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Isopycnal analysis of float 5145 data

Summary

Over the first 1.4-year of float data that are available, the isopycnal surfaces have shallowed by about 100 m and the nutrient concentration have decreased by about 1.5 mmol N /m^3. This is followed by a lowering of about 20 cm of the sea surface height. These trends need to be confirmed by statistical calculation but, if confirmed, they need also to be understood and removed before studying the relationship between nutrient upwelling and FSLE.

Results

The depth anomaly of and the nutrient anomaly along isopycnal surfaces have been calculated in the data of the float 5145. There is interannual, seasonal and intraseasonal variability (Figs. 1 and 2). Before studying the relationship between nutrient upwelling events and FSLE, one need to understand and remove first the low-frequency component of the signal.

../../../../../../_images/float_5145_depth_ano.png

Figure 1: Depth anomaly of isopycnals. All figures are made with float_analysis.m in /home/ipu1/userdirs/francois_moli5/RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/Johnson_etal_09.

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Figure 2: Nutrient anomaly along isopycnals

For σ between 24 and 25 kg/m^3 (80-180 m deep on average), there is an inverse relationship, about 70% of the time, between the depth and nutrient anomaly: when the isopycnal surfaces are shallow, the nutrient is lower (consistent with the idea of biological uptake) and when the isopycnal layer deepens, the nutrient is larger (consistent with which process?).

../../../../../../_images/float_5145_depth_ntr_ano_sigma_24to25.png

Figure 3: Nutrient anomaly versus depth anomaly for σ between 24 and 25 kg/m^3 (80-180 m deep on average).

Fig. 4 shows that for σ between 24.5 and 25 kg/m^3 (120-180 m deep on average), there is a 1.4-year linear trend during which nutrient concentration is lowering with the shallowing of isopycnal surfaces and the lowering of the sea surface height. Statsitical calculation of the trend is needed.

../../../../../../_images/float_5145_depth0_ntr0_ano_sigma24_5to25.png

Figure 4: Depth anomaly and nutrient anomaly averaged for σ between 24.5 and 25 kg/m^3 (120-180 m deep on average).

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Figure 5: Depth anomaly averaged for σ between 24.5 and 25 kg/m^3, and sea surface height anomaly (ssha).