Interannual spring bloom variability and Ekman pumping in the coastal Gulf of Alaska

A lower trophic level ecosystem model is coupled to a three-dimensional coastal ocean circulation model for the northwestern coastal Gulf of Alaska (CGOA) to investigate regional ecosystem dynamics on seasonal and interannual timescales. By including explicit growth limitation by light, nitrate, ammonium, silicic acid, and iron, the ecosystem model provides a comprehensive framework to investigate the combined role of macronutrients and micronutrients in shaping phytoplankton community structure. On the basis of comparisons with available in situ and remotely sensed observations for 1998 through 2002, the model reproduces the dominant modes of variability associated with the northwestern CGOA ecosystem dynamics. An empirical orthogonal function (EOF) analysis of biological and surface forcing fields suggests that, for the 5-year period, interannual variability in peak chlorophyll concentrations during the spring bloom is related to interannual variability in the wind stress curl over the northern CGOA during the previous winter. Positive wind stress curl anomalies during winter result in enhanced Ekman pumping and, thus, nutrient upwelling on the shelf and at the shelfbreak. Since phytoplankton growth is severely light-limited when Ekman pumping occurs, nutrient upwelling associated with the wind stress curl acts as a priming mechanism for the CGOA shelf, leading to higher peak chlorophyll concentrations during the following spring bloom. Enhanced wind stress curl and Ekman pumping are associated with the presence of a low-pressure system in the northern CGOA, and potentially connected to the large-scale variability of the Aleutian Low in the North Pacific.