Equatorial Sea Surface Temperature Sensitivity to Net Surface Heat Flux: Some Ocean Circulation Model Results

D. E. Harrison NOAA/PMEL, Seattle Washington

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Abstract

Several primitive-equation ocean general circulation model experiments have been carried out in order to explore the sensitivity of equatorial sea surface temperature (SST) results to uncertainty in the net surface heal flux (Q) imposed at the surface. Both climatological seasonal cycle experiments and hindcasts of the 1982/33 ENSO event are considered. It is found that regions of light winds, which typically reach values of SST in excess of 31°C using this ocean model and past Q parameterizations, attain more realistic SST values of 29°–30°C when Q is reduced by as little as 10 W m−2. Sensitivity in this regime is about 0.1–0.2°C (W m−2)−1 for low-frequency SST changes. In regions of easterly winds with their associated upwelling, horizontal advection, and stronger mixing, changes of Q in excess of 50 W m−2 produce SST changes typically of 0.7°C, for a sensitivity of about 0.02°C (W m−2)−1. These results apply equally well to the ENSO hindcasts and the seasonal cycle studies. The reasons for the large variation in sensitivity and the very large sensitivity under light winds are described. To the extent that these results are representative of oceanic conditions, very accurate Q information will be required for studies of the low-frequency variability of SST in light wind regions like the western Pacific; much less accurate fluxes appear needed for studies of comparable variability in upwelling regions.

Abstract

Several primitive-equation ocean general circulation model experiments have been carried out in order to explore the sensitivity of equatorial sea surface temperature (SST) results to uncertainty in the net surface heal flux (Q) imposed at the surface. Both climatological seasonal cycle experiments and hindcasts of the 1982/33 ENSO event are considered. It is found that regions of light winds, which typically reach values of SST in excess of 31°C using this ocean model and past Q parameterizations, attain more realistic SST values of 29°–30°C when Q is reduced by as little as 10 W m−2. Sensitivity in this regime is about 0.1–0.2°C (W m−2)−1 for low-frequency SST changes. In regions of easterly winds with their associated upwelling, horizontal advection, and stronger mixing, changes of Q in excess of 50 W m−2 produce SST changes typically of 0.7°C, for a sensitivity of about 0.02°C (W m−2)−1. These results apply equally well to the ENSO hindcasts and the seasonal cycle studies. The reasons for the large variation in sensitivity and the very large sensitivity under light winds are described. To the extent that these results are representative of oceanic conditions, very accurate Q information will be required for studies of the low-frequency variability of SST in light wind regions like the western Pacific; much less accurate fluxes appear needed for studies of comparable variability in upwelling regions.

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