Spiciness and dissolved oxygen variability in Oregon shelf, slope and offshore waters, from a 25-year model simulation

Over the past 50 years observations have identified a trend of decreasing dissolved oxygen (DO) in mid-depth slope waters and bottom shelf waters along the west coast of the United States. These long term trends in subsurface DO can be influenced by both local changes, such as increased respiration or less vigorous shelf flushing, and remote effects, including changes in circulation or the characteristics of the source waters that supply the region. A high-resolution, regional-scale, coupled physical biogeochemical model has been developed to simulate seasonal-to-decadal scale variability along the US west coast to discern the dynamics behind these spatial and temporal patterns. Simulations run from 1981 through 2006 with atmospheric forcing and open boundary conditions that incorporate the larger scale physical trends over this 25 year period are found to reproduce the reported pattern of strengthening and shoaling of the California Undercurrent (CUC) along the Oregon coast. Numerous observational datasets, including historical data from NODC, moorings, Argo floats and glider profiles (primarily along the NH-line off Oregon) have been used to verify the skill of the model in capturing these trends. Model results show the increased influence of equatorial relative to subarctic source waters as manifested in increased spiciness and reduction in dissolved oxygen on the slope at the depths of the undercurrent that is expected to coincide with a strengthening CUC. But a portion of the decrease in dissolved oxygen concentration along the Oregon and Washington coast is due to local biogeochemical processes. At undercurrent depths the cumulative effect of repeated low DO summers is a long-term trend of decreasing oxygen levels in slope and North Pacific waters, as far as several 100 km offshore. This causes a decrease in DO of the source waters that replenish the coastal waters at the beginning of each upwelling season.

Over the past 50 years observations have identified a trend of decreasing dissolved oxygen (DO) in mid-depth slope waters and bottom shelf waters along the west coast of the United States. These long term trends in subsurface DO can be influenced by both local changes, such as increased respiration or less vigorous shelf flushing, and remote effects, including changes in circulation or the characteristics of the source waters that supply the region. A high-resolution, regional-scale, coupled physical biogeochemical model has been developed to simulate seasonal-to-decadal scale variability along the US west coast to discern the dynamics behind these spatial and temporal patterns. Simulations run from 1981 through 2006 with atmospheric forcing and open boundary conditions that incorporate the larger scale physical trends over this 25 year period are found to reproduce the reported pattern of strengthening and shoaling of the California Undercurrent (CUC) along the Oregon coast. Numerous observational datasets, including historical data from NODC, moorings, Argo floats and glider profiles (primarily along the NH-line off Oregon) have been used to verify the skill of the model in capturing these trends. The model indicates the increased influence of equatorial relative to subarctic source waters as manifested in increased spiciness and reduction in dissolved oxygen on the slope at the depths of the undercurrent that is expected to coincide with a strengthening CUC. But a significant portion of the decrease in dissolved oxygen concentration along the Oregon and Washington coast is due to local biogeochemical processes. At undercurrent depths the cumulative effect of repeated low DO summers is a long-term trend of decreasing oxygen levels in slope and North Pacific waters, as far as several 100 km offshore, affecting the source waters that replenish the coastal waters at the beginning of each upwelling season.