Prof. Chang Ping
Department of Oceanography
Texas A&M University

Time: 9:00 am, 2016.10.28

Location: First floor conference room

in Yifu Building

Abstract:Generations of global coupled atmosphere-ocean climate models have been plagued by persistent sea-surface temperature (SST) biases in the tropical Atlantic. These biases are shown to hinder model representation of the West African monsoon and tropical cyclones, with seasonal Atlantic tropical cyclone activity reduced by 50%, motivating improvements to advance our capability to make reliable seasonal forecasts and future climate projections. Focus is on the particularly severe (up to 6-10°C) warm SST biases of the southeastern tropical Atlantic Benguela coastal upwelling system, which is characterized by a northward atmospheric low-level coastal jet parallel to the coast, called the Benguela jet. To investigate this problem, observational datasets and output from a suite of regional atmosphere model simulations with horizontal resolutions of 9, 27, 81, and 243 km were used to force a suite of regional ocean model imulations. The coastal SST and upwelling are shown to be sensitive to the surface wind stress and wind stress curl associated with the Benguela jet. Substantial mprovements in simulated coastal SST are achieved by forcing the ocean model with a fine-resolution representation of the Benguela jet. In particular, the shape of the near coastal wind profile, with a sudden drop of wind speed towards the coast, is extremely critical for simulating realistic upwelling and SSTs in the ocean model. Satellite wind products show that the Benguela jet has a detailed spatial structure with two near-shore preferred jet core regions, one near the Angola-Benguela Front at 17.5°S where the Kunene upwelling cell is located, and the other near 25-27.5°S where the Lüderitz upwelling cell is situated. The representation of the Benguela jet in atmospheric models depends highly on resolution and the geometry of the coastline, as the northern jet core is characterized by a hydraulic expansion fan. This work indicates that the inability of current generation of global climate models in resolving the Benguela jet is a primary cause of the longstanding tropical Atlantic SST bias problem. To resolve this issue, increasing resolutions of climate models is required.

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