Overview of the Region
Circulation in the upper layers of the North Pacific Ocean (about the upper 1500 meters) is driven principally by the wind. The ocean surface and near-surface atmosphere transfer heat, moisture, and momentum to one another. Therefore, the principal large-scale patterns of the North Pacific Ocean’s circulation have much in common with the major components of North Pacific winds (Figure 1.1). The northeast trade winds, a band of westward-flowing air, dominate the equatorial and tropical regions of the North Pacific. These trade winds extend across the entire Pacific Basin. In the subtropics the surface winds are dominated by the eastward-flowing westerlies, which extend in a band across the North Pacific. Together the trade winds and the westerlies drive a large-scale oceanic feature called the North Pacific subtropical anticyclone, which consists of a clockwise-flowing system of currents made up of the westward-flowing North Equatorial Current, the northward- and northeastward-flowing Kuroshio Current off Japan, the eastward-flowing North Pacific Current (sometimes called the West Wind Drift), and the southward-flowing California Current (Figure 1.2).
Many of the storms that traverse the North Pacific are generated in the Kuroshio Current region, where tremendous amounts of heat are lost to the atmosphere from the warm waters of tropical origin that make up the Kuroshio. Other areas where storms develop include the central Aleutian Archipelago and the eastern Gulf of Alaska, particularly in the vicinity of large glaciers that front the coastal region. These storm systems are important to the ecology of the coastal rain forest because of their direct influence on the physical environment of the eastern North Pacific. These storms affect streamflow, cloud cover, fog, ambient air, stream and upper ocean temperatures, and the amount of stored precipitation available as spring runoff. Storms are also important in the moderation of continental temperature fluctuations (and associated processes) in the coastal temperate rain forest. The region is temperate largely because of the moderating influence of the marine component of the climate system.
Between about 45 and 60 degrees north latitude, extending across the Pacific Basin is an atmospheric feature called the Aleutian Low pressure system (Figure 1.3), a region of statistically low barometric pressure, enhanced by a nearly continuous procession of storms (except during summer) that move generally northeastward after forming in the Kuroshio region, the Bering Sea, or the Gulf of Alaska. The Gulf of Alaska is one of the most active meteorological regions on earth (Wilson and Overland 1987), especially during winter, when it is not uncommon for storms to generate 15-meter-high seas and 100-knot winds. A storm moves through this region on the average of every four or five days throughout the winter months (Hartman 1974; Wilson and Overland 1987). The coastal rain forest owes many of its physical characteristics to the effects of atmospheric and oceanic processes associated with the Aleutian Low: frequent and intense coastal storms, strong and persistent winds, fog, rain, and snow. The physical morphology of the coastal region is literally shaped by the actions of wind and waves on the continental landmass.
Figure 1.1. Winds and mean atmospheric pressure highs and lows in the Indian and Pacific oceans: (a) February (northeast monsoon); (b) August (southwest monsoon).
Figure 1.2. Circulation in the subarctic Pacific Ocean south of the Aleutian Islands, showing the Alaskan and Western Subarctic gyres. The Kuroshio, California, and Subarctic currents (also known as the north branch of the North Pacific Current) are also shown. Source: Pearcy (1992), based on Dodimead et al. (1963). Reprinted by permission of the Washington Sea Grant Program.
Figure 1.3. Mean North Pacific sea level pressure: 1946–1988.
The Aleutian Low dominates the atmospheric circulation between 45 and 60 degrees north latitude during fall, winter, and early spring. During summer the storm activity abates and the low-pressure cells are displaced (they often move over land) by a high-pressure system called the North Pacific High. The North Pacific High is statistically present year round in the subtropical Pacific region between about 20 and 45 degrees north. It is the dominant factor influencing the weather on the west coast of North America during summer, when it intensifies and expands northward over the subarctic region of the Pacific Ocean.
The winds in the Aleutian Low drive two large ocean gyres, one in the Bering Sea and the other in the Gulf of Alaska (Figure 1.2). The predominant ocean circulation in both regions is counterclockwise, as are the wind systems that drive the currents. These wind systems are directly associated with the storms that frequently form in the regions or move there after forming off eastern Asia in warm waters of tropical Pacific origin. The Alaska gyre consists of the eastward-flowing North Pacific Current (which forms the southern boundary of the gyre) and the northwestward- and westward-flowing Alaska Current. The Bering Sea and the Gulf of Alaska are extremely important to the ecology of the temperate rain forest region because they play a major role in supporting the oceanic phases of the life histories of Pacific salmon. Pacific salmon characteristically spend more than half of their lives (depending on the species) feeding on zooplankton produced in these oceanic regions. In addition these regions are economically valuable to the coastal rain forest area because they support large stocks of commercially important fishes including halibut, sablefish, walleye pollock, Dungeness, king, and tanner crab, albacore, scallops, shrimp, sea urchins, and sea cucumbers, to name just a few.
The Alaska Coastal Current flows within about 50 kilometers of the coast of the entire Gulf of Alaska. It is a westward-flowing current driven by fresh water and wind. It carries the entire load of runoff from the rivers bordering the Gulf of Alaska (Royer 1979, 1981; Schumacher and Reed 1980). The strength of the coastal current is strongly influenced by seasonal variations in both surface winds and freshwater discharge from rivers. The ecology of the nearshore regions in the subtropical and subarctic North Pacific is quite distinct from the waters of the offshore ocean gyres largely because of continental effects associated with differential precipitation and changes in the winds and ocean currents associated with the continental margins. This is particularly true in the Gulf of Alaska region, where the coastal mountain ranges ringing the gulf act as a barrier to the inland passage of storms, resulting in large amounts of precipitation (2 to 3 meters per year) in the coastal region as the storm systems are forced against the high coastal mountains in the nearshore region of the gulf. The storm systems transport tremendous amounts of heat from the ocean, as well, affecting the temperature extremes in the coastal regions. The coastal rain forest probably extends so far poleward because of these moderating effects.
Coastal upwelling and downwelling are oceanic conditions that represent the interaction between wind stress, the rotation of the earth, and frictional effects between the ocean and the continental boundary. In the Northern Hemisphere, these interactions move water to the right of the prevailing winds. Thus upwelling is associated with equatorward winds along the coast and offshore movement of surface waters, while downwelling is associated with poleward winds along the coast and onshore motion of surface waters. During fall, winter, and spring, the coastal regions of northern British Columbia and Alaska are characterized by downwelling conditions that occur in response to prevailing southeasterly and easterly winds. The coastal region of the Gulf of Alaska is characterized as a coastal temperate environment because the prevailing winds moderate the cold subarctic climate through the importation of warm moist subtropical air masses and warm subtropically derived ocean currents. The continental shelf regions of the subarctic North Pacific support high densities of economically and ecologically important marine populations.
During spring and summer, the prevailing winds blow from the north along the west coast of North America, creating an upwelling along the west coast of the United States and southern British Columbia. This upwelling greatly moderates the climate and ecology of the coastal region by bringing relatively cold nutrient-rich subsurface waters to the surface. The nutrient-rich waters support high levels of primary production, which in turn supports tremendous zooplankton and fish biomass. Coastal upwelling regions are among the most productive marine ecosystems on the planet. The ecology of coastal upwelling systems is strongly modulated regionally by seasonal and interannual variability in the winds and by more distant variability in both atmospheric and oceanic conditions in the tropical Pacific region.
The southward-flowing California Current and the northward-flowing Alaska Current relate most directly to variability in the coastal rain forest region. As the North Pacific Current nears the North American continent, it splits and becomes the Alaska Current and the California Current. The relative amounts of water that flow poleward and equatorward in these systems are thought to be closely related to the strength and size of the Aleutian Low. When the Aleutian Low is intensified, it also tends to expand its region of influence further south. It is thought that under these conditions more warm water of subtropical origin flows poleward into the Alaskan Current than during periods of a weakened Aleutian Low. A weakened and contracted Aleutian Low would result in less water moving into the Alaska Current and more cold waters with subarctic physical properties moving south into the California Current. These processes have implications for fisheries. Shifts in the position of the Aleutian Low cause shifts in the positions of the large-scale North Pacific ocean frontal systems—the boundary regions of the large current systems in terms of their physical and ecological characteristics. On time scales of several years, large-scale displacements (to the north or south) of these current systems and associated frontal systems result in the expansion or contraction of the ranges of various phytoplankton, zooplankton, and fish populations.
