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Donald B. Percival, James E. Overland, and Harold O. Mofjeld

Abstract

A major difficulty in investigating the nature of interdecadal variability of climatic time series is their shortness. An approach to this problem is through comparison of models. In this paper a first-order autoregressive [AR(1)] model is contrasted with a fractionally differenced (FD) model as applied to the winter-averaged sea level pressure time series for the Aleutian low [the North Pacific (NP) index] and the Sitka winter air temperature record. Both models fit the same number of parameters. The AR(1) model is a “short-memory” model in that it has a rapidly decaying autocovariance sequence, whereas an FD model exhibits “long memory” because its autocovariance sequence decays more slowly.

Statistical tests cannot distinguish the superiority of one model over the other when fit with 100 NP or 146 Sitka data points. The FD model does equally well for short-term prediction and has potentially important implications for long-term behavior. In particular, the zero crossings of the FD model tend to be farther apart, so they have more of a “regimelike” character; a quarter century interval between zero crossings is 4 times more likely with the FD than the AR(1) model. The long-memory parameter δ for the FD model can be used as a characterization of regimelike behavior. The estimated δs for the NP index (spanning 100 yr) and the Sitka time series (168 yr) are virtually identical, and their size implies moderate long-memory behavior. Although the NP index and the Sitka series have broadband low-frequency variability and modest long-memory behavior, temporal irregularities in their zero crossings are still prevalent. Comparison of the FD and AR(1) models indicates that regimelike behavior cannot be ruled out for North Pacific processes.

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John M. Bane, Clinton D. Winant, and James E. Overland

A number of observational programs have been carried out on the United States continental shelf to describe coastal-ocean circulation with emphasis on mesoscale processes. In several of these studies the atmosphere was found to play a central role in determining the coastal circulation through either local or remote forcing. Because of these results, the Coastal Physical Oceanography (CoPO) planning effort has designated three coastal air-sea interaction areas to focus on in a national program to study the physical processes on the continental shelf. These areas are shelf frontogenesis, interaction of stable layers with topography, and forcing by severe storms. The long-term objective of the air-sea interaction component of CoPO is to better understand the structure, dynamics, and evolution of the various mesoscale and synoptic-scale processes that significantly affect coastal/shelf circulation through air-sea interactions. Within this body of knowledge will be an improved quantification of the air-sea exchanges of dynamically important quantities set in the framework of mesoscale and synoptic-scale processes.

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Nicholas A. Bond, Clifford F. Mass, and James E. Overland

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.

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James E. Overland, Jennifer Miletta Adams, and Nicholas A. Bond

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.

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Nicholas A. Bond, James E. Overland, and Philip Turet

Abstract

The surface wind stress over the Bering Sea is estimated for the period 1946–90 from sea level pressure analyses, empirical relationships between the geostrophic wind and the surface wind, and a bulk aerodynamic formula. The focus is on the propagation and variability of the stress and the curl of the stress as a function of frequency. The stress at high frequencies (>0.1 cpd) is dominated by northward- and eastward-propagating disturbances with mean wavelengths of ∼2500 and 10 000 km, respectively. At periods of ∼10–100 days, the mean propagation is near zero; there are, however, significant interannual variations in the zonal propagation. Wind-driven ocean transports estimated by the Sverdrup method for the deep Bering basin account for ∼6 Sv or roughly one-half of the observed transport within the western boundary current along the Kamchatka peninsula. A low-pass-filtered (retaining periods greater than 18 months) time series of the Sverdrup transport exhibits a standard deviation of 25% of the mean.

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Jennifer Miletta Adams, Nicholas A. Bond, and James E. Overland

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In the Arctic atmosphere, the fall cooling cycle involves the evolution of the zonally symmetric circulation in late summer into the asymmetric flow of winter. This paper uses historical reanalysis data to document how the dominant components of the Arctic heat budget influence the summer–winter transition. The spatial variability of 20-yr climatologies of 700-mb temperature and geopotential height, the net surface flux, and the horizontal convergence of eddy sensible heat fluxes are examined for September through February.

The development of the zonal asymmetries in the temperature and geopotential height fields in the Arctic is linked to the land–water–ice distribution that regulates the surface fluxes and the baroclinic zones in the hemispheric circulation, which lead to regional heating/cooling by the transient and standing eddies. These eddies serve to transport the heat energy gained via the surface fluxes over the North Atlantic and North Pacific to the continental and ice-covered regions of the central Arctic, where the net surface flux is small. The transient eddies are especially important in the Atlantic and Eurasian sectors of the Arctic, while the standing eddies play the larger role in the heat budget on the Pacific side of the Arctic in early to mid-winter.

The Arctic oscillation (AO) has a small effect on the basinwide pattern of heating and cooling by the eddy circulations, but on smaller spatial scales there are isolated regions where the AO influences the Arctic heat budget.

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James E. Overland, Jennifer Miletta Adams, and Nicholas A. Bond

Abstract

The surface temperature field in the Arctic winter is primarily controlled by downward longwave radiation, which is determined by local atmospheric temperature and humidity profiles and the presence of clouds. The authors show that regional differences in the atmospheric thermal energy budget are related to the tropospheric circulation in the Arctic. Data sources include several gridded meteorological datasets and surface and rawinsonde observational data. Four independent climatologies of mean January surface temperature show consistent spatial patterns: coldest temperatures in the western Arctic north of Canada and warmer regions in the Chukchi, Greenland, and Barents Seas. Data from the five winters of 1986–90 illustrate the coupling between the surface temperature, the downward longwave radiative fields, and the tropospheric temperature and humidity fields, with monthly surface–upper-air correlations on the order of 0.6. Upper-level circulation patterns reveal features similar to the surface temperature fields, notably a persistent low center located over northern Canada; the cyclonic flow around the low is a tropospheric extension of the polar vortex. Colder and drier conditions are maintained within the vortex and communicated to the surface through radiative processes. The polar vortex also steers transient weather systems, the most important mechanism for horizontal heat transport, into the eastern Arctic, which results in as much as 25 W m−2 more heat flux into the eastern Arctic than the western Arctic. A reason for the colder temperatures in the western Arctic is that the polar vortex tends to be situated downstream of the northern Rocky Mountains; this preferred location is related to orographic forcing of planetary waves. Monthly and interannual variability of winter temperatures is conditioned by the interaction of the Arctic and midlatitude circulations through the strength and position of the polar vortex.

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James E. Overland, Michael C. Spillane, Harley E. Hurlburt, and Alan J. Wallcraft

Abstract

A limited-area, primitive equation, three-layer hydrodynamic model, with realistic coastlines and bathymetry and ⅛° resolution, is used to investigate the circulation of the Bering Sea basin and the adjacent North pacific ocean. The westward flowing Alaskan Stream to the south of the Aleutian Island chain is specified as a boundary condition at inflow and outflow ports with a constant throughput of 15 Sv (Sv = 1 ×106 m3 s−1). Atmospheric forcing is based an the Hellerman and Rosenstein monthly climatological wind field. The model is spun up over 50 years and the statistics of the final decade are described. The general features of the model circulation as discussed below are consistent with available hydrographic and buoy drift observations. The model Alaskan Stream separates from the Aleutian Island chain near 175°; beyond this point there is strong interannual variability associated with meandering and occasional eddy shedding along the northern arm of the western subarctic gyre. There is a generally cyclonic, but spatially complex and nonstationary, circulation within the Bering See basin, fed by inflow through the Aleutian passes; outflow is confined to Kamchatka Strait and varies seasonally, between 8.5 Sv in summer and 13 Sv in winter. A region of intense eddy activity lies west-northeast of Bowers Ridge. The model predicts seasonal reversals in the Bering slope current that are not clearly evident in the temporally sparse observational database. The numerical study demonstrates that flow instabilities contribute to substantial interannual variability in the circulation of the Bering Sea and adjacent northwest Pacific Ocean.

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Muyin Wang, James E. Overland, Vladimir Kattsov, John E. Walsh, Xiangdong Zhang, and Tatyana Pavlova

Abstract

There were two major multiyear, Arctic-wide (60°–90°N) warm anomalies (>0.7°C) in land surface air temperature (LSAT) during the twentieth century, between 1920 and 1950 and again at the end of the century after 1979. Reproducing this decadal and longer variability in coupled general circulation models (GCMs) is a critical test for understanding processes in the Arctic climate system and increasing the confidence in the Intergovernmental Panel on Climate Change (IPCC) model projections. This study evaluated 63 realizations generated by 20 coupled GCMs made available for the IPCC Fourth Assessment for their twentieth-century climate in coupled models (20C3M) and corresponding control runs (PIcntrl). Warm anomalies in the Arctic during the last two decades are reproduced by all ensemble members, with considerable variability in amplitude among models. In contrast, only eight models generated warm anomaly amplitude of at least two-thirds of the observed midcentury warm event in at least one realization, but not its timing. The durations of the midcentury warm events in all the models are decadal, while that of the observed was interdecadal. The variance of the control runs in nine models was comparable with the variance in the observations. The random timing of midcentury warm anomalies in 20C3M simulations and the similar variance of the control runs in about half of the models suggest that the observed midcentury warm period is consistent with intrinsic climate variability. Five models were considered to compare somewhat favorably to Arctic observations in both matching the variance of the observed temperature record in their control runs and representing the decadal mean temperature anomaly amplitude in their 20C3M simulations. Seven additional models could be given further consideration. Results support selecting a subset of GCMs when making predictions for future climate by using performance criteria based on comparison with retrospective data.

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John E. Walsh, David H. Bromwich, James. E. Overland, Mark C. Serreze, and Kevin R. Wood

Abstract

The polar regions present several unique challenges to meteorology, including remoteness and a harsh environment. We summarize the evolution of polar meteorology in both hemispheres, beginning with measurements made during early expeditions and concluding with the recent decades in which polar meteorology has been central to global challenges such as the ozone hole, weather prediction, and climate change. Whereas the 1800s and early 1900s provided data from expeditions and only a few subarctic stations, the past 100 years have seen great advances in the observational network and corresponding understanding of the meteorology of the polar regions. For example, a persistent view in the early twentieth century was of an Arctic Ocean dominated by a permanent high pressure cell, a glacial anticyclone. With increased observations, by the 1950s it became apparent that, while anticyclones are a common feature of the Arctic circulation, cyclones are frequent and may be found anywhere in the Arctic. Technology has benefited polar meteorology through advances in instrumentation, especially autonomously operated instruments. Moreover, satellite remote sensing and computer models revolutionized polar meteorology. We highlight the four International Polar Years and several high-latitude field programs of recent decades. We also note outstanding challenges, which include understanding of the role of the Arctic in variations of midlatitude weather and climate, the ability to model surface energy exchanges over a changing Arctic Ocean, assessments of ongoing and future trends in extreme events in polar regions, and the role of internal variability in multiyear-to-decadal variations of polar climate.

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