Observed Yanai wave trajectories in the Equatorial Atlantic Ocean

The western equatorial Atlantic Ocean features a variety of dynamical processes. The upper-ocean circulation is characterized by both western boundary currents and the equatorial current system, which are important pathways of the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). On subseasonal to seasonal timescales, equatorial waves further influence local ocean dynamics. Yanai waves (sometimes referred to as mixed Rossby-gravity waves) are observed in all tropical ocean basins on timescales of 10 to 30 days. They are associated with meridional velocities at the equator, setting them apart from other equatorial waves. While Yanai waves have been thoroughly analyzed regarding their energy dissipation, generation mechanisms, and propagation characteristics, little observational evidence has been provided regarding their surface trajectories. This study investigates the trajectories of Lagrangian surface drifters with respect to the presence of Yanai waves. Only few surface drifters remain long enough at or close to the equator to offer insights into equatorial phenomena since the prevailing poleward Ekman flow near the equator typically drives drifters to higher latitudes fairly quickly which makes measurements difficult, but valuable. During a research cruise in May 2023, 8 surface drifters were deployed into the western boundary current system off Brazil along 35°W between the equator and 2.25°S. Three of these drifters became trapped within circling surface velocities centered around the equator which can be attributed to a Yanai wave. One commonly accepted generation mechanism of Yanai waves in the ocean is cross-equatorial fluctuation of the meridional velocity component of the wind. Evidence for fluctuations at the same period as the drifters oscillations was detected in current velocities driven by wind fluctuations. By conducting a series of numerical experiments with artificial drifters, combining the mean background flow of the area with theoretical Yanai wave-induced surface velocities, the observed trajectories can be reproduced. The Yanai wave is characterized by a 14-day period and velocity amplitudes of approximately 0.6 to 0.7 m/s.