troposphere.Furthermore,it is found that baroclinicity and baroclinic energy conversion play an important role in the formation of the NPST response to the KE multi-scale oceanic variations.

The combined effects of North Atlantic Oscillation and Western Pacific teleconnection on winter temperature in Eastern Asia during 1980−2021

As important atmospheric circulation patterns in Northern Hemisphere(NH),the North Atlantic Oscillation(NAO)and the Western Pacific teleconnection(WP)affect the winter climate in Eurasia.In order to explore the combined effects of NAO and WP on East Asian(EA)temperature,the NAO and WP indices are divided into four phases from 1980−2021:the positive NAO and WP phase(NAO+/WP+),the negative NAO and WP phase(NAO−/WP−),the positive NAO and negative WP phase(NAO+/WP−),the negative NAO and positive WP phase(NAO−/WP+).In the phase of NAO+/WP+,the low geopotential height(GH)stays in north of EA at 50°−80°N;the surface air temperature anomaly(SATA)is 0.8−1℃lower than Southern Asian.In the phase of NAO−/WP−,the center of high temperature and GH locate in the northeast of EA;the cold air spreads to Southern Asia,causing the SATA decreases 1−1.5℃.In the phase of NAO+/WP−,the high GH belt is formed at 55°−80°N.Meanwhile,the center of high SATA locates in the north of Asia that increases 0.8−1.1℃.The cold airflow causes temperature dropping 0.5−1℃in the south of EA.The SATA improves 0.5−1.5℃in south of EA in the phase of NAO−/WP+.The belt of high GH is formed at 25°−50°N,and blocks the cold air which from Siberia.The NAO and WP generate two warped plate pressure structures in NH,and affect the temperature by different pressure configurations.NAO and WP form different GH,and GH acts to block and push airflow by affecting the air pressure,then causes the temperature to be different from the north and south of EA.Finally,the multiple linear regression result shows that NAO and WP are weakened by each other such as the phase of NAO+/WP+and NAO−/WP−.

Interdecadal Enhancement in the Relationship between the Western North Pacific Summer Monsoon and Sea Surface Temperature in the Tropical Central-Western Pacific after the Early 1990s

This study reveals the strengthened interdecadal relationship between the western North Pacific summer monsoon(WNPSM)and tropical central-western Pacific sea surface temperature anomaly(SSTA)in summer after the early 1990s.In the first period(1979–91,P1),the WNPSM-related precipitation anomaly and horizontal wind anomaly present themselves as an analogous Pacific-Japan(PJ)-like pattern,generally considered to be related to the Niño-3 index in the preceding winter.During the subsequent period(1994–2019,P2),the WNPSM-related precipitation anomaly presents a zonal dipole pattern,correlated significantly with the concurrent SSTA in the Niño-4 and tropical western Pacific regions.The negative(positive)SSTA in the tropical western Pacific and positive(negative)SSTA in the Niño-4 region,could work together to influence the WNPSM,noting that the two types of anomalous SSTA configurations enhance(weaken)the WNPSM by the positive(negative)phase PJ-like wave and Gill response,respectively,with an anomalous cyclone(anticyclone)located in the WNPSM,which shows obvious symmetry about the anomalous circulation.Specifically,the SSTA in Niño-4 impacts the WNPSM by an atmospheric Gill response,with a stronger(weaker)WNPSM along with a positive(negative)SSTA in the Niño-4 region.Furthermore,the SSTA in the tropical western Pacific exerts an influence on the WNPSM by a PJ-like wave,with a stronger(weaker)WNPSM along with a negative(positive)SSTA in the tropical western Pacific.In general,SSTAs in the tropical western Pacific and Niño-4 areas could work together to exert influence on the WNPSM,with the effect most likely to occur in the El Niño(La Niña)developing year in P2.However,the SSTAs in the tropical western Pacific worked alone to exert an influence on the WNPSM mainly in 2013,2014,2016,and 2017,and the SSTAs in the Niño-4 region worked alone to exert an influence on the WNPSM mainly in Central Pacific(CP)La Niña developing years.The sensitivity experiments also can reproduce the PJ-like wave/Gill response associated with SSTA in the tropical western Pacific/Niño-4 regions.Therefore,the respective and synergistic impacts from the Niño-4 region and the tropical western Pacific on the WNPSM have been revealed,which helps us to acquire a better understanding of the interdecadal variations of the WNPSM and its associated climate influences.

Impacts of the extratropical North Pacific on boreal summer Arctic circulation

北极夏季大气环流的一个显着特征是以北冰洋为中心的异常反气旋环流,与北极增暖存在紧密的联系.先前的研究已将其形成的潜在机制与早春欧亚融雪,热带太平洋海表温度异常联系起来.本研究发现北极夏季反气旋环流异常与春季副热带北太平洋海表温度异常(SSTA)存在显着联系,表明副热带北太平洋与北极之间存在遥相关.该SSTA的特点是北太平洋中纬度地区有暖的SSTA,,周围有明显的冷SSTA,类似于太平洋年代际振荡(PDO)的负位相,但热带地区没有明显的信号(本研究称之为类负位相PDO),5月份,这种类负位相PDO可以激发从白令海传播到北极的罗斯贝波,导致北极出现反气旋环流异常.这种反气旋环流异常可以持续到夏季.同时,这种类负位相PDO的SSTA可以持续到夏季,并在夏季引起白令海上空的低压异常.这种低层的低压异常会导致异常的上升运动,并引起高层的辐散异常,从而进一步加剧夏季北极对流层上层的反气旋环流异常。这种反气旋环流异常可迫使北极上空出现异常绝热下沉运动,导致北极出现显著的绝热加热.于是,北极上空出现显著的暖异常,其暖中心位于北极对流层中部。数值模拟试验证实了5月份类负位相PDO的SSTA与夏季北极反气旋环流异常之间的联系.

Influence of the pace of El Nino decay on tropical cyclone frequency over the western north pacific during decaying El Nino summers

本文分析了El Nino事件衰减速度的差异对衰退年夏季西北太平洋热带气旋(tropical cyclone,TC)频数的不同影响。按照ElNiio事件衰减速度不同,将其划分为迅速衰减(rapid decaying,RD)和缓慢衰减(slowdecaying,SD)的El Nino事件.SD(RD)El Nino事件的衰退年夏季,赤道中东太平洋海温仍维持正异常(衰减为负异常).与SD El Nino事件相比,RDElNino事件衰退年夏季西北太平洋TC频数显著减少.进一步的分析揭示了导致TC频数差异的大尺度环境要素,指出热带印度洋-太平洋海温异常密切相关的西北太平洋低层反气旋异常在其中起到了关键作用。

The Catch Distribution of Ommastrephes batramii in Squid Jigging Fishery and the Relationship between Fishing Ground and SST in the North Pacific Ocean in 2004

Neon flying squid Ommastrephes batramii is widely distributed in the North Pacific Ocean, which has become the main fishing species for Chinese squid jigging fleets since 1993. Many authors have made the studies on the fields of fishing ground and its environment conditions. However, the squid catch per fishing vessel attained the highest level of about 550 t in 2004. In this paper, the catch and its distribution in 2004 would be compared with the previous year. Based on the catch data from Chinese squid jigging vessels and sea surface temperature with the format of 1 °latitude by 1 °longitude from May to November in 2004, the distribution maps were drawn by Marine explorer 4.0. The results show that the production in the east waters to 160°E was low during May and July. During October and November, the production in the waters from 150°E to 160°E was relatively higher, which occupied 62.5 percent of the total catch. During November, the production in the west waters to 150°E was also low. The highest CPUE area located in the west waters to 150°E, the next was the area from 150°E to 160°E and the lowest CPUE area located in the east waters to 160°E. The SST in the fishing ground seems to change seasonally. The suitable SST for each month is as follows： 12-14 ℃ in May, 15 ℃ - 16 ℃ in June, 14 ℃ - 16 ℃ in July, 18 ℃ - 19 ℃ in August, 16 ℃ -17 ℃ in September, 15 ℃- 16 ℃ in October and 12 ℃ - 13 ℃ in November. The result of K-S test shows that the above monthly suitable SST is considered as the indicator of looking for the main fishing ground.

Assessing the Influence of the ENSO on Tropical Cyclone Prevailing Tracks in the Western North Pacific

Using a statistical model for simulating tropical cyclone （TC） formation and a trajectory model for simulating TC tracks, the influence of the El Nino-Southern Oscillation （ENSO） on the peak-season （July-September） TC prevailing tracks in the western North Pacific basin is assessed based on 14 selected El Nino and 14 selected La Nina years during the period 1950-2007. It is found that the combination of statistical formation model and a trajectory model can simulate well the primary features of TC prevailing tracks on the interannual timescale. In the El Nino years, the significant enhancement of TC activity primarily occurs south of 20°N, especially east of 130°E. TCs that take the northwestward prevailing track and affect East Asia, including Taiwan Island, the Chinese mainland, Korea, and Japan, tend to move more westward in the El Nino years, while taking a more northward track in the La Nina years. Numerical simulations confirm that the ENSO-related changes in large-scale steering flows and TC formation locations can have a considerable influence on TC prevailing tracks.

Atmospheric Anomalies Related to Interdecadal Variability of SST in the North Pacific

Anomalous patterns of the atmospheric circulation and climate are studied corresponding to the two basic interdecadal variation modes of sea surface temperature (SST) in the North Pacific, namely, the 25-35-year mode and the 7-10-year mode. Results clearly indicate that corresponding to the positive and negative phases of the interdecadal modes of SST anomaly (SSTA) in the North Pacific, the anomalous patterns of the atmospheric circulation and climate are approximately out of phase, fully illustrating the important role of the interdecadal modes of SST. Since the two interdecadal modes of SSTA in the North Pacific have similar horizontal structures, their impacts on the atmospheric circulation and climate are also analogous. The impact of the interdecadal modes of the North Pacific SST on the atmospheric circulation is barotropic at middle latitudes and baroclinic in tropical regions.

Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the TropicalWestern North Pacific during Summer

There is a well-known seesaw pattern of precipitation between the tropical western North Pacific(WNP) and the Yangtze River basin(YRB) during summer. This study identified that this out-of-phase relationship experiences a subseasonal change;that is, the relationship is strong during early summer but much weaker during mid-summer. We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale. It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer, i.e., the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP. During mid-summer, the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere. In the lower troposphere, the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB. In the upper troposphere, the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall. The subseasonal change in the WNP–YRB precipitation relationship illustrated by this study has important implications for the subseasonalto-seasonal forecasting of the YRB rainfall.

Western North Pacific Tropical Cyclone Database Created by the China Meteorological Administration

This paper describes the access to, and the content, characteristics, and potential applications of the tropical cyclone(TC) database that is maintained and actively developed by the China Meteorological Administration, with the aim of facilitating its use in scientific research and operational services. This database records data relating to all TCs that have passed through the western North Pacific(WNP) and South China Sea(SCS) since 1949. TC data collection has expanded over recent decades via continuous TC monitoring using remote sensing and specialized field detection techniques,allowing collation of a multi-source TC database for the WNP and SCS that covers a long period, with wide coverage and many observational elements. This database now comprises a wide variety of information related to TCs, such as historical or real-time locations(i.e., best track and landfall), intensity, dynamic and thermal structures, wind strengths, precipitation amounts, and frequency. This database will support ongoing research into the processes and patterns associated with TC climatic activity and TC forecasting.

The Role of the Kuroshio in the Winter North Pacific Ocean-Atmosphere Interaction: Comparison of a Coupled Model and Observations

A comparative study between the output of the Flexible Global Climate Model Version 1.0 （FGCM- 1.0） and the observations is performed. At 500 hPa, the geopotential height of FGCM is similar to the observations, but in the North Pacific the model gives lower values, and the differences are most significant over the northern boundary of the Pacific. In a net heat flux comparison, the spatial patterns of the two are similar in winter, but more heat loss appears to the east of Japan in FGCM than in COADS. On the interannual timescale, strong （weak） Kuroshio transports to the east of Taiwan lead the increasing （decreasing） net heat flux, which is centered over the Kuroshio Extension region, by 1-2 months, with low （high） pressure anomaly responses appearing at 500 hPa over the North Pacific （north of 25°N） in winter. The northward heat transport of the Kuroshio is one of the important heat sources to support the warming of the atmosphere by the ocean and the formation of the low pressure anomaly at 500 hPa over the North Pacific in winter.