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2004 mean vertical flux of nitrate in the subtropical North Pacific in OFES

The 2004 mean vertical flux of nitrate at 120m in OFES is plotted in Fig. 1. This figure should be compared to Fig. 3a of Oschlies (2002) reproduced in Fig. 2. The contours are the same in both. As in the Atlantic, the subtropical gyre is the location of weak vertical flux of nitrate compared to the region to the north, the subpolar gyre, and the equatorial region. Although the mean in OFES was taken only over one year (5 year in Fig. 2), I am still surprised by its much larger noise-to-signal ratio. Am I computing the same quantity? Notice the zonal filaments of mean flux between 180E-140W, 25N-40N. Notice also the large area of downward flux south of the Kuroshio extension (140E-180E, 28N-32N).

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Figure 1: 2004 mean vertical flux of nitrate (vertical velocity times nitrate concentration) at 120 m in OFES.

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Figure 2: From Fig. 3 of Oschlies (2002): “(a) Annual nitrate input into the upper 126 m of the numerical model, averaged over a 5 year integration period. (b) Simulated nitrogen input via horizontal advection of particulate organic nitrogen (PON), i.e., phytoplankton, zooplankton, and detritus. (c) Sum of nitrate input (Figure 3a) and PON input (Figure 3b). Units are mol N m-2 yr-1. For reference, observational estimates for nitrate input by Jenkins [1988] and Lewis et al. [1986] are included as is the position of the Beta Triangle [Jenkins, 1982].”

I have also computed the vertical advection of nitrate at 120 m (vertical velocity times the vertical gradient of nitrate) but its 2004 OFES mean is noisy everywhere and does not have much of a structure; nothing compared to Fig. 4b of Oschlies (2002). Thus, I am wondering how ‘vertical advection’ is actually defined in this paper.

Additional notes (07/06/2010)

  • Fig. 1 needs to be compared to Fig. 4b, and not Fig. 3a, of Oschlies (2002). The comparison is slightly better as the pattern in Fig. 4b is noisier than in Fig. 3a. Furthermore, the smoother pattern in vertical advection in Oschlies’ model might be due also due to the lower horizontal model resolution (1/3-of-a-degree instead of 1/10-of-a-degree) and thus lower energy at the scale (mesoscale) at which the noise in predominant in the present simulation.
  • The mean vertical flux over the years 2004-06 is very similar to that in Fig. 1 so that we might need many more years to get ridd of the noisy pattern.
  • The downward nitrate flux seen in the northern part of the subtropical gyre might be due in part to the unrealistic negative N values that occur in the model. When N is corrected to 0 in these cases, the area of downward flux is reduced, although it still appears.