Measurements of the Cromwell current

Direct current measurements made along the equator in the central and eastern tropical Pacific during the IGY show that the subsurface eastward flow, first described in 1954, is a fast, thin, but major current. As defined by the 25 cm/sec contour, it is 300 km wide and about two-tenths of a kilometre thick; it is symmetrical about the equator. At 140°W the core of the current is at 100 m. Velocities up to 150 cm/sec (3 knots) were recorded. The average transport based on four sections is 39 × 10⁶ m³/sec. The current was traced from 140 °W to the Galapagos Islands at 92°W. It was not found east of the Galapagos at 89°W. The core velocity was 100–150cm/sec from 140°W to the Galapagos; the depth of the core shallowed from 100 to 40 m. At the surface and at 500 m the flow was to the west.

Hydrographic measurements suggest that the flow is in geostrophic equilibrium to within half a degree of the equator. The major features of the Pacific Equatorial Undercurrent (Cromwell Current), including its general shape, the depth of the core, which slopes upward to the east, can be accounted for by assuming a geostrophic flow in response to the perturbation in the horizontal pressure gradients caused by mixing through the thermocline at the equator.

Direct measurements of the Cromwell Current show that the current extends from at least 92°W to 150°W (3500 miles). Indirect evidence suggests that the current will be found to at least 170°W (4700 miles long) and perhaps as far as 160°E (6500 miles long).