The 1976 Pacific climate shift is examined, and its manifestations and significance in Alaskan climatology during the last half-century are demonstrated. The Pacific Decadal Oscillation index shifted in 1976 from dominantly negative values for the 25-yr time period 1951–75 to dominantly positive values for the period 1977–2001. Mean annual and seasonal temperatures for the positive phase were up to 3.1°C higher than for the negative phase. Likewise, mean cloudiness, wind speeds, and precipitation amounts increased, while mean sea level pressure and geopotential heights decreased. The pressure decrease resulted in a deepening of the Aleutian low in winter and spring. The intensification of the Aleutian low increased the advection of relatively warm and moist air to Alaska and storminess over the state during winter and spring. The regime shift is also examined for its effect on the long-term temperature trends throughout the state. The trends that have shown climatic warming are strongly biased by the sudden shift in 1976 from the cooler regime to a warmer regime. When analyzing the total time period from 1951 to 2001, warming is observed; however, the 25-yr period trend analyses before 1976 (1951–75) and thereafter (1977–2001) both display cooling, with a few exceptions. In this paper, emphasis is placed on the importance of taking into account the sudden changes that result from abrupt climatic shifts, persistent regimes, and the possibility of cyclic oscillations, such as the PDO, in the analysis of long-term climate change in Alaska.

During a ship voyage from Tasmania to Antarctica in summer 2000/01, radiative and meteorological measurements were continuously made, from which the surface energy budget was calculated. Sea conditions throughout the voyage ranged from open water to broken pack and finally to snow-covered unbroken sea ice in McMurdo Sound. The global radiation increased on average during the trip (to higher latitudes) as we travelled poleward. The net radiation, which was positive (toward the surface) on average, decreased however, mostly due to the increase in surface albedo. For open water, most of the net radiation is used for evapouration (61%), while for broken sea-ice conditions, nearly all energy is used for melting of the sea ice or heating of the ocean (96%). For unbroken snow-covered sea ice, the net radiation lies close to zero, due to the high surface albedo, which reached a mean value of 0.81. The sensible heat flux becomes the largest heat source and nearly all the energy is used for warming of the surface. Finally, a Radarsat image, on which the ship track was visible, was used to compare the ship observations with satellite derived ice types.

[1] Radiative and meteorological measurements were collected continuously during a cruise from Australia to Antarctica in austral summer 2000. On the average, the amount of fractional cloud cover was high (81%), reducing the incoming solar radiation. The albedo varied widely from over 80% for snow-covered undisturbed sea ice to below 10% for open water. In general, sea ice concentration was the strongest determining factor for the reflectivity. However, different ice types and snow cover also had a substantial influence on the reflectivity. When a highly reflecting surface was present (total snow-covered undisturbed ice pack with an albedo of 81%), the incoming global radiation under overcast conditions was 85% higher than for a water surface due to multiple reflections. The net radiation was found to be a strong function of both fractional cloud cover and surface albedo. For low albedo values, the net radiation increases with decreasing cloudiness, e.g., for a water surface (albedo 8%) the mean daily value increased from 88 Wm À2 for total cloud cover to 226 Wm À2 for clear skies. For highly reflecting surfaces, the net radiation decreases with decreasing cloud amount for most of the day. The mean daily value was slightly negative for clear skies (À7 Wm À2 ), but somewhat positive (23 Wm À2 ) for overcast conditions.

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