Comparison of unstable manifold and deep Chl maximum at station ALOHA

The time series of the vertical profile of Chl a has been extracted from the HOT website (Fig. 1) and the time series of Chl a averaged between 50 and 150 m is shown in Fig. 2.

Figure 1: Chl a observed at station ALOHA

Figure 2: 50-150 m averaged Chl a observed at station ALOHA

The deep Chl maximum has some interannual variability as well as monthly variability. The backward-in-time finite-size Lyapunov exponent (FSLE) has been calculated for a small region around station ALOHA from T/P sea surface height (SSH) and is shown in Fig. 3 against the Chl time series.

Figure 3: 50-150 m averaged Chl a observed at station ALOHA and backward-in-time finite-size Lyapunov exponent (FSLE) computed from T/P sea surface height abd averaged within 0.1°from station ALOHA (158°W, 22.75°N).

So far, I fail to find any obvious correlation. I tried correlations over portions of the time series, lag correlations and correlations using instead the surrounding FSLE, but every time to no avail. In conclusion, the correlation is not obvious, which should make us cautious about the results obtained with the float.

I do now ask some questions that may explain the apparent lack of correlation:

• Are the upwelling events of nutrients related or not to the events of Chl increase at station ALOHA? If no, then I should compare the FSLE to the upwelling events instead.
• Were upwelling events of nutrients, as those observed by the float, observed also at station ALOHA?

I will read Dore et al. (2008) to have an idea of the relationship between upwelling and Chl, and Lapeyre and Klein (2006) to have an idea of the relationship between upwelling and surface flow.

Also, I am not inclined to use remotely-observed surface Chl, given their apparent poor correlation with the deep Chl maximum in oligotrophic environment. Is there anybody who can disprove this statement? In particular, is there any study that looked at the relatonship between remotely-observed surface Chl and deep Chl maximum at station ALOHA and deduced that remotely-observed surface Chl is a good proxy for the deep maximum?