12.15.10: Horizontal velocity field, Rossby number and density anomaly during wh01a

Here, I plot the 1-day averaged horizontal velocity field measured during WHOTS-1 (wh01a) by a moored 300-kHz ADCP instrument (Fig. 1). The velocity field is quite independent of depth. Furthermore, it varies highly in time right when the short incursion of deep water into the surface layer occurs (compare Fig. 2c with Figs. 2a and 2b).

NOTA: The estimate of relative vorticity described below may not be good. A quick test using the 1/10th deg. OFES simulation (made with test_zonal_prog.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/WHOTS/analysis on ipu1) reveals that such estimate does not reproduce well the actual relative vorticity.

Also, at these times, the estimated Rossby number is the largest of the record (the relative vorticity was estimated from the zonal derivative of the meridional velocity alone and assuming that the energy is propagating uniformly westward in time with a speed of 8.46 cm/s ± 1.80 cm/s; the estimate using the two extreme speeds are also shown with red dashes). This suggests that the short events occuring in Spring during WHOTS-1 might be qualified as energetic submesoscale structure.

Figure 1: 1-day averaged (a) zonal (U) and (b) meridional (V) velocity measured during WHOTS-1.

Figure 2: Vertically averaged quantities: (a) U between 0 and 114 m, (b) V between 0 and 114, (c) “submesoscale” potential density σ (defined as the variability with periods smaller than one month) and (d) Rossby number estimated from the zonal gradient of V alone and by assuming the energy is propagating uniformly westward. In (d), we use the speed of propagation estimated in this note: the estimate obtained with the mean velocity is shown in plain blue, the estimates obtained with the two extreme speeds are shown in dash red.

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Rossby number computed with WHOTS_Rossby_number.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/WHOTS/analysis on ipu1.