Evolution of Large-Scale Circulation during TOGA COARE: Model Intercomparison and Basic Features

Abstract

An intercomparison study of the evolution of large-scale circulation features during TOGA COARE has been carried out using data from three 4D assimilation systems: the National Meteorological Center (NMC, currently known as the National Center for Environmental Prediction), the Navy Fleet Numerical Oceanography Center, and the NASA Goddard Space Flight Center. Results show that the preliminary assimilation products, though somewhat crude, can provide important information concerning the evolution of the large-scale atmospheric circulation over the tropical western Pacific during TOGA COARE. Large-scale features such as sea level pressure, rotational wind field, and temperature are highly consistent among models. However, the rainfall and wind divergence distributions show poor agreement among models, even though some useful information can still be derived. All three models shows a continuous background rain over the Intensive Flux Area (IFA), even during periods with suppressed convection, in contrast to the radar-estimated rainfall that is more episodic. This may reflect a generic deficiency in the oversimplified representation of large-scale rain in all three models. Based on the comparative model diagnostics, a consistent picture of large-scale evolution and multiscale interaction during TOGA COARF emerges. The propagation of the Madden and Julian Oscillation (MJO) from the equatorial Indian Ocean region into the western Pacific foreshadows the establishment of westerly wind events over the COARE region. The genesis and maintenance of the westerly wind (WW) events during TOGA COARE are related to the establishment of a large-scale east-west pressure dipole between the Maritime Continent and the equatorial central Pacific. This pressure dipole could be identified in part with the ascending (low pressure) and descending (high pressure) branches of the MJO and in part with the fluctuations of the austral summer monsoon. Accompanying the development of WW over the IFA and crucial to its maintenance is a robust meridional circulation, with strong cross-equatorial flow and rising motion near the entrance region of the WW and sinking motion in the extratropical Northern Hemisphere. The presence of a quasi-stationary equatorial heat source near the date line may have provided additional feedback mechanisms for the WWs. Surface pressure and wind surges related to cold air outbreaks off the East Asian continent play an important role in the rapid build up and/or termination of the WWs during TOGA COARE. The establishment of WWs in the near equatorial region may be linked to the modulation of North Pacific storm track activities.