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James E. Overland and Nicholas A. Bond

Abstract

Blocking of onshore flow by coastal mountains was observed south of Vancouver Island, British Columbia, by the NOAA P-3 aircraft on 1 December 1993. Winds increased from 10 m s−1 offshore to 15 m s−1 nearshore and became more parallel to shore in the blocked region, which had a vertical scale of 500 m and an offshore scale of 40–50 km. These length scale and velocity increases are comparable to theory. The flow was semigeostrophic with the coast being hydrodynamically steep; that is, the coast acts like a wall and the alongshore momentum balance is ageostrophic. This is shown by the nondimensional slope parameter—the Burger number, B = hmN/fLm—being greater than 1, where hm and Lm are the height and half-width of the mountain, N is the stability frequency, and f is the Coriolis parameter. The height scale is given by setting the local Froude number equal to 1—that is, hl = U/N ∼ 500 m, where U is the onshore component of velocity. This scale is appropriate when hl is less than the mountain height, hm; in this case hl/hm ∼ 0.4. The offshore scale is given by the Rossby radius LR = (Nhm/f)Fm = U/f ∼ 50 km for F m < 1, where the mountain Froude number F m = h l/h m = U/h m N ∼ 0.4. The increase in the alongshore wind speed due to blocking, &DeltaV, is equal to the onshore component of the flow, U ≈ 6 m s−1 or in this case about half of the near-coastal alongshore component. A second case on 11 December 1993 had stronger onshore winds and weak stratification and was in a different hydrodynamic regime, with F m ∼ 6. When F m > 1, L R = Nh m/f ∼ 200 km, and ΔV = h m N ∼ 2 m s−1, a small effect comparable to changes in the synoptic-scale flow. The authors expect a maximum coastal jet response when F m ∼ 1.

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James E. Overland and William H. Gemmill

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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.

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James E. Overland and Bernard A. Walter Jr.

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.

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Bernard A. Walter Jr. and James E. Overland

Abstract

Aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral (zi/L=-1.2, where zi 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.

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Gary M. Lackmann and James E. Overland

Abstract

Gap winds occur in topographically restricted channels when a component of the pressure gradient is parallel to the channel axis. Aircraft flight-level data are used to examine atmospheric structure and momentum balance during an early spring gap-wind event in Shelikof Strait, Alaska. Alongshore sea level pressure ridging was observed. Vertical cross sections show that across-strait gradients of boundary-layer temperature and depth accounted for the pressure distribution. Geostrophic adjustment of the mass field to the along-strait wind component contributed to development of the observed pressure pattern. Boundary-layer structure and force balance during this event was similar to that often observed along isolated barriers. However, the Rossby radius was lager than the strait width, and atmospheric structure in the strait exit region indicates transition of the flow to open coastline conditions. Two across-strait momentum budgets show that the Coriolis force and across-strait pressure gradient were an order of magnitude larger than other terms. Largest terms in the along-strait balance were the pressure gradient force, acceleration, entrainment, and friction. Boundary-layer acceleration in the along-strait direction was 55% of the potential Emit determined by the along-strait pressure gradient. Entrainment of air into the boundary layer was the largest retarding force and contributed to the along-strait profile of boundary-layer depth. Large horizontal divergence was observed within the strait, yet boundary-layer depth increased slightly following the flow. Entrainment at the inversion and sea surface fluxes accounted for along-strait variation of boundary-layer equivalent potential temperature.

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Kevin R. Wood and James E. Overland

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.

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James E. Overland and Carol H. Pease

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.

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James E. Overland and R. W. Preisendorfer

Abstract

A technique is presented for selection of principal components for which the geophysical signal is greater than the level of noise. The level of noise is simulated by repeated sampling of principal components computed from a spatially and temporally uncorrected random process. By contrasting the application of principal components based upon the covariance matrix and correlation matrix for a given data set of cyclone frequencies, it is shown that the former is more suitable to fitting data and locating the individual variables that represent large variance in the record, while the latter is more suitable for resolving spatial oscillations such as the movement of primary storm tracks.

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James E. Overland, Philip Turet, and Abraham H. Oort

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The authors investigate the climmological heating of the Arctic by the atmospheric moist static energy (MSE) flux from lower latitudes based on 25 years (November 1964–1989) of the GFDL dataset. During the five month winter period (NDJFM) the transport of sensible heat by transient eddies is the largest component (50%) at 70°N, followed by the transport of sensible but by standing eddies (25%), and the moist static energy flux by the mean meridional circulation (25%). The mean meridional circulation (MMC) changes from a Ferrel cell to a thermally direct circulation near 60°N; maximum horizontal velocities in the thermally direct circulation peak new 70°N. North of 60°N the sensible heat flux by the MMC is southward and opposes the greater northward transport of geopotential energy. The transport of energy is not uniform. Major pathways are the northward transport of positive anomalies through the Greenland and Barents Seas into the eastern Arctic and the southward transport of negative anomalies to the cast of the Siberian high. The Atlantic pathway in winter relates to transport by transient eddies, while the western Siberian flux relates to the standing eddy pattern. Interannual variability of northward MSE is concentrated in these two regions. The western Arctic Ocean from about 30° to 60°W receives about 50 W m−2 less energy flux convergence than the eastern Arctic. This result compares well with the observed minimum January surface air temperatures in the Canadian Basin of the western Arctic and implies that the greater observed ice thickness in this region may have a thermodynamic as well as a dynamic origin.

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James E. Overland, Muyin Wang, and Nicholas A. Bond

Abstract

The lower troposphere of the western Arctic (eastern Siberia to northern Canada) was relatively warm during spring in the 1990s. Based on the NCEP–NCAR reanalysis, supplemented by the Television Infrared Observational Satellite (TIROS) Operational Vertical Sounder (TOVS) Polar Pathfinder dataset, this warmth is a result of a recent increase in the frequency of warm months, compared to the previous four decades. The primary difference between four notably warm springs in the 1990s and four cold springs in the 1980s was the sense of the horizontal advection term in a lower-tropospheric heat budget for northern Alaska/southern Beaufort Sea. While the horizontal advection of heat was highly episodic, it was related to changes in the mean circulation at low levels, in particular a shift from anomalous northeasterly flow in the 1980s to anomalous southwesterly flow in the 1990s during March and April. This change in the low-level winds in the western Arctic coincided with a systematic shift in the Arctic Oscillation (AO) near the end of the 1980s, and reflects the equivalent barotropic nature of the AO. The stratospheric temperature anomalies associated with the AO were greatest in March; the low-level wind anomalies brought about near-surface temperature anomalies in northern Alaska that peaked in April. In addition to substantial decadal differences, there was considerable month-to-month and year-to-year variability within the last two decades.

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