Wind-driven Equatorial Oscillations in the Meridional Overturning Circulation

The great ocean conveyor presents a time-mean perspective on the interconnected network of major ocean currents, transporting key watermass properties around the globe. Zonally integrating the meridional velocities, either globally or across basin-scale domains reduces the conveyor to a 2D projection widely known as the meridional overturning circulation (MOC). Understanding the mean state and variability of the MOC on all timescales has motivated numerous high profile research programmes in the past 2 decades. Both observation and model studies have revealed variability in the MOC occurring over a wide range of timescales and originating from a multitude of processes. Studies using numerical models have shown interesting structure to the variability of the MOC, and also for the MOC to exhibit variability on near-inertial (12-72 hour) timescales. Such studies also indicate a region of enhanced variability near the Equator.

We present analysis of a set of integrations of a global configuration of the numerical ocean model, which show very large amplitude oscillations in the MOCs in the Atlantic, Indian and Pacific oceans confined to the equatorial region. The amplitude of these oscillations is proportional to the width of the ocean basin, typically about 100 (200) Sv in the Atlantic (Pacific). We show that these oscillations are driven by surface winds within 10\degree N/S of the Equator, and their periods (typically 4-10 days) correspond to a small number of low mode equatorially trapped planetary waves. These high frequency fluctuations in the MOC can be accurately simulated by linearising the equations of motion about a stably stratified state of rest and projecting the wind forcing onto the meridional and vertical normal modes. Although no direct observations of these MOC oscillations exist, previous studies using data from the TAO/TRITON identified the existence of equatorially trapped planetary waves in the Pacific basin, exciting vertical displacement of the 500 db dynamic height, giving confidence that this MOC variability occurs in reality.