Stratification, variance and snapshots of relative vorticity (model and eSQG) during 2004 at 160E and 140W¶
The following is a set of figures to understand better the prediction of eSQG method in the subtropics in the OFES model. All quantites are computed within 5 deg. x 5 deg. regions. We first describe the stratification as well as various plots of the variance in relative vorticity, first at 160E then at 140W. We followed with some snapshots of the relative vorticity from eSQG compared to that from the model first at 160E and 140W, then for the region southwest of Hawaii. All snapshots are given for two different times: March 27, 2004 and August 8, 2004. See on the right hand side for contents.
160E¶
The time series of buoyancy frequency in the northern (32.5N) and southern (17.5N) part of the subtropical gyre is shown in Fig. 1, and the 2004 mean is shown in Fig. 2. In the northern part, there is a strong shallow seasonal thermocline and a weak deep (near 600 m) permanent thermocline. In the southern part, there is a relatively weaker shallow seasonal thermocline and a relatively stronger and shallower (near 100 m) permanent thermocline. The transition near 25 N is in accord with the transition between shallow and deep accuracy of the eSQG prediction (see black contours in Fig. 2 as well as Fig. 2a in this post).
Figure 1: Buoyancy frequency squared. The frequency is spatially averaged within each region.
Figure 2: 2004 mean buoyancy frequency squared. The black contours are the 0.5 and 0.7 2004 mean correlation between the model and eSQG for the relative vorticity (see Fig. 2a in this post).
The 2004 mean variance of relative vorticity from the model and from eSQG is shown in Fig. 3. The time series of the variance at the surface, 100 m and 320 m are shown in Figs. 4, 5 and 6, respectively and snapshots of vertical profile are shown in Fig. 7. The eSQG succeeds to reproduce well the mean as well as the variability of the variance, especially north of 30N; at this longitude, this variability is associated with that of the Kuroshio extension (KE). The exponential decay of the variance from eSQG is similar to that in the model in the northern part (Figs. 7a and b) but is weaker in the southern part in winter (Figs. 7c and d). Notice, furthermore, that the eSQG prediction cannot reproduce the small-vertical-scale variation in variance seen at 17.5N.
Figure 3: 2004 mean (spatial) variance of relative vorticity.
Figure 4: Variance of relative vorticity at the surface.
Figure 5: Variance of relative vorticity at 100 m.
Figure 6: Variance of relative vorticity at 320 m.
Figure 7: Snapshots of variance of relative vorticity.
140W¶
The rest of the figures is the same set as previously but for 140W. At 140W, the permanent thermocline is shallow near 100 m in the northern and southern part of the gyre, while the seasonal thermocline in the southern part is more pronounced than at 160E (Figs. 8 and 9). As at 160E, the stratification is weaker in the northern part coinciding with a deeper penetration of the accuracy of eSQG prediction there (Fig. 9).
Figure 8: Buoyancy frequency squared. The frequency is spatially averaged within each region.
Figure 9: 2004 mean buoyancy frequency squared. The black contours are the 0.5 and 0.7 2004 mean correlation between the model and eSQG for the relative vorticity (see Fig. 2a in this post).
As in the east, the eSQG succeeds to reproduce the 2004 mean variance of the relative vorticiy (Fig. 10) as well as its temporal variability (Figs. 11, 12 13). A notable exception are the submaxima of variance that appear between 250 and 500 m near 17N, 27N and 32N that are not reproduced by the exponetially-decaying eSQG variance. The KE region does not exist at 140W and the variance is actually larger in the southern part of the gyre. The high variance near 17.5N is part of a zonal band of relatively high variance that is localized within the gyre (Fig. 15). I do not know why such zonal band exists. The snapshots shown in Fig. 14 show that, as at 160E, the eSQG predictions do not reproduce the small-vertical-scale variation in variance but does reproduce the overall decrease of the variance with depth, even if the local “effective” buoyancy frequency has not been used.
Figure 10: 2004 mean (spatial) variance of relative vorticity.
Figure 11: Variance of relative vorticity at the surface.
Figure 12: Variance of relative vorticity at 100 m.
Figure 13: Variance of relative vorticity at 320 m.
Figure 14: Snapshots of variance of relative vorticity.
Figure 15: 2004 mean variance of relative vorticity at the surface. High values are probably meaningless and linked to topography features such that the Hawaiian Islands or continents in the north-east and north-west.
Snapshots of relative vorticity at 162.5E and 137.5W¶
162.5E, 17.5N on 03/24/2004¶
Figure 16: Snapshot of relative vorticity near 162.5E, 17.5N on 03/24/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
162.5E, 17.5N on 08/12/2004¶
Figure 17: Snapshot of relative vorticity near 162.5E, 17.5N on 08/12/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
162.5E, 32.5N on 03/24/2004¶
Figure 18: Snapshot of relative vorticity near 162.5E, 32.5N on 03/24/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
162.5E, 32.5N on 08/12/2004¶
Figure 19: Snapshot of relative vorticity near 162.5E, 32.5N on 08/12/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
137.5W, 17.5N on 03/24/2004¶
Figure 20: Snapshot of relative vorticity near 137.5W, 17.5N on 03/24/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
137.5W, 17.5N on 08/12/2004¶
Figure 21: Snapshot of relative vorticity near 137.5W, 17.5N on 08/12/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
137.5W, 32.5N on 03/24/2004¶
Figure 22: Snapshot of relative vorticity near 137.5W, 32.5N on 03/24/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
137.5W, 32.5N on 08/12/2004¶
Figure 23: Snapshot of relative vorticity near 137.5W, 32.5N on 08/12/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
Snapshots of relative vorticity southwest of Hawaii¶
160W, 17.5N on 03/24/2004¶
Figure 24: Snapshot of relative vorticity near 160W, 17.5N on 03/24/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.
160W, 17.5N on 08/12/2004¶
Figure 25: Snapshot of relative vorticity near 160W, 17.5N on 08/12/2004 from the model (left) and eSQG (right): (upper) surface, (middle) 100 m, (lower) 320 m.