Understanding Summer Monsoon by GCM Experimentation
Shang-Ping Xie (IPRC, University of Hawaii, Honolulu, USA)
June-Yi Lee and In-Sik Kang (Seoul National University, Seoul, Korea)
Naomi Saito (Fukuoka Prefecture Department of Civil Engineering, Japan)
The world's greatest deserts and monsoons co-exist at the same latitudes on the subtropical Afro-Eurasian Continents. To investigate the mechanisms for these subtropical climates, we conduct atmospheric general circulation model (GCM) experiments under an idealized land-sea distribution featuring an Asian continent with a straight south coast at 20N. Sea surface temperature and land surface parameters are all set zonally uniform to avoid a priori longitudinal preference for rainfall.
The model with flat land surface produces a realistic zonal distribution of precipitation, with arid (wet) conditions over the western (eastern) part of the subtropical continent. The gigantic high pressure over the ocean is responsible for this distribution of western desert and eastern monsoon climates. Also crucial to the western desert is the slow precipitation-soil moisture interaction that keeps the northward advance of the monsoon to lag behind the sun.
The monsoon on the eastern continent starts abruptly in late June, three months after the spring equinox when a northward temperature gradient is established near the ground on the south coast. The onset is associated with the explosive growth of a westward-traveling moist baroclinic wave. Further analysis suggests a Geostrophic Monsoon concept: The earth's rotation effect resists and delays the formation of a thermally direct circulation, until baroclinic instability breaks the thermal wind balance and triggers the rapid onset of monsoon.
Recent experiments focus on the effects of north-south land-sea contrast and employ a circumpolar continent of zonally uniform surface conditions. By varying the latitude of the continent's south coast, a critical latitude is identified that separates the regime where the continental and equatorial rainbelts interact strongly from that where their interaction is minimum. The implications for the Indian monsoon's break-active cycle will be discussed.
