State estimation
The California Current System is located at the east of the north Pacific. Equatorward upwelling favorable wind draws nutrient-rich water from the interior to the surface making ocean very productive. In fact, approximately 35% of wild-caught fish come from here (Kudela et al., 2009). I investigated the interaction between wind, upwelling and marine ecosystem using ocean circulation model.
Along the California coast, equatorward wind transports water to offshore, creating divergence and upwelling at the coast. In the open ocean, the divergence and upwelling can occur when the wind-driven offshore transport of water increases as we go offshore. The latter mechanism is referred to the Ekman pumping upwelling and depends on the wind field structure. Equatorward wind can have an abrupt change near the coast, but cliamte models cannot resovle a sharp drop-off of the wind. I wanted to investigate how the upwelling source waters change if we can resolve the sharp drop-off of the wind near the coast.
When equatorward wind stress has a gradual change (left panel in Figure 4), wind stress curl is weakly positive over the broad region near the coast, causing weak and shallow upwelling. Hence, the sources for the upwelling are local waters. In constrast, the sharp drop-off of the wind stress creates strong positive curl near the coast (right panel in Figure 4). The coast upwelling become stronger with the Ekman pumping upwelling, and the deep enough to have the poleward undercurrent as a source. Equatorward flow is located nearer to the coast, and it also supplies water to the upwelling. This result has applications for predicting how interannual and decadal changes in wind conditions alter the source and properties of upwelled waters and, consequently, the marine ecosystem in the California Current. Please refer Song et al. (2010) for more details.
Following is the abstract of “Application of a data-assimilation model to variability of Pacific sardine spawning and survivor habitats with ENSO in the California Current System” in Journal of Geophysical Research, 2012:
The Pacific sardine (Sardinops sagax) showed significant differences in spawning habitat area, spawning habitat quality and availability of survivor habitat as the Pacific Ocean went through the La Niña state in April 2002 to a weak El Niño in April 2003. During another El Niño/Southern Oscillation transition period in 2006–2007 when the El Niño state retreated and the La Niña returned, a similar pattern in spawning habitat quality was seen. The coupling between the atmospheric forcing, the physical ocean states and the properties of the sardine egg spawning are investigated using dynamically consistent data-assimilation fits of the available physical oceanographic observations during these months. Fits were executed using the Regional Ocean Modeling System four-dimensional variational assimilation platform along with adjoint model runs using a passive tracer to deduce source waters for the areas of interest. Analysis using the data-assimilation model runs reveals that unusually strong equatorward wind-forcing drives offshore transport during the La Niña conditions, which extends the spawning habitat for sardine further offshore. A statistical model of sardine spawning habitat shows better habitat quality during the El Niño conditions, which is associated with higher egg densities and corresponded to higher daily egg production. Concentration of eggs is also increased by convergence of water. The results of the source waters analysis using the adjoint data assimilation model support the idea that offshore transport extends the spawning habitat, and show that higher levels of nutrient are brought into the spawning habitat with high concentration of sardine eggs.
Monthly averaged SST shows colder SST under the La Niña state (Figure 5). Since the offshore transport is stronger in La Niña years, upwelled water can be transported further offshore, resulting in gradual cross-shore SST changes. In contrast, the SST in El Niño years shows a pattern parallel to the coastline, which can be interpreted as an evidence of weaker offshore transport. Please refer Song et al. (2010) for more details.