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Bin Wang

Abstract

The latest two Pacific basinwide warm episodes (1982–83 and 1986–87) exhibit some common features in their development and vertical structure. These features are examined by multivariate empirical orthogonal function analysis of the interannual variability of the ocean-atmosphere system along equatorial Indian and Pacific oceans.

The updraft and downdraft branches of the anomalous Walker circulation originate over the western Pacific and the eastern Indian Ocean, respectively. The early development of basinwide warming is characterized by the strengthening of a cross-equatorial low-level southerly component over the eastern Pacific and the enhancement of convection and boundary-layer westerlies over the western Pacific.

The structure of the ENSO anomaly mode changes from the cold to the warm phase of the Southern Oscillation. This is attributed to its eastward migration and the intrinsic longitudinal dependence of the vertical structure. The latter results from the east–west contrast of the air–sea interaction processes. Over Indonesia and the western Pacific, the land–sea thermal contrast and high SST maintain a semipermanent convective action center, whose intensity is sensitively modulated by small SST fluctuation. Since moist static ability is small, the surface pressure responds sensitively to the heating, so that the anomalous low pressure and associated zonal wind convergence in the boundary layer are in phase with the enhanced convection. In contrast, over the central-eastern Pacific, large SST gradient-induced pressure gradient force drives boundary-layer flows whose beta convergence determines atmospheric heating, while the feedback of the free atmosphere to boundary-layer flows is weak due to large static stability. The enhanced convection is thus nearly in phase with anomalous boundary-layer westerlies, positive zonal SST gradient, and negative zonal surface pressure gradient.

It is possible that an individual ENSO event may result from different combinations of various sets of coupled processes, especially with regard to those that work in the eastern Pacific cold tongue and those in the western Pacific warm pool.

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Bin Wang

Abstract

Stability of the equatorial atmosphere to a quasi-zonal, low frequency (order of 10−6 s−1) disturbance is investigated, using a model that consists of a two-layer free atmosphere and well-mixed boundary layer. The inclusion of boundary layer convergence leads to a circulation-dependent heating more nearly in accord with the behavior of numerically simulated low-frequency waves.

Slowly eastward moving unstable waves were found in a parameter regime stable to inviscid wave-CISK. The instability depends crucially upon the vertical distribution of the moist static energy of the basic state. A derived instability criterion suggests that amplification occurs when the condensational heating supported jointly by interior wave convergence and frictional convergence dominates over the dissipations due to longwave radiation and boundary layer viscosity.

The unstable waves exhibit preferred planetary scales. Since the vertical distribution of moist static energy of the basic state is closely related to sea surface temperature (SST), with increasing SST the growth rate increases for all wavelength, but the preferred scales shifts to shorter wavelength. The boundary layer convergence plays an important role in spatial scale selection. It not only suppresses unbounded growth of short waves, but also couples barotropic and baroclinic components in such a way that the generation of wave available potential energy is most efficient for planetary scales.

In the presence of boundary layer friction, the amplitude of an unstable wave in the zonal wind (or geopotential) at the upper level is significantly larger than that at the lower level. The phases between the two levels differ by about 180°. The slow eastward movement results mainly from the interior wave-induced reduction of static stability and thermal damping.

Behaviors of model unstable waves appear to resemble the observed 40–50 day mode in many aspects, such as vertical structure, energy source, zonal phase propagations, characteristic zonal and meridional scales, and its relation to SST. Limitations of the model are also discussed.

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Bin Wang

Abstract

abstract not available.

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Bin Wang

Abstract

In spite of the fundamental difficulties in interpreting the growth of tropical storms, the basic idea of CISK remains valuable in understanding the instability resulting from the interaction between cumulus convection and large-scale flows. A generalized solution of a quasi-balanced continuous model, which can be applied to various types of vertical heating distribution and basic-state stratification, is derived and used to explore the behaviors of the CISK mode.

In the absence of cumulus momentum mixing, the CISK solution exhibits, in general, a scale selection. However, two types of unbounded growth rates associated with different closure assumptions may exist. Both of them take place in a common situation that is characterized by local warming at the top of the moist convergence layer in the area of rising motion. In these circumstances, the direct coupling between the heating and the large-scale moisture supply through the divergent wind component dominates over the indirect coupling through the rotational component. It is suggested that, for a feasible Ekman CISK mechanism, the dominant feedback of the convective heating to the low-level moisture convergence must be of an indirect nature. In this feedback process, planetary vorticity and/or preexisting relative vorticity play an essential role in converting heating-induced divergent kinetic energy to rotational kinetic energy, thus accelerating the spin-up of a large-scale vortex.

The cumulus momentum mixing destabilizes short waves by enhancing cyclonic circulation at the top of the Ekman layer and by reducing the vertical extent of the temperature disturbance; meanwhile, it stabilizes long waves by weakening the anticyclonic circulation in the upper levels.

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Bin Wang

Abstract

The low-level cyclonic vortices which form over the Tibetan Plateau in the summer monsoon season are major rain-producing systems and have the potential to trigger cyclogenesis on the lee side when they move off the plateau. Two cases of the plateau vortices which occurred in July 1979 are studied. The characteristics of the vertical structure in their developing stage, and the circulation condition favorable for the eastward movement in the mature stage are diagnosed and presented by use of FGGE IIIb datasets. Numerical simulations with real data were performed using the GFDL limited-area mesoscale simulation model. Results suggest that the latent heating is an essential driving force for the development of the vortices studied here.

The analysis of a continuous CISK model with a basic state resembling that actually observed over the summer plateau shows that the predicted unstable mode has a preferred scale, growth rate and vertical structure, all of which are qualitatively comparable to observations. The instability in the plateau environment is mainly attributed to 1) the relatively shallow vertical extent of heating located in the upper troposphere in which the heat capacity of the air column per unit surface area is relatively small; 2) the dramatic reduction of the static stability due to surface sensible heat flux; and 3) the significant increase of moisture content in the plateau boundary layer due to surface evaporation and monsoon transport of water vapor. Most of these favorable conditions are referred to as the dynamic and thermal effects of the elevated plateau terrain. In this sense, the development of the plateau vortices during the rainy season may be regarded as resulting from the interaction between the lane-sca1e circulation and the plateau topographic effect and from the release of convective latent heat.

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Bin Wang

Abstract

The characteristics of the onset of the Pacific basin-wide warming have experienced notable changes since the late 1970s. The changes are caused by a concurrent change in the background state on which El Niño evolves.

For the most significant warm episodes before the late 1970s (1957, 1965, and 1972), the atmospheric anomalies in the onset phase (November to December of the year preceding the El Niño) were characterized by a giant anomalous cyclone over east Australia whose eastward movement brought anomalous westerlies into the western equatorial Pacific, causing development of the basin-wide warming. Meanwhile, the trades in the southeastern Pacific (20°S–0°, 125°–95°W) relaxed back to their weakest stage, resulting in a South American coastal warming, which led the central Pacific warming by about three seasons. Conversely, in the warm episodes after the late 1970s (1982, 1986–87, and 1991), the onset phase was characterized by an anomalous cyclone over the Philippine Sea whose intensification established anomalous westerlies in the western equatorial Pacific. Concurrently, the trades were enhanced in the southeastern Pacific, so that the coastal warming off Ecuador occurred after the central Pacific warming.

It is found that the atmospheric anomalies occurring in the onset phase are controlled by background SSTs that exhibit a significant secular variation. In the late 1970s, the tropical Pacific between 20°S and 20°N experienced an abrupt interdecadal warming, concurrent with a cooling in the extratropical North Pacific and South Pacific and a deepening of the Aleutian Low. The interdecadal change of the background state affected El Niño onset by altering the formation of the onset cyclone and equatorial westerly anomalies and through changing the trades in the southeast Pacific, which determine whether a South American coastal warming leads or follows the warming at the central equatorial Pacific.

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Bin Wang

Abstract

Annual distribution and phase propagation of tropical convection are delineated using harmonic and amplitude-phase characteristics analysis of climatological pentad mean outgoing longwave radiation and monthly frequencies of highly reflective cloud.

An annual eastward propagation of peak rainy season along the equator from the central Indian Ocean (60°E) to Arafura Sea (130°E) is revealed. This indicates a transition from the withdrawal of the Indian summer monsoon to the onset of the Australian summer monsoon. Significant bimodal variations are found around major summer monsoon regions. These variations originate from the interference of two adjacent regimes.

The convergence zones over the eastern North Pacific, the South Pacific, and the southwest Indian Ocean are identified as a marine monsoon regime that is characterized by a unimodal variation with a concentrated summer rainfall associated with the development of surface westerlies equatorward of a monsoon trough. Conversely, the central North Pacific and North Atlantic convergence zones between persistent northeast and southeast trades are classified as trade-wind convergence zones, which differ from the marine monsoon regime by their persistent rainy season and characteristic bimodal variation with peak rainy seasons occurring in late spring and fall.

The roles of the annual march of sea surface temperature in the phase propagation and formation of various climatic regimes of tropical convection are also discussed.

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Bin Wang

Abstract

In the tropical eastern central Pacific Ocean, the annual cycle in sea surface temperature (SST), surface winds and pressure, and clouds are alternatively dominated by an antisymmetric (with respect to the equator) monsoonal mode in February and August and a quasi-symmetric equatorial-coastal mode in May and November, both having a period of one year. The monsoonal mode is forced by the differential insulation between the Northern and Southern Hemispheres. The surface wind variation of the monsoonal mode tends to lead SST variation in late spring/fall. The equatorial-coastal mode originates from atmosphere–ocean interaction. Its development is characterized by contemporaneous intensification and spatial expansion (westward and poleward phase propagation).

The interaction between the forced monsoonal mode and the coupled equatorial-coastal mode plays a critical role in the annual cycle. From October to February, the decline of the southern winter regime of the monsoonal mode initiates and sustains the amplification of the equatorial-coastal mode, causing annual weakening of the cold tongue. From April to June, the enhancement of the poleward SST gradient associated with the decay of the equatorial-coastal mode initiates the eastern North Pacific summer monsoon. Atmosphere-ocean interaction is directly responsible for the annual weakening and reestablishment of the cold tongue, whereas the annual cycle in insulation regulates the interaction indirectly through the forced monsoonal mode.

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LinHo
and
Bin Wang

Abstract

Despite the seemingly intricate and multifold time–space structure of the mean Asian–Pacific summer monsoon (APSM), its complexity can be greatly reduced once the significance of fast annual cycles has been recognized and put into perspective. The APSM climatology is characterized by a slowly evolving seasonal transition (slow annual cycle) superposed by pronounced singularities in the intraseasonal timescale, termed the “fast annual cycle” in this study. The fast annual cycles show nonrepetitive features from one episode to another, which are often divided by abrupt change events. The APSM fast annual cycles are composed mainly of two monsoon outbreaks, each marking a distinctive dry–wet cycle. The first cycle spans from the middle of May to early July and the second cycle from late July to early September. When the first cycle reaches its peak in mid-June, a slingshot-like convection zone, described as the grand-onset pattern, rules an area from the Arabian Sea to the Indochina Peninsula then bifurcates into a mei-yu branch and a tropical rain belt in the lower western North Pacific. After a brief recess during 20–29 July, the APSM harbors another rain surge in mid-August. This time a giant oceanic cyclone intensifies over the western North Pacific (around 20°N, 140°E); thus the rainy regime jumps 10°–15° north of the previous rain belt. This ocean monsoon gyre incubates numerous tropical cyclones. Meanwhile, the convection zone of the Indian monsoon intensifies and extends well into the subcontinent interior.

From the first to second cycle the major convection center has shifted from the adjacent seas in the northern Indian Ocean to the open ocean east of the Philippine Islands. The major cloud movement also switches from a northeastward direction in the Indian Ocean to a northwestward direction over the western North Pacific.

The two monsoon cycles turn out to be a global phenomenon. This can be shown by the coherent seasonal migration of upper-level subtropical ridgelines in the Northern Hemisphere. During the first cycle all the ridgelines migrate northward rapidly, a sign that the major circulation systems of boreal summer go through a developing stage. After 20–29 July, they reach a quasi steady state, a state in which all ridgelines stand still near their northern rim throughout the entire second cycle.

A reconstructed fast annual cycle based on four leading empirical orthogonal function modes is capable of reproducing most fine details of the APSM climatology, suggesting that the subseasonal changes of the mean APSM possess a limited number of degrees of freedom. A monsoon calendar designed on the basis of fast annual cycles (FACs) gives a concise description of the APSM climatology and provides benchmarks for validating climate model simulations.

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Bin Wang
and
LinHo

Abstract

To date, the monsoon-research community has not yet reached a consensus on a unified definition of monsoon rainy season or on the linkage between the onsets over the Asian continent and the adjacent oceans. A single rainfall parameter is proposed, and a suite of universal criteria for defining the domain, onset, peak, and withdrawal of the rainy season are developed. These results reveal a cohesive spatial–temporal structure of the Asian–Pacific monsoon rainy season characteristics, which will facilitate validation of monsoon hydrological cycles simulated by climate system models and improve our understanding of monsoon dynamics.

The large-scale onset of the Asian monsoon rainy season consists of two phases. The first phase begins with the rainfall surges over the South China Sea (SCS) in mid-May, which establishes a planetary-scale monsoon rainband extending from the south Asian marginal seas (the Arabian Sea, the Bay of Bengal, and the SCS) to the subtropical western North Pacific (WNP). The rainband then advances northwestward, initiating the continental Indian rainy season, the Chinese mei-yu, and the Japanese baiu in early to mid-June (the second phase). The heights of the rainy seasons occur primarily in three stepwise phases: in late June over the mei-yu/baiu regions, the northern Bay of Bengal, and the vicinity of the Philippines, in late July over India and northern China; and in mid-August over the tropical WNP. The rainy season retreats northward over east Asia, yet it moves southward over India and the WNP.

Clear distinctions in the characteristics of the rainy season exist among the Indian, east Asian, and WNP summer monsoon regions. Nevertheless, the rainy seasons of the three subsystems also show close linkage. The causes of the regional distinctions and linkages are discussed. Also discussed are the atypical monsoon rainy seasons, such as the skewed and bimodal seasonal distributions found in various places of Asian monsoon domain.

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