National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2004

Physical oceanographic conditions during GasEx 2001

Johnson, G.C., C.L. Sabine, K.E. McTaggart, and J.M. Hummon

J. Geophys. Res., 109(C8), C08S04, doi: 10.1029/2002JC001718 (2004)

GasEx-2001 is a study of air-sea gas exchange in a region of CO outgassing. The bulk of the experiment followed a drifting array of near-surface instruments deployed during the second half of February 2001 just south of the equator in the central Pacific Ocean. Physical oceanographic conditions including local currents, the seasonal cycle, Kelvin waves, and tropical instability waves are described using shipboard data and a variety of other data sources to set the large-scale oceanographic context for GasEx-2001. Local physical oceanographic conditions during GasEx-2001 are then analyzed using shipboard data and a simple one-dimensional mixed layer model. The thermocline shoals about 13 m over the 15-day experiment, implying an upwelling rate of 1 × 10 m s. Zonal velocity is surface-intensified and westward, with vertical shear mostly through the thermocline. Meridional velocity is also strongly sheared with a maximum equatorward flow in the thermocline that is much reduced by 17-m depth. The mixed layer model exhibits more near-surface warming over the course of the experiment than is observed. Prescribing upwelling in the model closes the heat budget within error estimates. Entrainment at the base of the mixed layer plays a limited role in the mixed layer budgets of carbon and other water properties. Vertical shear of horizontal velocity within the mixed layer and slippage of the array through the surface water also have small (but uncertain) roles in these budgets.

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