Attribution and Impacts of Upper-Ocean Biases in CCSM3

Abstract

The largest and potentially most important ocean near-surface biases are examined in the Community Climate System Model coupled simulation of present-day conditions. They are attributed to problems in the component models of the ocean or atmosphere, or both. Tropical biases in sea surface salinity (SSS) are associated with precipitation errors, with the most striking being a band of excess rainfall across the South Pacific at about 8°S. Cooler-than-observed equatorial Pacific sea surface temperature (SST) is necessary to control a potentially catastrophic positive feedback, involving precipitation along the equator. The strength of the wind-driven gyres and interbasin exchange is in reasonable agreement with observations, despite the generally too strong near-surface winds. However, the winds drive far too much transport through Drake Passage [Ͼ190 Sv (1 Sv ϵ 10 6 m 3 s Ϫ1)], but with little effect on SST and SSS. Problems with the width, separation, and location of western boundary currents and their extensions create large correlated SST and SSS biases in midlatitudes. Ocean model deficiencies are suspected because similar signals are seen in uncoupled ocean solutions, but there is no evidence of serious remote impacts. The seasonal cycles of SST and winds in the equatorial Pacific are not well represented, and numerical experiments suggest that these problems are initiated by the coupling of either or both wind components. The largest mean SST biases develop along the eastern boundaries of subtropical gyres, and the overall coupled model response is found to be linear. In the South Atlantic, surface currents advect these biases across much of the tropical basin. Significant precipitation responses are found both in the northwest Indian Ocean, and locally where the net result is the loss of an identifiable Atlantic intertropical convergence zone, which can be regained by controlling the coastal temperatures and salinities. Biases off South America and Baja California are shown to significantly degrade precipitation across the Pacific, subsurface ocean properties on both sides of the equator, and the seasonal cycle of equatorial SST in the eastern Pacific. These signals extend beyond the reach of surface currents, so connections via the atmosphere and subsurface ocean are implicated. Other experimental results indicate that the local atmospheric forcing is only part of the problem along eastern boundaries, with the representation of ocean upwelling another likely contributor.

Simulation of the Global Hydrological Cycle in the CCSM Community Atmosphere Model Version 3 (CAM3): Mean Features

“…The origins of the biased freshwater cycle are difficult to pinpoint, but are probably related to the lack of a damping mechanism which would inhibit air-sea freshwater exchange in ways comparable to heat exchange, when the ocean becomes too fresh or salty. Subtropical Sea Surface Salinity (SSS) and Sea Surface Temperature (SST) in the south are fresher and warmer than observed in CCSM3 Large and Danabasoglu (2005). This suggests that excess tropical precipitation transported to the subtropics by the ocean renders the midlatitude upper ocean too fresh and stable, thus inhibiting deep mixing which would lower the SST.…”

“…Anomalously high SST results in the enhanced evaporation rates which, after atmospheric transport back to the tropics, recurs as excessive precipitation. Process studies indicate that, at least in the Atlantic, correcting the west coast ocean SST bias greatly reduces excessive tropical precipitation Large and Danabasoglu (2005).…”

The Influence of the AMOC Variability on the Atmosphere in CCSM3

“…The mean oceanic fields are broadly realistic but exhibit some significant biases in the North Atlantic that are also commonly found in global climate models of similar resolution. Although the separation of the Gulf Stream (GS) is well located, the path of the Gulf Stream/North Atlantic Current (GS/NAC) is too zonal near the tail of the Grand Banks, resulting in a large cold and fresh bias near the surface around 408-508N, 508-208W (Large and Danabasoglu 2006;Danabasoglu 2008). The main deep convection site in the North Atlantic is centered in the western subpolar gyre near 548N, 458W.…”

Tropical Atlantic biases and their relation to surface wind stress and terrestrial precipitation

“…This bias exhibits comparatively little seasonal variability, in contrast to the JJA peak of the equatorial bias. Several model studies have attributed this error to excessive shortwave radiation associated with the under-representation of stratocumulus in GCMs (Large and Danabasoglu 2006;Huang et al 2007;RX08;Wahl et al 2009;Hu et al 2010). If stratocumulus is the only reason, however, it remains unclear why the maximum SST error occurs just off the coast, given that observed stratus decks extend far offshore, and why SST and stratocumulus errors peak in different seasons.…”