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12.22.10: Notes on “Role of late winter mesoscale events in the biogeochemical variability of the upper water column of the North Pacific Subtropical Gyre” by Letelier et al. (2000) and “Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre” by Letelier et al. (2004)

“During March and April 1997 these instruments recorded significant temporal changes in the upper water column light field, associated with a strong upward doming of the isothermal field.”

“Sediment-trap derived flux estimates, water column nutrient analyses, and inverted echo sounder records have independently documented sporadic mesoscale perturbations at Station ALOHA [Karl et al., 1996; Chiswell, 1996].”

The shoaling that they observe lasted more than one month. longer than the events seen in the WHOTS data (< 10 days). The diameter of the feature is 100-150 km, so a little larger than the scale of mesoscale features believed to cause the shoaling in the WHOTS data (this statement is based on the scale of the features seen in the HYCOM model and the relatively short period of the events). Maybe the feature observed in this paper was just very slow to propagate through station ALOHA.

An important observation is the peak in chlorophyll happens about 1 month after the moments of shallowest isopycnal field, thus suggesting that we cannot trust satellite-derived surface chlorophyll as a proxy for the input of nutrients into the surface layer.

“estimates of phytoplankton nutrient availability based on variations of the nutrient depth distribution must take into account the evolution of the light field resulting from temporal changes in phytoplankton biomass. In general, present models estimating nitrate injections into the euphotic zone as a consequence of eddy activity assume a fixed euphotic zone depth [McGillicuddya nd Robinson, 1997; Oschlies and Garcon, 1998].”

“Although month-to-month variability is significant during all times of the year, large nutrient injections into the upper 100 m are usually recorded as single cruise events during late winter.”


In Letelier et al. (2004), the seasonal variation of the depth of the nutricline at station ALOHA is shown and explained as due to the variation in light intensity (see p. 515-516 and their Fig. 9).

“If, instead of integrating to a fixed depth, we integrate from surface to the depth of this isolume, then the seasonal pattern in [NO21 NO2] disappears.”

I suggest I do this to remove the seasonal effect in the nutricline and focus on rapid incursion of nutrient-rich water into the euphotic layer.