Results

1. An observational test of the diffusive nature of eddy heat fluxes in the surface layer Arnaud Czaja, Imperial College, London. a.czaja@imperial.ac.uk Abstract Analysis of satellite-derived SST data suggests that Southern Ocean eddies evolve within a non uniform “environment”. This situation is in sharp contrast to that observed in the atmosphere and suggests that observed oceanic eddy heat fluxes in the surface layer can not simply be related to “environmental gradient”, i.e., are fundamentally non diffusive. The much larger heat capacity of the oceanic surface layer is proposed to be responsible for this difference. Motivation In the diffusive model, the perturbation temperature associated with the meridional displacement L of an eddy is simply related to the “environmental gradient”, One assumption behind this is that Fig. 1 Data & Method Seven years (2002-2009) of AMSR-E microwave SST were used (Wentz et al., 2000), with a spatial resolution of about 50km. The parameter ε was estimated using a 2D Taylor expansion rather than the 1D illustrative case described in Fig. 1. The eddy displacement was chosen along the SST gradient with a length scale L = 100km. For averaging purposes, the ACC region was defined according to geostrophic streamlines going through Drake Passage (black lines in Fig. 2). For the atmosphere, low level (850mb) temperatures were taken from the NCEP reanalyses over the period 1980-2008. The eddy displacement length scale was set to L = 1000km. For both ocean and atmosphere, the environmental temperature was set to the time mean temperature field in austral winter. which physically reflects that eddies “see” a uniform environmental gradient, or, equivalently, that there exists a scale separation between the eddies and the “environment”. This study is an investigation of the smallness of the expansion parameter ε for surface eddy heat fluxes in the ocean and the atmosphere. Results Time mean SSTs display regions of large fronts but also thermostats associated with mode waters (Fig. 2, upper panel). Large fronts are typically associated with dipolar curvature features (Fig. 2, bottom panel). The expansion parameter ε (i.e., the ratio of the lower and upper panels in Fig. 2) increases as one moves towards the ACC and its poleward flank (Fig. 3). About 40% of oceanic grid points have ε≥1/2 within the ACC (Fig. 4, green curve). This number goes beyond 50% poleward of the ACC (Fig.4, blue curve). As a reference calculation, the cumulative distribution of ε was computed for low level atmospheric temperatures (Fig. 4 magenta curve). In agreement with previous results (e.g., Held, 1999) only 10% of grid points are found with ε≥1/2. Fig. 3 Fig. 2 Interpretation Scale separation, and the resulting diffusive nature of surface eddy heat fluxes, is supported by analysis of atmospheric observations. This is attributed to the strong damping of atmospheric surface temperature anomalies which prevents the eddies to significantly alter their environment. The larger thermal inertia of the ocean leads to a very different situation, with temperature contours folding more easily by advection and ε≈1. References -Held, 1999: The macro-turbulence of the troposphere, Tellus, 59-70. -Wentz et al., 2000: Satellite measurements of SST through clouds, Science, 847-850. Fig. 4