Tim Li

Atmosphere-warm Ocean interaction and its impacts on Asian-Australian monsoon variation

Abstract Asian–Australian monsoon (A–AM) anomalies depend strongly on phases of El Nino (La Nina). 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 Nino (La Nina) from its development to decay. Two off-equatorial surface anticyclones dominate the A–AM anomalies during an El Nino—one 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 Nino, rapidly reaches its peak intensity in fall, and decays when El Nino matures. The WNP anticyclone, on the other hand, forms in fall, attains maximum intensity after El Nino 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 Nina resemble those durin...

Structures and Mechanisms of the Northward Propagating Boreal Summer Intraseasonal Oscillation

Abstract 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 couplin...

Decadal Change of the Spring Snow Depth over the Tibetan Plateau: The Associated Circulation and Influence on the East Asian Summer Monsoon*

Abstract The decadal change in the spring snow depth over the Tibetan Plateau and impact on the East Asian summer monsoon are investigated using station observations of snow depth data and the NCEP–NCAR reanalysis for 1962–93. During spring (March–April), both the domain-averaged snow depth index (SDI) and the first principal component of the empirical orthogonal function (EOF) analysis exhibit a sharp increase in snow depth after the late 1970s, which is accompanied by excessive precipitation and land surface cooling. The correlation between SDI and precipitation shows a coherent remote teleconnection from the Tibetan Plateau–northern India to western Asia. It is found that the increased snow depth over the plateau after the mid-1970s is concurrent with a deeper India–Burma trough, an intensified subtropical westerly jet as well as enhanced ascending motion over the Tibetan Plateau. Additional factors for the excessive snowfall include more moisture supply associated with the intensification of the south...

A Theory for the Indian Ocean Dipole-Zonal Mode*

Abstract Four fundamental differences of air–sea interactions between the tropical Pacific and Indian Oceans are identified based on observational analyses and physical reasoning. The first difference is represented by the strong contrast of a zonal cloud–SST phase relationship between the warm and cool oceans. The in-phase cloud–SST relationship in the warm oceans leads to a strong negative feedback, while a significant phase difference in the cold tongue leads to a much weaker thermodynamic damping. The second difference arises from the reversal of the basic-state zonal wind and the tilting of the ocean thermocline, which leads to distinctive effects of ocean waves. The third difference lies in the existence of the Asian monsoon and its interaction with the adjacent oceans. The fourth difference is that the southeast Indian Ocean is a region where a positive atmosphere–ocean thermodynamic feedback exists in boreal summer. A conceptual coupled atmosphere–ocean model was constructed aimed to understand th...

Interannual and Interdecadal Variations of the East Asian Summer Monsoon and Tropical Pacific SSTs. Part II: Meridional Structure of the Monsoon

Abstract The relationship between the interannual variations of the East Asian summer monsoon and that of the tropical SST shows considerable variations. In this study, rainfall in the southeastern coastal area of China (SEC) during 1951–96 is used to composite the tropical SST, 850-hPa wind, and 500-hPa height. The results relative to the May–June rainfall, which represents most of the SEC summer monsoon rainfall, are compared to the Yangtze River Valley (YRV) rainfall composites. It is shown that strong interdecadal changes in the Pacific may account for the observed variations in the meridional structure of the monsoon–SST relationship. The western Pacific 500-hPa subtropical ridge, which is influenced by the equatorial eastern Pacific SST, is crucial to these variations. During 1951–77 the SEC wet phase is produced by an anomalous anticyclone in the northern South China Sea, which tends to make the monsoon pre-Mei-yu and Mei-yu fronts quasi-stationary in the general area of both SEC and YRV, and also ...

Relative Contributions of the Indian Ocean and Local SST Anomalies to the Maintenance of the Western North Pacific Anomalous Anticyclone during the El Niño Decaying Summer

Abstract To investigate the relative role of the cold SST anomaly (SSTA) in the western North Pacific (WNP) or Indian Ocean basin mode (IOBM) in maintaining an anomalous anticyclone over the western North Pacific (WNPAC) during the El Nino decaying summer, a suite of numerical experiments is performed using an atmospheric general circulation model, ECHAM4. In sensitive experiments, the El Nino composite SSTA is specified in either the WNP or the tropical Indian Ocean, while the climatological SST is specified elsewhere. The results indicate that the WNPAC is maintained by the combined effects of the local forcing of the negative SSTA in the WNP and the remote forcing from the IOBM. The former (latter) contribution gradually weakens (enhances) from June to August. The negative SSTA in the WNP is crucial for the maintenance of the WNPAC in early summer. However, because of a negative air–sea feedback, the negative SSTA gradually decays, as does the local forcing effect. Enhanced local convection associated ...

Seasonally evolving dominant interannual variability modes of East Asian climate.

A season-reliant empirical orthogonal function (S-EOF) analysis is applied to seasonal mean precipitation over East Asia for the period of 1979-2004. The first two dominant modes account for 44% of the total interannual variance, corresponding to post-ENSO and ENSO turnabout years, respectively. The first mode indicates that in El Nino decaying summer, an anomalous anticyclone appears over the western North Pacific (WNP). This anticyclone is associated with strong positive precipitation anomalies from central China to southern Japan. In the following fall, enhanced convection appears over the WNP as a result of the un- derlying warm SST anomalies caused by the increase of the shortwave radiative flux in the preceding sum- mer. A dry condition appears over southeastern China. The anomalous precipitation pattern persists throughout the subsequent winter and spring. The second mode shows that during the El Nino developing summer the anomalous heating over the equatorial central Pacific forces a cyclonic vorticity over the WNP. This strengthens the WNP monsoon. Meanwhile, an anomalous anticyclone develops in the northern Indian Ocean and moves eastward to the South China Sea and the WNP in the subsequent fall and winter. This leads to the increase of precipitation over southeastern China. The anticyclone and precipitation anomalies are maintained in the following spring through local air-sea interactions. The diagnosis of upper-level velocity potential and midlevel vertical motion fields reveals a season- dependent Indian Ocean forcing scenario. The Indian Ocean basinwide warming during the El Nino mature winter and the subsequent spring does not have a significant impact on anomalous circulation in the WNP, because convection over the tropical Indian Ocean is suppressed by the remote forcing from the equatorial central-eastern Pacific. The basinwide warming plays an active role in impacting the WNP anomalous an- ticyclone during the ENSO decaying summer through atmospheric Kelvin waves or Hadley circulation.

A Theory for the Tropical Tropospheric Biennial Oscillation

The key questions of how the tropospheric biennial oscillation (TBO) maintains the same phase from northern summer in South Asia to southern summer in Australia, and how the reversed phase can last through three locally inactive seasons to the next monsoon, are studied by a simple tropical atmosphere‐ocean‐land model. The model has five boxes representing the South Asian and Australian monsoon regions and the equatorial Indian and western and eastern Pacific Oceans. The five regions interact with each other through the SST‐ monsoon, evaporation‐wind, monsoon‐Walker circulation, and wind stress‐ocean thermocline feedbacks. A biennial oscillation emerges in a reasonable parameter regime, with model SST and wind variations resembling many aspects of the observed TBO. Warm SST anomalies (SSTA) in July in the equatorial Indian Ocean cause an increase of surface moisture convergence into South Asia, leading to a stronger monsoon. The monsoon heating on one hand induces a westerly wind anomaly in the Indian Ocean, and on the other hand intensifies a planetary-scale east‐west circulation leading to anomalous easterlies over the western and central Pacific. The westerly anomaly over the Indian Ocean decreases the local SST, primarily by evaporation‐wind feedback. The easterly anomaly in the central Pacific causes a deepening of the ocean thermocline in the western Pacific therefore increasing the subsurface and surface temperatures. In addition, a modest easterly anomaly in the western Pacific opposes the seasonal mean westerlies so evaporation is reduced. These effects overwhelm those of the cold zonal advection and anomalous upwelling. The net result is warm SSTA persisting in the western Pacific through northern fall, leading to a stronger Australian monsoon. Meanwhile, the warming in the western Pacific also induces a stronger local Walker cell and thus a surface westerly anomaly over the Indian Ocean. This westerly anomaly helps the cold SSTA to persist through the succeeding seasons, leading to a weaker Asian monsoon in the following summer. During northern winter the westerly anomaly associated with the stronger Australian monsoon, through anomalous ocean downwelling and reduction of evaporation (when the seasonal mean wind is easterly), reinvigorates the warm SSTA in the western Pacific, which has been weakened by the slow cold advection from the eastern Pacific. This further intensifies the eastern Walker cell and helps to keep the eastern Pacific cold. The authors’ theory indicates that the TBO is an inherent result of the interactions between northern summer and winter monsoon and the tropical Indian and Pacific Oceans. Thus, it is an important component of the

A new paradigm for the predominance of standing Central Pacific Warming after the late 1990s

Canonical El Niño has a warming center in the eastern Pacific (EP), but in recent decades, El Niño warming center tends to occur more frequently in the central Pacific (CP). The definitions and names of this new type of El Niño, however, have been notoriously diverse, which makes it difficult to understand why the warming center shifts. Here, we show that the new type of El Niño events is characterized by: 1) the maximum warming standing and persisting in the CP and 2) the warming extending to the EP only briefly during its peak phase. For this reason, we refer to it as standing CP warming (CPW). Global warming has been blamed for the westward shift of maximum warming as well as more frequent occurrence of CPW. However, we find that since the late 1990s the standing CPW becomes a dominant mode in the Pacific; meanwhile, the epochal mean trade winds have strengthened and the equatorial thermocline slope has increased, contrary to the global warming-induced weakening trades and flattening thermocline. We propose that the recent predominance of standing CPW arises from a dramatic decadal change characterized by a grand La Niña-like background pattern and strong divergence in the CP atmospheric boundary layer. After the late 1990s, the anomalous mean CP wind divergence tends to weaken the anomalous convection and shift it westward from the underlying SST warming due to the suppressed low-level convergence feedback. This leads to a westward shift of anomalous westerly response and thus a zonally in-phase SST tendency, preventing eastward propagation of the SST anomaly. We anticipate more CPW events will occur in the coming decade provided the grand La Niña-like background state persists.

Interactions between the seasonal cycle and the southern oscillation-frequency entrainment and chaos in a coupled ocean-atmosphere model

Nonlinear interactions between the seasonal cycle and interannual variations in the coupled ocean-atmosphere system have recently been proposed as the cause of irregularity of El Nino-Southern Oscillation (ENSO). We investigated such a hypothesis using a coupled ocean-atmosphere model which allows coupling between total sea surface temperature (SST) and total surface winds. Numerical simulations indicate that the model is capable of capturing the essential SST variability on seasonal-to-interannual time scale. Furthermore, it is shown that, as the seasonal forcing amplitude is gradually increased from zero, the coupled model undergoes several transitions between periodic (frequency-locking) and chaotic states before it finally ‘gives up’ its intrinsic ENSO mode of oscillation entirely and acquires the frequency of the seasonal forcing. Chaotic response is found as the forcing amplitude approaches the observed value and the route to ENSO chaos is identified to be the period-doubling cascade. The study suggests that the response of a coupled system, coupled General Circulation Models of the ocean and atmosphere for example, can be very sensitive not only to changes in the internal model parameters but also to changes in the external forcing conditions.