Aspects of the large-scale tropical atmospheric circulation (original) (raw)
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Monsoons in a Moist Axially Symmetric Model of the Atmosphere
Large scale meridional overturning circulations such as the Asian monsoon and the Hadley Cell are a dominant feature of the dynamics of the tropical atmosphere. Our theoretical understanding of these circulations is largely based on idealized axisymmetric models, which describe them as steady-state and quasi-inviscid. In such theoretical models it can be shown [1] that meridional flow can only occur if it conserves angular momentum, entailing a condition on the thermal forcing of the atmosphere. In the case of the Asian monsoon, Xie and Saiki [2] have recently argued that this constraint can be only met once moisture has penetrated over the Indian sub-continent, and that this is the result of a zonally asymmetric instability that draws water over land from the Indian Ocean, marking the onset of the monsoon. Their argument entails that differences in vertical thermal structure between a moist convecting atmosphere and a dry one are critical for the existence of a monsoon. Furthermore...
Observed analysis of the 35 years of 1979-2013 reveals considerable interdecadal change and significant recent intensification in the difference of convective precipitation between the South Asian monsoon (SAM) and East Asian monsoon (EAM) systems during the major summer monsoon season (June-July). We propose that the recent strengthening of the zonal gradient of sea surface temperature (SST) between the Indian Ocean, western Pacific, and eastern Pacific is a possible cause for the intensification of the convective precipitation contrast. It is noted that the strengthening of the zonal SST gradient associated with the recent mega-La Niña trend tends to reinforce the negative connection between SAM and EAM systems by inducing enhanced convection over the maritime continent and then facilitating the northwestward emanation of Rossby waves. Consequently, a cyclonic circulation anomaly that effectively changes the local Hadley circulation has been formed over the SAM region, resulting in the noticeable difference between the SAM and EAM. The years 2013 and 1983 are further investigated as the strongest extreme years for positive and negative phases of submonsoon contrast, respectively. The result confirms that the meridional dipole height pattern along the Asian Jet stream, which is caused by the strong zonal gradient of tropical SST, serves as a key trigger in strengthening the submonsoon contrast.
Evolution of Large-Scale Circulation and Heating during the First Transition of Asian Summer Monsoon
Journal of Climate, 1999
This study investigates the characteristics of large-scale circulation and heating during the first transition of the Asian summer monsoon by a compositing technique. The first transition is characterized by a sudden change in large-scale atmospheric circulation and convective activity in South and Southeast Asia. The most notable features include 1) the development of the low-level cyclonic circulation and the upper-level anticyclone in South Asia, 2) the strong convection in the Bay of Bengal, the Indochina peninsula, and the South China Sea, and 3) the warming and the subsequent cooling of the SST in the Bay of Bengal.
SST-convection relation over tropical oceans
International Journal of Climatology, 2012
According to current knowledge, convection over the tropical oceans increases with sea surface temperature (SST) from 26 to 29 °C, and at SSTs above 29 °C, it sharply decreases. Our research shows that it is only over the summer warm pool areas of Indian and west Pacific Oceans (monsoon areas) where the zone of maximum SST is away from the equator that this kind of SST-convection relationship exists. In these areas (1) convection is related to the SST gradient that generates low-level moisture convergence and upward vertical motion in the atmosphere. This has modelling support. Regions of SST maxima have low SST gradients and therefore feeble convection. (2) Convection initiated by SST gradient produces strong wind fields particularly cross-equatorial low-level jetstreams (LLJs) on the equator-ward side of the warm pool and both the convection and LLJ grow through a positive feedback process. Thus, large values of convection are associated with the cyclonic vorticity of the LLJ in the atmospheric boundary layer. In the inter-tropical convergence zone (ITCZ) over the east Pacific Ocean and the south Pacific convergence zone (SPCZ) over the west Pacific Ocean, low-level winds from north and south hemisphere converge in the zone of maximum SST, which lies close to the equator producing there elongated bands of deep convection, where we find that convection increases with SST for the full range of SSTs unlike in the warm pool regions. The low-level wind divergence computed using QuikSCAT winds has large and significant linear correlation with convection in both the warm pool and ITCZ/SPCZ areas. But the linear correlation between SST and convection is large only for the ITCZ/SPCZ. These findings have important implications for the modelling of largescale atmospheric circulations and the associated convective rainfall over the tropical oceans.
Impact of Convective Activity in the Western Tropical Pacific on the East Asian Summer Circulation
Journal of the Meteorological Society of Japan. Ser. II, 1989
A statistical study is made of the relationship between the Northern Hemisphere atmospheric circulation and tropical convective activity during summer on intraseasonal time scales. An emphasis is placed on differences in the tropical-midlatitude interaction over East Asia between two 50-day periods: May 26-July 14 and July 15-September 2. Each period corresponds to the East Asian rainy season or the East Asian midsummer season, respectively. Barotropic instability of climatological monthly mean 300mb stream functions is also investigated to interpret the observational results. It is found from correlative analysis that the tropical convection is not strongly related to the midlatitude flow over East Asia during the rainy season, and there are no definite indications that the tropical convection forces the midlatitude flow. On the other hand, during the midsummer season convective activity around the Philippines and the Indo-China Peninsula has a large impact on the East Asian atmospheric circulation. In particular, geopotential height over the Yellow Sea is most strongly affected by the tropical convection. Active convection in the western tropical Pacific is accompanied by a wave train with zonal wavenumber 6 over the North Pacific, which originates from the western tropical Pacific. The location of the wave train does not depend strongly on the location of the convective activity. One of the major barotropic unstable modes for the climatological 300mb flow for August resembles the wave train observed over the North Pacific during the midsummer season. It behaves like a standing wave, and has a large amplitude around the Yellow Sea. The e-folding time of this mode is 11 days when dissipation is ignored. Barotropic unstable modes for June do not have large amplitudes over East Asia. These results are consistent with the above observational results and suggest that barotropic instability plays an important role in the tropical-midlatitude interaction over East Asia during the midsummer season.
Journal of the Atmospheric Sciences, 2013
In this paper, zonal momentum balances of the tropical atmospheric circulation during the global monsoon mature months (January and July) are analyzed in three dimensions based on the ECMWF Interim Re-Analysis (ERA-Interim). It is found that the dominant terms in the balance of the atmospheric boundary layer (ABL) in both months are the pressure gradient force, the Coriolis force, and friction. The nonlinear advection term plays a significant role only in the Asian summer monsoon regions within the ABL. In the upper troposphere, the pressure gradient force, the Coriolis force, and the nonlinear advection are the dominant terms. The transient eddy force and the residual force (which can be explained as convective momentum transfer over open oceans) are secondary, yet cannot be neglected near the equator. Zonal-mean equatorial upper-troposphere easterlies are maintained by the absolute angular momentum advection associated with the cross-equatorial Hadley circulation. Equatorial upper...
Shallow Meridional Circulations in the Tropical Atmosphere
Journal of Climate, 2008
A shallow meridional circulation (SMC) in the tropical atmosphere features a low-level (e.g., 700 hPa) flow that is in the opposite direction to the boundary layer monsoon or trade wind flow and is distinct from the meridional flow above. Representations of the SMC in three global reanalyses show both similarities and astonishing discrepancies. While the SMC over West Africa appears to be the strongest, it also exists over the eastern Atlantic and eastern Pacific Oceans, and over the Indian subcontinent, with different strength and structure. All SMCs undergo marked seasonal cycles. The SMCs are summarized into two types: one associated with the marine ITCZ and the other with the summer monsoon. The large-scale conditions for these two types of SMCs are similar: a strong meridional gradient in surface pressure linked to surface temperature distributions and an absence of deep moist convection. The processes responsible for these conditions are different for the two types of SMCs, as are their structures relative to moist convection, associated precipitation, and deep meridional overturning circulations. It is suggested that discrepancies among the representations of the SMC in the three global reanalyses stem from different treatment of physical parameterizations, especially for cumulus convection, in the models used for the data assimilation.