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

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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 Niño-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.

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

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