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WIREs Clim Change
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Climate model biases in the eastern tropical oceans: causes, impacts and ways forward

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The eastern boundaries of the tropical and subtropical oceans are regions of high biological productivity that support some of the world's largest fisheries. They also feature extensive stratocumulus cloud decks that play a pivotal role in the response of the climate system to greenhouse gas forcing. Global climate models experience great difficulties simulating eastern boundary regions, with one of the most notable shortcomings being warm sea‐surface temperature biases that often exceed 5 K. These model biases are due to several reasons. (1) Weaker than observed alongshore winds lead to an underrepresentation of upwelling and alongshore currents and the cooling associated with them. (2) Stratocumulus decks and their effects on shortwave radiation are underpredicted in the models. (3) The offshore transport of cool waters by mesoscale eddies is not adequately represented by global models due to insufficient resolution. (4) The sharp vertical temperature gradient separating the warm upper ocean layer from the deep ocean is too diffuse in the models. More work will be required to assess the relative importance of these error sources and to find ways of mitigating them. Coordinated multi‐model experiments are vital to achieve this goal, as are enhanced ocean and atmosphere observations of the eastern boundary regions. To what extent eastern ocean biases compromise the models' ability to produce accurate seasonal predictions, and climate change projections should be another focus of research efforts. WIREs Clim Change 2015, 6:345–358. doi: 10.1002/wcc.338

(a) Observed annual mean sea‐surface temperature (SST) from the optimally interpolated (OI) SST data set. (b) Annual mean bias of the CMIP5 ensemble relative to OISST. See Table for a list of models. The gray boxes denote the regions discussed in this article and their longitudinal and latitudinal extents correspond to the ranges plotted in Figures and , respectively. The text labels refer to the naming used in Figure .
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Fields from the CMIP5 ensemble mean (green line) and observations (black and gray lines) zonally averaged from the coast to 5° offshore for the Southwest African coast (first column from the left), the South American coast (second column), the Northwest African coast (third column), and the North American coast (fourth column). The four averaging areas encompass the Namibian, Peruvian, Canarian, and Californian stratus regions in that order. All data are averaged from 1984 to 2005. (a) Sea‐surface temperature (SST) (°C) from OISST (black line) and the Hadley Centre Sea Ice and Sea Surface Temperature (HADISST) (gray line). (b) Meridional surface winds (m s−1) from CCMP (black line) and ICOADS (gray line). (c) Meridional surface currents (m s−1) from the Simple Ocean Data Assimilation (SODA) (black line) and the Global Ocean Data Assimilation System (GODAS) (gray line) reanalyses. (d) Depth of the 15°C isotherm from SODA (black line) and GODAS (gray line). (e) Ocean vertical velocity (m s−1 × 10−5) from SODA (black line) and GODAS (gray line).
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Meridionally averaged fields from the CMIP5 ensemble mean (green line) and observations (gray and black lines) for the subtropical Southeast Atlantic (first column from the left; averaged 20–10°S), subtropical Southeast Pacific (second column; averaged 20–10°S), subtropical Northeast Atlantic (third column; averaged 15–25°N), and subtropical Northeast Pacific (fourth column; 20–30°N). Land points are masked out. The four averaging areas encompass the Namibian, Peruvian, Canarian, and Californian stratus regions in that order. All data are averaged from 1984 to 2005. (a) Low‐level cloud incidence (%) from the International Satellite Cloud Climatology Project (ISCCP) (black line) and ERA‐Interim reanalysis (gray line). (b) Net surface shortwave radiation (W m−2; upward positive) from the CORE2 (black line) and OAFlux (gray line) observationally derived products. (c) Surface latent data set heat flux (W m−2; upward positive) from CORE2 (black line) and OAFlux (gray line). (d) Surface meridional wind (m s−1) from the Cross‐Calibrated Multi‐Platform Ocean Surface Wind Vector (CCMP) data set (black line) and the International Comprehensive Ocean‐Atmosphere Data Set (ICOADS) (gray line).
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