Heat And Freshwater Controlling Processes On The Northern Gulf Of Alaska Shelf

Abstract

We examined conditions and processes that control the distribution of heat and freshwater on the northern Gulf of Alaska (GOA) shelf. Cross-shelf heat gradients are weak throughout the year, while salinity gradients are substantial due to the impact of coastal freshwater runoff. Outer shelf water properties are influenced by large anticyclonic eddies, while the inner and middle shelves may be regulated by wind and freshwater runoff dynamics around the Alaska Coastal Current (ACC). On the outer shelf, anticyclonic eddies propagate from the eastern GOA southwestward along the continental slope, where they favor on-shelf (off-shelf) transport of saline and nutrient-rich (fresh and iron-rich) waters Certain along-shelf locations are identified where low-salinity coastal waters are found near the shelfbreak within reach of eddies and may be regions of enhanced cross-shelf freshwater transport. The eddies have lifetimes of ~5 years and increase in size and sea level anomaly west of the Seward Line, which implies more vigorous eddy cross-shelf exchange in the northwestern GOA. By comparison, on the inner shelf the heat and freshwater distribution is dominated by large coastal river runoff, which forces the ACC and controls the vertical distribution of temperatures through stratification. In May 2007, the coastal GOA revealed some of the lowest ocean temperatures since the early 1970s, initiated by strong atmospheric cooling and reduced coastal runoff in November 2006. Stepwise regression shows that 81% of the variability of deep temperatures is explained by salinity stratification and air-sea heat fluxes. Weak baroclinic flow in May 2007 likely aided the cooling through reduced along-shore heat transport. A more detailed examination of heat transport indicated that along-shore heat flux convergence in the ACC may re-supply 10-35% of the heat removed by air-sea fluxes throughout the coastal GOA cooling season, while the annual mean cross-shore heat flux convergence is insignificant. Spatial gradients show increasing heat fluxes from off- to on-shore and from east to west. The cross-shore gradients result from wind speed gradients due to ageostrophic near-shore wind jets near coastal mountains, while the along-shore gradients result from larger-scale pressure systems. While the ACC advects coastal freshwater around the GOA shelf its waters are subjected to disproportional heat loss west of the Seward Line.