Modulation of land-sea thermal contrast on the energy source and sink of tropical cyclone activity and its annual cycle

Abstract

In general, the tropical cyclone (TC) activity is considered to be influenced by the heat content of underlying ocean, vertical shear of horizontal wind, vorticity in the low troposphere, moisture in the troposphere, and favorable condition for deep convection development. However, these factors by nature merely present the internal factors of either atmosphere or ocean which influence the TC activity. In fact, the energy budget of the Earth system and its variation, modulated by the land-sea thermal contrast, are the intrinsic reasons responsible for the variation of TC activity. Here we investigate the modulation of diabatic heating distribution associated with the land-sea thermal contrast on the distribution of TC activity energy source and sink as well as the seasonality. An accumulated energy increment index (AEI) is defined using the TC best track data, and the energy sources and sinks of TC activity are then diagnosed effectively and practically according to the distribution of AEI. Results show that the thermal contrast of land and ocean is the primary reason for asymmetric distribution of TC activity about the Equator as well as the zonally asymmetric distribution of TC activity. The energy sources of TC activity are dominated by condensation heating of deep convection or double-dominant heating, which includes the condensation heating and cooling of longwave radiation (LO), while the sink areas are dominated by LO. The large scale diabatic heating associated with land-sea thermal contrast results in more favorable conditions for TC activity over the west part of oceans than those over the east parts. Moreover, the intensity of interaction of different diabatic heating over the west and east parts of ocean is also affected by the zonal scale of the oceans, which induces the difference of TC activity over the western North Pacific (WNP) and North Atlantic (ATL). The favorable westerlies and anticyclonic vertical shear associated with the tropical zonally asymmetric diabatic heating also contribute to the most intense TC activity over the WNP. The variation of large scale diabatic heating modulates the annual cycle of TC energy sources and sinks. In particular, the annual cycle over the WNP is the most typical one among the three basins (the WNP, the south Indian Ocean, and western South Pacific) that are characterized by the meridional shift of the energy sources and sinks. However, sources over the eastern North Pacific tend to extend westward and withdraw eastward associated with the variation of LO, while over the ATL, sources always merge from small pieces into a big one as the different diabatic heating over its west and east parts interacts with each other. Over the boreal Indian Ocean, the subcontinental scale land-sea heating contrast modifies the large scale circulation, and consequently contributes to the bimodal annual cycle of TC activity. In summary, TC activities are closely related to the interaction among various components of the climate system more than the atmosphere and ocean.