A sensitivity study of the dense shelf water formation in the Okhotsk Sea
[1] Sensitivity of Dense Shelf Water (DSW) formation to tidal mixing around the Kuril Straits, wind stress, and river runoff in the Okhotsk Sea is examined by an ice-ocean coupled model. Horizontal resolution of the model is set to 3–8 km in the northern Okhotsk Sea for well resolving the coastal polynyas which is believed to be the principal region of DSW formation. The model shows a good performance in terms of sea ice production and the consequent DSW formation. DSW is also found to be formed in the offshore region apart from the coastal polynya. DSW is defined independently for each experiment such that it identifies the water influenced by brine. By introducing such definition the sensitivity of the DSW formation is assessed separately for change of density and that of formation rate. The density of DSW exhibits high sensitivity to all the elements considered herein, while its formation rate is sensitive only to winds. Winds affect the DSW formation rate mainly by influencing that occurs in the offshore region.
Assessment of the Effects of Tidal Mixing in the Kuril Straits on the Formation of the North Pacific Intermediate Water
Abstract To assess accurately the effect of tidal mixing in the Kuril Straits on the formation of the North Pacific Intermediate Water (NPIW), the spatial distribution of diapycnal diffusivity recently obtained by the present authors is incorporated into an eddy-permitting OGCM. It is shown that the NPIW is successfully reproduced, although the diapycnal diffusivity averaged over the entire Kuril Straits is an order of magnitude less than has previously been assumed as a tuning parameter to reproduce the NPIW in low-resolution OGCMs. This strongly suggests that the effect of tidal mixing in the Kuril Straits on the formation of the NPIW is relatively minor and that the physical processes omitted by the low-resolution OGCMs, such as isopycnal mixing along the Kuroshio Extension region, are much more important. This suggestion gives warning of the danger that some misleading conclusions might be derived from OGCMs that employ diapycnal diffusivity just as a tuning parameter to reproduce the observed features.
Velocity moments in alongshore bottom stress parameterizations
The time-averaged alongshore bottom stress is an important component of nearshore circulation models. In a widely accepted formulation the bottom stress is proportional to , the time average of the product of the instantaneous velocity magnitude |u→| and the instantaneous alongshore velocity component υ. Both mean and fluctuating (owing to random, directionally spread waves) velocities contribute to . Direct estimation of requires a more detailed specification of the velocity field than is usually available, so the term is parameterized. Here direct estimates of based on time series of near-bottom currents observed between the shoreline and 8-m water depth are used to test the accuracy of parameterizations. Common parameterizations that are linear in the mean alongshore current significantly underestimate for moderately strong alongshore currents, resulting in overestimation of a drag coefficient determined by fitting modeled (with a linearized bottom stress) to observed alongshore currents. A parameterization based on a joint-Gaussian velocity field with the observed velocity statistics gives excellent overall agreement with the directly estimated and allows analytic investigation of the statistical properties of the velocity field that govern . Except for the weakest flows, depends strongly on the mean alongshore current and the total velocity variance but depends only weakly on the mean wave angle, wave directional spread, and mean cross-shore current. Several other nonlinear parameterizations of are shown to be more accurate than the linear parameterizations and are adequate for many modeling purposes.