Blake Moore

Strong Temperature Increase and Shrinking Sea Ice in Arctic Alaska

Barrow, the most northerly community in Alaska, observed a warming of 1.51°C for the time period of 1921- 2012. This represents about twice the global value, and is in agreement with the well-known polar amplification. For the time period of 1979-2012, high quality sea ice data are available, showing a strong decrease in sea ice concentrations of 14% and 16% for the Beaufort and Chukchi Seas, respectively, the two marginal seas bordering Northern Alaska. For the same time period a mean annual temperature increase of 2.7°C is found, an accelerated increase of warming over the prior decades. Looking at the annual course of change in sea ice concentrations, there is little change observed in winter and spring, but in summer and especially autumn large changes were observed. October displayed the greatest change; the amount of open water increased by 44% and 46% for the Beaufort and Chukchi Seas, respectively. The large amount of open water off the northern coast of Alaska in autumn was accompanied by an increase of the October temperature at Barrow by a very substantial 7.2°C over the 34 year time period. Over the same time period, Barrows precipitation increased, the frequency of the surface inversion decreased, the wind speed increased slightly and the atmospheric pressure decreased somewhat.

The First Decade of the New Century: A Cooling Trend for Most of Alaska

During the first decade of the 21st century most of Alaska experienced a cooling shift, modifying the long-term warming trend, which has been about twice the global change up to this time. All of Alaska cooled with the exception of Northern Regions. This trend was caused by a change in sign of the Pacific Decadal Oscillation (PDO), which became dominantly negative, weakening the Aleutian Low. This weakening results in less relatively warm air being advected from the Northern Pacific. This transport is especially important in winter when the solar radiation is weak. It is during this period that the strongest cooling was observed. In addition, the cooling was especially pronounced in Western Alaska, closest to the area of the center of the Aleutian Low. The changes seen in the reanalyzed data were confirmed from surface observations, both in the decrease of the North-South atmospheric pressure gradient, as well as the decrease in the mean wind speeds for stations located in the Bering Sea area.

Recent sea ice increase and temperature decrease in the Bering Sea area, Alaska

We analyzed the sea ice conditions in the Bering Sea for the time period 1979–2012, for which good data based on microwave satellite imagery, being able to look through clouds and darkness, are available. The Bering Sea, west of Alaska, is ice-free in summer, but each winter, an extensive sea ice cover is established, reaching its maximum normally in March. We found a slight increase in ice area over the time period, which is in stark contrast to the significant retreat observed in the Beaufort Sea north of Alaska and the Arctic Ocean as a whole. Possible explanation might be found in the Pacific Decadal Oscillation (PDO), which went from dominantly positive values to more negative values in the last decade. The PDO is related to the sea surface temperature (SST) in the North Pacific, negative values indicated cooler temperatures and cooler SST weakening the semipermanent Aleutian Low. When comparing the circulation pattern obtained from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyzed data set for years of heavy ice against light ice years, an additional vectorial northerly wind component could be deduced from the pressure data. Hence, less relatively warm air is advected into the Bering Sea, which becomes of special importance in winter, when solar radiation is at its minimum. Surface observations confirmed these findings. Atmospheric pressure increased in Cold Bay, located close to the center of the semi-permanent Aleutian Low, the N–S pressure gradient (Nome–Cold Bay) in the Bering Sea decreased, wind speeds of the coastal stations became weakened, and the temperature of coastal stations decreased.

On the radiation characteristics of Antarctic Sea Ice

Abstract Radiative measurements were carried out continuously during a cruise from Australia to Antarctica during austral summer 1995/96. Both shortwave and longwave radiative fluxes were measured. Some of the results are: • The incoming solar radiation had a mean value of 217 W m–2; this was a relatively weak value due to the large amount of fractional cloud cover observed. The sun was, for a large part of the trip, above the horizon for 24 hours a day. • The reflectivity varied widely, not only as a function of sea‐ice concentration, but also as a function of ice type. • Snow covered pack ice gave the highest albedo values (<70%), while flooded sea ice and thin ice reflected much less (<30%). • For each sea‐ice type, short term observations showed a good relationship between albedo and ice concentration. • The albedo increased with decreasing solar elevation. • The net longwave radiation was negative (mean –27 W m–2); this small absolute value is due to a high amount of fractional cloud cover. There was a weak diurnal variation with a maximum loss (–33 W m–2) in the early afternoon. • On the average, the net radiation was positive for 17 hours, and negative for 7 hours a day. However, the duration of a positive balance depended strongly on the surface albedo. • For the observed albedo values, modelling results showed that the net radiation was always positive when averaged over a day. The magnitude, however, depended strongly on the surface albedo, varying by more than the factor of three.

On the surface energy budget of sea ice

The surface energy budget was investigated during a cruise through the pack ice in the Southern Ocean. The time of observation was close to mid-summer. Some of the more important findings were: The mean albedo varied from 11% for open water to 59% for 10/10 ice cover. Hourly values span the range from 6% (open water) to 76% (total ice cover). The net heat flux into the ocean (B) was on average 109 W m -2 . If this energy were used solely for melting of sea ice, 30 mm could be melted each day. For low surface albedos (ice concentration below 7/10), the net radiation increased with decreasing cloudiness. However, the opposite was the case for a high surface albedo. The last point shows the importance of clouds on the surface energy budget. Not only should their presence or absence be known but also the reflectivity of the underlying surface, as it might change the net radiation in opposite ways.

Weatherwatch: September/October 2010

T ropical systems played a key role in moistening parts of the nation but bypassed a broad area stretching from the central Gulf Coast states into the Ohio Valley. Eight named tropical systems (four tropical storms and four hurricanes) formed over the Atlantic Basin during the month, tying a September record originally set in 2002. Warm weather dominated the United States during September, except across portions of the nation’s northern tier. Late-season warmth was most impressive in the Southwest and Southeast, while unusually cool conditions stretched from the northern Plains into the Great Lakes States.

Weatherwatch: May/June 2012

SEPTEMBER / OCTOBER 2012 Drier-than-normal weather caused rapid development or expansion of drought from the central and southern Plains into the Mid-South and lower Midwest. In those areas, diminishing moisture reserves and above-normal temperatures set the stage for a crop disaster in some of the nation’s most fertile land. In contrast, beneficial showers eased or eradicated dry conditions across portions of the northern Plains, upper Midwest, and Atlantic Coast states. Some of the heaviest rain fell late in the month, when a series of cold fronts traversed the nation’s northern tier and Tropical Storm Beryl soaked the southern Atlantic region. Another region that received much-needed May rainfall was the Rio Grande Valley and neighboring areas in New Mexico and Southern and Western Texas. Meanwhile, a period of warm, dry weather in California and the Northwest followed a chilly start to spring. Cool, showery conditions returned, however, late in the month. Elsewhere, generally hot, dry weather in the Southwest brought an early end to the snowmelt season and fostered the spread of several wildfires. In fact, nearto above-normal temperatures covered the nation, except for some slightly cooler-than-normal conditions from the Pacific Northwest to the northern High Plains. Monthly temperatures averaged at least 5°F above normal at several Southwestern locations and in a broad swath stretching from the central and southern Plains into the Midwestern and northern MidAtlantic states. weatherwatch

Weatherwatch: July/August 2011

NOVEMBER / DECEMBER 2011 H eat and humidity expanded across the Midwest and Northeast, making for some mid-summer misery across some of the nation’s major population centers. Patchy dryness accompanied the heat, contributing to a browning of lawns and pastures. In the nation’s breadbasket, some of the most extensive short-term dryness stretched from southern Iowa into Central Indiana. Meanwhile, historically hot, dry conditions persisted in the South-Central United States. Texas experienced its hottest, second-driest July on record, sharply aggravating the effects of a 10-month drought. By early August, the United States Department of Agriculture rated more than 90 percent of the rangeland and pastures in both Oklahoma and Texas in very poor to poor condition. Drought also persisted in much of the Southeast, although locally heavy showers provided some relief in the central Gulf Coast states and the southern Atlantic region. Similarly, monsoon showers provided some limited relief to drought-affected areas in the Southwest. Elsewhere, cool, occasionally showery conditions lingered across the far West.

Weatherwatch: January/February 2011

MAY / JUNE 2011 I n a dramatic change from December, little or no precipitation fell in California during January. The dry regime also stretched eastward into the Four Corners states. In contrast, warmth and melting snow accompanied a period of heavy precipitation from the Pacific Northwest to the northern Rockies, leading to some flooding. Colder-than-normal weather dominated the eastern half of the nation during January, while nearto abovenormal temperatures covered most areas from the High Plains westward. Monthly temperatures averaged at least 5°F below normal in parts of the southern Atlantic region and a few locations in the upper Midwest.

Weatherwatch: November/December 2010

MARCH / APRIL 2011 N ovember rainfall provided drought relief from the central Gulf Coast States into the Ohio Valley. In fact, lower Midwestern drought was nearly eradicated by late-autumn precipitation. By month’s end, snow covered parts of the upper Midwest, consistent with an evolving La Niña. In contrast, drought developed or intensified in parts of the southern Atlantic region, particularly across Florida and Georgia. Also typical of La Niña, cold, stormy weather dominated the northern Plains and the Northwest, resulting in the development of an extensive snow cover. Meanwhile, unfavorably dry weather covered parts of the central and southern Plains. However, a single, renegade storm system provided much-needed precipitation from Texas’s northern panhandle into southeastern Nebraska. Elsewhere, a parade of storms helped to establish high-elevation snow packs from the Pacific coastal ranges to the Rockies, excluding parts of the Southwest.