Ekman revisited: Surface currents to the left of the winds in the Northern Hemisphere

Abstract

Ekman’s theory of wind-driven ocean currents on a rotating planet is central to our understanding of why surface currents are deflected to the right of the winds in the Northern Hemisphere and to the left of the winds in the Southern Hemisphere. The theory admits solutions for currents deflected in the opposite direction at periods shorter than the local inertial period, but Ekman did not mention these currents, and they have only rarely been observed. Here, we describe a prominent example of surface flow in the Bay of Bengal directed to the left of clockwise-rotating land breeze wind forcing using multiple years of data from a long-term deepwater surface moored buoy. We further refine Ekman’s theory so as to better reconcile it with our own and previous measurements and then conclude by discussing the broad implications of this work for understanding wind-forced ocean circulation.

Improving Coastal Circulation Using HF‐Radar Surface Currents

“…This assumption is too stringent to hold true even for the best state-of-the-art ocean models and possibly breaks down at all times. In addition, a recent study has shown that the known Ekman dynamics, wherein the ocean currents move to the right (left) of the winds in the northern (southern) Hemisphere, break down at subinertial frequencies (McPhaden et al, 2024). It is also inadvertently assumed within this formalism that contributions from other sources like non-hydrostatic contributions are negligible.…”

Improving Coastal Circulation Using HF‐Radar Surface Currents

“…This assumption is too stringent to hold true even for the best state-of-the-art ocean models and possibly breaks down at all times. In addition, a recent study has shown that the known Ekman dynamics, wherein the ocean currents move to the right (left) of the winds in the northern (southern) Hemisphere, break down at subinertial frequencies (McPhaden et al, 2024). It is also inadvertently assumed within this formalism that contributions from other sources like non-hydrostatic contributions are negligible.…”

Improving Coastal Circulation Using HF‐Radar Surface Currents

“…This assumption is too stringent to hold true even for the best state-of-the-art ocean models and possibly breaks down at all times. In addition, a recent study has shown that the known Ekman dynamics, wherein the ocean currents move to the right (left) of the winds in the northern (southern) Hemisphere, break down at subinertial frequencies (McPhaden et al, 2024). It is also inadvertently assumed within this formalism that contributions from other sources like non-hydrostatic contributions are negligible.…”

Improving Coastal Circulation Using HF‐Radar Surface Currents

“…This assumption is too stringent to hold true even for the best state-of-the-art ocean models and possibly breaks down at all times. In addition, a recent study has shown that the known Ekman dynamics, wherein the ocean currents move to the right (left) of the winds in the northern (southern) Hemisphere, break down at subinertial frequencies (McPhaden et al, 2024). It is also inadvertently assumed within this formalism that contributions from other sources like non-hydrostatic contributions are negligible.…”