Abstract

Characteristics of the Arctic Ocean’s Beaufort Sea high are examined using fields from the NCEP–NCAR reanalysis. At a 2-hPa contour interval, the Beaufort Sea high appears as a closed anticyclone in the long-term annual mean sea level pressure field and in spring. In winter, the Beaufort Sea region is influenced by a pressure ridge at sea level extending from the Siberian high to the Yukon high over northwestern Canada. As assessed from 6-hourly surface winds, the mean frequency of anticyclonic surface winds over the Beaufort Sea region is fairly constant through the year. While for all seasons a strong closed high can be interpreted as the surface expression of an amplified western North American ridge at 500 hPa, there is some suggestion of a split flow, where the ridge linked to the surface high is separated from the ridge to the south that lies within the main belt of westerlies. The Aleutian low in the North Pacific tends to be deeper than normal when there is a strong Beaufort Sea high. In all seasons but autumn, a strong Beaufort Sea high is associated with positive lower-tropospheric temperature anomalies covering much of the Arctic Ocean; positive anomalies are especially pronounced in spring. Seasons with a weak anticyclone show broadly opposing anomalies. A strong high is found to be a feature of the negative phase of the summer northern annular mode, the positive phase of the Pacific–North American wave train, and, to a weaker extent, the positive phase of the summer Arctic dipole anomaly and Pacific decadal oscillation. The unifying theme is that, to varying degrees, the high-latitude 500-hPa ridge associated with the Beaufort Sea high represents a center of action in each teleconnection pattern.

Abstract

An improved monthly precipitation climatology for the Arctic is developed by blending the Legates and Willmott gridded product with measurements from Russian “North Pole” drifting stations and gauge-corrected station data for Eurasia and Canada. The improved climatology is used to examine the accuracy of mean precipitation forecasts from the National Centers for Environmental Prediction (NCEP) and European Reanalysis Agency (ERA) reanalysis models, based on data for the period 1979–88. Both models capture the major spatial features of annual mean precipitation and general aspects of the seasonal cycle but with some notable errors. Both underestimate precipitation over the Atlantic side of the Arctic. NCEP overestimates annual totals over land areas and to a somewhat lesser extent over the central Arctic Ocean. Except for the North Atlantic–Scandinavia sector, the NCEP model also depicts the seasonal precipitation maximum consistently one month early in July. Overall, the ERA predictions are better. Both models perform best during winter and worst during summer.

The most significant problem with the NCEP model is a severe oversimulation of summer precipitation over land areas, due to excessive convective precipitation. Further investigation for July reveals that both the NCEP analyses and 12-h forecasts are too wet below about 850 mb and have more negative low-level temperature gradients as compared to available rawinsonde profiles. This suggests that low-level observations are not being effectively incorporated in the analyses. Given this finding, the high humidities are consistent with excessive surface evaporation rates. This problem may in turn relate to soil moisture, which NCEP updates by the modeled precipitation. If soil moisture is too high, this would favor excessive evaporation and high low-level humidities, fostering excessive precipitation, in turn keeping soil moisture and evaporation rates high. The NCEP downwelling shortwave fluxes are also much too high, contributing to excessive evaporation and possibly influencing the low-level temperature gradients. By comparison, soil moisture in the ERA model is adjusted using the difference between the model first guess and analysis value (the analysis increment) of low-level humidity, which prevents model drift. The ERA downwelling shortwave fluxes are also closer to observations. These attributes are consistent with the superior ERA precipitation forecasts in summer and suggest avenues for improving the performance of the NCEP model.

Abstract

The Arctic frontal zone (AFZ) is a narrow band of strong horizontal temperature gradients that develops along the Arctic Ocean coastline each summer in response to differential heating of the atmosphere over adjacent land and ocean surfaces. Past research has linked baroclinicity within the AFZ to summer Arctic cyclone development, especially by intensifying storms that migrate northward from the Eurasian continent. This study uses the Community Earth System Model Large Ensemble in conjunction with an advanced cyclone detection and tracking algorithm to assess how the AFZ, summer Arctic cyclone activity, and the relationship between them respond to warming under the representative concentration pathway 8.5 (RCP8.5) scenario. Under this strong warming scenario, the AFZ remains a significant cyclone intensifier. Changes to the AFZ are largely restricted to June, when earlier snowmelt leads to strengthening of land–ocean temperature contrasts. This strengthening is accompanied by enhanced cyclogenesis along the east Siberian coast, but no change is observed for overall cyclone frequency over the Arctic Ocean. However, simultaneous changes to subpolar storm tracks impact Arctic cyclone activity in all summer months, sometimes in opposition to the impact of the AFZ. In June, the storms migrating poleward to the Arctic Ocean become weaker under RCP8.5, leading to lower Arctic cyclone intensity. In July and August, the poleward shift of the North Pacific storm track enhances cyclone activity in the Beaufort and Chukchi Seas.

Abstract

Extratropical cyclone activity over the central Arctic Ocean reaches its peak in summer. Previous research has argued for the existence of two external source regions for cyclones contributing to this summer maximum: the Eurasian continent interior and a narrow band of strong horizontal temperature gradients along the Arctic coastline known as the Arctic frontal zone (AFZ). This study incorporates data from an atmospheric reanalysis and an advanced cyclone detection and tracking algorithm to critically evaluate the relationship between the summer AFZ and cyclone activity in the central Arctic Ocean. Analysis of both individual cyclone tracks and seasonal fields of cyclone characteristics shows that the Arctic coast (and therefore the AFZ) is not a region of cyclogenesis. Rather, the AFZ acts as an intensification area for systems forming over Eurasia. As these systems migrate toward the Arctic Ocean, they experience greater deepening in situations when the AFZ is strong at midtropospheric levels. On a broader scale, intensity of the summer AFZ at midtropospheric levels has a positive correlation with cyclone intensity in the Arctic Ocean during summer, even when controlling for variability in the northern annular mode. Taken as a whole, these findings suggest that the summer AFZ can intensify cyclones that cross the coast into the Arctic Ocean, but focused modeling studies are needed to disentangle the relative importance of the AFZ, large-scale circulation patterns, and topographic controls.

Abstract

At least four different modeling studies indicate that variability in snow cover over Asia may modulate atmospheric circulation over the North Pacific Ocean during winter. Here, satellite data on snow extent for east Asia for 1971–95 along with atmospheric fields from the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis are used to examine whether the circulation signals seen in model results are actually observed in nature. Anomalies in snow extent over east Asia exhibit a distinct lack of persistence. This suggests that understanding the effects of east Asian snow cover is more germane for short- to medium-range weather forecasting applications than for problems on longer timescales. While it is impossible to attribute cause and effect in the empirical study, analyses of composite fields demonstrate relationships between snow cover extremes and atmospheric circulation downstream remarkably similar to those identified in model results. Positive snow cover extremes in midwinter are associated with a small decrease in air temperatures over the transient snow regions, a stronger east Asian jet, and negative geopotential height anomalies over the North Pacific Ocean. Opposing responses are observed for negative snow cover extremes. Diagnosis of storm track feedbacks shows that the action of high-frequency eddies does not reinforce circulation anomalies in positive snow cover extremes. However, in negative snow cover extremes, there are significant decreases in high-frequency eddy activity over the central North Pacific Ocean, and a corresponding decrease in the mean cyclonic effect of these eddies on the geopotential tendency, contributing to observed positive height anomalies over the North Pacific Ocean. The circulation signals over the North Pacific Ocean are much more pronounced in midwinter (January–February) than in the transitional seasons (November–December and March–April).

Abstract

A fascinating feature of the northern high-latitude circulation is a prominent summer maximum in cyclone activity over the Arctic Ocean, centered near the North Pole in the long-term mean. This pattern is associated with the influx of lows generated over the Eurasian continent and cyclogenesis over the Arctic Ocean itself. Its seasonal onset is linked to the following: an eastward shift in the Urals trough, migration of the 500-hPa vortex core to near the pole, and development of a separate region of high-latitude baroclinicity. The latter two features are consistent with differential atmospheric heating between the Arctic Ocean and snow-free land. Variability in the strength of the cyclone pattern can be broadly linked to the phase of the summer northern annular mode. When the cyclone pattern is well developed, the 500-hPa vortex is especially strong and symmetric about the pole, with negative sea level pressure (SLP) anomalies over the pole and positive anomalies over middle latitudes. Net precipitation tends to be anomalously positive over the Arctic Ocean. When poorly developed, the opposite holds.

Abstract

Synoptic activity in the Arctic basin from 70°–907deg;N is examined for the period 1979–85, using improved pressure analyses incorporating data from a network of drifting buoys. Geographical and seasonal variations in cyclone and anticyclone frequencies, mean cyclone pressures and other cyclone characteristics are determined. Results, in general, compare favorably with those from earlier studies.

The atmospheric circulation of the Arctic is characterized by strong seasonality, but with considerable year-to-year variability, evidenced through large-scale seasonal shifts in the position and intensity of cyclone and anticyclone frequency maxima, the types of systems comprising these pattern, and major cyclone tracks. In winter and spring, cyclonic activity is largely restricted to the eastern Arctic. Local frequency maxima are found near Svalbard, the northern tip of Novaya Zemlya, and, in winter, also near the Pole at about 90°E. The systems comprising these patterns are migratory, the majority entering store the North Atlantic and Barents Sea. Corresponding anticyclone frequency maxima occur within the Canada basin, in a broad zone from about 160°E to 140°W, extending up to 85°N. By contrast, the Canada basin in summer is the region of highest cyclone frequencies. Systems migrate into this region primarily from along the Siberian coast and subsequently stall, resulting in a persistent persistent of low pressure analogous to the Ieclandic low. This summer cyclone pattern is detailed in a case study. The summer anticyclone pattern consists of four distinct cells at about 78°N from 60°E to 150°W. Autumn is a transitional season, with cyclone frequency patterns similar to summer and winter, and anticyclone patterns similar to winter and spring.

Abstract

Seasonal and regional variations in characteristics of the Arctic low-level temperature inversion are examined using up to 12 years of twice-daily rawinsonde data from 31 inland and coastal sites of the Eurasian Arctic and a total of nearly six station years of data from three Soviet drifting stations near the North Pole. The frequency of inversions, the median inversion depth, and the temperature difference across the inversion layer increase from the Norwegian Sea eastward toward the Laptev and East Siberian seas. This effect is most pronounced in winter and autumn, and reflects proximity to oceanic influences and synoptic activity, possibly enhanced by a gradient in cloud cover. East of Novaya Zemlya during winter, inversions are found in over 95% of all soundings and tend to be surface based. For all locations, however, inversions tend to he most intense during winter due to the large deficit in surface net radiation. The strongest inversions are found over eastern Siberia, and reflect the effects of local topography. The frequency of inversions is lowest during summer, but is still >50% at all locations. Most summer inversions are elevated, and are much weaker than their winter counterparts. Data from the drifting stations reveal an inversion in every sounding from December to April. The minimum frequency of 85% occurs during August. While the median inversion depth is over 1200 m during March, it decreases to approximately 400 m during August, with median temperature differences across the inversion layer of 12.6° and 2.8°C, respectively. The median depth of the summertime mixed layer below inversions at the drifting stations ranges from 300 to 400 m. Seasonal changes in these inversion characteristics show a strong relationship with seasonal changes in cloud cover.

Abstract

One of the hypothesized effects of global warming from increasing concentrations of greenhouse gases is a change in the frequency and/or intensity of extratropical cyclones. In this study, winter frequencies and intensities of extratropical cyclones in the Northern Hemisphere for the period 1959–97 are examined to determine if identifiable trends are occurring. Results indicate a statistically significant decrease in midlatitude cyclone frequency and a significant increase in high-latitude cyclone frequency. In addition, storm intensity has increased in both the high and midlatitudes. The changes in storm frequency correlate with changes in winter Northern Hemisphere temperature and support hypotheses that global warming may result in a northward shift of storm tracks in the Northern Hemisphere.

Abstract

Calculations of a thermal front parameter using NCEP–NCAR reanalysis data over the period 1979–98 reveal a relative maximum in frontal frequencies during summer along northern Eurasia from about 60° to 70°N, best expressed over the eastern half of the continent. A similar relative maximum is found over Alaska, which is present year-round although best expressed in summer. These high-latitude features can be clearly distinguished from the polar frontal zone in the midlatitudes of the Pacific basin and collectively resemble the summertime“Arctic frontal zone” discussed in several early studies. While some separation between high- and midlatitude frontal activity is observed in all seasons, the summer season is distinguished by the development of an attendant mean baroclinic zone aligned roughly along the Arctic Ocean coastline and associated wind maxima in the upper troposphere. The regions of maximum summer frontal frequency correspond to preferred areas of cyclogenesis and to where the summertime contribution to annual precipitation is most dominant. Cyclones generated in association with the Eurasian frontal zone often track into the central Arctic Ocean, where they may have an impact on the sea-ice circulation. Development of the summertime Eurasian frontal zone and the summertime strengthening of the Alaskan feature appear to be largely driven by differential heating between the cold Arctic Ocean and warm snow-free land. Frontal activity also shows an association with orography. Several studies have argued that the location of the summer Arctic frontal zone may be in part determined by discontinuities in energy exchange along the tundra–boreal forest boundary. While such a linkage is not discounted here, a vegetation forcing is not required in this conceptual model.