1. Regional circulation in the Monterey Bay region using a nonhydrostatic model Yu-Heng Tseng 1, Larry Breaker 2 and D. E. Dietrich 3 1 Computational Research Division, Lawrence Berkeley National Laboratory, USA (yhtseng@lbl.gov) 2 Moss Landing Marine Laboratory, USA (lbreaker@mlml.calstate.edu), 3 Acu Sea Inc., USA (dietrich@nmia.com) Seasonal variability Seasonal mean velocity fields at several depths are shown in Figures 2-5 for a simulation year (vertical velocity is represented by color contours). In the spring, defined as March to May (Figure 2), the equatorward flow strengthens and dominates the flow from surface to depth 100m. This equatorward flow is upwelling-favorable. A weak cyclonic eddy is observed within MB. There is also a large-scale, anti- cyclonic eddy around 50 km from the coast that extends down to depth 400 m where the eddy is stretched significantly by the coastal bathymetry. Acknowledgements This work is partially supported by a DOE SciDAC climate project and a NERSC Program. Abstract The regional circulation in the vicinity of Monterey Bay, California is complex and fully of variability. We use a one-way coupled, non-hydrostatic DieCAST ocean model to simulate the regional circulation in this area. It has been found that seasonally varying local wind stress, topographic irregularities, coastal upwelling and open ocean condition are significantly important. Satellite images often show a cyclonic eddy in the bay and an anti-cyclonic eddy outside the bay during spring and summer. An anti-cyclonic eddy is also observed in the deep Canyon all year round (located approximately 50 km offshore of Monterey Bay). The simulation compares well with observed HF radar-derived velocity data and satellite image during a typical upwelling event. It is found that the upwelled water flows along the Monterey Submarine Canyon walls and then quickly spreads and mixes with surrounding water. The deep circulation enhances the mixing significantly. We further investigated the regional circulation by comparing with the case where the deep canyon has been removed. Stronger cyclonic circulation and weaker vertical velocity is observed in the case without the deep canyon during upwelling season. Where does the upwelled water go during upwelling events? Reference [1] Tseng, Y.-H., D.E. Dietrich and J.H. Ferziger. 2005. Regional circulation of the Monterey Bay region: hydrostatic versus nonhydrostatic modeling. J. Geophys. Res.-Ocean, 110: doi:10.1029/2003JC002153. [2] Tseng, Y. H. and J. H. Ferziger. 2001. Effects of coastal geometry and the formation of cyclonic/anti-cyclonic eddies on turbulent mixing in upwelling simulation J. Turbul., 2: 014. [3] Tseng, Y. H. and J. H. Ferziger. 2003. A ghost-cell immersed boundary method for flow in complex geometry. J. Comput. Phys., 192: 593-623. [4] Rosenfeld, L.K., Schwing, F.B., Garfield, N. and Tracy, D.E., Bifurcated flow from an upwelling center – a cold-water source for Monterey Bay, Cont. Shelf Res., 14, 931-964, 1994 [5] Paduan, J.D. and L.K. Rosenfeld, 1996. Remotely sensed surface currents in Monterey Bay from shore-based HF radar (CODAR), J. Geophys. Res., 101: 20669-20686. Introduction Monterey Bay (MB) is located 100 km south of San Francisco between 36.5° and 37° N along the central California coast. Many studies have examined the circulation and its related processes (e.g. upwelling) due to its high biological productivity. When coastal upwelling occurs, vertical motion is the most important aspect of the process. Particularly, when cold and dense water is upwelled to the surface, an upwelling front forms and density inversion subsequently occurs. It is found that hydrostatic models can not handle the density inversion and resulting instability which is fully nonhydrostatic. Our previous work found that mixed baroclinic /barotropic instability and Reyleigh-Taylor instability contribute significantly to the development of filaments/meanders [1,2]. Using a nonhydrostatic ocean model, we study the seasonal flow pattern in the vicinity of MB using realistic topography, and then investigate the role of Monterey Submarine Canyon (MSC). We also focus on the impact of MSC on an upwelling event and the vertical velocity. We used the non-hydrostatic version of Monterey Bay area regional model (MBARM). Figure 1 shows the model domain. The MBARM is based on the z-level, mixed Arakawa A and C grid, fourth-order accurate Dietrich/Center for Air-Sea Technology (DieCAST) ocean model. The MBARM is one-way coupled Figure 1 Model setup to a larger scale California Current System (CCS) model and uses the immersed boundary method [3] to represent the coastal geometry and bathymetry. The model details can be found in [1]. Figure 2 Figure 3 Figure 4 Figure 5 The modeled spring circulation pattern is similar to that observed from the satellite image [4] and HF radar-derived velocity field [5]. The subsurface northward flow exists below depth 200 m, which is consistent with the year round northward California Undercurrent (CU). In summer (Figure 3), the southward flow in the upper ocean strengthens and tends to move offshore, forming the filaments observed in the satellite images. Point Sur is the location where the offshore flow is most significant. Our simulation shows this to be an effect of local topography on the enhancement of flow toward steep bathymetry and the steering effect of the Monterey headland. Autumn is the season in which the dominant flow changes from equatorward to poleward in the upper ocean (Figure 4). By October, upwelling favorable circulation occurs much less frequently, and near-surface flow along the central coast is under the influence of the northward flowing Davidson Current (DC). The seasonal-averaged velocity fields show that the northward flow intensifies and occurs at all depths. Figure 5 shows the mean flow in wintertime. The deep anti- cyclonic eddy still persists in the MSC while the DC and CU flows close to the California coast. Note that larger vertical velocity (either downwelling or upwelling) is observed in the MSC during upwelling season particularly. Figure 6. x: offshore distance (°); y: depth (m) (a) (b) (c) (d) (e) Figure 6 shows the contiguous 12 hours averaged vertical velocity at the cross section cut through the MSC center (dash line in Figure 1) during spring upwelling event. The panels (a)-(e) represent different time periods. Vertical velocity larger than 0.01 cm/s is marked as red while that smaller than -0.01 cm/s is marked as blue. Such velocity has been observed in Monterey Canyon during upwelling Figures 7-9 show the instantaneous horizontal temperature and velocity fields at several depths for three different days during the same time periods. Not only vertical overturning is important as shown in Figure 6, but also the mixing at all layers appears dominated by horizontal advection. It is clear that the upwelled water spreads and mixes at all depths. Particularly, the anticyclonic eddy inside the MSC significantly enhances the mixing. It is clear that the water would enter MB and follow the topography within the interior of the canyon until certain levels (cyclonic eddy is observed inside the bay, e.g. day 10). This is also associated with a strong anticyclonic eddy offshore. The deep circulation and deep upwelling confirms the early observation results. In the coastal ocean, the circulation is strongly affected by the topography. With the numerical model, we can assess the importance of such effects. The influence of the MSC is particularly important due to its steep topography. In order to emphasize the importance of the canyon on the general circulation in the vicinity of MB, we performed a nonhydrostatic simulation with everything exactly the same except for the bottom bathymetry. We eliminated MSC by seasons and could potentially bring the deep water up a few hundred meters within one day. The contour interval is 0.001 cm/s. Positive vertical velocity is seen along the canyon slope frequently. The upwelling could occur (1) inside the MB; (2) in the MSC; (3) both inside MB and MSC. The occurrence depends on the strength/direction of the upwelling flow and also is associated with larger downwelling offshore and density inversion. Some local vertical overturns are found inside MB and MSC. Note that downwelling flow is found near the upstream rim of the canyon. Downwelling flow in this region is frequently observed in the field [6] during upwelling season. The overturning implies complicated flow patterns and Figure7: day 5 Figure 8: day 10 Figure 9: day 15 enhanced mixing. The current study shows that the topography of MSC may have significant influence on the local circulation, including vertical transport, and thus an enhancing turbulent mixing. In the next section, we further compare the numerical simulation of MB with and without the MSC. Canyon effects-with/without canyon replacing it with a flat bottom at depth 580 m. The bathymetry in the absence of the canyon is illustrated by the bold red line (Figure 1). Figure 9 shows the spring upwelling season mean flow configuration at several depths without the Canyon. No significant difference is observed, as expected, except a stronger cyclonic eddy inside Figure 10 the MB and smaller vertical velocity. Less horizontal energy is transfer to vertical energy inside the MSC, which reduce the mixing strength. The vertical velocity magnitude is also smaller. This implies the existence of canyon indeed increase the vertical velocity (overturning) and vertical mixing.