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Bin Wang and Isidoro Orlanski

Abstract

A case of the heavy rain vortex which occurred during the period 14–15 July 1979 is studied using a limited-area mesoscale numerical model. This is a representative example of a group of warm southwest vortices that often form over the eastern flank of the Tibetan Plateau after the onset of the summer Indian monsoon.

Some common features of the dynamic structures exhibited both by the simulation and by observations are discussed. The developing vortex is noticeably detached from the polar frontal zone. A 180° phase shift exists between the upper and lower layer vorticity fields. In the boundary layer, a pronounced northward transport of mass and moisture is connected with an intense upward motion near and to the east of the 700-mb vortex center, whole the southward cold advection is insignificant.

The vortex originated and rapidly developed in a stagnation region on the lee side of the plateau. The presence of the stagnation region not only removes local dynamical energy sources from the environmental flow, but also diminishes topographic generation of vorticity by reducing the vortex stretching in the wind component flowing over the plateau and the horizontal convergence in the component moving around the plateau. Without latent heating, dynamic instability and/or forcing of the large-scale flow interacting with the Tibetan Plateau is not sufficient to generate the observed disturbance.

On the other hand, the plateau blocking effect favors the establishment of a conditionally unstable environment. The simulation indicates that a sudden onset of vigorous deep convection, following by a rapid growth of relative vorticity in the lower troposphere, takes place once the dynamic forcing associated with a mesoscale plateau disturbance was positioned over the western stagnation region. Our principle result is that the warm heavy rain vortex in this can study is triggered by a migratory plateau boundary layer disturbance and basically driven by cumulus convective heating. The thermal influence of the elevated plateau topography may appreciably affect the vortex initiation through changing the intensity of the forcing associated with the triggering mechanism.

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Liguang Wu and Bin Wang

Abstract

The vertical coupling and movement of an adiabatic baroclinic tropical cyclone (TC) are investigated through two numerical experiments in which the TC is affected by either a vertical environmental shear or a differential beta drift. In both cases, the initial response of the symmetric vortex is to tilt in the vertical. In response to the vertical tilt, a three-dimensional asymmetric circulation with a typical radius of 100 km develops within the TC core region. In addition, the wavenumber-one potential vorticity (PV) anomalies develop with positive anomalies downtilt (uptilt) above (below) the maximum PV level in order to maintain a balanced state between the thermal and dynamical fields. On a beta plane, in contrast to the beta gyres, the mesoscale asymmetric circulation is a pair of counterrotating inner gyres centered at the radius of maximum wind. As a result, the resulting three-dimensional mesoscale asymmetric circulation, not the penetration flow, plays an important role in the vertical coupling of adiabatic baroclinic vortices. In both cases, the TC motion is not simply due to the advection of the symmetric PV component by the asymmetric (ventilation) flow. The horizontal advection of the asymmetric PV anomalies by the symmetric cyclonic flow and the vertical PV advection associated with the asymmetric vertical motion also considerably contribute to the TC motion. The latter two processes also play a critical role in the vertical coupling of the baroclinic TC due to the presence of the vertical PV gradient.

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Bin Wang and Zhen Fan

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.

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Baozhen Zhu and Bin Wang

Abstract

The tropical Indian and western Pacific oceans are two prominent action centers for tropical 30–60-day convective variability. When convection is enhanced over the equatorial Indian Ocean, the tropical western Pacific often experiences an abnormal dry condition (phase I), whereas the development of the convection over the tropical western Pacific tends to be accompanied by suppressed convection in the equatorial Indian Ocean (phase II). This convection seesaw is a fundamental characteristic of the tropical 30–60-day oscillation.

The seesaw is intimately associated with the activity of propagating low-frequency convective systems (LFCSs). Its formation process is season dependent. Typical boreal summer seesaw results from a time-lagged development of two systems: a western system that originates in the equatorial Indian Ocean and moves eastward and/or northward and an eastern system that develops in the western Pacific monsoon region and moves westward and/or northward. The boreal winter seesaw, on the other hand, is caused by the longitudinal dependence of the evolution of eastward-moving LFCSs that strongly amplify in the equatorial Indian Ocean, weaken and/or split when rapidly passing over the maritime continent, and reintensify in the South Pacific convergence zone (SPCZ).

There are two phases of the seesaw. During the first phase, the LFCSs interact with the Indian monsoon in boreal summer and Indonesian–Australian monsoon in boreal winter. Likewise, during the second phase, the LFCSs interplay with monsoon circulations over the western Pacific monsoon trough in boreal summer and over the SPCZ in boreal winter. The convection seesaw activity is closely tied to the corresponding active-break monsoon cycles over the two polar regions of the seesaw.

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Liguang Wu and Bin Wang

Abstract

The influence of convective heating on movement and vertical coupling of tropical cyclones (TCs) is investigated using a hurricane model with different environmental flows. The authors identify two processes by which convective heating may affect TC motion. One is the advection of symmetric potential vorticity (PV) by heating-induced asymmetric flow. The other is the direct generation of a positive PV tendency by asymmetric heating, which acts to shift a TC to the region of maximum downward gradient of asymmetric heating. A steering level exists that is located at the level where the direct influence of asymmetric heating vanishes, normally in the lower troposphere. At that level, a TC moves with the asymmetric flow averaged within a radius of 200 km, because the influence of asymmetric flows on TC motion is weighted by the horizontal PV gradient that is primarily confined within the TC core. Although the vertical shear in the asymmetric flow (including environmental and heating-induced flows) could tilt the vortex, the influence of asymmetric heating tends to offset the vertical tilt caused by the vertical shear through a fast adjustment between the asymmetric wind and diabatic heating. Therefore, diabatic heating enhances the vertical coupling.

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Bin Wang and Qin Zhang

Abstract

The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Niño to east Asian climate during the mature and decay of an El Niño (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Niño; its strength increases with the El Niño intensity and its sign reverses during a La Niña. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment.

The development of the PSAC is attributed to combined effects of the remote El Niño forcing, tropical–extratropical interaction, and monsoon–ocean interaction. The developing El Niño induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical–extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Niño–induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind–evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphereocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.

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Juan Li and Bin Wang

Abstract

The present study aims to explore the origins of decadal predictability of East Asian land summer monsoon rainfall (EA-LR) and estimate its potential decadal predictability. As a preliminary study, a domain-averaged EA-LR index (EA-LI) is targeted as it represents the leading mode of variability reasonably well. It is found that the decadal variations of EA-LI are primarily linked to a cooling over the central-eastern tropical Pacific (CEP) and a warming over the extratropical North Pacific and western tropical Pacific (NWP) during May–October. Two numerical experiments suggest that the CEP cooling may be a major driver of EA-LR, while the NWP warming, which is largely a response, cannot be treated as a forcing to EA-LR. However, this does not mean that the NWP sea surface temperature anomalies (SSTAs) play no role. To elaborate on this point, a third experiment is conducted in which the observed cooling is nudged in the CEP but the SST is nudged to climatology in the NWP (i.e., atmosphere–ocean interaction is not allowed). The result shows anomalous northerlies and decreased rainfall over East Asia. Results of the three experiments together suggest that both the forcings from the CEP and the atmosphere–ocean interaction in the NWP are important for EA-LR. Assuming that the tropical and North Pacific SSTAs can be “perfectly” forecasted, the so-called perfect prediction of EA-LI, which is achieved by a physics-based empirical model, yields a significant temporal correlation coefficient skill of 0.70 at a 7–10-yr lead time during a 40-yr independent hindcast (1968–2009), providing an estimation of the lower bound of potential decadal predictability of EA-LI.

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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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Fei Liu and Bin Wang

Abstract

This study investigates the moisture and wave feedbacks in the Madden–Julian oscillation (MJO) dynamics by applying the general three-way interaction theoretical model. The three-way interaction model can reproduce observed large-scale characteristics of the MJO in terms of horizontal quadrupole-vortex structure, vertically tilted structure led by planetary boundary layer (PBL) convergence, slow eastward propagation with a period of 30–90 days, and planetary-scale circulation. The moisture feedback effects can be identified in this model by using diagnostic thermodynamic and momentum equations, and the wave feedback effects are investigated by using a diagnostic moisture equation. The moisture feedback is found to be responsible for producing the MJO dispersive modes when the convective adjustment process is slow. The moisture feedback mainly acts to reduce the frequency and growth rate of the short waves, while leaving the planetary waves less affected, so neglecting the moisture feedback is a good approximation for the wavenumber-1 MJO. The wave feedback is shown to slow down the eastward propagation and increase the growth rate of the planetary waves. The wave feedback becomes weak when the convective adjustment time increases, so neglecting the wave feedback is a good approximation for the MJO dynamics during a slow adjustment process. Sensitivities of these two feedbacks to other parameters are also discussed. These theoretical findings suggest that the two feedback processes, and thus the behaviors of the simulated MJO mode, should be sensitive to the parameters used in cumulus parameterizations.

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Qinghua Ding and Bin Wang

Abstract

This study investigated the most recurrent coupled pattern of intraseasonal variability between midlatitude circulation and the Indian summer monsoon (ISM). The leading singular vector decomposition (SVD) pattern reveals a significant, coupled intraseasonal variation between a Rossby wave train across the Eurasian continent and the summer monsoon convection in northwestern India and Pakistan (hereafter referred to as NISM). The wave train associated with an active phase of NISM rainfall displays two high pressure anomalies, one located over central Asia and the other over northeastern Asia. They are accompanied by increased rainfall over the western Siberia plain and northern China and decreased rainfall over the eastern Mediterranean Sea and southern Japan. The circulation of the wave train shows a barotropic structure everywhere except the anomalous central Asian high, located to the northwest of India, where a heat-induced baroclinic circulation structure dominates. The time-lagged SVD analysis shows that the midlatitude wave train originates from the northeastern Atlantic and traverses Europe to central Asia. The wave train enhances the upper-level high pressure and reinforces the convection over the NISM region; meanwhile, it propagates farther toward East Asia along the waveguide provided by the westerly jet. After an outbreak of NISM convection, the anomalous central Asian high retreats westward. Composite analysis suggests a coupling between the central Asian high and the convective fluctuation in the NISM. The significance of the midlatitude–ISM interaction is also revealed by the close resemblance between the individual empirical orthogonal functions and the coupled (SVD) modes of the midlatitude circulation and the ISM.

It is hypothesized that the eastward and southward propagation of the wave train originating from the northeastern Atlantic contributes to the intraseasonal variability in the NISM by changing the intensity of the monsoonal easterly vertical shear and its associated moist dynamic instability. On the other hand, the rainfall variations over the NISM reinforce the variations of the central Asian high through the “monsoon–desert” mechanism, thus reenergizing the downstream propagation of the wave train. The coupling between the Eurasian wave train and NISM may be instrumental for understanding their interaction and can provide a way to predict the intraseasonal variations of the Indian summer monsoon and East Asian summer monsoon.

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