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Animation fof backward-in-time FSLE, SSHA and nitrate observed by the float

In this animation, the upper panel shows the backward-in-time FSLE as calculated from DT-ref SSH, the anomaly of which is shown in the middle panel. The vertical profile of nitrate as observed by the float is shown in the lower panel. The float trajectory is shown in dash white and dark lines in the upper and middle panel respectively. The vertical white dash lines in the lower panel shows the nitrate at the time of the FSLE and SSHA shown in the two other panels.

The nitrate averaged between 0 and 200 m is compared to the backward-in-time FSLE averaged within 0.2° from the float in Fig. 1. Given the animation and Fig. 1, it seems that the nitrate level and FSLE can arguably be correlated.

Poor correlation is found at the beginning of the record (between days 10 and 90) and in the middle (between days 290 to 375). With respect to the later, the high level of nitrate not only corresponds to low level of FSLE but also weak gradient of SSHA. What can nonetheless explain these high values of nitrate? A wind mixing event?

Good correlation seems, however, to be found during the rest of the record:

  • The high values around day 100 corresponds to the passage of a cyclonic eddy,
  • There are some isolated events of high nitrate that corresponds to high value of FSLE (for instance, near days 175 and 275),
  • An interesting part of the observations is between days 200 and 230, best seen in the animation: for about 30 days, the float is moving back west just to the west of a filament that is moving also west at the same speed. Thus, the float is quasi-stationary with respect to the front. During this period, the level of nitrate measured is indeed relatively high and stationary while the level of FSLE is high but declining slowly with time.
  • The five peaks in nitrate from days 410 to 550 seem to corresponds to the five peaks in FSLE that occur either before, after or at the same. This is again best seen in the animation. Before day 400, the float is right on the filament corresponding to the high value of FSLE. Only when the float leaves the filament and travels to its east, the nitrate level becomes high, consistent with the idea that upwelling/downwelling area can be on each side of a filament. The second peak seems to be due to the fact that the float stays close and to the east of the filament. The third peak is due an approaching set of filaments. The fourth and fifth peaks seem to be due to one isolated filament in each case.

Figure 1: 0-200 m nitrate from float observations and backward-in-time FSLE (from RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/Johnson_etal_09/large_files/unstable_manifold_around_float_traj_ref2.mat) averaged within 0.2° from the float.