Some figures related to Johnson et al. (2009, submitted to Nature)

Johnson et al. (2009, submitted to Nature) report observations of vertical profiles of oxygen and nitrate taken by a float around station ALOHA during 21 months. It is reported that events of upwelled nutrient occur 5 to 10 times a year which appears to be consistent with the frequency of high finite-size Lyapunov exponent (FSLE); see this note and Fig. 1 from Paulo.

Figure 1: Unstable manifold near station ALOHA deduced from T/P sea surface height anomaly (from Paulo).

I also computed FSLE for the year 2008 (Fig. 2); I will need to check my computation of FSLE against Francesco’s one.

Figure 2: Unstable manifold at station ALOHA deduced from T/P sea surface height anomaly.

The strongest upwelling event reported by Johnson et al. was on Feb. 11, 2009 and was located at the edge of a cyclonic eddy. Calculation of FSLE shows that this location could be associated with an area of relatively high FSLE (Fig. 3) but calculation of FSLE along the full trajectory of the path is needed before concluding anything.

Figure 3: (a) T/P sea surface height anomaly and (b) unstable manifold deduced from T/P sea surface height anomaly on Feb. 11, 2009. The red dot shows the location of the strong upwelling event observed by the float on that day.