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Bin Wang and Xihua Xu

Abstract

Using climatological pentad mean outgoing longwave radiation (OLR) and European Centre for Medium-Range Weather Forecasts analysis winds, the authors show that the Northern Hemisphere summer monsoon displays statistically significant climatological intraseasonal oscillations (CISOs). The extreme phases of CISO characterize monsoon singularities—monsoon events that occur on a fixed pentad with usual regularity, whereas the transitional phases of CISO represent the largest year-to-year monsoon variations.

The CISO results from a phase-locking of transient intraseasonal oscillation to annual cycle. It exhibits a dynamically coherent structure between enhanced convection and low-level convergent (upper-level divergent) cyclonic (anticyclonic) circulation. Its phase propagates primarily northward from the equator to the northern Philippines during early summer (May–July), and westward along 15°N from 170°E to the Bay of Bengal during August and September.

The propagation of CISO links monsoon singularities occurring in different regions. Four CISO cycles are identified from May to October. The first cycle has a peak wet phase in mid-May that starts the monsoon over the South China Sea and Philippines. Its dry phase in late May and early June brings the premonsoon dry weather over the regions of western North Pacific summer monsoon (WNPSM), Meiyu/Baiu, and Indian summer monsoon (ISM). The wet phase of Cycle II peaking in mid-June marks the onsets of WNPSM, continental ISM, and Meiyu, whereas the dry phase in early to mid-July corresponds to the first major breaks in WNPSM and ISM, and the end of Meiyu. The wet phase of Cycle III peaking in mid-August benchmarks the height of WNPSM, which was followed by a conspicuous dry phase propagating westward and causing the second breaks of WNPSM (in early September) and ISM (in mid-September). The wet phase of Cycle IV represents the last active WNPSM and withdrawal of ISM in mid-October.

The relationships among ISM, WNPSM, and East Asian Subtropical Monsoon (EASM) are season dependent. During Cycle II, convective activities in the three monsoon regions are nearly in phase. During Cycle III, however, the convective activities are out of phase between ISM and WNPSM; meanwhile, little linkage exists between WNPSM and EASM. The causes of unstable relationships and the phase propagation of CISO are discussed.

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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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Baoqiang Xiang and Bin Wang

Abstract

Understanding the variability and change of monsoon onset is of utmost importance for agriculture planning and water management. In the last three decades, the Asian summer monsoon onset (ASMO) has remarkably advanced, but the physical mechanisms underlying the change remain elusive. Since the overall ASMO occurs in May, this paper focuses on the change of mean fields in May and considers enhanced mean precipitation and monsoon westerly winds as signs of advanced ASMO. The results reveal that the advanced ASMO mainly represents a robust decadal shift in the mid-to-late 1990s, which is attributed to the mean state change in the Pacific basin characterized by a grand La Niña–like pattern. The La Niña–like mean state change controls the ASMO through the westward propagation of Rossby waves and its interaction with the asymmetric background mean states in the Indian Ocean and western Pacific, which intensifies the Northern Hemispheric perturbations and westerly winds. Intriguingly, the abrupt decadal shifts of monsoon onset in the Arabian Sea and Bay of Bengal occur in 1999, in contrast to the South China Sea with a decadal shift in 1994. Numerical experiments with a coupled climate model demonstrate that the advanced monsoon onset in the Arabian Sea and Bay of Bengal is governed by the enhanced zonal sea surface temperature (SST) gradients in the equatorial Pacific, while that in the South China Sea is primarily determined by the abrupt SST warming near the Philippine Sea.

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Xiouhua Fu and Bin Wang

Abstract

The boreal-summer intraseasonal oscillation (BSISO) simulated by an atmosphere–ocean coupled model is validated with the long-term observations [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, ECMWF analysis, and Reynolds' SST]. This validation focuses on the three-dimensional water vapor cycle associated with the BSISO and its interaction with underlying sea surface. The advantages of a coupled approach over stand-alone atmospheric approaches on the simulation of the BSISO are revealed through an intercomparison between a coupled run and two atmosphere-only runs.

This coupled model produces a BSISO that mimics the one presented in the observations over the Asia– western Pacific region. The similarities with the observations include 1) the coherent spatiotemporal evolutions of rainfall, surface winds, and SST associated with the BSISO; 2) the intensity and period (or speed) of the northward-propagating BSISO; and 3) the tropospheric moistening (or drying) and overturning circulations of the BSISO. However, the simulated tropospheric moisture fluctuations in the extreme phases (both wet and dry) are larger than those in the ECMWF analysis. The simulated sea surface cooling during the wet phase is weaker than the observed cooling. Better representations of the interaction between convection and boundary layer in the GCM and including salinity effects in the ocean model are expected to further improve the simulation of the BSISO.

The intercomparison between a coupled run and two atmospheric runs suggests that the air–sea coupled system is the ultimate tool needed to realistically simulate the BSISO. Though the major characteristics of the BSISO are very likely determined by the internal atmospheric dynamics, the correct interaction between the internal dynamics and underlying sea surface can only be sustained by a coupled system. The atmosphere-only approach, when forced with high-frequency (e.g., daily) SST, introduces an erroneous boundary interference on the internal dynamics associated with the BSISO. The implications for the predictability of the BSISO are discussed.

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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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Qinghua Ding and Bin Wang

Abstract

Extreme active and break phases of the Indian summer monsoon (ISM) often bring about devastating floods and severe draughts. Here it is shown that these extreme phases exhibit distinctive precursory circulation conditions in both the tropics and extratropics over a range of antecedent periods. The extremely active monsoon over northern India is preceded by a strengthening of the upper-tropospheric central Asian high and enhancement of the tropical convection over the equatorial Indian Ocean and the South China Sea. The concurrent buildup of the anomalous high over central Asia and the arrival of tropical convection over northern India increase the likelihood of occurrence of a heavy rainy period there. Similarly, the concurrent anomalous low over central Asia and the arrival of suppressed convection originating from the equatorial Indian Ocean and the South China Sea precede extremely strong monsoon breaks over northern India. Two predictors can be used to predict the extreme active/break phases of the northern ISM: normalized 200-hPa geopotential height over central Asia and outgoing longwave radiation over southern India. Once the mean of the two predictors exceeds a threshold unit (1.0), an extreme phase is anticipated to occur over northern India after 4–5 days and reach peak intensity after an additional 2 days. In general, an event forecast by this simple scenario has a 40% probability of developing into an extreme phase, which is normally a small probability event (a less than 4% occurrence).

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Fei Liu and Bin Wang

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The eastward-propagating tropical low-frequency disturbances, such as the moist Kelvin waves or the Madden–Julian oscillation (MJO), are often observed to experience convective enhancement when meeting with the westward-propagating 2-day waves. A scale interaction (SI) model is built to understand the nature of the interaction between the 2-day waves and moist Kelvin waves or MJO. In this model, the convective complex of moist Kelvin waves modulates the strength and location of the 2-day waves, which feed back through the upscale eddy transfer. An ageostrophic model describing the 2-day waves is first solved, and the resultant westward-propagating, backward-tilted disturbances are consistent with the observed 2-day waves. An explicit representation of eddy momentum transfer (EMT), eddy heating transfer (EHT), and eddy moisture transfer (EQT) arising from the 2-day waves is then formulated. The SI model shows that the 2-day waves in front of moist Kelvin waves produce an EMT accelerating the low-frequency easterly in the lower troposphere, an EHT cooling down the middle troposphere, and an EQT moistening the middle troposphere. These three transfer terms have comparable magnitude. Although the negative EHT tends to damp the moist Kelvin waves, both the EMT and EQT provide instability sources for the moist Kelvin waves. The 2-day waves also slow down the moist Kelvin waves, mainly through the advective effects of the EMT. So the unstable moist Kelvin waves may exhibit convective enhancement when meeting with the 2-day waves. The theoretical results presented here point to the need to further observe the multiscale structures within the moist Kelvin waves and the MJO.

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Fei Liu and Bin Wang

Abstract

During late boreal summer (July–October), the intraseasonal oscillation (ISO) exhibits maximum variability over the western North Pacific (WNP) centered in the South China Sea and Philippine Sea, but many numerical models have difficulty in simulating this essential feature of the ISO. To understand why this maximum variability center exists, the authors advance a simple box model to elaborate the potential contribution of the mean-state-dependent atmosphere–ocean interaction. The model results suggest that the WNP seasonal mean monsoon trough plays an essential role in sustaining a strong stationary ISO, contributing to the existence of the maximum intraseasonal variability center. First, the monsoon trough provides abundant moisture supply for the growing ISO disturbances through the frictional boundary layer moisture convergence. Second, the cyclonic winds associated with the monsoon trough provide a favorable basic state to support a negative atmosphere–ocean thermodynamic feedback that sustains a prominent stationary ISO. In an active phase of the ISO, anomalous cyclonic winds enhance the monsoon trough and precipitation, which reduce shortwave radiation flux and increase evaporation; both processes cool the sea surface and lead to an ensuing high pressure anomaly and a break phase of the ISO. In the wintertime, however, the wind–evaporation feedback is positive and sustains the Philippine Sea anticyclone. The result here suggests that accurate simulation of the boreal summer climatological mean state is critical for capturing a realistic ISO over the WNP region.

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Bin Wang and Zheng Fang

Abstract

Based on first principles, a theoretical model for El Niño-Southern Oscillation (ENSO) is derived that consists of prognostic equations for sea surface temperature (SST) and for thermocline variation. Considering only the large-scale, equatorially symmetric, standing basin mode yields a minimum dynamic system that highlights the cyclic, chaotic, and season-dependent evolution of ENSO.

For a steady annual mean basic state, the dynamic system exhibits a unique limit cycle solution for a fairly restricted range of air-sea coupling. The limit cycle is a stable attractor and represents an intrinsic interannual oscillation of the coupled system. The deepening (rising) of the thermocline in the eastern (western) Pacific leads eastern Pacific warming by a small fraction of the cycle, which agrees well with observation and plays a critical role in sustaining the oscillation. When the nonlinear growth of SST anomalies reaches a critical amplitude, the delayed response of thermocline adjustment provides a negative feedback, turning over warming to cooling or vice versa.

When the basic state varies annually, the limit cycle develops a strange attractor and the interannual oscillation displays inherent deterministic chaos. On the other hand, the transition phase of the oscillation tends to frequently occur in boreal spring when the basic state is most unstable. The strongest boreal spring instability is due to the weakest mean upwelling and largest vertical temperature difference across the mixed layer base. The former minimizes the negative feedback of mean upwelling, whereas the latter maximizes the positive feedback of anomalous upwelling effects on SST; both favor spring instability. It is argued that the season-dependent coupled instability may be responsible for the tendencies of ENSO phase locking with season and period-locking to integer multiples of the annual period, which, in turn, create irregularities in oscillation period and amplitude.

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Bin Wang and Xiaosu Xie

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Over the warm pool of the equatorial Indian and western Pacific Oceans, both the climatological mean state and the processes of atmosphere–ocean interaction differ fundamentally from their counterparts over the cold tongue of the equatorial eastern Pacific. A model suitable for studying the coupled instability in both the warm pool and cold tongue regimes is advanced. The model emphasizes ocean mixed layer physics and thermodynamical coupling that are essential for the warm pool regime. Different coupled unstable modes are found under each regime.

In contrast to the cold tongue basic state, which favors coupled unstable low-frequency SST mode, the warm pool regime (moderate mean surface westerlies and deep thermocline) is conducive for high-frequency (intraseasonal timescale) coupled unstable modes. The wind–mixed layer interaction through entrainment/evaporation plays a central role in the warm pool instability. The cloud-radiation feedback enhances the instability, whereas the ocean wave dynamics have little impact. The thermodynamic coupling between the atmosphere and ocean mixed layer results in a positive SST anomaly leading convection, which provides eddy available potential energy for growing coupled mode. The relatively slow mixed layer response to atmospheric forcing favors the growth of planetary-scale coupled modes. The presence of mean westerlies suppresses the low-frequency SST mode.

The characteristics of the eastward-propagating coupled mode of the warm pool system compares favorably with the large-scale features of the observed Madden–Julian Oscillation (MJO). This suggests that, in addition to atmospheric internal dynamic instability, the ocean mixed layer thermodynamic processes interacting with the atmosphere may play an active part in sustaining the MJO by (a) destabilizing atmospheric moist Kelvin waves, (b) providing a longwave selection mechanism, and (c) slowing down phase propagation and setting up the 40–50-day timescale.

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