Southcentral Alaska Climate

  Southcentral Climate

The climate of Southcentral Alaska falls into three of the four climatic zones. Obviously, all coastal area and islands fall into the Maritime Zone. With no terrain influences involved, annual precipitation amounts to about 60 inches, including snow. In some areas the same air pushed up mountainsides may produce more than 200 inches of moisture. Temperatures usually reach the upper 50s for mean maximums during summer and drop into the low 20s for mean lows during winter. Temperature extremes often appear out of place with the mean. These extremes result from outbreaks of extremely cold air in winter and relatively hot air in summer from the interior of Alaska. The strongest surface winds in this region, both in averages and extremes, occur in the coastal area. Offshore speeds average between 12 and 18 knots; slightly less on shore because of surface friction. Extremes of 50 to 75 knots are common in winter, and can exceed 100 knots when channeled.

The interior climate of the region is in the Continental Zone. Mean maximum temperatures in summer are mostly in the mid- to upper-60s and mean lows in winter range from 10 to nearly 30 degrees below zero F. Annual precipitation is generally between 10 and 15 inches. Where terrain is a factor, annual totals may exceed 20 inches. Heavier amounts usually occur at elevations above those now inhabited by people. Surface winds are light compared to those of the coast. Channeling through mountain valleys results in narrow bands of strong winds in some local areas.

In the Transition Zone weather conditions average between the Continental and Maritime Zones. Temperature extremes most resemble those of the Continental Zone, while precipitation amounts tend to range from light (near 10 inches) to heavy enough to maintain glaciers. Surface winds range between coastal and interior conditions, including channeled winds.


Marine Environment

Sea Ice

Sea ice in the Southcentral Region is found primarily in Cook Inlet, north of the Forelands (Figure 36 ). Occasionally, small icebergs break off coastal glaciers and are found along the rim of the Gulf. Water temperatures of about 40 degrees F limit their existence. Shore fast ice is found in bays and inlets, particularly where streams flow into them.

In general, upper Cook Inlet ice moves slowly south where water temperatures are higher. As a result, large ice concentrations south of the Forelands are rare, occurring only when air Temperatures are extremely low for prolonged periods. The ice is thicker and more extensive than usual under these conditions.

Temperature controls ice growth in upper Cook Inlet (Hutcheon 1972). Research directly related to Cook Inlet ice growth is very recent. Ice research done in other locations, principally the Russian and Canadian Arctic, forms the basis for forecast procedures now used routinely on Cook Inlet ice. Figure 37 shows the relationship between ice thickness, freezing degree days, and snow cover of the ice. Even one inch of snow forms an effective insulation layer. For example, with no snow, 470 freezing degree days are required for an ice thickness of 30 inches. With one inch of snow, 880 degree days, or almost twice as many are required. Figure 37 also shows the difference in requirements between forming and melting sea ice. Almost twice as many degree days are needed to form ice as to melt it.

The amount of snow cover is an important factor in the growth of ice. Decreases in thickness require fewer thawing degree days than were necessary to form the same thickness.

Tidal Action

If there were no tidal action in Cook Inlet, a solid sheet of ice would form at freezeup in the fall and remain until breakup in the spring. Tidal action and tidal currents keep the Inlet ice in shattered condition. The tidal range measured at Anchorage is one of the largest in the world. Figure 38 shows typical tide curves for Anchorage and Ketchikan. The maximum range shown on the curve for Anchorage is 34 feet with one exception, a range of 38.9 feet on January 9-10, 1974. Strong tidal currents generated by unusual tides are not as spectacular as the tides themselves. The narrowest point of the Inlet, between the East and West Forelands, has mid-channel speeds of one to six knots with an average of 3.8 knots. Unverified reports from ship captains say that currents of up to nine knots have been encountered. Figure 39 shows a typical current curve for the mid-channel point between the Forelands.