The spatial variability of vertical velocity in an Iceland basin eddy dipole

Pidcock, Rosalind, Martin, Adrian, Allen, John, Painter, Stuart C. and Smeed, David (2013) The spatial variability of vertical velocity in an Iceland basin eddy dipole. Deep Sea Research Part I: Oceanographic Research Papers, 72, 121-140. (doi:10.1016/j.dsr.2012.10.008).

Abstract

This paper quantitatively assesses the mesoscale spatial variability in vertical velocity associated with an open ocean eddy dipole. High-resolution, in situ data were collected during a research cruise aboard the NERC research ship RRS Discovery to the Iceland Basin in July/August 2007. A quasi-synoptic SeaSoar spatial survey revealed a southeastward flowing jet with counter-rotating eddies on either side. The anti-cyclonic component was identified as a mode water eddy, characterised by a homogenous core (?35.5 psu and 12 °C) centred at a depth of ?600 m. Vertical velocities were calculated by inverting the quasi-geostrophic (QG) Omega equation at each point in a three-dimensional grid encompassing the dipole. The strongest vertical velocities (up to 5 m day?1) were found primarily in the central jet between the eddies, as fast flowing water was forced over raised isopycnals associated with the large potential vorticity anomaly of the mode water eddy. Weaker upward (downward) vertical velocity was diagnosed ahead of the cyclonic (mode water) eddy in the direction of propagation, reaching 0.5 m day?1 (2.5 m day?1) at the depth of maximum potential vorticity (PV) anomaly. The results demonstrate that the mesoscale velocity field cannot be accurately reconstructed from analysis of individual isolated eddy features and that detailed three-dimensional maps of potential vorticity are required to quantify the cumulative effects of their interactions. An examination of potential sources of error associated with the vertical velocity diagnosis is presented, including sampling strategy, quasi-synopticity, sensitivity to interpolation length scale and the unquantified effect of lower boundary conditions. The first three of these errors are quantified as potentially reaching 50%, ?20% and ?25% of the calculated vertical velocity, respectively, indicating a potential margin of error in the vertical velocity diagnosis of order one.

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