Comparison of the structure of the ITCZ in the West Pacific during the boreal summers of 1989-93 using AMIP simulations and ECMWF reanalysis

The dynamic structure and interannual variability of the ITCZ in the western North Pacific and east Asia during the boreal summers of 1989-93 are investigated. European Centre for Medium-Range Weather Forecasts (ECMWF) reanalyses are used to characterize the dynamical structure of the ITCZ. The ITCZ structure in the National Center for Atmospheric Research (NCAR) Community Climate Model 3 (CCM3.6) and the National Aeronautics and Space Administration (NASA) Goddard Institute of Space Studies (GISS) S12000 GCM is also explored. The ITCZ axis can be partitioned into three sections in the western North Pacific and east Asia: the continental monsoon trough (CMT) in India, the Bay of Bengal, and Indo-China; the oceanic monsoon trough (OMT) in the Pacific warm pool; and the trade wind trough (TWT) in the central Pacific. The confluence point (CP) is defined as an area where the broad cyclonic flow in OMT changes to the easterly trades of the TWT. East of the CP, both GCMs simulate a weaker easterly trade wind regime with a broader ITCZ due to the dislocation of the Pacific high. To the west of the CP, both GCMs simulate a weaker cross-equatorial flow and OMT. Both models show skill in simulating the interannual variability of the location of the CP; however, for the peak ENSO warm phase years (1992-93), the spread among ensemble members of the CP longitude is comparable to the shift of ensemble mean CP location relative to 1989. That is, the internal variability of each GCM is on the same order as the simulated interannual variability in this region.