Observational evidence is presented to show a teleconnection between the central Pacific and East Asia during the extreme phases of ENSO cycles. This Pacific-East Asian teleconnection is confined to the lower troposphere. The key system that bridges the warm (cold) events in the eastern Pacific and the weak (strong) East Asian winter monsoons is an anomalous lower-tropospheric anticyclone (cyclone) located in the western North Pacific. The western North Pacific wind anomalies develop rapidly in late fall of the year when a strong warm or cold event matures. The anomalies persist until the following spring or early summer, causing anomalously wet (dry) conditions along the East Asian polar front stretching from southern China northeastward to the east of Japan (Kuroshio extension). Using atmospheric general circulation and intermediate models, the authors show that the anomalous Philippine Sea anticyclone results from a Rossby-wave response to suppressed convective heating, which is induced by both the in situ ocean surface cooling and the subsidence forced remotely by the central Pacific warming. The development of the anticyclone is nearly concurrent with the enhancement of the local sea surface cooling. Both the anticyclone and the cooling region propagate slowly eastward. The development and persistence of the teleconnection is primarily attributed to a positive thermodynamic feedback between the anticyclone and the sea surface cooling in the presence of mean northeasterly trades. The rapid establishment of the Philippine Sea wind and SST anomalies implies the occurrence of extratropical-tropical interactions through cold surge-induced exchanges of surface buoyancy flux. The central Pacific warming plays an essential role in the development of the western Pacific cooling and the wind anomalies by setting up a favorable environment for the anticyclone-SST interaction and midlatitude-tropical interaction in the western North Pacific.

An analysis of 35-yr (1965-99) data reveals vital impacts of strong (but not moderate) El Niño and La Niña events on tropical storm (TS) activity over the western North Pacific (WNP). Although the total number of TSs formed in the entire WNP does not vary significantly from year to year, during El Niño summer and fall, the frequency of TS formation increases remarkably in the southeast quadrant (0Њ-17ЊN, 140ЊE-180Њ) and decreases in the northwest quadrant (17Њ-30ЊN, 120Њ-140ЊE). The July-September mean location of TS formation is 6Њ latitude lower, while that in October-December is 18Њ longitude eastward in the strong warm versus strong cold years. After the El Niño (La Niña), the early season (January-July) TS formation in the entire WNP is suppressed (enhanced). In strong warm (cold) years, the mean TS life span is about 7 (4) days, and the mean number of days of TS occurrence is 159 (84) days. During the fall of strong warm years, the number of TSs, which recurve northward across 35ЊN, is 2.5 times more than during strong cold years. This implies that El Niño substantially enhances poleward transport of heat-moisture and impacts high latitudes through changing TS formation and tracks. The enhanced TS formation in the SE quadrant is attributed to the increase of the low-level shear vorticity generated by El Niño-induced equatorial westerlies, while the suppressed TS generation over the NW quadrant is ascribed to upper-level convergence induced by the deepening of the east Asian trough and strengthening of the WNP subtropical high, both resulting from El Niño forcing. The WNP TS activities in July-December are noticeably predictable using preceding winter-spring Niño-3.4 SST anomalies, while the TS formation in March-July is exceedingly predictable using preceding October-December Niño-3.4 SST anomalies. The physical basis for the former is the phase lock of ENSO evolution to the annual cycle, while for the latter it is the persistence of Philippine Sea wind anomalies that are excited by ENSO forcing but maintained by local atmosphere-ocean interaction.

Analysis of the 56-yr NCEP–NCAR reanalysis data reveals a recurrent circumglobal teleconnection (CGT) pattern in the summertime midlatitude circulation of the Northern Hemisphere. This pattern represents the second leading empirical orthogonal function of interannual variability of the upper-tropospheric circulation. The CGT, having a zonal wavenumber-5 structure, is primarily positioned within a waveguide that is associated with the westerly jet stream. The spatial phases of CGT tend to lock to preferred longitudes. The geographically phase-locked patterns bear close similarity during June, August, and September, but the pattern in July shows shorter wavelengths in the North Pacific–North America sector. The CGT is accompanied by significant rainfall and surface air temperature anomalies in the continental regions of western Europe, European Russia, India, east Asia, and North America. This implies that the CGT may be a source of climate variability and predictability in the above-mentioned midlatitude regions. The CGT has significant correlations with the Indian summer monsoon (ISM) and El Niño–Southern Oscillation (ENSO). However, in normal ISM years the CGT–ENSO correlation disappears; on the other hand, in the absence of El Niño or La Niña, the CGT–ISM correlation remains significant. It is suggested that the ISM acts as a “conductor” connecting the CGT and ENSO. When the interaction between the ISM and ENSO is active, ENSO may influence northern China via the ISM and the CGT. Additionally, the variability of the CGT has no significant association with the Arctic Oscillation and the variability of the western North Pacific summer monsoon. The circulation of the wave train shows a barotropic structure everywhere except the cell located to the northwest of India, where a baroclinic circulation structure dominates. Two possible scenarios are proposed. The abnormal ISM may excite an anomalous west-central Asian high and downstream Rossby wave train extending to the North Pacific and North America. On the other hand, a wave train that is excited in the jet exit region of the North Atlantic may affect the west-central Asian high and, thus, the intensity of the ISM. It is hypothesized that the interaction between the global wave train and the ISM heat source may be instrumental in maintaining the boreal summer CGT.

Monsoon rainfall and tropical storms (TSs) impose great impacts on society, yet their seasonal predictions are far from successful. The western Pacific Subtropical High (WPSH) is a prime circulation system affecting East Asian summer monsoon (EASM) and western North Pacific TS activities, but the sources of its variability and predictability have not been established. Here we show that the WPSH variation faithfully represents fluctuations of EASM strength (r = -0.92), the total TS days over the subtropical western North Pacific (r = -0.81), and the total number of TSs impacting East Asian coasts (r = -0.76) during 1979-2009. Our numerical experiment results establish that the WPSH variation is primarily controlled by central Pacific cooling/warming and a positive atmosphere-ocean feedback between the WPSH and the Indo-Pacific warm pool oceans. With a physically based empirical model and the state-of-the-art dynamical models, we demonstrate that the WPSH is highly predictable; this predictability creates a promising way for prediction of monsoon and TS. The predictions using the WPSH predictability not only yields substantially improved skills in prediction of the EASM rainfall, but also enables skillful prediction of the TS activities that the current dynamical models fail. Our findings reveal that positive WPSH-ocean interaction can provide a source of climate predictability and highlight the importance of subtropical dynamics in understanding monsoon and TS predictability. S ummer monsoons and tropical storms (TSs) affect billions of people's livelihoods over East Asia including China, Japan, Korea, Indo-China peninsula, and Philippines. Prediction of the East Asian summer monsoon (EASM) rainfall and the TS in the western North Pacific (WNP) is a forefront scientific challenge of great societal importance and economic value. The latest assessment of the world-class climate models' performance clearly demonstrates the models' poor skills in prediction of the monsoon rainfall (1) and their inability to predict WNP TS variations.The western Pacific Subtropical High (WPSH) has profound effects on (and interact with) EASM and WNP TS activities (2-6); it also has far reaching influence on the summer rainfall over the Great Plains of the United States through atmospheric teleconnection (7,8). Understanding the mechanism and predictability of the WPSH is a prerequisite for better prediction of the EASM and WNP TS.It has been noticed decades ago that an enhanced WPSH occurs during El Niño decaying summer, but the physical interpretation was not offered until the turn of 21st century (9-11). Recently, the influence of the Indian Ocean (IO) warming (12) has been revived to explain why the WPSH is abnormally strong after a peak El Niño (13-17). Note, however, approximately onehalf of the strong anomalous WPSH years do not concur with decaying El Niño (Fig. S1) or IO warming (Fig. S2). Thus, it is necessary to reshape the conventional thinking on the causes of the interannual variation of the WPSH.Here, we reveal two funda...

In the south Asian region, two of the major precipitation maxima associated with areas of intensive convective activity are located near the Bay of Bengal and in the vicinity of the Philippines. The variations of monthly mean outgoing longwave radiation in the two regions are poorly correlated, particularly in the decade of 1980s. The enhanced convection over the Bay of Bengal and Indian subcontinents is coupled with reinforced monsoon circulation west of 80°E over India, the western Indian Ocean, and the tropical northern Africa. In contrast, the enhanced convection in the vicinity of the Philippines corresponds to intensified monsoon circulation primarily east of 80°E over southeast Asia including the Indochina peninsula, South China Sea, Philippine Sea, and the Maritime Continent. To better reflect regional monsoon characteristics, two convection indices (or associated circulation indices that are dynamically coherent with the convection indices) are suggested to measure the variability of the Indian summer monsoon (ISM) and the southeast Asian summer monsoon, respectively. The change in the Bay of Bengal convection (the ISM) has planetary-scale implications, whereas the change in Philippine convection has primarily a regional impact including a linkage with the east Asia subtropical monsoon. The equatorial western Pacific winds exhibit a considerably higher correlation with the ISM convection than with the Philippine convection. During the summers when a major Pacific warm episode occurs (e.g., 1982-83, 1986-87, 1991-92, and 1997), the convection and circulation indices describing the ISM often diverge considerably, causing inconsistency among various normally coherent monsoon indices. This poses a primary difficulty for using a single monsoon index to characterize the interannual variability of a regional monsoon. The cause of the breakdown of the coherence between various convection and circulation indices during ENSO warm phase needs to be understood.

The spatial and temporal structures of the northward-propagating boreal summer intraseasonal oscillation (BSISO) are revealed based on the analysis of both the ECHAM4 model simulation and the NCEP-NCAR reanalysis. The BSISO structure and evolution characteristics simulated by the model bear many similarities to those derived from the NCEP-NCAR reanalysis. The most notable features are the remarkable meridional asymmetries, relative to the BSISO convection, in the vorticity and specific humidity fields. A positive vorticity perturbation with an equivalent barotropic structure appears a few latitude degrees north of the convection center. The maximum specific humidity also shows a clear northward shift in the lower troposphere. Two internal atmospheric dynamics mechanisms are proposed to understand the cause of the northward propagation of the BSISO. The first is the vertical shear mechanism. The key process associated with this mechanism is the generation of barotropic vorticity due to the coupling between the free-atmosphere baroclinic and barotropic modes in the presence of the vertical shear of the mean flow. The induced barotropic vorticity in the free atmosphere further causes a moisture convergence in the planetary boundary layer (PBL), leading to the northward shift of the convective heating. The second mechanism is the moisture-convection feedback mechanism. Two processes contribute to the northward shift of the low-level moisture. One is the moisture advection by the mean southerly in the PBL. Another is the moisture advection by the BSISO wind due to the mean meridional specific humidity gradient. The asymmetric specific humidity contributes to the northward shift of the convective heating. A theoretical framework is constructed to investigate the instability of the northward-propagating BSISO mode and the relative roles of various mechanisms including air-sea interactions. An eigenvalue analysis indicates that the northward propagation of the BSISO is an unstable mode of the summer mean flow in the monsoon region. It has a typical wavelength of 2500 km. While the easterly shear contributes to the northward propagation primarily north of 5ЊN, the moisture feedback and the air-sea interaction also contribute significantly, particularly in the region near and south of the equator. The internal atmospheric dynamics are essential in causing the northward propagation of the BSISO over the tropical Indian Ocean.

Asian-Australian monsoon (A-AM) anomalies depend strongly on phases of El Nin ˜o (La Nin ˜a). Based on this distinctive feature, a method of extended singular value decomposition analysis was developed to analyze the changing characteristics of A-AM anomalies during El Nin ˜o (La Nin ˜a) from its development to decay. Two off-equatorial surface anticyclones dominate the A-AM anomalies during an El Nin ˜oone over the south Indian Ocean (SIO) and the other over the western North Pacific (WNP). The SIO anticyclone, which affects climate conditions over the Indian Ocean, eastern Africa, and India, originates during the summer of a growing El Nin ˜o, rapidly reaches its peak intensity in fall, and decays when El Nin ˜o matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Nin ˜o matures, and persists through the subsequent spring and summer, providing a prolonged impact on the WNP and east Asian climate. The monsoon anomalies associated with a La Nin ˜a resemble those during an El Nin ˜o but with cyclonic anomalies. From the development summer to the decay summer of an El Nin ˜o (La Nin ˜a), the anomalous sea level pressure, low-level winds, and vertical motion tend to reverse their signs in the equatorial Indian and western Pacific Oceans (10ЊS-20ЊN, 40Њ-160ЊE). This suggests that the tropospheric biennial oscillation is intimately linked to the turnabouts of El Nin ˜o and La Nin ˜a.The remote El Nin ˜o forcing alone can explain neither the unusual amplification of the SIO anticyclone during a developing El Nin ˜o nor the maintenance of the WNP anticyclone during a decaying El Nin ˜o. The atmosphere-ocean conditions in the two anticyclone regions are similar, namely, a zonal sea surface temperature (SST) dipole with cold water to the east and warm water to the west of the anticyclone center. These conditions result from positive feedback between the anomalous anticyclone and the SST dipole, which intensifies the coupled mode in the SIO during El Nin ˜o growth and maintains the coupled mode in the WNP during El Nin ˜o decay. The interactions in the two anticyclone regions share common wind evaporation/entrainment and cloud-radiation feedback processes but they differ with regard to the oceanic dynamics (vertical and horizontal advection and thermocline adjustment by oceanic waves). The outcome of the interactions in both regions, however, depends crucially on the climatological surface winds. The SIO-coupled mode is triggered by El Nin ˜o-induced subsidence and alongshore winds off the coast of Sumatra. However, other independent El Nin ˜o local and remote forcing can also trigger this coupled mode.The traditional view has regarded SST anomalies in the Indian and western Pacific Oceans as causing the A-AM variability. The present analysis suggests that the SST anomalies in these warm ocean regions are, to a large extent, a result of anomalous monsoons. Thus, the atmosphere-warm ocean interaction may significantly modify the impacts of remote El Nin ˜o forcing and should be ...

Defining the intensity of the East Asian summer monsoon (EASM) has been extremely controversial. This paper elaborates on the meanings of 25 existing EASM indices in terms of two observed major modes of interannual variation in the precipitation and circulation anomalies for the 1979-2006 period. The existing indices can be classified into five categories: the east-west thermal contrast, north-south thermal contrast, shear vorticity of zonal winds, southwesterly monsoon, and South China Sea monsoon. The last four types of indices reflect various aspects of the leading mode of interannual variability of the EASM rainfall and circulations, which correspond to the decaying El Niño, while the first category reflects the second mode that corresponds to the developing El Niño.The authors recommend that the EASM strength can be represented by the principal component of the leading mode of the interannual variability, which provides a unified index for the majority of the existing indices. This new index is extremely robust, captures a large portion (50%) of the total variance of the precipitation and three-dimensional circulation, and has unique advantages over all the existing indices. The authors also recommend a simple index, the reversed Wang and Fan index, which is nearly identical to the leading principal component of the EASM and greatly facilitates real-time monitoring.The proposed index highlights the significance of the mei-yu/baiu/changma rainfall in gauging the strength of the EASM. The mei-yu, which is produced in the primary rain-bearing system, the East Asian (EA) subtropical front, better represents the variability of the EASM circulation system. This new index reverses the traditional Chinese meaning of a strong EASM, which corresponds to a deficient mei-yu that is associated with an abnormal northward extension of southerly over northern China. The new definition is consistent with the meaning used in other monsoon regions worldwide, where abundant rainfall within the major local rain-bearing monsoon system is considered to be a strong monsoon.