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Percentage of area where the relative vorticity changes sign between the surface and depth across the equatorial and North Pacific ocean at 140W in OFES

The percentage of points (=area) where the relative vorticity ζ changes between the surface and depth has been calculated along 140W from the equatorial region to the North Pacific in OFES. See the caption of Fig. 1 for the details of the calculation.

The mean stratification is strong within 10 deg. from the equator, is weaker in the subtropics and moderate north of 45N (Fig. 1a). On average between 2 and 20 deg. from the equator, ζ changes sign over about 40 to 50% of the total area (Fig. 1b). This proportion reduces to 30% at the equator and between 10N and 45N and to less than 10% north of 45N. Thus south of 45N, the pattern follows that of the stratification (Fig. 1b) suggesting that the model of Hoskins and Bretheron (1972) with ζ changing sign between the surface and the depth of the thermocline could be relevant for the equatorial ocean, although not entirely: there is still 50% of the area where ζ does not change sign. Notice that the presence of the North Equatorial Countercurrent (NECC) and its strong stratification at 7N-9N do not correspond to a much larger proportion of ζ changing sign between the surface and the depth.

During 2004, the percentage varies with time at the weekly to monthly time scale but the averaged pattern described from Fig. 1is respected most of the time (Fig. 2).

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Figure 1: (a) Mean N^2 where N is the buoyancy frequency. N^2 is averaged between 140W and 137.5W and over 2004. (b) Maximum of N^2 from (a) in red and the percentage of area where ζ changes sign between the surface and depth in blue. The percentage has been calculated for each 3-day snapshots over a series of 2.5 deg. by 2.5 deg. along 140W. The percentage plotted in (b) is the 2004 average. ζ at the surface is defined as the vertical average between the surface and 55 m and ζ at depth as the vertical average between 150 and 320 m. Computed with step=4 using /home/francois/RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/eSQG/analysis/test_OFES/rel_vort_HB72.

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Figure 2: Percentage of area where ζ changes sign between the surface and depth. See details of the calculation in the caption of Fig. 1.