EVOLUTION OF LARGE-SCALE CIRCULATION DURING TOGA COARE - MODEL INTERCOMPARISON AND BASIC FEATURES

An intercomparison study of the evolution of large-scale circulation f eatures during TOGA COARE has been carried our using data from three 4 D assimilation systems: the National Meteorological Center (NMC, curre ntly known as the National Center for Environment Predication), the Na vy Fleet Numerical Oceanography Center, and the NASA Goddard Space Fli ght Center. Results show that the preliminary assimilation products, t hough somewhat crude, can provide important information concerning the evolution of the large-scale atmospheric circulation over the tropica l western Pacific during TOGA COARE. Large-scale features such as sea level pressure, rotational wind field, and temperature are highly cons istent among models. However, the rainfall and wind divergence distrib utions show poor agreement among models, even though some useful infor mation can still be derived. All three models shows a continuous backg round rain over the Intensive Flux Area (IFA), even during periods wit h suppressed convection, in contrast to the radar-estimated rainfall t hat is more episodic. This may reflect a generic deficiency in the ove rsimplified representation of large-scale rain in all three models. Ba sed on the comparative model diagnostics, a consistent picture of larg e-scale evolution and multiscale interaction during TOGA COARE emerges . The propagation of the Madden and Julian Oscillation (MJO) from the equatorial Indian Ocean region into the western Pacific foreshadows th e establishment or westerly wind events over the COARE region. The gen esis and maintenance of the westerly wind (WW) events during TOGA COAR E are related to the establishment of a large-scale east-west pressure dipole between the Maritime Continent and the equatorial central Paci fic. This pressure dipole could be identified in part with the ascendi ng (low pressure) and descending (high pressure) branches of the MJO a nd in part with the fluctuations of the austral summer monsoon. Accomp anying the development of WW over the IFA and crucial to its maintenan ce is a robust meridional circulation, with strong cross-equatorial fl ow and rising motion near the entrance region of the WW and sinking mo tion in the extratropical Northern Hemisphere. The presence of a quasi -stationary equatorial heat source near the date line may have provide d additional feedback mechanisms for the WWs. Surface pressure and win d surges related to cold air outbreaks off the East Asian continent pl ay an important role in the rapid build up and/or termination of the W Ws during TOGA COARE. The establishment of WWs in the near equatorial region may be linked to the modulation of North Pacific storm track ac tivities.