Module 3c:
TEXT ABOUT ANIMATION
Aleutian Low
From September through April, the Aleutian Low Pressure System (Aleutian Low) is a
semi-permanent winter weather feature of the North Pacific. Forming off of the Aleutian
Islands, this large-scale storm system is consequently surrounded by higher pressure.
Since air behaves like a fluid, it flows from high to low pressure areas, just like water
(when you open the nozzle on a hose, water flows out of the hose, not into it!). But
because this is on such an extensive scale, the air movement is deflected by the Coriolis
effect so that it veers to the right in the Northern Hemisphere. The low-pressure center is
thus surrounded by air moving to the right or generating a counterclockwise flow (also
called “cyclonic” as in a really bad storm or cyclone). The Aleutian Low brings high
winds and a lot of precipitation to the Gulf of Alaska, which has sometimes been called
“the graveyard of storms”.
Due to the predominant high-altitude winds for this latitude, the low-pressure zone moves
from west to east across the Bering Sea and Gulf of Alaska. The first winds to hit the
southern and southeastern coasts of Alaska are moving counterclockwise and oriented
approximately parallel to the coastline. Thus, in the northern Gulf of Alaska winds
typically blow from east to west and are called easterly winds. Due to Ekman transport,
ocean waters are moved by the wind not in the same direction as the wind, but at a right
angle to the wind direction. This deflection steers the seas from a westward to a
northward direction. If the Ekman transport is to the north, water will tend to “pile up”
on the Alaskan coast. This coastal water mounding or convergence forces near shore
waters to react by having their accumulated volume flow down and outwards from
beneath the pile-up in a process called downwelling . In this ensuing flow, the water is
again deflected to the right, this time along the coast. Downwelling tends to vertically
mix waters, taking higher density water away from the surface and replacing it with
slightly lower density water. This process occurs most of the year on the Gulf of Alaska
shelf, but it does relax or even reverse itself (upwelling) in the summer in response to the
North Pacific high-pressure system weakening the Aleutian Low. The freshwater runoff
from the rivers and other land sources can also spread across the shelf and mix with the
upwelling water.
Near surface waters are often stripped of their “fertilizer and food” by spring and summer
plant growth known as algal blooms. As observed along the Oregon and California
coasts, upwelling brings nutrient-rich water up from the deep and replenishes these
constituents. All this organic activity supports abundant fisheries. But a major
unanswered question in Alaskan oceanographic research persists: Why does Alaska have
one of the nation’s most productive fisheries when most of the time its waters are in a
downwelling system, supposedly reducing its vitality?