Analysis of ARGO float with climatologies

NOTE #1: I realized that the increase of nitrate between days 200 and 500 (see Fig. 1a) does not repeat one year after (days 565 to 865) suggesting that this increase might not be due to seasonal variability (except if that variability disappears further east, which is highly doubtful). Thus, no climatology might have such increase. It would be interesting to know if this increase also occur in potential density. Due to the strong seasonal cycle and the eastward increase, it is hard to tell.

NOTE #2: I cannot reproduce the time series of the vertically-averaged potential density shown in Fig. 1b. I may have done a mistake. Disregard it.

The goal, here, is to try to see if the annual/intraseasonal variability in nitrate and isopycnal surfaces measured by the ARGO float 5145 (Fig. 1) are present in any climatology (WOA05, HOT, APDRC ARGO) in order to remove it and isolate the nitrate events. The trajectory of the ARGO float is reminded in Fig. 2.

Figure 1: (a) nitrate, (b) potential density σ and (c) minus dissolved oxygen measured by the ARGO float 5145Hawaii and vertically averaged over the upper 200 m. Notice that the dissolved oxygen does not follow well the nitrate variability. The 100% saturation needs to be remove and, given Johnson et al. (2010)’s figures, I expect the variability of the two time serie to be much closer.

WOA05

We start first with the WOA05 climatology. It was found that sampling the monthly climatology as in the float data results in a noisy time series; this is because the climatology has large month to month variation in regions of low nitrate such as in the North Pacific subtropical gyre. I, thus, calculated instead the mean nitrate climatology and sampled it in space as the float data. This time series is compared in Fig. 3 to the nitrate measured by the float. We see that, if the climatology is right, the low-frequency changes in the float time series are not due to the float displacement.

However, we do see in the mean climatology, that the eastward float displacement should result into a shallowing of isopycnal surfaces (Fig. 4) so that this trend should be remove to define the anomalies in potential density σ. I confirm that in the OFES model (which was initialized with WOA98), the time-mean isopycnal surfaces shallow eastward while the nitrate surfaces do not (not shown). The same is true for dissolved oxygen that increases eastward (Fig. 5). We can see that increase in the flota data (Fig. 6; units are different between Figs. 5 and 6). There is also a clear seasonal cycle in dissolved oxygen that needs to be removed.

Figure 2: Trajectories of the ARGO float 5145Hawaii up to Aug. 2010. Its starting point is shown with an open circle.

Figure 3: Nitrate (a) measured by ARGO float 5145Hawaii up to Aug. 2010 and (b) from mean WOA05 climatology sampled in space as the float data. See float_analysis_2.m and float_analysis_2.mat in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/FSLE/analysis/Johnson_etal_09.

Figure 4: Potential density σ (a) measured by ARGO float 5145Hawaii up to Aug. 2010 and (b) from mean WOA05 climatology sampled in space as the float data.

Figure 5: Dissolved oxygen from mean WOA05 climatology sampled in space as the float data.

Figure 6: Dissolved oxygen measured by ARGO float 5145Hawaii.