107 Evidence of Jet-Scale Overturning Circulations in an Eddy-Resolving Ocean Model and Observed Argo Trajectories

Tuesday, 27 June 2017
Salon A-E (Marriott Portland Downtown Waterfront)
Qian Li, The Pennsylvania State University, State College, PA; and S. Lee

In a recent study with an eddy-resolving model, a new, jet-scale overturning circulation (JSOC) was discovered in the model’s Indo-western Pacific Southern Ocean (90°E-145°E). The model’s JSOC is driven by eddy momentum flux convergence, has a meridional scale of only a few degrees, and is most notably associated with the Subantarctic Front (SAF). Being driven mechanically, the JSOC is thermally indirect, resembling the Ferrel Cell in the atmosphere, with sinking (rising) motion on the equatorward (poleward) flank of the jet. These sinking and rising motions are evident between ~500 m and the ocean floor, and are connected by poleward motion across the jet.

In this study, Argo trajectories and salinity fields are used to investigate if the JSOC also exists in nature. Utilizing the fact that the Argo floats are parked at ~1 km depth, it is investigated if their trajectories show evidence of pronounced poleward drift across the observed SAF. In the mid-latitude Southern Ocean, the magnitude of the meridional velocity at a depth of 1 km is generally less than 1 cm s-1. However, in the sector of 120E-150E where the jets are persistent and zonal, hence the JSOC is theoretically expected to exist, the meridional velocities calculated from the Argo trajectories reveal pronounced poleward motion across the SAF with magnitudes exceeding 2 cm s-1. To the north and south of the SAF core, ~2 degrees or more distant from it, the same Argo trajectories indicate much weaker meridional motions. This variation in meridional velocity across the SAF is consistent with the presence of the JSOC. The Argo salinity field provides additional support of the JSOC, because it shows a SAF-scale meridional structure consistent with sinking (rising) motion on the equatorward (poleward) flank of the SAF. These findings suggest that the JSOC is present not only in the eddy-resolving model but also in nature, at least in the Indo-western Pacific Southern Ocean.

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