Comparison between OFES at Station ALOHA and ARGO float 5145

We compare here time series of nitrate, salinity and potential density at Station ALOHA in the 1/10th-of-a-deg. OFES simulation and measured by the ARGO float 5145, the trajectory of which is shown in Fig. 1. Here is a list of observations:

1. The nutricline is deeper by about 25-50 m in the OFES simulation than in the HOT and ARGO data (Fig. 2).
2. There are in OFES events of shallowed nutritate, isopycnal surfaces and salinity (compared to the mean depth over these time series) that are similar in magnitude to those observed in the ARGO float (Figs. 3-4, 6-11 and Fig. 12b).
3. The lifetime and frequency of the events in OFES seems to be larger and lower, respectively, than in the observations. One possible explanation may be due to the difference in sampling. The float drifts between two observations over a distance that may be larger than the scale of the process responsible for the nitrate events, thus reducing their lifetime. Furthermore, because the float is drifting eastward, observations may be Doppler-shifted toward higher frequency.
4. Fig. 5 shows the nitrate in OFES sampled as the ARGO observations. The frequency of the events in this time series seems to be still lower than in the observations. This is confirmed by its power spectrum (Fig. 14), the high-frequency end of it has the same slope than the OFES of Fig. 4 and has lower energy than in observations. Although more spectra would be needed to assess the error and variability in the OFES spectra in Fig. 14, these spectra already suggest that if the events exist in OFES, their frequency is lower than observed.
5. The probability distribution of moderate to large positive and high-frequency nitrate anomalies is remarkably similar in both the OFES and ARGO data (Figs. 11 and 12).

In conclusion, nitrate events similar in magnitude but lower in frequency to those observed in the ARGO data are present in OFES. As in the observations, these events involve up/downwelled isopycnal surfaces. The OFES simulation may thus offer a possible mechanism for such events. However, given that the events are part of a noisy time series (in both the model and observations), one cannot yet conclude that the cause of the events is the same in both, unless some property of the process at play in OFES could be identifed in the observations.

Concerning that process, this note shows that most of the events (both positive and negative nitrate anomalies) in OFES are due to the passage of large and positive nitrate anomalies stirred horizontally by the eddies. Another animation, shown in this note, reveals that local upwelling events of nitrate do occur once in while but most of the anomalies are due to lateral eddy stirring.

Figure 1: Trajectory of ARGO float 5145 from Dec. 23, 2007 to Aug. 20, 2010. Its starting position is shown with an open circle.

Figure 2: Time-mean nitrate measured by the ARGO float (mean from Dec. 23, 2007 to Aug. 20, 2010) and in the

Figure 3: Nitrate measured by the ARGO float.

Figure 4: Nitrate in the 1/10th-of-a-deg. OFES simulation at the location of station ALOHA.

Figure 5: Nitrate in the 1/10th-of-a-deg. OFES simulation with the same temporal and spatial sampling as the ARGO float.

Figure 6: Nitrate and isopycnal surfaces measured by the ARGO float.

Figure 7: Nitrate and isopycnal surfaces in the 1/10th-of-a-deg. OFES simulation at the location of station ALOHA.

Figure 8: Salinity measured by the ARGO float.

Figure 9: Salinity in the 1/10th-of-a-deg. OFES simulation at the location of station ALOHA.

Figure 10: Salinity and isopycnal surfaces measured by the ARGO float.

Figure 11: Salinity and isopycnal surfaces in the 1/10th-of-a-deg. OFES simulation at the location of station ALOHA.

Figure 12: Vertical averaged nitrate measured by the ARGO float and in the 1/10th-of-a-deg. OFES simulation at the location of station ALOHA. The depth range used is 0-200 m for the ARGO data and 0-250 m for the OFES data. In (a), the full temporal structure is shown while in (b) only periods shorter than 200 days are shown.

Figure 13: Probability distribution of the two quantities plotted in Fig. 12b.

Figure 14: Power spectra for the vertically averaged time series of Fig. 12a and the 0-250 m vertical average OFES time series sampled as ARGO shown in Fig. 5.