Kinematic and Thermodynamic Characteristics of the Flow over the Western Pacific Warm Pool during TOGA COARE

Abstract

Rawinsonde and satellite infrared radiation (IR) data from the Tropical Ocean Global Atmosphere (TOGA) Coupled Ocean Atmosphere Response Experiment (COARE) are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western Pacific warm pool. Infrared data for the 4-mo Intensive Observing Period (IOP) and vertical motion fields indicate that the intensity of convection, height of maximum upward motion, and SST all increased from west (140°E) to east across the COARE domain. IOP-mean IR data show a double ITCZ (Intertropical Convergence Zone) structure north and south of the Intensive Flux Array (IFA, centered at 2°S, 156°E), although marked variability in the patterns occurred on a month to month basis. Three prominent westerly wind bursts occurred over the IFA during the 4-mo IOP in association with the intraseasonal oscillations (ISOs). Strong upward motion usually occurred 1–3 weeks prior to the peak low-level westerlies. Subsidence dominated when the westerly winds prevailed. COARE data reveal that the vertical wind shear (more than 50 m s−1 from 850 to 100 hPa) and the vertical extent of westerlies during the peak westerly wind bursts were far greater than previously recognized. The mean low-level equatorial flow over the western Pacific was westerly, interrupted occasionally by brief periods of easterly flow. The perturbation westerlies to the west of the disturbance associated with the ISO were usually stronger than the perturbation winds to the east. Maximum surface latent beat flux usually occurred during the peak westerlies, whereas the surface sensible heat flux peaked prior to the strongest westerlies. The IOP-mean divergence profile over the IFA shows a very weak divergence near the surface and weak convergence at middle and low levels. The ITCZ-band divergence profiles show strong low-level convergence from the surface to about 700 hPa. The striking difference between the divergence profiles along the equator over the IFA and those north and south in the ITCZ bands suggests that, although the divergence and vertical motion profiles tend to look alike whenever and wherever the convection is strong, great care should be exercised in generalizing divergence and vertical motion profiles from one region to another over the western Pacific warm pool. Correlations between cold clouds and vertical motion indicate that cold clouds are a good indicator of upper-level upward vertical motion but not low-level vertical motion. In a significant number of cases, low-level downward motion occurred under very cold cloud tops over the warm pool, indicating extensive optically thick anvil cloud and nonprecipitating high cirrus are a common occurrence over the warm pool. The IOP-mean relative humidity profile over the IFA shows a primary peak at low levels at the top of the mixed layer and a secondary peak near 550 hPa (near the 0°C level). The secondary peak is not present in either ECMWF or NMC operational analyses, and the midtroposphere is much drier in the two model-assimilated results. A synthesis of the kinematic and thermodynamic characteristics of the December–early January westerly wind burst as it passed the IFA is presented.