Interannual variations of Atlantic tropical instability waves

Abstract

Observations are used to develop metrics for interannual tropical instability wave (TIW) variability in the Atlantic and to relate that variability to larger scale processes. The analysis is partitioned into different latitude bands to distinguish between off‐equatorial (5°S, 2°N, and 5°N) and near‐equatorial (2°S and 0°) TIWs. TIW metrics based on sea surface temperature (SST) and sea level anomaly (SLA) fluctuations are compared against interannual anomalies of SST in the cold tongue region. To examine the role of barotropic shear instabilities in modulating the intensity of a TIW season, wind stress and near‐surface current indices are developed in regions where the shear between the Equatorial Undercurrent (EUC) and the northern branch of the South Equatorial Current (nSEC) and between the nSEC and the North Equatorial Countercurrent (NECC) are expected to be largest. Good agreement is found between the SST and SLA TIW metrics along the off‐equatorial latitude bands, and interannual variations of both metrics can largely be attributed to barotropic shear instabilities. In particular, years with low (high) TIW variance along the off‐equatorial latitude bands are associated with anomalously warm (cold) SSTs in the cold tongue region, weak (strong) wind stress divergence and curl in the EUC‐nSEC region, and weak (strong) zonal current shear in the nSEC‐NECC region. In contrast, in the near‐equatorial latitude bands, poor agreement is found between interannual TIW activity based on the SST and SLA metrics, and near‐equatorial TIW variability cannot be explained by the large‐scale SST, wind stress divergence and curl, and current shear indices.