Hydrographic responses at a coastal site in the northern Gulf of Alaska to seasonal and interannual forcing

Abstract

Abstract Three decades (1970–2000) of hydrographic (temperature–salinity–depth) sampling at the coastal site, GAK1, near (60°N, 149°W) in the northern Gulf of Alaska provides an opportunity to investigate the seasonal and interannual variability within this water column. Over this relatively deep shelf (260 m), the temperature and salinity are forced by solar heating, coastal freshwater discharge, winds and El Nino-Southern Oscillation (ENSO) events. Seasonally, the water temperatures at the surface and bottom change out of phase with one another. From about November until April, there are temperature inversions with thermal stratification increasing from April to August. The upper-layer (0–100 m) salinity closely follows the seasonal freshwater discharge with the annual minimum in October and maximum in March. However, this is in sharp contrast with the lower-layer (100–250 m) salinity that has an April minimum and October maximum. Water density cycles follow the salinity changes at this site, not the temperature changes. The lower-layer salinity cycle is the sum of responses to buoyancy and wind forcing. Maximum freshwater discharge in autumn should enhance the entrainment through the strengthening of both cross-shelf and alongshore pressure gradients, causing a deep intrusion onto the shelf of relatively high-salinity offshore water. The contributions of the very weak, summer upwelling winds to the increased lower-layer salinity are uncertain. The summer is a period when the hydrography on the shelf relaxes, since the non-summer winds are the downwelling-type. They force less saline water downward, diminishing the lower-layer salinity especially in late winter. This downwelling will force relatively warm water downward until the temperature inversion occurs in November. After that, the downwelling will be forcing cooler water downward. This leads to the maximum lower-layer temperature in November. The interannual anomalies of temperature and salinity give insight into the potential forcing of this ecosystem. Correlations between the forcing phenomena of local winds, freshwater discharge, Southern Oscillation Index or patterns of sea-surface temperature (Pacific Decadal Oscillation (PDO)) suggest that interannual subsurface temperature anomalies are linked to ENSO events with a propagation time from the equator to the Gulf of Alaska of about 8–10 months. There are no significant interannual temperature variations in the surface layers (0–50 m) correlated with ENSO events. However, the interannual temperature variability throughout the water column does respond to the interannual variability of coastal freshwater discharge and also follows the PDO. Additionally, the water-column temperature anomalies are well correlated with local winds and regional winds up to about 1000 km eastward, with delayed responses of 3–8 months. The salinity anomalies in the upper layer (0–100 m) correlate inversely with coastal freshwater discharge anomalies with a 1-month delay. In contrast, the behavior of the salinities in the lower layer (150–250 m) is opposite to the surface layers. The deep interannual salinity anomalies increase with increasing freshwater runoff, reflecting a possible strengthened cross-shelf circulation. The salinity anomalies do not follow PDO or ENSO. Winds over the eastern Gulf of Alaska are well correlated with the salinity anomalies, though lags approach 5 years. Interdecadal trends in these coastal temperatures and salinities are consistent with a general warming of the upper layer (0–100 m) of the water column, with temperatures increasing by about 0.9 °C since 1970 and 0.8 °C in the lower layer (100–250 m). During this same time period, the sea-surface salinity decreased by about 0.3 and the upper-layer salinity decreased by 0.06, while the lower-layer salinity increased by about 0.04. Consequently, there is a tendency for the stratification to increase. This has been accompanied by a tendency for less downwelling and increased freshwater discharge. Both of these influences will tend to increase the coastal stratification.