On the Geostrophic Flow at the Surface of the Pacific Ocean with Respect to the 1,000-decibar Surface

The geopotential anomaly at the surface of the Pacific Ocean with respect to the 1,000-decibar surface is computed. Horizontal variations in geopotential at greater depth are shown to be smaller and are neglected. The geostrophic flow at the sea surface with respect to the 1,000-decibar surface is shown to agree in general with the averages of currents estimated from the set and drift of vessels and tabulated in various atlases. The major gyres of the ocean are clearly shown and one new feature, a South Equatorial Countercurrent, is revealed in the western Pacific.

Since there is meridional flow in the ocean, the flow is not entirely geostrophic and the contours of geopotential anomaly may lie at some angle to the streamlines. This is most evident where the contours intersect the coast and cross the equator. A qualitative consideration of wind stress indicates that where wind and current are in the same direction geopotential anomaly will rise along a streamline and where they are opposed geopotential anomaly will decrease along a streamline. This effect can at least qualitatively account for the rise of geopotential anomaly to the west in the region of the trades and toward the east in the northern hemisphere westerlies.