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This study reveals an interdecadal change in boreal winter dominant ENSO atmospheric teleconnection and demonstrates its underlying physical mechanism. During the period 1960–2020, the ENSO teleconnection was characterized by the Western Pacific (WP) pattern and the Pacific–North American (PNA) pattern, but the dominant pattern of ENSO teleconnection witnessed an alternation at around early 1980s. In the first epoch of 1960–1979 (E1), the ENSO teleconnection was mainly dominated by the WP pattern with an inhibited PNA pattern, whereas in the second epoch of 1987–2020 (E2), the ENSO teleconnection was dominated by the PNA pattern without a prominent WP pattern. It is further suggested that the changing strength of the westerly jet, rather than the pattern of the ENSO-related tropical convection, played the critical role in the alternation of the winter dominant ENSO teleconnection. During E1, because the westerly jet stream in the East Asia–North Pacific sector was relatively weak, poleward propagation of the Rossby wave train from the western Pacific was evident, forming the dominant WP pattern related to ENSO. In E2, because the westerly jet in the region largely strengthened, the PNA pattern was dominant owing to the enhanced zonal waveguide of the westerly jet. Evidence from a set of numerical experiments and the simulations of CMIP6 models supports the important role of the westerly jet strength in shaping the ENSO teleconnection. [ABSTRACT FROM AUTHOR]

The spring persistent rainfall (SPR) is a unique synoptic and climatic phenomenon before the onset of the Asian summer monsoon over Southeast China (SEC). The warming climate has caused notable interdecadal changes of various climate systems, such as the Asian summer monsoon. However, it is still unclear whether there is a regime shift within the SPR band. To fill this knowledge gap, the boreal spring (March–May) rainfall data across SEC from the Climate Research Unit TS 4.05 are used herein to reveal its spatio‐temporal variations for 1951–2019 with the empirical orthogonal function (EOF) analysis. The EOF1 pattern exhibits a spatial monotonic variability of the SPR over SEC, which is jointly impacted by the El Niño‐Southern Oscillation and the Arctic Oscillation. The EOF2 pattern, a south–north dipole pattern, highlights a marked decrease (increase) of the SPR approximately to north (south) of the 26°N. We find that the decrease in the northern SPR band is closely modulated by western Pacific teleconnection pattern (WPP). Recent weakening WPP is coincident with a decline of the East Asian subtropical jet over this area, causing anomalous descending motions and thus accounts for persistent decline of the SPR over north of 26°N. [ABSTRACT FROM AUTHOR]

The predictability of the wintertime Western Pacific (WP) pattern is evaluated based on seasonal predictions from five models participating in the Asia-Pacific Economic Cooperation (APEC) Climate Center (APCC) multi-model ensemble (MME) for the winters from 1982/1983 to 2021/2022. The temporal correlation coefficient (TCC) between the observed and MME-predicted WP indices was 0.61 (0.37–0.54 for individual models) for the entire series. However, when only three Super El Niño (SEN) years (Niño3.4 ≥ 2.0) out of the 40-year series were excluded, the TCC dropped down to 0.54 (0.27–0.42). During the SEN years, the WP was strongly affected by the SEN-excited anomalies via the PNA. In observations from non-SEN years, the WP pattern was strongly related to the dipole pattern in Northwestern Pacific SST (TCC = 0.8), for the description of which we suggested a Northwestern Pacific (NWP) index, and it was significantly weakly related to the ENSO and IOD, whereas in the model simulations, the main role was played by the ENSO (TCC = 0.6). The NWP index was well predictable in MME (TCC = 0.73) and individual models (0.56–0.71). We showed that the prediction of the WP index polarity is reliable when both predicted WP and NWP anomalies are significant and indicate the same WP sign that has implications for the seasonal forecasting. [ABSTRACT FROM AUTHOR]

Using multiple reanalysis datasets, this study reveals that the variability in the western Pacific pattern (WP) in boreal winter has shown notable changes during recent decades. The variability in the winter WP exhibited a marked weakening trend before the early 2000s and increased slightly thereafter. Two epochs with the highest and lowest WP variabilities are selected for a comparative analysis. Winter WP-related meridional dipole atmospheric anomalies over the North Pacific were stronger and had a broader range during the high-variability epoch than during the low-variability epoch. Correspondingly, the winter WP had larger impacts on surface temperature variations over Eurasia and North America during the high-variability epoch than that during the low-variability epoch. We find that the shift in the winter WP variability is closely related to changes in the connection between the winter WP and the El Niño-Southern Oscillation (ENSO) and to changes in the amplitude of the North Pacific storm track. Specifically, ENSO had a closer connection with the WP during the high-variability epoch, when the amplitude of the North Pacific storm track was also stronger. During the high-variability epoch, the extratropical atmospheric anomalies generated by the tropical ENSO shifted westward and projected more on the WP-related atmospheric anomalies, thus contributing to an increase in WP variability. In addition, the larger amplitude of the North Pacific storm track that occurred during the high-variability epoch led to the stronger feedback of synoptic-scale eddies to the mean flow and contributed to stronger WP variability. Further analysis indicates that the change in the connection of ENSO with the WP may be partly related to the zonal shift of the sea surface temperature anomaly in the tropical Pacific associated with ENSO. [ABSTRACT FROM AUTHOR]

The western Pacific pattern (WP), characterized by a meridional dipole structure in geopotential height anomalies centred at approximately 50°N over the North Pacific, is one of the most prominent teleconnection patterns over the Northern Hemisphere (NH) in boreal winter. Several methods to identify the WP exist in the literature. Based on eight WP indices defined by different methods, this study analyses their dominant frequencies, spatial structures, climatic impacts, and relations to the Arctic Oscillation (AO) and El Niño‐Southern Oscillation (ENSO). The correlation coefficients of the eight WP indices show a considerable spread, but most of them are statistically significant. Most of the WPIs capture a pronounced power peak at approximately 5 years. The meridional dipole structure of the WP can be captured by all the WPIs, but it shows a large spread in the location of the centre. Several WPIs are significantly correlated with the winter AO, with marked signals of atmospheric anomalies over the Arctic region. The connections of the WP with the simultaneous winter ENSO depend largely on their definitions. The impacts of the WP on the surface air temperature over many parts of Eurasia and North America are also sensitive to its definition. Differences in surface air temperature anomalies are related to differences in the spatial structure of the WP as captured by the range of WPIs. [ABSTRACT FROM AUTHOR]

ABSTRACT Northeast China cold vortex ( NCCV) circulation associated with a blocking-type circulation over the Yakutsk/Okhotsk ( YO) region brings persistent cool weather over Northeast China from May to mid-June and often causes a considerable reduction of local crop production. This study investigated the dynamic features of the YO-NCCV event on intraseasonal time scale and its relation with climate background circulation through analyzing two typical cases of YO-NCCV events. The first event (E1, from 25 May to 4 June, 2008) selected in this study was a long-lived (12 days) case, while the second event (E2, 16-18 May, 1972) was a short-lived one (only 3 days). Though two typical YO-NCCV events both developed from the southeastward movement of the polar high potential vorticity ( PV) air, they showed different life cycle features. In the E1, the formation of the NCCV circulation was associated with the poleward intrusion of two low- PV tongues over the Kamchatka Peninsula and the Ural Mountains. The low-frequency Rossby waves propagating from the Scandinavia Peninsula along the westerly to Northeast China mainly contributed to the maintenance of the NCCV circulation. In contrast, for the E2, the low- PV tongue over the Ural Mountains was absent and the PV field over Eurasian continent exhibited a synoptic wave feature. The transient eddy feedback forcing ( TEFF) anomalies contributed greatly to the formation and development of the YO-NCCV circulation. From the climatic view, the circulation anomalies associated with the E1 may be regarded as an intensive development of the climatological mean eddy field on the intraseasonal time scale. Results also show that the YO-NCCV event, among the four main types of NCCV event, was most closely related to the negative phase of western Pacific (WP) pattern. [ABSTRACT FROM AUTHOR]

The East Asian subtropical jet (EAJ) and the closely related Western Pacific pattern (WP) are among the most important features in global atmospheric dynamics, but little is known about their long-term variability. This study presents reconstructions of the Spring EAJ index (EAJI) and the Spring WP index (WPI) based on significant relationships identified between mean values for these features and a millennial length tree-ring width chronology of Chamaecyparis obtusa var. formosana, a high-mountain cloud forest species from northeastern Taiwan. Tree-ring based reconstructions of high pass filtered versions of the EAJI and WPI (EAJI 5YR and WPI 5YR) presented herein explain 42 and 31 % of the WPI 5YR and EAJI 5YR, respectively, and display acceptable reliability back to A.D. 1237. A significant trend present in the long-term variance of the reconstructed EAJI and WPI after A.D. 1860 suggests long-term increasing variability in the spring mean latitudinal placement and/or the strength/breadth of the EAJ core region near Taiwan and Japan and in the trajectory of the EAJ over the North Pacific. Related features affected by changes in the EAJ include the North Pacific storm track and Asian Dust transport. [ABSTRACT FROM AUTHOR]