Observed Yanai wave trajectories in the Western Equatorial Atlantic

Abstract. 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. On subseasonal to seasonal timescales, equatorial waves such as Yanai waves further influence local ocean dynamics. Yanai waves, also referred to as mixed Rossby-gravity waves, exist in all tropical ocean basins predominantly 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 in the western equatorial Atlantic. 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 sparse but particularly valuable. During a research cruise in May 2023, eight 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 got trapped within circling surface movements centered around the equator. Our analysis suggests that this circular movement can be attributed to a Yanai wave. First, we find that the drifter oscillations coincided with cross-equatorial fluctuations of the meridional velocity component of the wind, a commonly accepted generation mechanism of Yanai waves. Additionally, we could reproduce the observed trajectories 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 Yanai wave is characterized by a 14-day period and velocity amplitudes of approximately 0.6 to 0.7 m s^-1.