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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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Chao Ji, Qinghe Zhang, and Yongsheng Wu
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Chao Ji, Qinghe Zhang, and Yongsheng Wu

Abstract

A new approach has been proposed to derive the expressions for three-dimensional radiation stress using solutions of the pressure and velocity distributions and the coordinate transformation function that are derived from a Lagrangian description wherein the pressure is zero (relative to the atmospheric pressure) at the sea surface. Using this approach, analytical expressions of horizontal and vertical depth-dependent radiation stress are derived at a uniform depth and for a sloping bottom, respectively. The results of the depth integration of the expressions agree well with the theory of Longuet-Higgins and Stewart. In the case involving a sloping bottom, the radiation stress expressions from this study provide a better balance of the net momentum compared to those from previous studies.

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Yuefeng Wu, Qinghe Zhang, Yongsheng Wu, and Chao Ji

Abstract

We commented on the empirical adjustment term recently introduced by Mellor. The purpose of the adjustment term is to include the effect of concentrating momentum at the surface, and the adjustment significantly improves the comparisons between modeled and measured velocity profiles. We found that the concentrating momentum from the adjustment term is largely overestimated relative to the commonly used parameterization approaches of concentrating momentum. The overestimated concentrating momentum leads to stronger velocity shear at the surface, which is partially canceled out by the vertical mixing caused by wave breaking. The model results also agree well with the measured velocity profiles if the fractions of the adjustment and the vertical mixing are simultaneously reduced. We also discussed an alternative method that includes the vertical radiation stress gradient term. The method exhibits no empiricism or uncertainty under the given wave conditions.

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Yongsheng Zhang, Tim Li, Bin Wang, and Guoxiong Wu

Abstract

The temporal and spatial structures of the atmospheric circulation associated with the climatology and interannual variations of the summer monsoon onset over the Indochina Peninsula were studied using the observed daily rainfall at 30 stations and the NCEP–NCAR reanalysis from 1951 to 1996. The climatological monsoon onset over Indochina is on 9 May, with a standard deviation of 12 days. The monsoon onset is characterized by the pronounced northeastward progression of the low-level southwesterlies over the Indian Ocean and the intensification and northward extension of the tropical convection from Sumatra. It coincides with the weakening of the midlatitude westerly over south Asia, and the westward propagation of the intraseasonal oscillation (ISO) originated in the South China Sea (SCS) and the western Pacific with a dominant timescale of 12–25 days.

A close relationship between the interannual variations of the monsoon onset and El Niño/La Niña was identified. Years with warm (cold) sea surface temperature (SST) anomalies in the western Pacific and cold (warm) SST anomalies in the central-eastern Pacific in the preceding spring have an early (late) onset. For an early onset year, strong convective activities appear over the southern Indochina Peninsula and the southern SCS in the preceding winter and spring. Associated with the changes of the Walker circulation and the local Hadley circulation related to La Niña, strong convective activities were maintained by the convergence between the anomalous southwesterlies in the Indian Ocean and northeasterlies over the northern SCS. The anomalous southwesterlies in the Indian Ocean were induced by both the anomalous Walker circulation associated with La Niña and anomalous land–sea thermal contrast. The anomalous northeasterlies over the northern SCS were originated in northern winter due to the combined effects of the cold east China land and the warm Philippine Sea, and further maintained by a positive thermodynamic air–sea feedback mechanism related to La Niña. An opposite scenario is found for a late onset year with warm SST anomalies in the central-eastern Pacific (El Niño).

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