Pacific Equatorial Undercurrent
The Pacific Equatorial Undercurrent (EUC), also known as the Cromwell Current, is a narrow, swift, subsurface eastward flow in the equatorial Pacific Ocean. It extends across much of the equatorial Pacific from the western basin toward South America, flowing at subsurface Thermocline depths with high transport rates measured in Sv.
Dynamics
The EUC is driven by the zonal pressure gradient established by the trade winds. While the surface trade winds push water westward (creating the South Equatorial Current), the resulting sea surface height gradient generates an eastward pressure force at depth. Because the Coriolis effect vanishes at the equator, this pressure gradient drives the undercurrent directly eastward without deflection.
The EUC carries cold, nutrient-rich thermocline water from the western Pacific warm pool toward the eastern Pacific. As it shoals approaching the eastern boundary, this water is brought closer to the surface, fueling upwelling.
Role in Galapagos climate
When the EUC encounters the Galapagos Archipelago, the submarine topography of the islands deflects and splits the current. This interaction causes intense local upwelling, particularly around the western islands (Isabela and Fernandina), creating the Galapagos Cold Pool — a region of anomalously low SSTs that profoundly influences the regional climate.
The upwelling driven by the EUC:
- Cools SSTs well below surrounding tropical waters
- Stabilizes the lower atmosphere, promoting a persistent temperature inversion
- Creates conditions for the Garua season (stratocumulus and fog)
- Supplies nutrients that support the rich marine ecosystems of the archipelago
ENSO modulation
The strength of the EUC is strongly modulated by ENSO:
- During El Nino, the trade winds weaken and the zonal pressure gradient diminishes. The EUC slows or can even reverse locally. Upwelling weakens, SSTs rise, and the Garua season is disrupted.
- During La Nina, trade winds and the pressure gradient strengthen. The EUC intensifies, upwelling increases, SSTs drop, and Garua conditions strengthen.
This modulation makes the EUC a key mechanism linking large-scale Pacific variability to local climate in the Galapagos.
See also: El Nino-Southern Oscillation, Galapagos Cold Pool, Galapagos Islands, Humboldt Current, El Nino and the Galapagos