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Bravo Mooring

The Bravo mooring at ( $56^{\circ} N, 51^{\circ} W$) kept temperature and salinity measurements throughout the winter of 1996-97. The model produces a slow deepening of the mixed layer and a rapid restratification in spring as seen in observation (see Fig. 12). The timing of the deepest convection, mid-March, is consistent within 10 days between the model and real observations. The depth of convection is almost 1500 meters in the data but approximately 1000 meters in the model. Further investigation of the model shows that convection as deep as 1500 meters occurred in the region directly west of Bravo, but not at that site. Also, the surface layers of the model react to the atmospheric forcing too quickly and strongly. A pool of fresh, cold water is present during the onset of convection. A closer look at the salinity restoring field indicates that virtual salt flux is freshening the ocean at this time. In fact, the spring restoring field is very fresh because it is weighted by a summer field with salinity of only 34.2 at Bravo. Even with the very fresh surface properties, this patch of water is statically unstable for 20 days at the height of winter. Although such a cold, fresh patch of water is not observed in data, it has no effect on the continuing increase of mixed layer depth. Previous model runs which completely mixed unstable profiles instantaneously always produced homogeneous patches of convected water (C. Herbaut, personal communication). The convective scheme in this model partially mixes statically unstable profiles every 30 minutes. For the unstable profile found a Bravo in mid-March, the convective scheme would take approximately 120 iterations (2.5 days) to remove the instability in the absence of other forces. The convective scheme can lower the density of surface water by a maximum of $0.01 kg~ m^{-3}$ every 30 minutes. However, a heat loss of $1000 W ~m^{-2}$ directly taken from the upper 15 meters would raise density by roughly $0.03 kg~ m^{-3}$ every 30 minutes. In the absence of strong vertical diffusivity in the model, supercooled surface layers of only 15-30 meters deep are present. Observed convection homogenizes on timescales of 12-24 hours, so an increased convective adjustment frequency is suggested. Secondly, increased vertical diffusivity is recommended for the immediate surface layers. By the end of March, the model water column is homogeneous to 1000 meter depth, although this process takes much longer than observed. Use of a more frequent convective adjustment is advised in order to homogenize this upper layer. By late spring, the model's surface temperature increases rapidly in the upper 50 meters. The temperature is over 6 degrees Celsius while only 3.5 degrees was observed. These problems do not extend to any great depth and are a result of the mixed layer physics again. The KPP model should be able to deepen the seasonal mixed layer in spring and summer by parameterizing the effect of wind stirring. On the other hand, KPP would probably not help homogenize the water column during winter, because it has a very crude convective scheme. In the case of a multi-year model run, the compacted warm layer after restratification may affect convection in subsequent years, but for our purposes here the timing and nature of convection is consistent with data .


next up previous
Next: PALACE Floats Up: Density Structure Previous: WOCE Hydrographic Cross-sections
Jake Gebbie 2003-04-10