Tropical Instability Waves and Vortices

Tropical Instability Waves and Tropical Instability Vortices (TIV) exert major controls on the ocean dynamics, thermodynamics and biology on intraseasonal to seasonal time scales. To understand the fundamental mechanisms at play, we opted for a Lagrangian analysis of the 3-D circulation of westward propagating TIVs entities in a high resolution Atlantic ocean simulation. The model reproduces the main temperature and velocity features of the tropical Atlantic mean state and the TIVs. Lagrangian diagnostics were used to track the water masses transported in vortices and exchanged with surrounding waters. The 3-D circulation within vortices is consistent with previous observations, dominated by anticyclonic rotation with downwelling and upwelling near the leading and trailing edges of the vortex, respectively. This convergent flow creates sharp gradients at the TIV south-western edge, where vertical mixing is most efficient. While TIVs remain highly dynamically coherent throughout their lifetime, significant exchanges occur with their surrounding, 50% of their water being renewed over one rotation cycle. A detailed investigation of the eddies sources and sinks reveals that they mostly transport southern water zonally, while northern water are mostly passing through or fluxed southward in their lee. A notable source of entrained water is the Equatorial Undercurrent.