Characteristics of diurnal and seasonal cycles in global monsoon systems

Abstract

In this paper, we characterize the climatological diurnal cycles over monsoon land and ocean regions, and seasonal cycles for six major monsoon systems around the world, from observations and outputs of the NASA Seasonal-to-Interannual Prediction Project (NSIPP) general circulation model outputs. Over monsoon land regions, the diurnal cycle has a pronounced late afternoon peak in rainfall and over the oceans a much weak diurnal signal with an early morning peak. The NSIPP model produces a daily peak in rainfall about 2-3 hours earlier than observed, possibly attributed to a cumulus parameterization which does not include cloud life cycle effects, and which lack detailed treatment of boundary layer processes. The seasonal cycles of all monsoon rain systems are controlled by an ITCZ and a subtropical and/or extratropical component due to the presence of contiguous large continental land mass. Strong climatological intraseasonal oscillations (CISO) are found in the monsoon systems of South Asia and East Asia, Australia, and South America, and to a lesser degree for West Africa and Mexico/North America. The NSIPP model captures the slow component seasonal component reasonably well, but the CISO's are not well simulated. Analysis with reference to a new monsoon index shows that the major monsoon systems can be classified into three major groups (M1-3) according to the relative strength of continental vs. oceanic controls. East Asian and South Asian monsoons belong to a group (MI), where the continental influence is stronger than that from the ITCZ. The North America and South America monsoons show similar characteristics with the ITCZ influence slight stronger than continental influence (M2). The West Africa and Australia monsoons are found to be essentially ITCZ-monsoon systems (M3), with strong oceanic control, and limited poleward excursion of the monsoon rainbelt. We also find a clear trend associated with an increase in continentality for M3 monsoons, i.e., the West African and the Australian monsoons in the last two decades. We speculate that the former is linked to the partially recovery of the Sahel drought since the 1980's.