11.12.2012: Water exchange between eddies and their surrounding during HD 2012

I study the exchange of waters between eddies and their surrounding during HD 2012. We focus on two particular periods: June 30-July 30 during HD5 (Figs. 1 and 2) and Aug. 16-Sept. 16 during HD8 and HD9 (Figs. 3 and 4). I also study the exchange at two different depths: within the surface mixed layer (SML; 30 m) and below the SML (200 m).

For this, I use the data-assimilating global HYCOM model. The eddies we are focusing on are the strong anticyclonic eddies that were observed during the whole HD 2012. I defined being inside the eddies using a SSHA criterion: SSHA is large than 10 cm. I release parcels inside the eddies (white) and outside the eddies (black) and I follow them for a month.

I also compute the “filament index” (finite-size Lyapunov exponent) that indicate barriers of mixing.

For the first period, there were two anticyclonic eddies located to the north of station ALOHA. I observe that most of the water sampled during HD5 can be considered outside the eddies and not influenced much by the eddies. Around July 14 at 30 m (Fig. 1), water from the western anticyclonic eddy almost arrived at the location of the observations but a barrier to horizontal mixing forms that prevented this.

At 200 m, the eddies stay more coherent than within the SML (Fig. 1). As at 30 m, most of the water sampled during HD5 can be considered from outside the eddies. Unlike 30 m, no strong barrier formed.

Figure 1: Water exchange starting on June 30, 2012 at 30 m. Anticyclonic eddies are defined by SSHA > 10 cm. Parcels inside the eddies are in white, parcels outside the eddies are in black. The background is SSHA. The red contours indicate the “filament index” (finite-size Lyapunov exponent larger than 0.25 1/day) that indicate barrier to mixing. The magenta dot indicates the location of the HD1 observations.

Figure 2: Same as Fig. 1 except at 200 m.

For the second period, we observe a similar situation where water from the anticyclonic eddy approached the location of observations but never reached it because of strong barrier layer (Fig. 3). No qualitative different at 200 m (Fig. 4).

Figure 3: Same as Fig. 1 except for the period Aug. 16-Sept. 16 (HD8 and HD9).

Figure 4: Same as Fig. 3 except at 200 m.