Influence of Surface Currents on the Sea Surface Temperature in the Tropical Pacific Ocean

Fabrice Bonjean Laboratoire d’Océanographie Dynamique et de Climatologie, Paris, France

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Abstract

Currents at 15-m depth derived from buoy drifts and current meter records for the 1987–93 period are used to study the influence of thermal advection on SST variations. Horizontal advection is found to be important for maintaining the yearly mean SST distribution in the central-western Pacific and in the northeastern region. At the equator, zonal advection compensates for and exceeds the semiannual autumn peak in air–sea heat flux, consistent with seasonal cooling of the ocean surface throughout this period. In the eastern equatorial region, zonal and meridional advection are much weaker than net air–sea heat flux, implicitly revealing the crucial role played by vertical advection and mixing. The residual, certainly dominated by vertical heat tranfers, plays a dominant role in the eastern area, mostly compensating for the mean annual air–sea heat flux, and, throughout the year, combining with air–sea heat flux to balance the SST tendency. The effect of horizontal advection on the anomalous SST changes is also studied, using the observational data as well as the response of the Laboratoire d’Oceanographie Dynamique et de Climatologie (LODYC) and Hadley Center GCMs to prescribed winds during the same period, 1987–93. SST advection by anomalous zonal currents dominates zonal advection and is most influential near the equator, in the east-central region. It is particularly important during part of the 1988–89 La Niña and 1991–92 El Niño. Compared to the observations, zonal advection plays a lesser role in the LODYC model and a larger role in the Hadley model. The effect of meridional advection is difficult to derive from the observations because of the larger noise in meridional velocity. However, observations and GCMs show that it is dominated by mean advection of anomalous SST; it had a strong impact in the eastern off-equatorial areas between 1987 and 1988, but was otherwise less important than zonal advection.

* Current affiliation: Earth and Space Research, Seattle, Washington.

Corresponding author address: Fabrice Bonjean, Earth and Space Research, 1910 Fairview Ave. E, Suite 102, Seattle, WA 98102.

Email: bonjean@esr.org

Abstract

Currents at 15-m depth derived from buoy drifts and current meter records for the 1987–93 period are used to study the influence of thermal advection on SST variations. Horizontal advection is found to be important for maintaining the yearly mean SST distribution in the central-western Pacific and in the northeastern region. At the equator, zonal advection compensates for and exceeds the semiannual autumn peak in air–sea heat flux, consistent with seasonal cooling of the ocean surface throughout this period. In the eastern equatorial region, zonal and meridional advection are much weaker than net air–sea heat flux, implicitly revealing the crucial role played by vertical advection and mixing. The residual, certainly dominated by vertical heat tranfers, plays a dominant role in the eastern area, mostly compensating for the mean annual air–sea heat flux, and, throughout the year, combining with air–sea heat flux to balance the SST tendency. The effect of horizontal advection on the anomalous SST changes is also studied, using the observational data as well as the response of the Laboratoire d’Oceanographie Dynamique et de Climatologie (LODYC) and Hadley Center GCMs to prescribed winds during the same period, 1987–93. SST advection by anomalous zonal currents dominates zonal advection and is most influential near the equator, in the east-central region. It is particularly important during part of the 1988–89 La Niña and 1991–92 El Niño. Compared to the observations, zonal advection plays a lesser role in the LODYC model and a larger role in the Hadley model. The effect of meridional advection is difficult to derive from the observations because of the larger noise in meridional velocity. However, observations and GCMs show that it is dominated by mean advection of anomalous SST; it had a strong impact in the eastern off-equatorial areas between 1987 and 1988, but was otherwise less important than zonal advection.

* Current affiliation: Earth and Space Research, Seattle, Washington.

Corresponding author address: Fabrice Bonjean, Earth and Space Research, 1910 Fairview Ave. E, Suite 102, Seattle, WA 98102.

Email: bonjean@esr.org

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