Tropical Atlantic Biases in CCSM4 Semyon A. Grodsky 1, James A. Carton 1, Sumant Nigam 1, and Yuko M. Okumura 2 1 Department of Atmospheric and Oceanic Science University of Maryland, College Park, MD; 2 National Center for Atmospheric Research, Boulder, CO Figure 3. Upper 200m annual mean meridional currents (shading), water temperature (contours), meridional and vertical currents (arrows) averaged 2 o off the west coast of South Africa. Although the MSLP in CCSM4 is erroneously high by a few mbar in the subtropical highs and surface winds in the tropics are ~1 ms -1 too strong, the excess winds cause depressed SSTs only north of the equator. In contrast south of the equator SST is erroneously high due to the presence of additional warming effects. The region of highest SST bias is close to Southern Africa near the mean latitude of the Angola-Benguela Front (ABF, Fig 1). Comparison of CCSM4 to ocean simulations of various resolutions (Table 1) suggests that insufficient horizontal resolution leads to insufficient northward transport of cool water along this coast and an erroneous southward stretching of the ABF (Figs. 2, 3). A similar problem arises in the coupled model if the atmospheric component produces alongshore winds that are too weak. Equatorward transport of cool southern hemisphere water south of 20 o S (coastal branch of the Benguela Current, BC) is present only in the 1/4 o POP_0.25 forced by the 20CR winds (Figs. 2, 3). Coastal BC is missing in the 1 o CCSM4 and stand alone ocean (POP/NYF) due to weak coastal winds and insufficient horizontal resolution in the eastern ocean boundary. But, coastal BC is also too weak in the 1/10 o POP_0.1/NYF due to abnormally weak coastal wind stress provided by the Normal Year Forcing (NYF, Fig. 4). Erroneously warm coastal SSTs spread westward through a combination of advection and positive air-sea feedback involving marine stratocumulus clouds. Erroneously warm SST in the southeastern tropical Atlantic along with erroneously cold SST in the northern tropical Atlantic produce erroneous southward SST gradient (persistent meridional mode) that shifts the ITCZ southward, and thus affects the climate of the entire tropical Atlantic sector. Improvements of the processes, including winds and ocean model resolution, that lead to errors in seasonal SST along the southwestern Africa require horizontal resolution much finer than the 1 o currently used in climate models. Figure 1. Annual mean SST bias in (a) CCSM4, (b) CCSM3, and (c) ocean stand alone component forced by the normal year forcing (POP/NYF) of Large and Yeager. Figure 2. Annual mean surface currents (arrows) and SST (contours) in (a) POP_0.25, (b) POP_0.1/NYF, (c) CCSM4, and (d) POP/NYF. Northward/southward currents are blue/red, respectively. Horizontal dashed line is the annual mean latitude of the Angola- Benguela front. Figure 4. Annual mean wind stress (arrows) and wind stress magnitude (shading) in the Benguela region. Panel (f) shows wind stress magnitude averaged 2 o off the coast (red line in b). Two analyses of QuikSCAT wind stress are shown: (solid) Bentamy et al. (2008) and (dashed) Risien and Chelton (2008). CCSM4 and NYF wind stress are erroneously weak along the Benguela coast. CCSM4 Coupled, 20-th century run with historical gas forcing 1.25°x1° ATM 1.125°x0.5° OCN (in tropics) POP/NYF Ocean component of CCSM4 Repeating annual cycle of Normal Year Forcing (NYF, Large and Yeager, 2009) 1.125°x0.5° (in tropics) POP_ CR v.2 fluxes ( Compo et al., 2011). 0.4°x0.25° (in tropics, Carton and Giese, 2008 ) POP_0.1/ NYF Repeating annual cycle of NYF 0.1°x0.1° (eddy resolving POP of Maltrud et al.,2010) Table 1. List of experiments NameForcingResolution Grodsky, S. A., J. A. Carton, S. Nigam, and Y. M Okumura, 2012: Tropical Atlantic Biases in CCSM4, J. Clim., in press.