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06.30.11: mean, std and snapshot of PV along a particular isopycnal in OFES

I study here the isopycnal σ = 39.28 kg/m^3 that has an averaged depth of 2000 m between 2004 and 2006 and long 60°W (see the study along 60|deg|W). The mean depth of the isopycnal as well as the mean and standard deviation (STD) of planetary and total potential vorticity (PV) along the isopycnal are shown in Fig. 1 to 5. A typical snapshot of planetary PV is shown in Fig. 6. The mean flow along that isopycnal with the mean planetary PV plotted in the background are provided in Fig. 7.

The depth of the isopycnal is consistent with the depth of the isopycnals studied by Gary et al. (2011; their Fig. 4): a dip extending from Cape Hatteras on the western boundary to the western edge of the mid-Atlantic ridge (MAR), with shallow depths in the Labrador Sea (Fig. 1). As in Gary et al. (2011; their Fig. 5), especially the 1/12°FLAME model, the mean planetary and total PV field is homogenized from the 25-30°N to the Labrador Sea (Figs. 2 and 4). It is stratified significantly only in the subtropics and along the western boundary of the Labrador Sea. The standard deviation is weak (Figs. 3 and 5) consistent with the fact that a snapshot of planetary PV (Fig. 6) resembles that of its mean.

Thus, consistent with the explanation of Gary et al. (2011), a parcel could travel from the Labrador Sea to the subtropics with its PV being nearly conserved. The questions that remain are: 1. Why the PV is homogenized over a region much larger than that covered by maximum of eddy kinetic energy? Is it because weak eddy kinetic energy is enough to produce homogenization? Is it because the homogenization is supported also by the low PV of the recently-ventilated water itself? 2. How does water cross the stratified PV region to reach the DWBC of the Labrador Sea? More generally, if water is prevented from reaching the western boundary because of the stratified PV there, why is there a DWBC at all? 4. The NADW parcels ought to change their PV at the southern edge of the homogenized PV region in order to pursue their course to the south within the DWBC. What controls that change, vertical or horizontal friction? How does the change occur, episodically or continuously? 5. do we have the same patterns for the deeper isopycnals?


Figure 1: 2004-2006 mean depth of σ = 39.28 kg/m^3 in the OFES model.


Figure 2: 2004-2006 mean planetary PV along σ = 39.28 kg/m^3 in the OFES model.


Figure 3: Same as in Fig. 2 but for the standard deviation.


Figure 4: Same as in Fig. 2 but for total PV.


Figure 5: Same as in Fig. 2 but for total PV.


Figure 6: A snapshot of planetary PV along σ = 39.28 kg/m^3 in the OFES model.


Figure 7: As in Fig. 2 with the mean along-isopycnal flow shown with arrows. Maximum speed is about 65 cm/s.

computed with OFES_PV_section_along_iso.m in RESEARCH/PROPOSALS/EDMF/Jun11/figures on the main disk. Note that to compute the thickness, I use an increment of 0.2 kg/m^3 which corresponds to the layer going on average from about 1200 to 2600 m. That is a thick layer. It would be interesting to use a much smaller increment.