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Juan FENG and Wen CHEN

Abstract

The El Niño-related anomalous western North Pacific anticyclone (WNPAC) shows different latitudinal extensions during the El Niño decaying summer, which determines the moisture transport to different regions and leads to distinct climate impacts over East Asia. It is known that both the north Indian Ocean (NIO) sea surface temperature (SST) and the tropical North Atlantic (TNA) SST can generate a WNPAC in summer. However, the difference between the NIO SST-forced WNPAC and the TNA SST-forced WNPAC has hardly been noted before now. This study shows that the NIO SST warming makes the WNPAC contract southward, whereas the TNA SST warming makes the WNPAC extend northward. The NIO SST warming generates the WNPAC via a Kelvin wave response. Owing to the limited domain of Kelvin wave activity, the Kelvin wave-induced suppressed convection over the western Pacific is confined south of 20°N, resulting in the WNPAC being concentrated in the low latitudes. In contrast, the TNA SST warming generates the WNPAC via a Rossby wave-induced divergence/convergence chain response over the Pacific. The Rossby wave-induced suppressed convection over the central-eastern Pacific north of the Equator leads to enhanced convection on its southwest side, which further generates the low-level anomalous divergent winds over the western North Pacific and suppresses convection there. In this process, the suppressed convection over the western North Pacific is pushed more northward, thus producing a WNPAC extending northward. Further study finds that there are good precursors for predicting the WNPAC latitudinal extension based on the El Niño spatial pattern and the NIO/TNA SST intensity in the previous winter and spring.

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Lin Wang and Wen Chen

Abstract

The thermal contrast between the Asian continent and the adjacent oceans is the primary aspect of the East Asian winter monsoon (EAWM) that can be well represented in the sea level pressure (SLP) field. Based on this consideration, a new SLP-based index measuring the intensity of the EAWM is proposed by explicitly taking into account both the east–west and the north–south pressure gradients around East Asia. The new index can delineate the EAWM-related circulation anomalies well, including the deepened (shallow) midtropospheric East Asian trough, sharpened and accelerated (widened and decelerated) upper-tropospheric East Asian jet stream, and enhanced (weakened) lower-tropospheric northerly winds in strong (weak) EAWM winters. Compared with previous indices, the new index has a very good performance describing the winter-mean surface air temperature variations over East Asia, especially for the extreme warm or cold winters. The index is strongly correlated with several atmospheric teleconnections including the Arctic Oscillation, the Eurasian pattern, and the North Pacific Oscillation/western Pacific pattern, implying the possible internal dynamics of the EAWM variability. Meanwhile, the index is significantly linked to El Niño–Southern Oscillation (ENSO) and the sea surface temperature (SST) over the tropical Indian Ocean. Moreover, the SST anomalies over the tropical Indian Ocean are more closely related to the index than ENSO as an independent predictor. This adds further knowledge to the prediction potentials of the EAWM apart from ENSO. The predictability of the index is high in the hindcasts of the Centre National de Recherches Météorologiques (CNRM) model from Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER). Hence, it would be a good choice to use this index for the monitoring, prediction, and research of the EAWM.

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Baode Chen, Wen-wen Tung, and Michio Yanai

Abstract

The authors examined the maintenance mechanisms of perturbation kinetic energy (PKE) in the tropical regions for multiple time scales by computing and analyzing its budget equation. The emphasis has been placed on the mean features of synoptic and subseasonal variabilities using a 33-yr dataset. From analysis of the contributions from u-wind and υ-wind components, the PKE maximum in the Indian Ocean is attributed less to synoptic variability and more to intraseasonal variability in which the Madden–Julian oscillation (MJO) dominates; however, there is strong evidence of seasonal variability affiliated with the Asian monsoon systems. The ones in the eastern Pacific and Atlantic Oceans are closely related to both intraseasonal and synoptic variability that result from the strong MJO and the relatively large amplitude of equatorial waves.

The maintenance of the PKE budget mainly depends on the structure of time mean horizontal flows, the location of convection, and the transport of PKE from the extratropics. In the regions with strong convective activities, such as the eastern Indian Ocean to the western Pacific, the production of PKE occurs between 700 and 200 hPa at the expense of perturbation available potential energy (PAPE), which is generated by convective heating. This gain in PKE is largely offset by divergence of the geopotential component of vertical energy flux; that is, it is redistributed to the upper- and lower-atmospheric layers by the pressure field. Strong PKE generation through the horizontal convergence of the extratropical energy flux takes place in the upper troposphere over the eastern Pacific and Atlantic Ocean, and is largely balanced by a PKE loss due to barotropic conversion, which is determined solely by the sign of longitudinal stretching deformation. However, over the Indian Ocean, there is a net PKE loss due to divergence of energy flux, which is compensated by PKE gain through the shear generation.

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Michio Yanai, Baode Chen, and Wen-wen Tung

Abstract

During the TOGA COARE Intensive Observing Period (November 1992–February 1993), two pronounced Madden–Julian oscillation (MJO) events associated with super cloud clusters and westerly wind bursts were observed. This paper presents a global view of the MJOs including the origin of the super clusters in the Indian Ocean, their migration into the Maritime Continent and the TOGA COARE large-scale soundings array (LSA) in the western equatorial Pacific, and their rapid decay over cold water of the eastern Pacific. The structure and evolution of the MJO are examined with emphasis on the coupling between large-scale motion and convection. Because of differences in propagation speeds, the positions of maximum zonal wind perturbations relative to deep convection undergo systematic changes during the travel of the MJO. However, the centers of deep convection always coincide with those of large-scale ascent. The super cloud cluster accompanies a wide area of warm air in the upper troposphere. Over the warm pool region the perturbation kinetic energy of the motion in the 30–60-day period range is maintained by the conversion of perturbation available potential energy generated by convective heating. Over the central-eastern Pacific, there is strong horizontal convergence of wave energy flux entering the equatorial upper-tropospheric westerly duct from the extratropical latitudes, suggesting interactions of the MJO with midlatitude disturbances.

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Shangfeng Chen, Wen Chen, and Renguang Wu

Abstract

Previous studies suggested that the boreal spring Arctic Oscillation (AO) exerts a pronounced influence on the following East Asian summer monsoon (EASM) variability. This study reveals that the relationship of spring AO with the following EASM experienced a significant interdecadal change in the early 1970s. The influence of spring AO on the following EASM is weak during the 1950s and 1960s but strong and significant during the mid-1970s through the mid-1990s. The spring AO-related sea surface temperature (SST), atmospheric circulation, and heating anomalies are compared between 1949–71 and 1975–97. Results show that the spring AO-related cyclonic circulation anomaly over the tropical western North Pacific is weaker and located more northward in the former epoch than in the latter epoch. Correspondingly, SST, atmospheric circulation, and heating anomalies over the tropical North Pacific are located more northeastward in the former than latter epoch from spring to summer. In the following summer, the spring AO-related cyclonic circulation anomalies over the tropical North Pacific are located farther away from East Asia in the former epoch. This interdecadal change in the AO–EASM connection may be attributed to a significant change in the intensity of spring North Pacific synoptic-scale eddy activity around the early 1970s from a weak regime to a strong regime, which induces a stronger eddy feedback to the low-frequency mean flow after the early 1970s. This may explain a stronger spring AO-related cyclonic circulation over the tropical western North Pacific and thus a closer relationship between the spring AO and the following EASM in the latter than former epoch.

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Zhang Chen, Renguang Wu, and Wen Chen

Abstract

The present study investigated the impacts of autumn Arctic sea ice concentration (SIC) changes on the East Asian winter monsoon (EAWM) and associated climate and circulation on the interannual time scale. It is found that the Arctic SIC anomalies have little impact on the southern mode of EAWM, but the northern mode is significantly associated with both western and eastern Arctic SIC anomalies. When there is less (more) SIC in eastern (western) Arctic, the EAWM tends to be stronger. The concurrent surface air temperature anomalies are induced both locally due to the direct effect of ice cover and in remote regions through anomalous wind advection. Analysis showed that eastern Arctic SIC anomalies have a larger effect on local atmospheric stability of the lower troposphere than western Arctic SIC anomalies. Winter temperature over the midlatitudes of East Asia is lower when there is more (less) SIC in the western (eastern) Arctic. The atmospheric response to the Arctic SIC anomalies is dominantly barotropic in autumn, and changes to baroclinic over the midlatitudes of Asia, but remains barotropic in other regions in winter. The mid- to high-latitude circulation systems, including the Siberian high, the East Asian trough, and the East Asian westerly jet stream, play important roles in connecting autumn Arctic SIC anomalies and the northern mode of the EAWM variability. No obvious concurrent sea surface temperature anomalies accompany Arctic SIC variations on the interannual time scale, indicating that the Arctic SIC anomalies have independent impacts on the East Asian winter climate.

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Hasi Aru, Wen Chen, and Shangfeng Chen

Abstract

The western Pacific pattern (WP) is one of the most important atmospheric teleconnections over the Northern Hemisphere (NH) in boreal winter, which plays key roles in regulating weather and climate variations over many parts of the NH. This study evaluates ability of the coupled models participated in CMIP5 and CMIP6 in capturing the spatial pattern, dominant frequency, and associated climate anomalies of the winter WP. Ensemble means of the CMIP5 and CMIP6 models well capture spatial structures of the WP, with slightly higher skills for the CMIP6. However, the northern (southern) centre of the WP is shifted westward (eastward) relative to the observations, and the strength of the northern centre is overestimated in most CMIP5 and CMIP6 models. CMIP6 shows an improvement in simulating the dominant periodicity of the WP. WP-related climatic anomalies in most parts of the NH can be well simulated. However, there exists a large spread across the models in simulating surface air temperature (SAT) anomalies in Russian Far East and Northwest North America, which is attributable to the diversity of the intensity of the WP’s northern lobe. Most CMIP5 and CMIP6 models largely overestimate the WP-related precipitation anomalies over Siberia, which is partly due to the overestimation of mean precipitation there. Furthermore, most models simulate a close relation of the WP and Arctic Oscillation (AO), which does not exist in observation. The CMIP5 and CMIP6 models with weak WP-AO relations have better ability than the models with strong WP-AO relations in capturing the WP-related SAT and precipitation anomalies over the NH, especially over Eurasia.

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Shangfeng Chen, Renguang Wu, and Wen Chen

Abstract

This study reveals a marked enhancement in the relationship between the North Atlantic Oscillation (NAO) and North Atlantic tripole (NAT) sea surface temperature (SST) anomaly pattern during boreal spring since the late 1980s. A comparative analysis is conducted for two periods before and after the late 1980s to understand the reasons for the above interdecadal change. During both periods, SST cooling in the northern tropical Atlantic during the positive phase of the NAT SST pattern results in an anomalous anticyclone over the subtropical western North Atlantic via a Rossby wave–type atmospheric response. The westerly wind anomalies along the north flank of the anomalous anticyclone are accompanied by a marked decrease in synoptic-scale eddies over the midlatitudes as well as cyclonic (anticyclonic) vorticity forcings at the north (south) side. As such, an NAO-like dipole atmospheric anomaly is induced over the North Atlantic, which in turn helps to maintain the NAT SST anomaly via modulating surface heat fluxes. The intensity of the synoptic-scale eddy feedback to mean flow is stronger after than before the late 1980s, which is related to interdecadal increase in the intensity of North Atlantic synoptic-scale eddies. This is followed by a stronger NAO-like atmospheric response to the NAT SST anomaly since the late 1980s. Further analysis shows that changes in the spatial structure of the spring NAO may also partly contribute to changes in the spring NAO–NAT SST connection around the late 1980s. In particular, spring NAO-related atmospheric anomalies are weaker and shift northward before the late 1980s, which reduces the contribution of the NAO to a tripole SST anomaly pattern in the North Atlantic.

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Zhang Chen, Renguang Wu, and Wen Chen

Abstract

The East Asian winter monsoon (EAWM)-related climate anomalies have shown large year-to-year variations in both the intensity and the meridional extent. The present study distinguishes the interannual variations of the low-latitude and mid- to high-latitude components of the EAWM to gain a better understanding of the characteristics and factors for the EAWM variability. Through composite analysis based on two indices representing the northern and southern components (modes) of the EAWM variability, the present study clearly reveals features unique to the northern and southern mode. The northern mode is associated with changes in the mid- to high-latitude circulation systems, including the Siberian high, the Aleutian low, the East Asian trough, and the East Asian westerly jet stream, whereas the southern mode is closely related to circulation changes over the global tropics, the North Atlantic, and North America. A strong northern mode is accompanied by positive, negative, and positive surface temperature anomalies in the Indochina Peninsula, midlatitude Asia, and northeast Russia, respectively. A strong southern mode features lower temperature over tropics and higher temperature over mid- to high-latitude Asia. While the southern mode is closely related to El Niño–Southern Oscillation (ENSO), the northern mode does not show an obvious relation to the tropical sea surface temperature (SST) change or to the North Atlantic Oscillation (NAO)/Arctic Oscillation (AO) on the interannual time scale. Distinct snow cover and sea ice changes appear as responses to wind and surface temperature changes associated with the two modes and their effects on the EAWM variability need to be investigated in the future.

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Renguang Wu, Jilong Chen, and Wen Chen

Abstract

Observational analysis reveals three types of El Niño–Southern Oscillation (ENSO) influences on the Indian summer monsoon (ISM): indirect influence of the preceding winter [December–February (DJF)] eastern equatorial Pacific (EEP) sea surface temperature (SST) anomalies (DJF-only cases), direct influence of the concurrent summer [June–September (JJAS)] EEP SST anomalies (JJAS-only cases), and coherent influence of both the preceding winter and concurrent summer EEP SST anomalies (DJF&JJAS cases). The present study distinguishes the three types of ENSO influences and investigates the processes connecting ENSO to the ISM separately.

In the DJF-only cases, the preceding winter EEP SST anomalies induce north Indian Ocean (NIO) SST anomalies through air–sea interaction processes in the tropical Indian Ocean. The SST anomalies over the western Indian Ocean alter the surface air humidity there. Both processes favor an anomalous ISM. In the JJAS-only cases, an anomalous ISM is directly induced by ENSO through large-scale circulation changes. The meridional thermal contrast may also contribute to an anomalous ISM. In the DJF&JJAS cases, the preceding winter EEP SST anomalies induce NIO SST anomalies and change the surface air humidity over the western Indian Ocean. Concurrent summer EEP SST anomalies induce large-scale vertical motion anomalies over South Asia. Together, they lead to an anomalous ISM. The anomalous meridional thermal contrast may contribute to an anomalous ISM in late summer.

Impacts of the preceding winter EEP SST anomalies in the DJF and JJAS cases may contribute to the contemporaneous correlation between ISM and EEP SST. There are more DJF&JJAS cases before than after the late 1970s. This provides an alternative interpretation for the observed weakening in the ISM–ENSO relationship around the late 1970s.

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