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Animation of 125-175-m nitrate in 1/10th-deg. OFESΒΆ

The “upwelling events” of nitrate seen at station ALOHA in OFES between in-situ density contours 25.4 and 25.8 kg/m^3 appear to be not events but rather part of the background nitrate that fills up between various anticylocnic structures that have relatively low nitrate level. A large contribution to the background seems to be bue to lateral advection followed by diffusion from the closeby regions rich in nutrients. A vertical-meridional section of nitrate would maybe reveal if the nitrate from the surrounding regions enters the center of the gyre poorer in nitrate along or across isopycnals.

Coming from the rich regions into the poor region are also cyclonic structures that propagate westward and can provide a strong signal in nitrate as shown for instance at a location further east of station ALOHA (lower panel). These observations are confirmed by the fact that relative vorticity and nitrate are positively correlated in space (Fig. 4a).At the depths shown, the nitrate does not seem to be consumed by primary production and appears to be rather conservative.

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Figure 1: (upper) 125-175-m nutrate in OFES for the year 2004. The red dot shows the location of station ALOHA. (middle) Time series of nitrate (colors) and in-situ density (white contours) at station ALOHA (158W, 22.75N). (lower) Same as in the middle panel but further east (150W, 22.5N). These two locations are shown with a black dot in the upper panel and the vertical white dash lines in the middle and lower panels show the time of the snapshot plotted in the upper panel. Computed with RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/OFES/anim_ntr_ALOHA/make_anim.m.