Equatorial Counter Current - Theoretical Background

Theoretical Background

The NECC is a direct response to the meridional change of the wind stress curl near the Intertropical Convergence Zone (ITCZ). In part the NECC owes its existence to the fact that the ITCZ is not located at the equator, rather several degrees latitude to the north. The change in the coriolis parameter (a function of latitude) moving across the equator combined with the ITCZ being located north of the equator leads to areas of convergence and divergence in the oceanic mixed-layer. This result is obtained from Ekman theory. Using the larger Pacific basin as an example, the resulting dynamic height pattern consists of a trough at the equator, and ridge near 5° degrees north, a trough a 10°N, and finally a ridge closer to 20°N. From geostrophy (the perfect balance between the mass field and velocity field), the NECC is located between the ridge and trough at 5°N and 10°N, respectively.

Sverdrup theory succinctly summarizes this phenomenon mathematically by defining a geostrophic mass transport per unit latitude, M, as the east-west integral of the meridional derivative of wind stress curl, minus any Ekman transport. The Ekman transport into the current is typically negligible, at least in the Pacific NECC. The total NECC is found by simply integrating M over the relevant latitudes.