Oceanographic Anomaly in the Bay of Bengal Poses Challenges to Ekman’s Century-Old Theory

For more than a century, Ekman's theory has been a cornerstone of oceanography, explaining how surface currents are influenced by winds and the Earth's rotation. Yet new research challenges this established understanding. An international team of researchers, affiliated with NOAA, the Indian National Center for Ocean Information Services and the University of Zagreb, observed a unique phenomenon in the Bay of Bengal: marine currents that move to the left of the winds in the northern hemisphere, defying traditional models.

Published in Science Advances, this discovery is based on a decade of data from a buoy anchored off the Indian coast. These results could have major implications for climate models and the understanding of complex interactions between the atmosphere and the ocean.

Ekman's century-old theory

Formulated in 1905 by Swedish oceanographer Vagn Walfrid Ekman, the theory that bears his name is a pillar of modern oceanography. Ekman, then a researcher influenced by work on fluid mechanics and Earth rotation, developed this theory in a context where scientists sought to better understand ocean movements and their role in global climate. His theory explains how near-surface ocean currents are influenced by winds and the Earth's rotation, via the Coriolis effect.

Average diurnal wind speed amplitude (m/s) between 2010 and 2019: (A) December-January, (B) July-August. Vertical hydrographic profiles (CE) at the buoy, average over 6 years (2011, 2013, 2014, 2017, 2018, 2019). © Michael J. McPhaden et al., 2024

In the Northern Hemisphere, this effect deflects currents to the right of the prevailing winds, while in the Southern Hemisphere they are deflected to the left. This mechanism creates a spiral profile known as the “Ekman spiral”, where the direction of the currents gradually changes with depth, until their influence disappears. This discovery not only provided a better understanding of the dynamics of surface currents, but also laid the foundations for modern models that explain the interactions between the atmosphere and the ocean.

An anomaly in surface currents, Ekman revisited

But today this theory is being called into question. The authors of the new study analyzed data collected by a buoy anchored at 13.5°N in the Bay of Bengal. They then surprisingly observed an unexpected deviation. Indeed, surface currents move to the left of the winds, thus defying established predictions. This buoy, deployed several hundred kilometers from the Indian coast, made it possible to collect data over a period of several years. It therefore covered different seasonal cycles and climatic conditions, reinforcing the robustness of this discovery.

The phenomenon is particularly visible during the southwest monsoon, between July and August. At this time, daytime land breezes, of remarkable regularity, extend 400 to 500 kilometers offshore. The winds, reaching speeds between 1 and 2 m/s, thus contribute up to 15% of the total wind speed in the region. The interaction between these winds and the specific characteristics of the ocean in the Bay of Bengal is based on marked stratification, where layers of water overlap depending on their density. A stable thermocline, characterized by a rapid transition between warm surface waters and cold waters at depth, acts as a natural barrier. These conditions reinforce the effects of surface winds while limiting exchanges with deeper layers, contributing to the formation of these atypical currents observed.

Unlike the patterns predicted by Ekman, where currents should follow a spiral oriented to the right of the winds in the Northern Hemisphere, these data suggest the existence of unique local and dynamic forces capable of modifying these dynamics. These results actually reveal gaps in current oceanographic models.

A new perspective on ocean-atmosphere interactions

This discovery highlights the complexity of interactions between the atmosphere and the ocean, in particular, super inertial flows. They are characterized by oscillations at frequencies higher than the local inertial period. The latter corresponds to the time necessary for an object subjected to the Coriolis force to complete a complete oscillation.

These currents, influenced by diurnal winds rotating clockwise, therefore show a unique dynamic. In the case of the Bay of Bengal, the marked stratification of the ocean, with a shallow mixed layer overlying a stable thermocline, plays a central role, as mentioned previously. This vertical gradient prevents deep mixing and promotes amplified surface responses. Responses contradict the Ekman spirals usually observed in more homogeneous oceans.

Land breeze vectors in the Bay of Bengal (July-August monsoon). Red: winds above the daily average; blue: lower. Box: currents (red) and wind stress (black) near the buoy (blue dot). © Michael J. McPhaden et al., 2024

By refining Ekman's original equations to include these specific conditions, the researchers were able to explain their observations. Currents influenced by clockwise diurnal winds can be directed to the left if the period of these winds is much shorter than the local inertial period. This also highlights the role of turbulent friction and pressure gradients in the formation of these flows. By examining variations in temperature, salinity and density collected by the buoy, the team was able to associate these phenomena with specific atmospheric conditions. And in particular the regular land breezes. They confirm the existence of dynamic processes hitherto ignored in standard models.

Implications for science and climate models

The implications of this discovery extend far beyond regional oceanography. Michael McPhaden, senior scientist at NOAA, insists that it sheds light on previously little-known ocean dynamics that could have profound implications for several disciplines. Nearly a third of the world's population depends on agricultural rains in Asia, largely influenced by the monsoons. A better understanding of wind-current interactions could refine their forecasts.

Beyond its theoretical interest, this discovery could also transform practical applications in critical areas. Marine biogeochemical cycles, which regulate processes such as primary production and nutrient transport, could be better integrated into models with more accurate predictions of ocean fluxes.

Furthermore, disaster management tools, such as oil spill response plans or search and rescue operations, could benefit from this progress. Unexpectedly deviating currents can influence the dispersion of pollutants or debris, making emergency responses more complex.

Finally, the researchers anticipate that data from future satellite missions, such as NASA's “Ocean Dynamics and Surface Exchange with the Atmosphere” mission, will help validate and complement these observations. These satellites, capable of simultaneously measuring winds and currents at a resolution of 5 km, will provide critical data to understand and anticipate these anomalies in other regions of the globe.

Source: Michael J. McPhaden et al., “Ekman revisited: Surface currents to the left of the winds in the Northern Hemisphere”, Science Advances (2024).