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12.28.10: Relationship between nitrate, potential density and SSH in the 1/10th OFES simulation

A set of animation is available:

  • animation of nitrate anomalies and potential density anomalies at 163 m: Anomalies are defined as deviations from the time mean.

    In this animation, we see coherent cyclonic eddies transporting from the southeast to the northwest large amount of nitrate. There is about one event a year that passes through the location of the ALOHA station. The density anomaly of the eddy changes from positive to negative during their course: this is because the density inside the eddy is at first order constant while the mean density increases to the north so that the difference decreases when the eddy is propagating to the north (up the mean gradient): This illustrates that a local negative anomaly in density can nonetheless be associated with a cyclonic eddy (positive anomaly at the formation of the eddy).

    There is an interesting event that occurs in Sept.-Oct. 2004 near 25°N and 145°W where the positive nitrate anomaly that has been initially advected from the south is beging reinforced maybe by the local eddy stirring. This type of event is rather rare.

    We might need smaller contours to see the more often but weaker anomalies that are at the latitude of the station ALOHA, the ones that the Hawaii5145 ARGO float has sampled.

  • animation of nitrate anomalies and potential density anomalies at 163 m with smaller contours.

    Notice the difference in regime between south of 22°N dominated by eddies and filaments and north of that line dominated by larger, less idealized structure.

    There are some events in nitrate near the location of station ALOHA that could be due to frontal processes: end of Nov. 2003 near 22°N and 155°W and during Sept.-Oct. 2004 along 155°W between 20°N and 22°N. But again, these events are rare and, at the latitudes of station ALOHA, the nitrate field is mostly correlated with the density field and are dominated by broad and weak mesoscale structures. If the nitrate and density fields behave as in the ARGO and WHOTS data, this suggests that the contribution of frontal processes at station ALOHA may be small due to the nitrate and density contours being parallel there; otherwise, the model underestimates the mesoscale variability and/or incorrectly reproduce the slope of the nitrate surfaces relative to that of density surfaces. See this note ................ for a discussion.

Following Klein et al. (1998), the more the contours of a tracer is slanted with respect to density contours, the larger the tracer gradient along the isopycnal layers and the more efficient the eddy stirring. Fig. 1 and Fig. 2 show the mean σ and nitrate at 163 m. Together with Fig. 2 in this note, we see that there is overall a southward gradient of nitrate along isopycnal layers with the gradient increasing southward; this means that 1) the eddies that mix nitrate downgradient, mix it northward and 2) the efficiency of the stirring and mixing decreases northward where it becomes nearly zero at the latitude of station ALOHA.


Figure 1: 2003-2004 potential density σ at 163 m depth in the 1/10th OFES simulation.


Figure 2: 2003-2004 nitrate at 163 m depth in the 1/10th OFES simulation.

Fig. 3 is a snapshot from the second animation. It shows how well correlated the nitrate and σ fields are, especially at the latitudes of station ALOHA:


Figure 3: Nitrate and σ on Dec. 7, 2003 at 163 m.

See link_ntr_ntr_grad_dens_SSH.m in RESEARCH/PROJECTS/MARINE_BIOLOGY/SUBMESOSCALE_PROCESSES/OFES/link_ntr_ntr_grad_dens_SSH in ipu1 and subdirectories therein.