WOCE Hydrographic Cross-sections

A WOCE hydrographic cross-section in the Labrador Sea was most recently occupied in June 1995 and Feb-Mar 1997. First, I will compare model hydrographic data taken in Feb-Mar 1997 with model output. Secondly, I will compare the temperature and salinity measurements taken in June 1995 with the model output for June 1997 despite the two year time difference. The WOCE cross sections took 2 weeks to complete, but I will compare it with the 10-day average field during the most applicable time. The cross-sections give a quasi-snapshot of the convective region. For convection to occur in the Labrador Sea, heat loss to the atmosphere must erode the salinity-stabilized upper layers of the ocean. Warm, salty Irminger Sea Water at depths of 200-700 meters lies directly below the surface-cooled surface water in the early winter. The temperature profile appears unstable, but it is stabilized by the salinity profile. The model shows a mid-depth local maximum in temperature and salinity at 700 meters depth at this time of the year. As the winter progresses, cold and dense surface water gradually erodes the mid-depth temperature maximum. The convection scheme does not leave a density jump at the base of the mixed layer, and therefore acts like nonpenetrative convection. Most observations (Lilly el al., 1999) show this type of convection in the Labrador Sea. As the mixed layer deepens, the mid-depth local maximum is moved to greater depth until it is completely removed in the southwest side of the basin.

Late Winter Hydrography
Late February and early March is the time of greatest convection in the model run, and can be directly compared to the WOCE hydrographic cross-section of Feb. 26-Mar. 7, 1997 (courtesy R. Pickart). It is instructive to compare the model's performance separately in three depth levels: 0-500 meters, 500-1500 meters, and the deeper waters (see Figs. 4-7). Due to a lack of mixed layer physics, the surface temperature is too cold uniformly across the cross-section. The observed cross-section shows a distinct asymmetry in the surface cooling which is not captured by the model. On the other hand, the model's surface salinity has similar values to the observed salinity distribution. The Irminger Current in the northeast side of the transect is slightly too deep and large, but its general T-S values are correct. As previously mentioned, the addition of mixed layer physics may improve the upper 500 meters in a future run. At depths of 500-1500 meters, the model does accurately predict the strongest convection to be in the southwest side of the transect. The newly-formed Labrador Sea Water has the right water mass characteristics : 2.78 degrees Celsius and 34.84 salinity. The structure of the convection above 1500 meters is reasonable. Below 1500 meters, the water mass characteristics are not greatly affected by convection. The model is 0.2 degrees colder than the actual values between 2300-2600 meters depth. This temperature discrepancy is due to the initial conditions. The climatology of M. Visbeck does include Iceland-Scotland Overflow Water (ISOW) as an incursion of relatively warm, saline water at 2300-2600 meters. However, the temperature inversion is not quite strong enough in the initial conditions. Therefore, the model ISOW is colder and more dense than the true ISOW. This will serve as a ``basement'' for any convection in the model. However, Labrador Sea convection has never been observed to break through this layer of ISOW. Deepest convection was observed in 1992 to a depth of 2200 meters (Lab Sea Group, 1998). I think that the model ISOW will not affect the depth of convection in any reasonable situation, and the initial conditions do not need to be ammended at this depth. At depths below 2500 meters, water tends to warm throughout the year because of a slumping of isopycnals from the deep boundary currents. This results from abnormally high levels of friction necessary in GCM's. The deep boundary currents should not play a role in transporting LSW, although the transport of Denmark Strait Overflow Water will be affected, and simple resolution of the problem is not available. The salinity structure has a homogeneous layer from 1500-2000 meters deep at 34.85 salinity. The increased salinity of ISOW is also present. The observed structure is very similar, except for a plume of fresh water in the convective region which extends to 2000 meters. Plumes are typically of very small scale, and are not likely to be captured by a relatively course climatology. I believe that the initialized salinity structure at depth is adequate. In conclusion, attention needs to be focussed on improving the model above 500 meters depth, although the characteristics of the deeper ocean are quite reasonable.

Early Summer Hydrography
Spring is a time of rapid restratification in the upper layers of the Labrador Sea. The model quickly restratifies and then settles into a summer state. June is sometime after the convective season and most of the restratification, so the T-S characteristics should not change drastically during the duration of the transect. There is a thin upper layer of restratified fluid due to the seasonal forcing in both model and observed fields (see Figs. 8-11). Below the surface layer, there is a nearly homogeneous layer of fluid to a depth of 2000 meters. Finally, the deepest waters are restratified to the bottom depth of 3500 meters. Due to the previously documented lack of mixed layer physics, the model's upper layer is too warm and shallow at the surface. Model surface temperature of 6.5 degrees C exceeds observation by nearly 2 degrees. If the model more strongly diffused heat downward in the mixed layer, both problems would be fixed. It appears that the total heat flux from the atmosphere is perhaps too strong in the model. Moving downward in the water column, a nearly homogeneous layer of fluid sits between 500 and 2000 meters. The lateral boundary of the homogeneous layer is nearly vertical, and the northeast side of the transect is colder than observation. The coldness of the northwest side should not be completely considered an artifact of the model's aversion to restratification. Although the model preferentially cools the southwest side of the transect in winter, the northwest side is also cooled considerably and its effect remains in early summer. However, a strong horizontal density gradient should eventually be smoothed. According to one hypothesis, geostrophic eddies are very important in transferring boundary water properties into the interior (Khatiwala and Visbeck, 2000). An ``eddy-induced'' velocity of $0.5 cm/s$ can restratify most of the Labrador Sea within a few months. Because this model does not resolve all eddies, it is probable that a lack of eddies does allow the vertical boundary of LSW to remain until June. Another mechanism for lateral exchange is simple lateral advection, but its effect relative to eddy diffusion is uncertain here (Spall and Pickart, 2000). Not surprisingly, the deepest bottom waters (2500-3500 meters) have almost exactly the same properties in both model and observations. In the salinity fields, the mixed layer has the right depth and correct value (Figs. ). The model salinity field also captures the asymmetry of processes in the Labrador Sea. However, the northwest side of the salinity cross-section also shows the model's difficulty in restratification. In conclusion, the WOCE hydrographic sections prove that the effect of convection in the model is reasonable, but questions the representation of surface layer temperature. Also, the model should produce a greater rate of mixing from the boundary into the interior.