Deep Sea Research Part II: Topical Studies in Oceanography
Hydrographic responses at a coastal site in the northern Gulf of Alaska to seasonal and interannual forcing
Introduction
Coastal hydrographic observations spanning nearly three decades at 59°50.7′N, 149°28.0′W (Gulf of Alaska, GAK1) in the northern North Pacific Ocean (Fig. 1) allow the investigation of hydrographic time scales that range from seasonal to interannual. Relatively large seasonal signals in the winds and moisture fluxes are present whereas the interannual time scales might respond to large-scale atmospheric forcing and remote equatorial forcing from El Niño-Southern Oscillation (ENSO) events. The seasonal atmospheric forcing of the northern North Pacific Ocean changes from a strong low-pressure system in winter to a weak high-pressure system in summer (Wilson and Overland, 1986). In winter, high-latitude storms spawn in the western North Pacific Ocean as dry, cold air outbreaks. These storms propagate eastward across the Pacific into the Gulf of Alaska, gaining heat and moisture from the ocean. Along the Pacific Northwest coastline, these storms encounter an extensive barrier of coastal mountains. As they attempt to continue their passage eastward over the mountains, their relatively warm, moist air masses are lifted adiabatically causing very high rates of precipitation, occasionally exceeding 800 cm year−1 (Wilson and Overland, 1986). The steep coastal terrain and relatively narrow coastal drainage area in Alaska does not allow the establishment of major river networks. Instead of entering the ocean as large river discharges, the freshwater enters the coastal waters through a myriad of small coastal streams (Royer, 1982). These sources sum to an annual average of more than 23,000 m3 s−1 for the Alaskan coastline alone, from its southern boundary with British Columbia to 150°W.
In concert with the seasonal precipitation variations, the coastal winds over the Gulf of Alaska undergo large seasonal changes. Low-pressure domination in winter assures the presence of downwelling winds and coastal convergences of the upper-layer waters at that time of the year. This winter convergence helps to maintain a band of low-salinity water along the shore (Xiong and Royer, 1984) that has been identified as the Alaska Coastal Current (Schumacher and Reed, 1980; Royer, 1981). As summer approaches, the storm tracks move northward into the Bering Sea (Whittaker and Horn, 1982) and the strong winter downwelling in coastal Gulf of Alaska is replaced by very weak upwelling.
The relatively low water temperatures (averaging less than 10 °C at the surface) and the range of temperatures enable the density to be more responsive to salinity changes rather than temperature changes. High rates of coastal freshwater discharge in combination with downwelling winds throughout most of the year create nearshore horizontal and vertical coastal stratifications that drive this alongshore flow cyclonically around the basin, averaging about 0.25 Sv (Royer, 1981; Schumacher and Reed, 1980) (1 Sv=1×106 m3 s−1). This coastal current has a width comparable to the internal Rossby radius of deformation, about 10–20 km (Johnson and Royer, 1986). The hydrographic station, GAK1, is located within this coastal current. Shelf depths here are relatively deep; usually greater than 100 m within less than a kilometer off the coast, increasing to several hundred meters across the shelf. This is quite unusual for a shelf to have such a rapidly increasing bathymetry; it is nearly a vertical wall. Farther offshore, bottom topography variations might exert significant control on the circulation, as onshore–offshore transports could be influenced by the numerous troughs and canyons on this shelf.
Section snippets
Heat flux
The paucity of direct measurements of many of the atmospheric parameters over the Gulf of Alaska requires the use of proxy data sets. The solar heat-flux variability is assumed to follow the seasonal changes in solar declination with the maximum of about 250 W m−2 day−1 in late June at the latitude of GAK1 (Bryant, 1997). While the seasonal pattern of sensible heat flux generally follows the solar flux, in winter there can be cold air outbreaks over the region that can extract more than 1000 W m−2 day
Seasonal variability of water temperature and salinity
This description and explanation of coastal temperature and salinity in the northern Gulf of Alaska is an update of an earlier paper that deals with the first look at the seasonal variability in the Gulf of Alaska using less than a decade of data (Xiong and Royer, 1984). Water-column temperature and salinity at GAK1 in 263 m of water have been measured since 1970 at irregular temporal sampling intervals ranging from hours to months. Since 1990, the sampling has been more regular, approximately
Temperature and salinity anomalies
To investigate further the relationships between the potential forcing functions and the hydrography at GAK1, the monthly means were removed from the data to yield the anomalies of temperature (Fig. 12) and salinity (Fig. 13). The temperature anomalies versus time and depth (Fig. 12) have a pattern of negative (lower than normal) anomalies throughout the water column from 1970 until about 1977 when alternating warm and cold episodes were established. The largest temperature anomalies,
Linear trends in water properties
Did significant changes in the water properties at GAK1 occur during the period of these observations? To simplify this analysis, the two-layer water column (0–100 and 100–250 m) is revisited. Linear fits to the temperature and salinity anomalies and the freshwater discharge and upwelling indices (Table 6) reveal highly significant changes in the temperature and freshwater anomalies. The upper-layer (0–100 m) linear temperature anomaly has a slope 0.032 °C year−1, that accounts for 14% of the
Conclusions
Water-column temperature and salinity in the northern Gulf of Alaska respond to seasonal changes of heat flux, wind and freshwater discharge. The responses of the water column to these changes are complicated by the high rates of freshwater discharge and relatively low water temperatures. The maximum freshwater discharge in fall precedes the maximum downwelling winds in winter. Thus, the maximum vertical density structure is created about 3 months prior to the peak alongshore wind stress. They
Acknowledgements
Appreciation is extended to all the scientists and crew who have assisted in making the observations that comprise this data set, but especially the scientists and crew of R./V. ACONA and R./V. ALPHA HELIX. The personnel aboard these ships gathered the vast majority of the measurements and have made the occupation of this hydrographic station part of the seagoing ritual at the University of Alaska. Captains Ken Turner, Mike Demchenko and Bill Rook are singled out for their dedication to this
References (34)
- et al.
Evidence of change in the winter mixed layer in the Northeast Pacific Ocean
Deep-Sea Research I
(1997) - et al.
A comparison of two current meters on a surface mooring
Deep-Sea Research
(1986) - et al.
Interdecadal variability of northeast Pacific coastal freshwater and its implications on biological productivity
Progress in Oceanography
(2001) - et al.
The northern oscillation indexa new climate index for the Northeast Pacific
Progress in Oceanography
(2002) - et al.
Spectral characteristics of sea level variability along the west coast of North America during the 1982–83 and 1997–98 El Niño events
Progress in Oceanography
(2001) - et al.
El Niño-Southern Oscillation & Climate Variability
(1996) - et al.
How does the El Niño generated coastal current propagate past the Mendocino escarpment?
Journal of Geophysical Research
(1997) - et al.
Rapid wastage of Alaska glaciers and their contribution to rising sea level
Science
(2002) - Bakun, A., 1973. Coastal upwelling indices, west coast of North America, 1946–71. US Department of Commerce, NOAA...
Climate Process and Change
(1997)