Abstract
Most of the coupled models contain a strong warm bias in sea surface temperature (SST) over the Atlantic Cold Tongue (ACT) region (10° S–3° N, 20° W–10° E) during June–August (JJA) and September–November (SON). In this study, the origins of the ACT SST bias and their relative contributions to the bias are explored by conducting a set of sensitivity experiments, which are based on an ocean-ice model, and by ignoring the nonlinear effects of each origin. The origins for the warm bias over the ACT in the coupled climate model during JJA are estimated as follows: westerly wind bias along the equator (5° S–5° N) during March–May (MAM; contributes approximately 32.6% of the warm bias), northerly bias over the southern tropical Atlantic (25° S–3° N, 40° W–20° E) during MAM and JJA (21.4%), bias in the surface specific humidity and surface air temperature (11.9%), and downward shortwave radiation bias (6.5%). The origins of the ACT bias during SON are as follows: northerly bias over the southern tropical Atlantic during SON (31.2%), bias in the surface specific humidity and surface air temperature (27.9%), downward shortwave radiation bias (17.4%), and zonal wind bias (13.4%). Note that these contribution ratios of these origins may be model-dependent. In addition, the local and non-local effects of the zonal wind bias are explored explicitly, while those of all the other biases are examined implicitly. Therefore, a better-performing atmospheric component is crucial when simulating zonal winds during MAM along the equator (5° S–5° N) and meridional winds during MAM, JJA, and SON over the southern tropical Atlantic, which will alleviate the warm bias over the ACT region in the coupled climate model.
Highlights
Most of the coupled climate models (CCMs) show a severe warm sea surface temperature (SST)bias for the tropical Atlantic [1,2,3,4,5]
The data nudged to the climate model FGOALS-g2 is from NCEP-NCAR for the period from 1979 to 2005
The northerly bias is over the Atlantic cold tongue (ACT) region year-round (Figure 4b) and this bias is strongest during MAM and weakest during JJA (Figure 4b)
Summary
Most of the coupled climate models (CCMs) show a severe warm sea surface temperature (SST). The underestimation in the equatorial easterly winds during MAM leads to a deeper thermocline than observed, which inhibits cold tongue development during the season [29] (JJA) This results in a summertime warm SST bias in the ACT region, which was further confirmed in studies by Chang et al [30] and Wahl et al [8]. Previous studies have pointed out the leading role of the MAM westerly wind bias in inducing the SST bias over the equatorial Atlantic [4,5,28,32,36,37] based on the results of a multi-model ensemble mean or a CCM These studies mainly focused on the zonal wind bias and rarely considered the effects of biases in other atmospheric forcings.
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