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Hualan Rui and Bin Wang

Abstract

The development and dynamical structure of intraseasonal low-frequency convection anomalies in the equatorial region are investigated using 10 years (1975–85) of outgoing longwave radiation (OLR) and 7 years (1979–85) of 200 and 850 mb wind data.

The composite OLR anomalies for 36 cases show a four-stage development process: initiation over equatorial Africa, rapid intensification when passing through the Indian Ocean, mature evolution characterized by a weakening in the maritime continent and redevelopment over the western Pacific, and dissipation near the date line in moderate events or emanation from the equator toward North America and southeastern Pacific in strong events.

A noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases. Equatorial upper- (lower-) level easterly (westerly) anomalies and associated twin anomalous anticyclonic (cyclonic) circulation anomalies couple with equatorial convection anomalies. The wind anomalies, however, generally lag convection anomalies in development and early mature phases, but nearly overlap in late mature phase and slightly lead the convection anomalies in dissipation phase. The upper-level twin cyclonic cells associated with the westerly anomalies in front of the convection travel across eastern Pacific after the convection ceases in the central Pacific, while the low-level wind anomalies die out east of the date line.

The implications of the findings in relation to theoretical hypotheses on low-frequency motion are discussed.

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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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Xiouhua Fu and Bin Wang

Abstract

A series of small-perturbation experiments has been conducted to demonstrate that an atmosphere–ocean coupled model and an atmosphere-only model produce significantly different intensities of boreal summer intraseasonal oscillation (BSISO) and phase relationships between convection and underlying SST associated with BSISO. The coupled model not only simulates a stronger BSISO than the atmosphere-only model, but also generates a realistic phase relationship between intraseasonal convection and underlying SST. In the coupled model, positive (negative) SST fluctuations are highly correlated with more (less) precipitation with a time lead of 10 days as in the observations, suggesting that intraseasonal SST is a result of atmospheric convection, but at the same time, positively feeds back to increase the intensity of the convection. In the atmosphere-only model, however, SST is only a boundary forcing for the atmosphere. The intraseasonal convection in the atmosphere-only model is actually less correlated with underlying SST. The maximum correlation between convection and SST occurs when they are in phase with each other, which is in contrast to the observations. These results indicate that an atmosphere–ocean coupled model produces a more realistic ISO compared to an atmosphere-only model.

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Hiroyuki Murakami and Bin Wang

Abstract

Possible future change in tropical cyclone (TC) activity over the North Atlantic (NA) was investigated by comparison of 25-yr simulations of the present-day climate and future change under the A1B emission scenario using a 20-km-mesh Meteorological Research Institute (MRI) and Japan Meteorological Agency (JMA) atmospheric general circulation model. The present-day simulation reproduces many essential features of observed climatology and interannual variability in TC frequency of occurrence and tracks over the NA. For the future projection, the model is driven by the sea surface temperature (SST) that includes a trend projected by the most recent Intergovernmental Panel on Climate Change (IPCC) multimodel ensemble and a year-to-year variation derived from the present-day climate. A major finding is that the future change of total TC counts in the NA is statistically insignificant, but the frequency of TC occurrence will decrease in the tropical western NA (WNA) and increase in the tropical eastern NA (ENA) and northwestern NA (NWNA). The projected change in TC tracks suggests a reduced probability of TC landfall over the southeastern United States, and an increased influence of TCs on the northeastern United States. The track changes are not due to changes of large-scale steering flows; instead, they are due to changes in TC genesis locations. The increase in TC genesis in the ENA arises from increasing background ascending motion and convective available potential energy. In contrast, the reduced TC genesis in the WNA is attributed to decreases in midtropospheric relative humidity and ascending motion caused by remotely forced anomalous descent. This finding indicates that the impact of remote dynamical forcing is greater than that of local thermodynamical forcing in the WNA. The increased frequency of TC occurrence in the NWNA is attributed to reduced vertical wind shear and the pronounced local warming of the ocean surface. These TC changes appear to be most sensitive to future change in the spatial distribution of rising SST. Given that most IPCC models project a larger increase in SST in the ENA than in the WNA, the projected eastward shift in TC genesis is likely to be robust.

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Xiouhua Fu and Bin Wang

Abstract

This paper reveals major deficiencies of the existing intermediate climate models for tropical surface winds and elaborates the important roles of cloud-longwave radiational forcing and boundary layer thermodynamics in driving the tropical surface winds.

The heat sink associated with the cloud-longwave radiation is demonstrated as an important driving force for boreal summer northeast trades and Indian Ocean southwest monsoons. Over the western North Pacific and Atlantic Oceans, low cloudiness and high sea surface temperature enhance longwave radiation cooling, strengthening subtropical high and associated trades. On the other hand, in the regions of heavy rainfall over South Asia, reduced cloud-longwave radiation cooling enhances monsoon trough and associated southwest monsoons. The boundary layer thermodynamic processes, primarily both the surface heat fluxes and the vertical temperature advection, are shown to be critical for a realistic simulation of the intertropical convergence zone, the equatorial surface winds, and associated divergence field.

To successfully simulate the tropical surface winds, it is essential for intermediate models to adequately describe the feedback of the boundary layer frictional convergence to convective heat source, cloud-longwave radiation forcing, boundary layer temperature gradient forcing, and their interactions. The capability and limitations of the intermediate tropical climate model in reproducing both climatology and interannual variations are discussed.

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Bin Wang and Hualan Rui

Abstract

A simple theoretical analysis on the stability of a resting tropical atmosphere to semigeostrophic perturbations is given using a free atmosphere–boundary layer coupled model on an equatorial β-plane.

An unstable mode emerges when sea surface temperature is higher than a critical value. The growing mode is a moist Kelvin wave modified through coupling with a Rossby wave of the lowest meridional index. The modified Rossby modes, however, remain damped even for high SST. The unstable mode selection can be explained in terms of wave energy generation due to the latent heating induced by frictional moisture convergence.

The horizontal mode-coupling has profound impacts on wave instability. It favors the amplification of long planetary-scale waves, slows down eastward propagation, and suppresses unrealistically fast growth of the uncoupled moist Kelvin mode by creating substantial meridional flows. These effects make the coupled unstable mode more resemble observed equatorial intraseasonal disturbances.

The results also demonstrate that when maximum SST moves from the equator to 7.5°N, the growth rate of the unstable wave is significantly reduced, suggesting that the annual march of the “thermal equator” and associated convective heating is likely responsible for annual variations of the equatorial 40–50 day wave activity.

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Liguang Wu and Bin Wang

Abstract

The recently reported increase in the proportion of intense hurricanes is considerably larger than those projected by the maximum potential intensity (MPI) theory and the results of numerical simulation. To reconcile this discrepancy, the authors examined the best-track datasets for the North Atlantic (NA), western North Pacific (WNP), and eastern North Pacific (ENP) basins. It was found that the changes in the tropical cyclone formation locations and prevailing tracks may have contributed to the changes in the proportion of the intense hurricanes over the past 30 yr. The authors suggest that the changes in the formation locations and prevailing tracks have a profound impact on the basinwide tropical cyclone intensity. Thus, how the atmospheric circulation in the tropical cyclone basins responds to the global warming may be a critical factor in understanding the impacts of global warming on tropical cyclone intensity.

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Fei Liu and Bin Wang

Abstract

The impact of horizontal advection of seasonal-mean moisture (SMM) on Madden–Julian oscillation (MJO) dynamics is investigated here using a theoretical model that includes moisture advection processes. The zonal advection of SMM with an eastward gradient is found to produce planetary-scale instability and promote slow eastward propagation corresponding to an intraseasonal periodicity. This is because the SMM advection by an anomalous easterly of the Kelvin waves generates a moisture source to the east of precipitation, which favors eastward propagation and unstable growth. On the other hand, the advection of SMM with a westward gradient results in a westward-propagating unstable mode. For a realistic SMM distribution, the simulated eastward propagation is enhanced over the Indo-Pacific warm pool, while the westward propagation prevails over the central-eastern Pacific. In contrast to the zonal advection of SMM, the meridional advection of SMM only affects short waves and leaves planetary waves nearly unaffected.

The effect of zonal advection of SMM suggests an important mechanism for explaining the eastward propagation and growth of the MJO over the Indo-Pacific warm pool when the SMM increases eastward. However, this mechanism alone produces unrealistic Kelvin wave–like structure and strong westward propagation in the central-eastern Pacific; both disagree with observations. These caveats, however, can be remitted if the planetary boundary layer (PBL) moisture convergence feedback is included, which couples the Kelvin wave and the Rossby wave via precipitation heating, producing a realistic horizontal structure and also substantially suppressing the unrealistically growing, westward-propagating mode in the central-eastern Pacific.

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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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Qinghua Ding and Bin Wang

Abstract

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.

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