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G. W. K. Moore
and
Gerald Holdsworth

Abstract

In late May 2005, three climbers were immobilized at 5400 m on Mount Logan, Canada’s highest mountain, by the high-impact weather associated with an extratropical cyclone over the Gulf of Alaska. Rescue operations were hindered by the high winds, cold temperatures, and heavy snowfall associated with the storm. Ultimately, the climbers were rescued after the weather cleared. Just prior to the storm, two automated weather stations had been deployed on the mountain as part of a research program aimed at interpreting the climate signal contained in summit ice cores. These data provide a unique and hitherto unobtainable record of the high-elevation meteorological conditions associated with an intense extratropical cyclone. In this paper, data from these weather stations along with surface and sounding data from the nearby town of Yakutat, Alaska, satellite imagery, and the NCEP reanalysis are used to characterize the synoptic-scale conditions associated with this storm. Particular emphasis is placed on the water vapor transport associated with this storm.

The authors show that during this event, subtropical moisture was transported northward toward the Mount Logan region. The magnitude of this transport into the Gulf of Alaska was exceeded only 1% of the time during the months of May and June over the period 1948–2005. As a result, the magnitude of the precipitable water field in the Gulf of Alaska region attained values usually found in the Tropics. An atmospheric moisture budget analysis indicates that most of the moisture advected into the Mount Logan region was preexisting water vapor already in the subtropical atmosphere and was not water vapor evaporated from the surface during the evolution of the storm. Implications of this moisture source for understanding of the water isotopic climate signal in the Mount Logan ice cores will be discussed.

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Hongxu Zhao
and
G. W. K. Moore

Abstract

Although the Arctic Oscillation (AO) and North Atlantic Oscillation (NAO) have been identified as important modes of climate variability during the Northern Hemisphere (NH) winter, whether the AO or the NAO is more fundamental to the description of this variability, especially in the North Pacific, is still an open question. An important contributor to this uncertainty is the lack of knowledge of the low-frequency linkages between the North Atlantic and North Pacific Oceans. This paper explores the linkage between the two oceanic basins on interdecadal time scales using the sea level pressure (SLP) field during the twentieth century. In particular, it is shown that the winter mean SLP in the North Pacific was positively correlated with the sign of the NAO during the periods of 1925–50 and 1980–98, which resulted in the classical AO pattern being the dominant mode in the NH. In contrast, during the period of 1951–79, the winter mean SLP in the two basins was decoupled, resulting in a dominant mode that more closely resembled the NAO. Using paleoclimate reconstructions, it is also shown that this interdecadal variability in the North Pacific climate began around 1850, which is nominally considered to be the end of the Little Ice Age.

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G. W. K. Moore

Abstract

Cape Farewell, Greenland’s southernmost point, is characterized by a number of low-level jets that are the result of topographic flow distortion associated with passing extratropical cyclones. The heavy seas associated with these wind events are a hazard to maritime traffic in the region. In addition, the air–sea heat flux associated with these weather systems plays an important role in the climate system by contributing to the forcing of the lower limb of the Atlantic meridional overturning circulation. In this paper, the North American Regional Reanalysis will be used to generate a higher-resolution climatology of these mesoscale jets as compared to previous studies. Through the use of a diagnostic that partitions the occurrence frequency of high-speed wind events by wind direction, the author shows that there are four different types of Cape Farewell tips jets that are characterized as having either northwesterly, southwesterly, northeasterly, or southeasterly wind direction. All four types have distinct regions in the vicinity of Cape Farewell where their respective occurrence frequencies and air–sea heat fluxes are at a maximum. The southwesterly and northeasterly jets closely resemble the wind systems previously identified as being westerly and easterly tip jets. There are also instances where one type evolves into another and so it is possible to view westerly tip jets as a continuum with the northwesterly and southwesterly events identified in this paper representing end members with a similar picture for easterly tip jets. The position of a particular event along these continua will determine its impact on local weather and the coupled climate system.

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S. Sathiyamoorthy
and
G. W. K. Moore

Abstract

A novel way of quantifying the variance of a time series is presented. The method first involves filtering the time series using filters with different temporal characteristics, and then using a moving window to calculate the variances in each filtered time series. The use of a moving window allows the original temporal resolution to be retained, as well as allowing one to study how the variance changes with time. Air–sea interaction time series from Ocean Weather Station (OWS) Bravo in the Labrador Sea are analyzed as an example. High-pass, bandpass, and low-pass filters are used to isolate the diurnal signal, the storm/cyclone signature, and the weather regime transition signal, respectively. The variance during the winter months is found to be strongly influenced by weather systems in the bandpass and the low-pass frequency range. The variance during the summer months, on the other hand, is dominated by the shortwave radiation in the high-pass frequency range.

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Mariusz Pagowski
and
G. W. K. Moore

Abstract

In this paper, the ability of the MM5 mesoscale forecast model to simulate the air–sea interaction, boundary layer development, and mesoscale structure associated with a cold-air outbreak over the Labrador Sea is investigated. The case chosen was one for which research aircraft data and satellite imagery are available for validation. The default surface-layer parameterization included in the model is shown to grossly overestimate the magnitude of the air–sea interaction resulting in forecasts of boundary layer growth and mesoscale development that differ substantially from observations. It is also shown that a representation of the inhomogeneities in sea-ice cover results in a significant improvement in simulations of the air–sea interaction, boundary layer development, and mesoscale structure both within the marginal ice zone and downstream over the open ocean. Finally, the mesoscale cyclones or polar lows observed in the wake of the cold-air outbreak are shown to be coupled to the evolution of an upper-tropospheric potential vorticity anomaly that was advected over the region. The model simulations suggest that MM5, most probably due to inaccuracies in and the limited resolution of the analyzed fields that supply the initial and boundary conditions to the model, was unable to correctly simulate the development and track of this anomaly and this ultimately led to an incorrect forecast of the polar lows’ finite-amplitude behavior.

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S. Sathiyamoorthy
and
G. W. K. Moore

Abstract

Deep water formation at high latitudes is believed to be the driving mechanism behind the ocean's thermohaline circulation. The exchange of heat and water with the atmosphere causes the density of the surface waters to change, with subsequent downwelling and upwelling resulting as the system relaxes toward convective equilibrium. The characteristics of this atmosphere–ocean exchange are examined by studying the temporal variability of the buoyancy flux at OWS Bravo, a location where deep water formation is known to occur. The authors find that there is significant high-frequency variability in the buoyancy flux attributable to the passage of synoptic weather systems, variability that is masked by monthly analyses. At high latitudes, precipitation plays a significant role in the buoyancy flux. If it is ignored, the buoyancy loss is overestimated (positive coordinate is downward). Precipitation also causes the buoyancy flux to become positive during the passage of a cyclone. The timescale for this change in buoyancy flux is found to be similar to the timescale for the convective plumes in the ocean, suggesting a link between the two. In addition, a strong negative correlation is found to exist between the sensible heat flux at Bravo and the North Atlantic Oscillation.

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Ian A. Renfrew
and
G. W. K. Moore

Abstract

Observational data from two research aircraft flights are presented. The flights were planned to investigate the air–sea interaction during an extreme cold-air outbreak, associated with the passage of a synoptic-scale low pressure system over the Labrador Sea during 8 February 1997. This is the first such aircraft-based investigation in this remote region. Both high-level dropsonde and low-level flight-level data were collected. The objectives were twofold: to map out the structure of the roll vortices that cause the ubiquitous cloud streets seen in satellite imagery, and to estimate the sensible and latent heat fluxes between the ocean and atmosphere during the event. The latter was achieved by a Lagrangian analysis of the flight-level data. The flights were part of the Labrador Sea Deep Convection Experiment, investigating deep oceanic convection, and were planned to overpass a research vessel in the area.

The aircraft-observed roll vortices had a characteristic wavelength of 4–5 km, particularly evident in the water vapor signal. Unlike observations of roll vortices in other regions, a roll signature was absent from the temperature data. Analysis of satellite imagery shows the cloud streets had a characteristic wavelength of 7–10 km, indicating a multiscale roll vortex regime. There was a dramatic deepening of the boundary layer with fetch, and also with time. Off the ice edge, surface sensible heat fluxes of 500 W m−2 and surface latent heat fluxes of 100 W m−2 were measured, with uncertainties of ±20%. The very cold air is thought to be responsible for the unusually high Bowen ratio observed.

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G. W. K. Moore
and
I. A. Renfrew

Abstract

The high topography of Greenland results in a number of orographically induced high wind speed flows along its coast that are of interest from both a severe weather and climate perspective. Here the surface wind field dataset from the NASA–JPL SeaWinds scatterometer on board the Quick Scatterometer (QuikSCAT) satellite is used to develop a wintertime climatology of these flows. The high spatial resolution and the twice-daily sampling of the SeaWinds instrument allows for a much more detailed view of the surface winds around Greenland than has been previously possible. Three phenomena stand out as the most distinctive features of the surface wind field during the winter months: the previously identified tip jets and reverse tip jets, as well as the hitherto unrecognized barrier flows along its southeast coast in the vicinity of the Denmark Strait. Peak surface wind speeds associated with these phenomena can be as large as 50 m s−1 with winds over 25 m s−1 occurring approximately 10%–15% of the time at each location.

A compositing technique is used to show that each type of flow is the result of an interaction between a synoptic-scale parent cyclone and the high topography of Greenland. In keeping with previous work, it is argued that tip jets are caused by a combination of conservation of the Bernoulli function during orographic descent and acceleration due to flow splitting as stable air passes around Cape Farewell, while barrier winds are a geostrophic response to stable air being forced against high topography. It is proposed that reverse tip jets occur when barrier winds reach the end of the topographic barrier and move from a geostrophic to a gradient wind balance, becoming supergeostrophic as a result of their anticyclonic curvature.

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Vladimir V. Smirnov
and
G. W. K. Moore

Abstract

The transport of water vapor through the Mackenzie River basin, a typical high-latitude river basin, is examined for the years 1979–93, with the European Centre for Medium-Range Weather Forecasts reanalysis dataset (ERA). It is shown that the transport of water vapor through the Mackenzie basin is highly variable in space and time. This transport has two distinct modes. During the autumn, winter, and spring, moisture is transported into the basin from the southwest by extratropical cyclones. The source of this moisture is argued to be the subtropical and midlatitude central Pacific Ocean. During the summer, moisture enters the basin from the northwest, with the source region being the Arctic Ocean. The values of monthly water vapor budgets obtained with the objectively analyzed fields are compared with those obtained from interpolated radiosonde data and with other known quantitative information about the water budget of the basin. It is found that the ERA data seriously overestimate the values of monthly water budget. A discussion of various potential sources of this discrepancy is provided.

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G. W. K. Moore
and
John L. Semple

Scientific interest in Mount Everest has been largely focused on the physiology of hypoxia caused by the summit's low barometric pressure. Although weather is recognized as a significant risk for climbers on the mountain, it has not been extensively studied. In this paper, we reconstruct the meteorological conditions associated with the deadly outbreak of high-impact weather on Mount Everest that occurred in May 1996 and was the subject of the best-selling book Into Thin Air. The authors show that during this event, two jet streaks—an upper-level shortwave trough and an intrusion of stratospheric air into the upper troposphere—were present in the vicinity of Mount Everest. Meanwhile, in the lower troposphere, there was convergence of water vapor transport from both the Arabian Sea and the Bay of Bengal into the region to the south of Mount Everest. The authors propose that the ageostrophic circulation associated with the upper-level features resulted in a region of large-scale ascent near Mount Everest that, in combination with the anomalous availability of moisture in the region, triggered convective activity. The resulting high-impact weather trapped over 20 climbers on Mount Everest's exposed upper slopes leading to the deaths of 8. These synoptic-scale characteristics provide some expectation of predicting life-threatening high-altitude storms in the Himalayas. In addition, the authors argue that the falling barometric pressure and the presence of ozone-rich stratospheric air that occurred near the summit of Mount Everest during this event could have shifted a coping climber from a state of brittle tolerance to physiological distress.

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