Aleutian Climate
The windswept islands of the Aleutian Archipelago experience a wet and stormy
maritime climate. Wind, fog, and rain are ubiquitous while sunny days are rare. The
average temperature range during the summer is 7 – 14°C (45 – 57°F) and during the
winter is -3 – 3°C (27 – 37°F). Precipitation is highly variable with annual averages
ranging between 75 and 160 cm per year depending on location. Wettest conditions
normally occur during the winter1. The storms that frequently batter the Aleutian Islands
typically originate east of Japan, moving northeastward along the Aleutian Chain toward
the Gulf of Alaska. These storms result in high winds, often in excess of 22 m/s (50
mph), during all but the summer months2.
Regime shifts, where the climate changes abruptly from one mean state to another, have
had substantial impacts on the marine ecosystems of the North Pacific3. In particular, the
regime shift that occurred in 1977 resulted in a southward displacement of the Pacific
storm track, lower sea surface temperatures and deeper ocean mixed layer in the central
North Pacific and higher sea surface temperatures along the coast of North America after
the shift. The shift in the environment was coincident with many biological changes
including an increase in the Alaskan catch of many species of salmon4. However, it
appears that, while these regime shifts have had dramatic consequences for the Gulf of
Alaska, impacts to the Aleutian Islands weather have been insignificant5. Instead, from at
least 1956 to 2002, the Aleutian Islands have experienced a long-term cooling trend. At
the same time, variability in surface air temperatures has increased6.
Oceanography
East of Samalga Pass (170°W), the Alaska Coastal Current flows southwestward along
the southern side of the Aleutian Islands. This relatively fresh and shallow current hugs
the shoreline and turns northward entering the Bering Sea through the eastern Aleutian
Passes (Unimak, Akutan, Umnak, and Samalga)7. West of Samalga Pass, the shelf south
of the islands is much narrower. This narrow shelf allows the Alaskan Stream, the deep
current that flows along the continental slope in the western Gulf of Alaska, to come
close to the islands. The Alaskan Stream flows southwestward along the southern side of
the islands, connecting the Gulf of Alaska to the Aleutian Islands region8. Waters from
the Alaskan Stream flow northward through the central and western Aleutian Passes to
feed the Aleutian North Slope Current, which flows northeastward along the northern
side of the islands9 (Figure 1: use Fig. 1 from Stabeno, et al., 2005?).
Due to the influence of the Alaska Coastal Current, the shallow, narrow passes east of
Samalga Pass (170°W) can be classified as a coastal environment with a strong influence
of coastal freshwater discharge. These waters are warmer, fresher, more strongly
stratified, and nitrate poor compared with the Aleutian waters west of Samalga Pass.
West of Samalga Pass, the passes are deeper and wider. The marine environment can be
classified as oceanic with primary influence from the Alaskan Stream10. The wider
passes allow bidirectional currents with mean flow to the north (from the Pacific to the
Bering) on the eastern side of the passes and to the south on the western side11. However,
the northward flow is generally stronger, more consistent, and occurs over most of the
cross-section of the passes so, except in Kamchatka Strait far to the west, the net
transport through the Aleutian Passes is northward from the Pacific Ocean to the Bering
Sea.
Within the passes, fierce tidal currents often exceeding 100 cm/s12, present hazards to
navigation and equipment. The tides result in substantial mixing within the passes. As
the tidal current pushes water over the shallow sills of the passes, salt, nutrients, and
plankton from deeper water can be mixed into the surface waters. The influence of tidal
mixing on surface nutrient concentrations depends on the depth of the pass. Passes with
depths between 120 and 200 m, such as Seguam and Tanaga Passes, are shallow enough
to mix top to bottom but deep enough that the mixing can access the deep nutrient
reservoir. Thus, these passes are most efficient at mixing nutrients into the euphotic zone.
In contrast, nutrient concentrations at the bottom of shallower passes, such as Unimak
and Akutan Passes, are lower so mixing does not result in substantially increased surface
concentrations. In deeper passes, such as Amukta Pass, the interaction of tidal currents
and the bottom topography can not result in mixing that reaches the surface.
Although tidal mixing can result in high surface nutrients in the passes, it can hinder the
development of phytoplankton blooms by limiting access to light13. Thus, blooms often
occur north of the passes, away from the intense mixing in the passes, but utilizing the
nutrients supplied by the mixing14. The vertical circulations created by interactions of
tidal currents with steep and variable bathymetry can also result in surface convergences
creating concentrations of prey for seabirds15 and other predators.
1
Western Regional Climate Center. http://www.wrcc.dri.edu/summary/Climsmak.html.
2
Rodionov, S.N., Overland, J.E., Bond, N.A., 2005. Spatial and temporal variability of the Aleutian
climate. Fisheries Oceanography 14 (s1), 3-21.
3
Hare, S.R., Mantua, N.J., 2000. Empirical evidence for North Pacific regime shifts in 1977 and 1989.
Progress in Oceanography 47 (2-4), 103-145.
4
Hare, S.R., Mantua, N.J., 2000. Empirical evidence for North Pacific regime shifts in 1977 and 1989.
Progress in Oceanography 47 (2-4), 103-145.
5
Rodionov, S.N., Overland, J.E., Bond, N.A., 2005. Spatial and temporal variability of the Aleutian
climate. Fisheries Oceanography 14 (s1), 3-21.
6
Rodionov, S.N., Overland, J.E., Bond, N.A., 2005. Spatial and temporal variability of the Aleutian
climate. Fisheries Oceanography 14 (s1), 3-21.
7
Ladd, C., Hunt, G.L., Jr., Mordy, C.W., Salo, S.A., Stabeno, P.J., 2005. Marine environment of the
eastern and central Aleutian Islands. Fisheries Oceanography 14 (Suppl. 1), 22-38.
8
Favorite, F., Dodimead, A.J., Nasu, K., 1976. Oceanography of the subarctic Pacific region, 1960-71.
International North Pacific Fisheries Commission, Bulletin 33, Vancouver, Canada, p. 187.
9
Reed, R.K., Stabeno, P.J., 1999. The Aleutian North Slope Current. In: Loughlin, T.R., Ohtani, K. (Eds.),
Dynamics of the Bering Sea. University of Alaska Sea Grant, Fairbanks, pp. 177-191.
10
Ladd, C., Hunt, G.L., Jr., Mordy, C.W., Salo, S.A., Stabeno, P.J., 2005. Marine environment of the
eastern and central Aleutian Islands. Fisheries Oceanography 14 (Suppl. 1), 22-38.
11
Stabeno, P.J., Schumacher, J.D., Ohtani, K., 1999. The physical oceanography of the Bering Sea. In:
Loughlin, T.R., Ohtani, K. (Eds.), Dynamics of the Bering Sea. UNIV ALASKA SEA GRANT, Fairbanks,
pp. 1-28.
12
Stabeno, P.J., Kachel, D.G., Kachel, N.B., Sullivan, M.E., 2005. Observations from moorings in the
Aleutian Passes: temperature, salinity and transport. Fisheries Oceanography 14 (Suppl. 1), 39-54.
13
Sverdrup, H.U., 1953. On conditions for the vernal blooming of phytoplankton. J. Conseil Exp. Mer 18,
287-295.
14
Mordy, C.W., Stabeno, P.J., Ladd, C., Zeeman, S., Wisegarver, D.P., Hunt Jr., G.L., 2005. Nutrients and
primary production along the eastern Aleutian Island Archipelago. Fisheries Oceanography 14 (Suppl. 1),
55-76.
15
Hunt, G.L., Jr., Russell, R.W., Coyle, K.O., Weingartner, T., 1998. Comparative foraging ecology of
planktivorous auklets in relation to ocean physics and prey availability. Marine Ecology-Progress Series
167, 241-259.