HR: 10:45h
AN: A22B-03    [Abstracts]
TI: Monsoon transitions in the seasonal cycle simulated with aquaplanet GCMs
AU: * Bordoni, S
EM: bordoni@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, Ca 91125, United States
AU: Schneider, T
EM: tapio@caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, Ca 91125, United States
AB: Traditionally, Earth's large-scale monsoons have been interpreted as a planetary-scale sea-breeze driven by the thermal contrast between land and ocean. However, simulations with an aquaplanet GCM show that, in the course of the seasonal cycle, the tropical mean meridional circulation undergoes transitions that resemble the onset and end of Earth's large-scale monsoons even in the absence of land masses, provided the heat capacity (mixed-layer depth) of the underlying ocean is sufficiently low. A transition in early summer is associated with a rapid rearrangement in the subtropics and intensification of the major precipitation zone, an abrupt strengthening and broadening of the cross-equatorial Hadley cell, and wind reversals in both the upper and lower levels. A reverse transition occurs in late summer. It is found, consistent with similar dry simulations (Schneider and Bordoni, 2007), that the rapid rearrangements of the meridional circulations mark shifts between an equinox and summer regime, in which eddy momentum flux divergence dominates the vertically averaged zonal momentum balance near the center of the Hadley cell, and a winter, "monsoon" regime, in which the eddy momentum flux divergence is small and the mean momentum flux dominates. The summertime precipitation zones form just equatorward of the lower lever moist static energy maximum, which is colocated with the boundary between the winter and the summer cell. Factors controlling the exact locations of these precipitation zones are examined. Sensitivity to changes in the mixed layer depth and implications for Earth's monsoons will also be discussed.
DE: 1620 Climate dynamics (0429, 3309)
DE: 1626 Global climate models (3337, 4928)
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting