On the dynamics and climatology of surface flow over the Equatorial Oceans

The surface circulation over the Equatorial Atlantic and Eastern Pacific is investigated using 60-year average ship data with a one-degree square resolution. At the height of the Northern summer, a zonally oriented speed maximum extends across the Eastern part of the oceans at about 4–5 N, near where the cross-equatorial flow recurves from Southeast to Southwest. This separates a band of strong divergence immediately to the North of the Equator from the convergence belt further poleward. Strongest convergence occurs about 350 km South of the confluence between Northern and Southern Hemisphere trades, and within a region of high directional steadiness of wind. A negative wind stress curl induces upwelling and low sea surface temperature immediately South of the Equator, and downwelling to the North.

In most of the Northeast trades and the Southeast trades of the Southern Hemisphere during July–August, frictional energy dissipation not offset by cross-isobaric flow produces a gradual slowing of the current and downstream convergence. As the Southern Hemisphere trades flow North from the Equator over the Eastern part of the oceans, the Coriolis acceleration increases acting to the right, enhancing the cross-isobaric angle and resulting in net kinetic energy generation, the aforementioned speed maximum and attendant zone of strong divergence. Continuation of this Coriolis effect recurves the flow from Southeast to Southwest, which further downstream diminishes the cross-isobaric angle, thus impairing kinetic energy generation and resulting in a band of maximum convergence. A positive relation is found between the latitude of the discontinuity separating Northern and Southern Hemisphere trades and the latitude and intensity of the zonally oriented convergence/divergence bands, both seasonally and on a year-to-year basis.