GENERAL vs. LOCAL CIRCULATION

Prince William Sound winds tend to be influenced by the general circulation from the Gulf of Alaska. During the winter months, strong winds associated with polar highs and Aleutian lows tend to dominate. Summer months experience weak circulation from the East Pacific High occasionally punctuated by the passage of weak to moderate Aleutian lows. Winter circulation tends to be from the south, southeast, or east during the frequent passages of low pressure systems and from the north, northwest, or northeast during outbreaks of polar high pressure. During the summer months, weak clockwise circulation around the East Pacific High normally brings southwesterly winds. Occasional summer lows create moderate to strong winds from the south, southeast or east. Summer highs over the interior sometimes create gusty winds from northerly and westerly quadrants.

This pattern can be understood in terms of the differential heating and cooling and heat capacities of land and water. During the winter months, warm water creates low pressure over the Gulf while the rapid cooling of the continent creates high pressure. Except when gulf lows are making a landfall, northerly offshore winds tend to prevail flowing from colder land to warmer water. During the winter months, temperature and pressure differences between ocean and land can be extreme creating strong winds. Summers are just the opposite. Air over the water becomes cooler and at a slightly higher pressure than air over the sun-warmed interior. A thermal low develops over the continent. Because the summer temperature and pressure differences are not so extreme, a weak onshore flow predominates.

While this general circulation winds in the Gulf are quite predictable, winds in the Sound are locally variable in both direction and strength. The high mountains, forests, islands, passages, fiords, glaciers and ice fields impede, accelerate and redirect the general circulation. Local temperature differences between land, water and ice often create local winds that can overpower the general circulation.

Mountains tend to impede and redirect the wind flow. Also, forested ridges and islands create considerable friction that slows wind speeds. Examining the wind speeds for Middleton Island, Valdez and Cordova presented in Appendix B readily demonstrates this. Taking Middleton Island as representative of the Gulf and Cordova and Valdez of the Sound let us examine three representative months ­ October, January and June. Whereas October winds average 13.2 knots in the Gulf with only 4% calms, Cordova averages 5.1 knots and 30% calms and Valdez 5.1 knots with 23% calms. January reveals Middleton Island with an average wind speed of 14.8 knots and 4% calms; Cordova 4.5 knots, and 42% calms; and Valdez 6.9 knots with 21% calms. In July Middleton Island winds average 7.4 knots with 7% calms; Cordova 3.3 knots with 38% calms and Valdez 4.4 knots and 23% calms.

Ridges, valleys, islands, passages, and steep sided fiords channel and block general circulation winds. Thus, winds tend to blow either up or down passages and fiords but rarely across them. As the winds twist and turn through channels and in and out of bays and coves, frictional drag creates extensive areas of calm as do blocking islands and ridges.

Mountains and restricted channels may also accelerate wind speeds. In some locations, strong winds dam up on the windward side of steep mountain ridges then sweep down the leeward side in fierce gusts known as "williwaws." In the northwest Sound fierce "katabatic" or "fall winds" can accelerate down glacier valleys blasting the ordinarily calm fiords. Also, constricted valleys can accelerate winds just as a nozzle on a garden hose increases water flow. This is known as "venturi funneling."

We noted that temperature differences between masses of air are equivalent to pressure differences. Winds are created by the flow of air from areas of high pressure to low pressure. Cold air tends to pool over the high mountains on the northern perimeter of the Sound. The presence of glaciers and ice fields augment this altitudinal cooling. The air over the relatively warmer waters of the Sound is warmer and less dense and hence at a lower pressure. A pressure gradient augmented by gravity forms creating local northerly drainage winds. Furthermore, when warm air accompanying a Gulf low moves in from the southeast, the temperature (and hence pressure) difference increases. The result is a northerly flow out of Valdez Arm and Port Wells even though the general Gulf circulation and winds in the mid and southerly Sound are from the southeast. In this instance, local temperature (pressure) differences totally overwhelm the general circulation pattern.

Local solar heating of forested hillsides or bare mountain slopes create summer sea breezes in certain areas. Sea breezes blow inland when the air over the water is cooler (at a higher pressure) than air over the warming shores. During the summer months, these local sea breezes can both reenforce and mask weak prevailing southwesterlies.

Williwaws Winds:

Williwaws occur in Prince William Sound where unimpeded wind over water encounters a mountain ridge perpendicular to its path. Hawkins, Hinchinbrook, and Montague Islands offer such barriers to Gulf southeasterlies. The coves and bays on the lee side of these islands are notorious for strong, gusty winds during the passage of Gulf lows. Thirty knot winds in the Gulf may create 60 knot gusts in these areas. Williwas are caused by the damming up of wind on the windward side of the ridge. Because air is a compressible fluid, the dammed up flow acts like a spring storing up energy of compression. The compressed air is forced up and over the summit imparting a wave to the overlying airstream. The stream then accelerates downslope on the lee side releasing the stored energy. The pull of gravity further adds to this acceleration. Rather than flowing smoothly down the lee side this spring-like flow encounters obstacles such as boulders and trees causing it to further compress and expand, creating gusty, turbulent winds.

Gravity and compression are probably only part of the explanation for the doubling of wind speeds during willawaw conditions. As the air flow pushes up and over the mountain ridge, it compresses the air streams above so that a venturi funnel is formed between these compressed air streams and the mountain ridge below. The airstreams in the wave are then squeezed and accelerated through this restriction. Furthermore, higher level winds are entrained into the downgoing wave bringing this stronger windflow to the surface.

Watching water flowing over a submerged rock in a stream can be instructive in imagining these processes. One can observe the wavelike dip in the water's surface and watch the stream accelerate as it plunges down the other side of the obstruction. Likewise, the induced turbulence on the lee side is quite evident.

The wavelike motion of air flowing over mountain ranges is often experienced as turbulence and lift when flying. Lenticular, lee-wave clouds, as explained above, are also observed under these conditions. However, these clouds do not always signal williwaws. Sometimes the wave motion continues to undulate beyond the summit forming harmless stationary lee waves and high level lenticulars . At other times it causes srtong gusts at the surface. Williwaws seem to occur when the waves become so large that they actually break like ocean waves. This decouples of the flow from the overlying stream allowing the fast-moving, compressed stream to reach the surface.

Gap Winds:

"Venturi" or "gap" winds occur where winds flow through restricted passages. The effect is similar to restricting the flow of water through a garden hose by placing your finger over the outlet. Hinchinbrook Entrance, for example, can be quite windy when strong southerly winds from the Gulf are squeezed into the Sound. Conversely, strong winter winds from the Sound are sometimes accelerated passing out through this channel. Similarly, northerly winds tend to accelerate through Valdez Narrows into Valdez Arm during the winter and from Valdez Arm into the Port in the summer.

(Continued)