On Eddy Characteristics, Eddy Transports, and Mean Flow Properties

Abstract

Estimates of eddy energy and eddy scales obtained previously from TOPEX/POSEIDON (T/P) altimeter data are interpreted in the context of a baroclinically unstable flow field. From the observations an integral timescale Talt can be defined that?combined with estimates of the eddy kinetic energy?sets a mixing length scale. Results are compared with theories of a baroclinically unstable flow field. For such conditions, the Eady theory predicts a timescale Tbc = Ri/f from the mean-flow Richardson number Ri, which shows some qualitative agreement with T/P results in terms of a geographical distribution. A factor of 2 difference between the timescales can be explained in terms of a systematic difference between the specific definitions of scale estimates. Although transfer length and velocity scales emerging out of scaling arguments lack resemblance with observations, a transfer length scale based on Tbc and the observed eddy kinetic energy is strikingly consistent with observed eddy scales. Primarily independent of the energetic state of the ocean, they are to first order largest in low latitudes and decrease toward high latitudes. Invoking a ?mixing length? hypothesis, an eddy transfer ? for a scalar tracer in the ocean can be estimated from eddy statistics as a function of geographical position. Two different estimates of ? can be obtained from altimetric data: (i) ? = αKELbc and (ii) ?? = α?(g/f)σ(SSH), where α and α? are scaling factors, and σ(?) is the rms sea-surface height variability estimated from T/P data. The approaches lead to similar estimates of a meridional eddy-induced heat and salt flux inferred from climatological meridional temperature and salinity gradient FT = ?cp??T/?y and FS = ???S/?y. Results are consistent with previous knowledge but, because estimates are based on mean meridional gradients, they have to be considered a lower bound on instantaneous eddy transports in the ocean.