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Guoxiong Wu and Huanzhu Liu

Abstract

Three sets of numerical experiments based on a GFDL GCM were developed to investigate the response of the large-scale tropical circulation and precipitation to the tropical and equatorial sea surface temperature (SST) anomalies. Specified SST anomaly (SSTA) with a small latitudinal scale of 13.5° was imposed in different regions of the Pacific Ocean in different sets of experiments and added to the climate-mean August SST to form a lower boundary forcing. Each set is composed of two experiments in which the SSTA possesses the same coverage and intensity but opposite sign. Anomalies of meteorological fields are calculated as the differences between the results of the warm and cold SSTA experiments.

In all experiments, prominent anomalous low-level convergence and high-level divergence are observed over the warm SSTA regions. For experiments with warm SSTA placed at the equator, responses of the tropical streamfunction are similar to corresponding results found in other studies. When the warm SSTA is placed in the warmest SST region in the western North Pacific away from the equator, the excited anomalous streamfunction is different from that in the equatorial SSTA cases. A strong anomalous low-level cyclone and high-level anticyclone are generated, and strong anomalous westerly–southwesterly flow at lower levels and northeasterly–easterly flow at upper levels sweep through the southeastern part of the region.

Anomalous rainfall is shown to be balanced mainly by anomalous convergence of stationary water flux; transient flux and anomalous evaporation from the warm water surface are secondary. Advection of water vapor by the large-scale flow and its anomaly were found to be significant in determining the rainfall pattern. Anomalous precipitation occurs in regions where the mean flow is down the SSTA gradient, or the anomalous flow is down the mean SST gradient. Mainly due to advection of water vapor by the divergent wind component and its anomaly, abnormal rainfall near the equator is biased toward the hemisphere where near-equatorial SST is warmer. Advection of anomalous water vapor by strong low-level equatorial easterlies in the eastern equatorial Pacific causes anomalous rainfall associated with the warm SSTA in the region to shift westward. Away from the equator, advection of water vapor by the anomalous rotational wind becomes important. It is this contributor that causes anomalous rainfall to shift away from the warmest SST region in the western North Pacific.

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Guo-Xiong Wu

Abstract

The subject of large-scale mountain waves is reviewed briefly. Existing mountain wave theory based on a linear system is shown to give an inadequate description of the balance of angular momentum. The response of the atmosphere to mechanical forcing in a nonlinear framework is then discussed, using a two-level quasi-geostrophic long-wave spectral model based on spherical coordinates, including diabatic heating, surface friction and mountains. The nonlinear theory shows that there exists a critical mountain height Hc, which is a function of the frictional coefficient as well as the phase difference between the mountain and the surface pressure field. If, and only if, the mountain height is less than this critical value, can the deflection effect of the mountain be neglected and the response regarded as approximately linear. This critical mountain height is only about 1 km. Thus most of the atmospheric response to large-scale mountains must be nonlinear.

In the nonlinear case, as the mountain height is increased the deflection effect becomes more and more important. Therefore, although in the upper atmosphere the mountain wave is intensified, at the surface the pressure perturbation decreases and the zonal surface winds become dominant and approach an asymptotic value.

It is also shown that the combined effect of mechanical and thermal forcing is nonlinear. Despite the fact that the formation of surface pressure systems is mainly a result of thermal forcing, orography affects, to some extent their intensities and locations. Considering the balance requirement of angular momentum, it is concluded that purely mechanical forcing cannot be the case in the real atmosphere. Although the mountain torque owes its existence to unevenness of the earth's surface, its sign and intensity depend critically upon the relative locations of mechanical and thermal forcing.

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Guoxiong Wu and Yongsheng Zhang

Abstract

Observations were employed to study the thermal characteristics of the Tibetan Plateau and its neighboring regions, and their impacts on the onset of the Asian monsoon in 1989. Special attention was paid to the diagnosis of the temporal and spatial distributions of surface sensible and latent heat fluxes. Results show that the whole procedure of the outbreak of the Asian monsoon onset is composed of three consequential stages. The first is the monsoon onset over the eastern coast of the Bay of Bengal (BOB) in early May. It is followed by the onset of the East Asian monsoon over the South China Sea (SCS) by 20 May, then the onset of the South Asian monsoon over India by 10 June. It was shown that the onset of the BOB monsoon is directly linked to the thermal as well as mechanical forcing of the Tibetan Plateau. It then generates a favorable environment for the SCS monsoon onset. Afterward, as the whole flow pattern in tropical Asia shifts westward, the onset of the South Asian monsoon occurs.

Finally, the timing of the onset of the Asian monsoon in 1989 was explored. It was shown that the onset of the Asian monsoon occurs when the warm or rising phase of different low-frequency oscillations reach the “East Asian monsoon area” (EAMA) concurrently. These include the warm phase of the eastward propagating two- to three-week oscillation (TTO) of the upper-layer temperature in middle latitudes, the rising phase of the northward propagating Madden–Julian oscillation of the southern tropical divergence, and the rising phase of the westward propagating TTO of the western Pacific divergence. It was concluded that the timing of the Asian monsoon onset is determined when the favorable phases of different low-frequency oscillations are locked over the EAMA.

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Guoxiong Wu and Yimin Liu

Abstract

Professor Yanai is remembered in our hearts as an esteemed friend. Based on his accomplishments in tropical meteorology and with his flashes of insight he led his group at the University of California, Los Angeles, in the 1980s and 1990s to explore the thermal features of the Tibetan Plateau (TP) and its relation to the Asian monsoon, and he brought forward the TP meteorology established by Ye Duzheng et al. in 1957 to a new stage. In cherishing the memory of Professor Yanai and his great contribution to the TP meteorology, the authors review their recent study on the impacts of the TP and contribute this chapter as an extension of their chapter titled “Effects of the Tibetan Plateau” published by Yanai and Wu in 2006 in the book The Asian Monsoon.

The influence of a large-scale orography on climate depends not only on the mechanical and thermal forcing it exerts on the atmosphere, but also on the background atmospheric circulation. In winter the TP possesses two leading heating modes resulting from the relevant dominant atmospheric circulations, in particular the North Atlantic Oscillation and the North Pacific Oscillation. The prevailing effect of the mechanical forcing of the TP in wintertime generates a dipole type of circulation, in which the anticyclonic gyre in the middle and high latitudes contributes to the warm inland area to the west, and the cold seashore area to the east, of northeast Asia, whereas the cyclonic gyre in low latitudes contributes to the formation of a prolonged dry season over central and southern Asia and moist climate over southeastern Asia. Such a dipole circulation also generates a unique persistent rainfall in early spring (PRES) over southern China.

In 1980s, Yanai and his colleagues analyzed the in situ observation and found that the constant potential temperature boundary layer over the TP can reach about 300 hPa before the summer monsoon onset. This study supports these findings, and demonstrates that such a boundary layer structure is a consequence of the atmospheric thermal adaptation to the surface sensible heating, which vanishes quickly with increasing height. The overshooting of rising air, which is induced by surface sensible heating, then can form a layer of constant potential temperature with a thickness of several kilometers.

The thermal forcing of the TP on the lower tropospheric circulation looks like a sensible heat–driven air pump (SHAP). It is the surface sensible heating on the sloping sides of the plateau that the SHAP can effectively influence the Asian monsoon circulation. In spring the SHAP contributes to the seasonal abrupt change of the Asian circulation and anchors the earliest Asian summer monsoon onset over the eastern Bay of Bengal. In summer, this pumping, together with the thermal forcing over the Iranian Plateau, produces bimodality in the South Asian high activity in the upper troposphere, which is closely related to the climate anomaly patterns over South and East Asia. Because the isentropic surfaces in the middle and lower troposphere intersect with the TP, in summertime the plateau becomes a strong negative vorticity source of the atmosphere and affects the surrounding climate and even the Northern Hemispheric circulation via Rossby wave energy dispersion. Future prospects in related TP studies are also addressed.

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Anmin Duan and Guoxiong Wu

Abstract

In Part I the authors have shown that heating sources in spring over the Tibetan Plateau (TP), and in particular the sensible heat flux (SHF), exhibit a significant weakening trend since the mid-1980s that is induced mainly by decreased surface wind speed. The possible reason of such a change is further investigated in Part II by analyzing historical observations and the NCEP/Department of Energy (DOE) reanalysis. The steady declining trend in the surface wind speed over the TP after the 1970s arises mainly from the zonal component. Since the mean altitude of the TP is about 600 hPa and the surface flow is controlled by the East Asian subtropical westerly jet (EASWJ) for most parts of the year, the substantial tropospheric warming in the mid- and high latitudes to the north of the plateau results in a decrease of the meridional pressure gradient in the subtropics. As a result, the EASWJ and the surface winds over the TP are decelerated. Moreover, changes of the general circulation in the twentieth century simulated by 16 coupled climate models driven by natural and anthropogenic forcings are examined. Intercomparison results suggest that sulfate aerosol indirect effects and ozone may be important in reproducing the weakening trend in EASWJ. Although nearly half of the models can successfully reproduce the observed trends in the EASWJ during the last two decades, there is an obvious spread in simulation of the spatial patterns of twentieth-century tropospheric temperatures, suggesting significant room still exists for improvement of the current state-of-the-art coupled climate models.

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Anmin Duan and Guoxiong Wu

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The trend in the atmospheric heat source over the Tibetan Plateau (TP) during the last four decades is evaluated using historical observations at 74 meteorological stations in the period of 1961–2003 and satellite radiation data from 1983 to 2004. It is shown that in contrast to the strong surface and troposphere warming, the sensible heat (SH) flux over the TP exhibits a significant decreasing trend since the mid-1980s. The largest trend occurs in spring, a season of the highest SH over the TP. The subdued surface wind speed contributes most to the decreasing trend. At the same time, the radiative cooling effect in the air column enhances persistently. Despite the fact that the in situ latent heating presents a weak increasing trend, the springtime atmospheric heat source over the TP loses its strength during two recent decades. Further investigation suggests that the weakened SH over the TP may be part of the global circulation shift.

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Guoxiong Wu and Ngar-Cheung Lau

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No abstract available.

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Guoxiong Wu and Ngar-Cheung Lau

Abstract

A low-resolution Geophysical Fluid Dynamics Laboratory (GFDL) general circulation model has been integrated for 15 years. In the course of this experiment, the observed month-to-month sea surface temperature (SST) variations in the tropical Pacific Ocean were incorporated in the lower boundary condition. The output from this model run was used to investigate the influence of El Niñ-Southern Oscillation (ENSO) events on the variability of tropical-storm formation.

Criteria for detecting tropical cyclogenesis and tropical-storm formation were developed for the model. Tropical storms appearing in the model atmosphere exhibit many typhoonlike characteristics: strong cyclonic vorticity and convergence in the lower troposphere, strong anticyclonic vorticity and divergence near the tropopause, and intense precipitation. It is demonstrated that, despite its coarse resolution, the model is capable of reproducing the observed geographical distribution and seasonal variation of tropical-storm formation.

The relationship between simulated tropical-storm formation and ENSO was explored using correlation statistics, composite fields for the warm and cold phases of ENSO, and individual case studies. Significant correlations were found between eastern equatorial Pacific SST anomalies and tropical-storm formation over the western North Pacific, western South Pacific, and western North Atlantic. In these areas, below-normal frequency of tropical-storm formation was simulated in warm El Niño years, whereas more tropical storms occurred in La Niña years. The correlation between tropical-storm formation and equatorial SST changes is particularly high for fluctuations on time scales of less than 3–4 years. During the boreal summer months (June–October), there exists a seesaw in the frequency of tropical-storm formation between western and central North Pacific: while more tropical storms were generated over western North Pacific during La Niña years, less tropical storms were detected over central North Pacific. The reverse situation prevails in El Niño years. Over the Indian Ocean, the relationship between storm formation and ENSO exhibits a seasonal dependence.

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Yimin Liu, Guoxiong Wu, and Rongcai Ren

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Monthly mean reanalysis data and numerical experiments based on a climate model are employed to investigate the relative impacts of different types of diabatic heating and their synthetic effects on the formation of the summertime subtropical anticyclones. Results show that the strong land surface sensible heating (SE) on the west and condensation heating (CO) on the east over each continent generate cyclones in the lower layers and anticyclones in the upper layers, whereas radiative cooling over oceans generates the lower-layer anticyclone and upper-layer cyclone circulations. Such circulation patterns are interpreted in terms of the atmospheric adaptation to diabatic heating through a potential vorticity–potential temperature view. A Sverdrup balance is used to explain the zonally asymmetric configuration of the surface subtropical anticyclones. The strong deep CO that is maximized in the upper troposphere over the eastern continent and the adjacent ocean is accompanied by upper-tropospheric equatorward flow and weaker lower-tropospheric poleward flow, whereas the very strong longwave radiative cooling (LO) that is maximized near the top of the planetary boundary layer over the eastern ocean is accompanied by strong surface equatorward flow and weaker upper-layer poleward flow. The center of the surface subtropical anticyclone is then shifted toward the eastern ocean, and its zonal asymmetry is induced. This study concludes that in the summer subtropics over each continent and its adjacent oceans LO, SE, CO, and a double-dominant heating (D) from west to east compose a LOSECOD heating quadruplet. A specific zonal asymmetric circulation pattern is then formed in response to the LOSECOD quadruplet heating. The global summer subtropical heating and circulation can then be viewed as “mosaics” of such quadruplet heating and circulation patterns, respectively.

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B. N. Goswami, Guoxiong Wu, and T. Yasunari

Abstract

Factors responsible for limited predictability of the Asian summer monsoon (ASM) are investigated within a conceptual framework for predictability. Predictability of the seasonal mean depends on the interannual variability (IAV) of the monsoon annual cycle (MAC) and is determined by relative contribution of the predictable “external” component of IAV compared to the unpredictable “internal” IAV. Contributions of slow processes such as those involving air–sea interactions associated with the El Niño–Southern Oscillation (ENSO) or local warm ocean–atmosphere interactions in generating IAV of the MAC are reviewed. Empirical evidence that these air–sea interactions modulate the MAC is presented. Estimates of internal IAV have been made from observations as well as atmospheric model simulations. In contrast to a large part of the Tropics where the summer climate is predictable, with the internal variability being much smaller than the external one, the limited predictability of the Asian monsoon appears to be due to the fact that the contribution from the external IAV over the region is relatively weak and comparable to that from internal IAV.

Cause for large internal IAV over the ASM region is investigated, and it is proposed that the internal IAV of the MAC is primarily due to interaction between the MAC and the summer intraseasonal oscillations (ISOs). Two mechanisms through which ISOs lead to internal IAV of the MAC are unraveled. The seasonal bias of the ISO anomalies can influence the seasonal mean if the spatial structure of the ISO has significant projection on that of the seasonal mean and if frequency of occurrence of positive and negative phases is unequal. Evidence supporting this is presented. In addition, it is demonstrated that the chaotic summer ISOs modulated by the annually varying forcing associated with the “slow annual cycle” can lead to IAV of the seasonal mean. Empirical evidence that IAV of ISO activity is related to IAV of the seasonal mean or MAC is also presented.

Thus, the Asian monsoon would remain a difficult system to predict. To exploit the predictable signal, however, it is imperative that systematic bias of the models is improved and the space–time structure of the summer ISOs is simulated accurately.

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