Reconstructability of Open‐Ocean Upper‐Layer Dynamics From Surface Observations Using Surface Quasigeostrophy (SQG) Theory

Abstract

AbstractAlthough previous studies suggest that the surface quasigeostrophy (SQG) theory is practically applicable for subsurface reconstruction from sea surface observations, they mostly examine the reconstructability of density but seldom evaluate the velocities. Meanwhile, these studies all focus on the regions of energetic kinetic energy, and observational validation of the SQG reconstructability has been lacking in relatively quiescent mid‐ocean regions. We perform such validation in a typical open‐ocean area of the Northeast Atlantic, using a SQG‐based “interior plus surface quasigeostrophic” (isQG) method. The inputs to isQG include satellite‐derived sea surface height, temperature and salinity. The reconstructed density and horizontal velocities are assessed against long‐term in situ observations. Overall, reasonable isQG reconstructability is found during the period with elevated mesoscale activities. In particular, when satellite‐derived sea surface density anomaly (SSDA) is consistent with its in situ counterpart, the reconstructed density captures the observed mesoscale signals reasonably well, including the period with the density anomaly changing sign with depth. However, the density reconstruction is considerably degraded when the satellite‐derived SSDA has large uncertainties. The horizontal velocity reconstruction is less affected by the SSDA uncertainty, and shows a better resemblance to the observation. Using the reconstructed variables, we further diagnose vertical velocities, wisQG, via the three‐dimensional QG omega equation, and compare them to the mooring‐based vertical velocities, wmo, estimated through the nondiffusive density equation. During a 1‐month period with the best reconstructions of density and horizontal velocity, wisQG well reproduce the persistent downwelling pattern of wmo associated with the geostrophic kinematic deformation forcing.