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Shallow-deep events between 210-220E and 20-25N in 2004 - First analysis

Conclusion

Results

This is the first analysis of the shallow and deep events for a set of isopycnal surfaces in OFES for the year 2004. The domain of study is the same as for the ARGO float: 210-220E and 20-25N.

Fig. 1 shows the depths of each isopycnal for the subdomain 214-216E and 22-23N and their respective spatial and time mean. As in the ARGO observations (Fig. 1 of this note), the standard deviation of the depth anomalies has a minimum near 200 m depth.

../../../../../../_images/OFES_depth_isopycnal_214E216E_22N23N_2004.png

Figure 1: All depths of each isopycnal surface for the subdomain 214-216E and 22-23N (black dots) and their spatial and time mean (red line). See analysis_1_script.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/OFES/iso_shallow_deep_events_survey.

Fig. 2 shows the nitrate concentration (N) for the same subdomain and for each isopycnal surface. At depth, the standard devation is small suggesting that N follows potential density there. The standard devation is maximum within the nutricline (100-200 m depth). The distribution of N for each surface is highly skewed; that means the high N values observed are rare. Indeed, animation of N along 24.5 kg/m^3 shows that the high values occured during one single event. This event corresponds to the passage during July of a large deep nitrate event (Fig. 6). This event does not have a signature in SSHA (Fig. 5a) and the nitrate was a maximum trapped inside a cyclonic subsurface eddy (Fig. 5b).

What is interesting is the absence, at least in this subdomain, of large negative N anomalies. At depth, below the euphotic zone (like along 26 kg/m^3), this means that any coherent vortices are bringing water that has a larger nitrate concentration than the local region, consistent with the idea that the subtropical gyre is surrounded by region of high N to the north and to the south.

../../../../../../_images/OFES_ntr_isopycnal_214E216E_22N23N_2004.png

Figure 2: Nitrate concentration (N) for each isopycnal surface for the subdomain 214-216E and 22-23N (black dots) and their spatial and time mean (red line).

Figs. 3 and 4 shows the depth anomaly (DA) versus SSHA for σ = 24.5 kg/m^3 and 26.5 kg/m^3 respectively for the subdomain 214-216E and 22-23N. Note that all large negative DA for σ = 24.5 kg/m^3 and positive DA for σ = 26.5 kg/m^3 that occur with negative weak SSHA occur during the July event. Fig. 5 shows together SSHA and DA for surfaces at the center of the subdomain (215E, 22.5N). Notice that only in few cases, the DA and SSHA covary and not even for the largest events (like the July’s one).

../../../../../../_images/OFES_DA_SSHA_sig24_5_214E216E_22N23N_2004.png

Figure 3: Depth anomly (DA) versus sea surface height anomaly (SSHA) for σ = 24.5 kg/m^3 for the subdomain 214-216E and 22-23N.

../../../../../../_images/OFES_DA_SSHA_sig26_5_214E216E_22N23N_2004.png

Figure 4: Depth anomly (DA) versus sea surface height anomaly (SSHA) for σ = 26.5 kg/m^3 for the subdomain 214-216E and 22-23N.

../../../../../../_images/OFES_DA_SSHA_allsig_215E_25N_2004.png

Figure 5: (a) SSHA and (b) DA for all isopycnal surfaces @ 215E, 22.5N.

../../../../../../_images/OFES_ntr_allsig_215E_25N_2004.png

Figure 6: N for all isopycnal surfaces @ 215E, 22.5N.

../../../../../../_images/OFES_DA_NA_sig24_5_214E216E_22N23N_2004.png

Figure 7: Depth anomly (DA) versus nitrate anomaly (NA) for σ = 24.5 kg/m^3 for the subdomain 214-216E and 22-23N. Red circles show the July observations.

../../../../../../_images/OFES_DA_NA_sig24_5_214E216E_22N23N_2004_2.png

Figure 8: Same as Fig.7 but focused on a different range of NA values. Red circles show the July observations.

../../../../../../_images/OFES_DA_NA_sig26_5_214E216E_22N23N_2004.png

Figure 9: Depth anomly (DA) versus nitrate anomaly (NA) for σ = 26.5 kg/m^3 for the subdomain 214-216E and 22-23N. Red circles show the July observations.

Animations

A set of animation is available: