Traditional quasi‐geostrophic modes and surface quasi‐geostrophic solutions in the Southwestern Atlantic

Abstract

We investigate whether the Quasi‐geostrophic (QG) modes and the Surface Quasi‐geostrophic (SQG) solutions are consistent with the vertical structure of the subinertial variability off southeast Brazil. The first‐order empirical orthogonal function (EOF) of current meter time series is reconstructed using different QG mode combinations; the first EOF is compared against SQG solutions. At two out of three moorings, the traditional flat‐bottom barotropic (BT) and first baroclinic (BC1) mode combination fails to represent the observed sharp near‐surface decay, although this combination contains up to 78% of the depth‐integrated variance. A mesoscale broad‐band combination of flat‐bottom SQG solutions is consistent with the near‐surface sharp decay, accounting for up to 85% of the first EOF variance. A higher‐order QG mode combination is also consistent with the data. Similar results are obtained for a rough topography scenario, in which the velocity vanishes at the bottom. The projection of the SQG solutions onto the QG modes confirms that these two models are mutually dependent. Consequently, as far as the observed near‐surface vertical structure is concerned, SQG solutions and four‐QG mode combination are indistinguishable. Tentative explanations for such vertical structures are given in terms of necessary conditions for baroclinic instability. “Charney‐like” instabilities, or, surface‐intensified “Phillips‐like” instabilities may explain the SQG‐like solutions at two moorings; traditional “Phillips‐like” instabilities may rationalize the BT/BC1 mode representation at the third mooring. These results point out to the presence of a richer subinertial near‐surface dynamics in some regions, which should be considered for the interpretation and projection of remotely sensed surface fields to depth.