Abstract

A monthly storm-track climatology is derived from monthly maps of cyclone tracks for the winter season, October through March, averaged over 23 years, 1957/58–1979/80, for a 2° latitude×4° longitude grid bounded by 51°N, 65°N, 157°W and 171°E. There is a decrease in the number of cyclones with latitude in all months and division into two storm tracks, one propagating north-northeast along the Siberian peninsula and one entering the southern Bering Sea on a northeasterly course and either curving northward into the central Bering Sea or continuing parallel to the Aleutian Island chain.

Monthly average ice extents are established for February and March 1958–80 along a line from Norton Sound southwest toward the ice edge, perpendicular to the average maximum extent. Comparison of composite cyclone charts summed over the winter season and over the five heaviest and five lightest ice years shows a shift in cyclone centers toward the west in light ice years. The correlation between maximum seasonal ice extent and the difference between the number of cyclone centers in the eastern minus the western part of the basin over each winter season is 0.71. The relation of sea ice extent and the location of cyclone tracks is consistent with previous observations that advance of the ice edge in the Bering Sea is dominated by wind-driven advection and that southerly winds associated with cyclone tracks to the west inhibit this advance. These results indicate that the interannual variability in seasonal sea-ice extent in the Bering Sea is controlled by an externally determined variation in storm-track position related to large-scale differences in the general circulation. A skewed distribution of ice extents toward heavy ice years, however, suggests the possibility of an oceanographic constraint on the magnitude of extreme seasonal ice extents, such as the inability of melting ice to cool the mixed layer beyond the continental shelf to the freezing point or the increased influence of the northwestward flowing, continental slope current.

Abstract

Comparison is made between wind velocity measurements at two NOAA buoys, EB34 and EB41, located in the New York Bight, and winds extrapolated from nearby coastal stations and inferred from sea level pressure analysis at the National Meteorological Center. The comparison covers 0000 and 1200 GMT observations for November 1975 through March 1976. Surface winds are obtained from gradient winds by means of the analytic single-point boundary layer model proposed by Cardone (1969) and simple empirical relations.

Buoy wind speeds in excess of 10 m s⁻¹ accounted for 28% of the observations. For these strong winds, pressure-gradient based estimates provided adequate specifications of surface winds for 81% of the cases, defined by vector error <5 m s⁻¹, and were in general superior to estimates extrapolated from single coastal stations.

Rapid changes in wind speed and direction recorded in hourly buoy data indicate that resolution of winter storms requires pressure analyses on at least a 6 h cycle. The presence of moving storm systems also suggests that the use of coastal station reports can be improved by extrapolation in time as well as space.

Abstract

There were extensive regions of Arctic temperature extremes in January and February 2016 that continued into April. For January, the Arctic-wide averaged temperature anomaly was 2.0°C above the previous record of 3.0°C based on four reanalysis products. Midlatitude atmospheric circulation played a major role in producing such extreme temperatures. Extensive low geopotential heights at 700 hPa extended over the southeastern United States, across the Atlantic, and well into the Arctic. Low geopotential heights along the Aleutian Islands and a ridge along northwestern North America contributed southerly wind flow. These two regions of low geopotential height were seen as a major split in the tropospheric polar vortex over the Arctic. Warm air advection north of central Eurasia reinforced the ridge that split the flow near the North Pole. Winter 2015 and 2016 geopotential height fields represented an eastward shift in the longwave atmospheric circulation pattern compared to earlier in the decade (2010–13). Certainly Arctic amplification will continue, and 2016 shows that there can be major Arctic contributions from midlatitudes. Whether Arctic amplification feedbacks are accelerated by the combination of recent thinner, more mobile Arctic sea ice and occasional extreme atmospheric circulation events from midlatitudes is an interesting conjecture.

Abstract

The last decade shows increased variability in the Arctic Oscillation (AO) index for December. Over eastern North America such increased variability depended on amplification of the climatological longwave atmospheric circulation pattern. Recent negative magnitudes of the AO have increased geopotential thickness west of Greenland and cold weather in the central and eastern United States. Although the increased variance in the AO is statistically significant based on 9-yr running standard deviations from 1950 to 2014, one cannot necessarily robustly attribute the increase to steady changes in external sources (sea temperatures, sea ice) rather than a chaotic view of internal atmospheric variability; this is due to a relatively short record and a review of associated atmospheric dynamics. Although chaotic internal variability dominates the dynamics of atmospheric circulation, Arctic thermodynamic influence can reinforce the regional geopotential height pattern. Such reinforcement suggests a conditional or state dependence on whether an Arctic influence will impact subarctic severe weather, based on different circulation regimes. A key conclusion is the importance of recent variability over potential trends in Arctic and subarctic atmospheric circulation. Continued thermodynamic Arctic changes are suggested as a Bayesian prior leading to a probabilistic approach for potential subarctic weather linkages and the potential for improving seasonal forecasts.

A unique glimpse of the Arctic from a period before the present era of climate warming is found in the records of the first International Polar Year (IPY) of 1882–83. Inspired by the Austrian scientist and explorer Carl Weyprecht, the purpose of the IPY was to discover the fundamental laws governing global meteorological and geophysical phenomena. It was understood that new discoveries would depend upon a program of simultaneous observations that encompassed the polar regions. The collection and analysis of the first series of coordinated meteorological observations ever obtained in the Arctic was one of the principal objects of the IPY. The field program was successfully completed and a vast body of data was collected, but afterward it fell into obscurity with little analysis completed.

We have analyzed for the first time the synchronous meteorological observations recorded during the first IPY. This analysis contributes to the goal of the upcoming fourth IPY scheduled for 2007–08: to understand the climate changes currently unfolding in the Arctic/Antarctic within the context of the past. We found that surface air temperature (SAT) and sea level pressure (SLP) observed during 1882–83 were within the limits of recent climatology, but with a slight skew toward colder temperatures, and showed a wide range of variability from place to place over the course of the year, which is a feature typical of the Arctic climate today. Monthly SAT, SLP, and associated phenological anomalies were regionally coherent and consistent with patterns of variability in the atmospheric circulation such as the North Atlantic Oscillation (NAO). Evidence of a strong NAO signature in the observed SAT anomalies during the first IPY highlights the impact of large-scale atmospheric circulation patterns on regional climate variability in the Arctic, both today and in the past.

Abstract

Aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral (z_i/L=-1.2, where z_i is the inversion height and L is the Monin-Obukhov length) boundary layer over the ice-covered Bering Sea during February. Steam fog, formed over cracks and leads in the ice, was used as a tracer to delineate the various scales of roll motion seen in satellite images. The satellite information combined with aircraft data collected by the NOAA P-3 indicated the presence of a hierarchy of roll vortex motions. It is suggested that interactions of the various scales of motion resulted in certain scales dominating in one area and other scales dominating in another. Two-kilometer wavelength variations an attributed to the inflection point instability mechanism while 12–15 km variations seen to have been reinforced by the upstream topography on the Chukotka Peninsula. Organization of the fog banks on scales of 30 km was also present and may be attributable to resonant subharmonics of the basic boundary layer instability or to a mesoscale entrainment instability.

Abstract

Gap winds can be defined as a flow of air in a sea level channel which accelerates under the influence of a pressure gradient parallel to the axis of the channel. In February 1980 two distinct cases of gap winds were observed in the Strait of Juan de Fuca between western Washington State and British Columbia during a study that measured spatial variation of low-level marine winds and other parameters from the NOAA P-3 research aircraft and a dense network of surface stations which included eight meteorological buoys. These two cases were a high-pressure region over central British Columbia and a low-pressure system propagating northward, seaward of the Washington coast. Both cases produced strong easterly winds of 13–15 m s⁻¹ at the western end of the Strait of Juan de Fuea. The high-pressure region provided a drainage air mass from the interior of British Columbia which flowed through the Straits of Georgia and Juan de Fuca and eventually into the Pacific Ocean. This air mass remained nearly homogeneous and was capped by a well-defined inversion. For the offshore low-pressure center, the lower atmosphere was stably stratified throughout the region, and weak winds were observed at the eastern end of the Strait of Juan de Fuca with strong winds at the western end. Although the features of the flow fields were complex, major characteristics of the wind fields can be accounted for by the combined effect of topography and the synoptic pressure field. Local winds were in approximate ageostrophic equilibrium between the inertia term and the imposed sea level pressure gradient.

Abstract

The behavior of stratified air flowing around an isolated mountain is dependent on an internal Froude number (F), which indicates the relative importance of upstream velocity and vertical stratification. Three cases of the flow in the lee of the Olympic Mountains in the State of Washington are studied where the measured F was in the range 1.0–1.4 but apparently dominated by stable stratification. This study combined measurements of spatial variation of low-level winds and other parameters from a NOAA P-3 research aircraft with a dense network of surface stations including eight meteorological buoys and six upper-air stations. Results from these cases show the presence of an area of light winds in the lee of the Olympic Mountains. The characteristics of the flow are shown to be similar to laboratory results for low Froude number flow around an isolated obstacle where the flow is confined to quasi-horizontal planes. These cases are contrasted with a situation which led to the formation of a mesoscale low-pressure area and high surface winds in the lee of the mountains. The latter case was the Hood Canal Bridge storm on 13 February 1979 where local winds in the lee of the Olympic Mountains were in excess of 50 m s⁻¹. The flow at the surface was produced by down-pressure-gradient acceleration in the confined channels of Puget Sound toward the orographically produced low-pressure center. The measured internal Froude number in this situation was 4.6, and the pressure fields are shown to agree with the linear hydrostatic model developed by Smith (1980) for F > 1. It is suggested that the Froude number calculated from routine, upper-air sounding data is an index that forecasters can use to determine the potential for severe wind conditions over the inland waters of Puget Sound.

Abstract

The northerly winds that predominate along the U.S. west coast during April–September are interrupted periodically by abrupt reversals to southerly flow. The climatology and composite temporal evolution of these reversals from Point Conception to the Canadian border are documented using hourly data from moored coastal buoys and Coastal-Marine Automated Network stations for the period 1981–91. The reversals are divided into two categories: coastally trapped reversals, in which the southerly flow is highly ageostrophic and restricted to the coastal zone, and synoptic reversals, which are associated with landfalling troughs or fronts. Coastally trapped events occur on average about 1.5 times per month along the central and northern California coast, about twice a month near the California–Oregon border, and about once a month near the Oregon–Washington border. The ratio of coastally trapped reversals to synoptic reversals is higher during July–September and lower during April–June, particularly in the north. Roughly one-quarter of the coastally trapped reversals have a southerly wind component that exceeds 5 m s⁻¹. Reversals along the California coast are gradual; the changes in the alongshore winds usually occur over a period of 6 h or longer, and the maximum southerlies are less than 8 m s⁻¹. In contrast, roughly one-half of the reversals north of the California–Oregon border feature abrupt changes with southerly winds reaching approximately 10–12 m s⁻¹ within 2–3 h of the wind shifts. These stronger northern events often include substantial decreases in air temperature and rises in pressure. The southerlies associated with coastally trapped reversals persist for an average of about 30 h at a particular location. There is a strong tendency for coastally trapped reversals to occur during the night or morning. North of Monterey Bay, the reversals typically advance poleward (but not necessarily in a smoothly continuous manner) at a mean speed of 7–8 m s⁻¹ and maintain significant amplitude for an alongshore distance of 500–1000 km.

Abstract

The January–February mean central pressure of the Aleutian low is investigated as an index of North Pacific variability on interannual to decadal timescales. Since the turn of the century, 37% of the winter interannual variance of the Aleutian low is on timescales greater than 5 yr. An objective algorithm detects zero crossings of Aleutian low central pressure anomalies in 1925, 1931, 1939, 1947, 1959, 1968, 1976, and 1989. No single midtropospheric teleconnection pattern is sufficient to capture the variance of the Aleutian low. The Aleutian low covaries primarily with the Pacific–North American (PNA) pattern but also with the Arctic Oscillation (AO). The change to a prominent deep Aleutian low after 1977 is seen in indices of both the PNA and AO; the return to average conditions after 1989 was also associated with a change in the AO. The authors’ analysis suggests an increasing covariability of the high- and midlatitude atmosphere after 1970.