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Jeffery C. Rogers

Abstract

The North Atlantic Oscillation (NAO) and Southern Oscillation (SO) are compared from the standpoint of their association with Northern Hemisphere winter mean distributions of sea-level pressure (SLP) and 500 mb height. The NAO and SO are associated with significant SLP differences over much of the hemisphere except for Siberia and western North America. Significant SLP and 500 mb height differences occur in the NAO over the Atlantic Ocean and near Baja California, while in the SO they occur over the Pacific Ocean, India and the western Atlantic. Only over the latter region do large pressure and height variations consistently occur in the extremes of both oscillations; these are also associated with winter temperature variability over the southeastern United States. For example, during winter 1982–83, when the two oscillations simultaneously reached extremes, the NAO was associated with record December warmth east of the Mississippi River, but during January and February the SO dominated the height and air temperature distributions over the United States.

The cospectrum of the NAO index and Darwin (Australia) pressure is largest at intermediate frequencies with periods of about 6 years, although the NAO itself has peak energy at 7.3 years. The NAO is characterized by a large trend toward lower index in the twentieth century through the 1960s; this is not associated with variations in the SO. In the 80 winters of data, simultaneous occurrences of particular modes of one oscillation with those of the other seem to occur by chance.

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Jeffery C. Rogers

Abstract

The spatial variability of seasonal mean sea level pressure (SLP) and 500 mb height anomalies are determined using eigenvector analysis. Previously, spatial variability had only been analyzed in such a manner during winter months, and the time coefficients of the eigenvectors of SLP and 500 mb heights were not compared. The eigenvectors of SLP and 500 mb heights obtained here are compared during each season, and the vectors are compared between seasons. Surface and 500 mb level eigenvector coefficients are found to be correlated and they are correlated in some seasons to air temperatures at surface stations. The data used were National Meteorological Center analyses of daily SLP and 500 mb heights between 1946 and 1977. The eigenvectors are determined for the covariance matrices of seasonal mean pressure and height departures from the 31 or 32 year long term normal over a 132 grid-point network.

The coefficients of the first eigenvector of 500 mb heights are most highly correlated to the coefficients of both the first and second winter SLP eigenvectors and they are highly correlated to mean winter air temperatures at stations in the southeastern United States. The first two winter SLP eigenvectors qualitatively resemble patterns of pressure variability associated with the North Atlantic and North Pacific oscillations. Spring and autumn eigenvectors suggest that 500 mb anomalies will lie over surface anomalies and have the same sign and configuration. This tendency for upper level anomalies to have the same location and configuration as those at the surface is not suggested by the summer eigenvectors and the time coefficients of the eigenvectors at both levels are not highly correlated either. The coefficients of the first summer 500 mb eigenvector shift to more positive values around 1963, a change which is correlated to observed air temperature variability in the eastern Canadian Arctic. In winter, spring, and autumn both the SLP and 500 mb height eigenvectors repeatedly indicate that an opposition in pressure exists between the northern North Pacific and the south-central North Pacific regions.

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Jeffery C. Rogers

Abstract

The relationship between meteorological factors, measured at Barrow, Alaska, and summertime Beaufort Sea ice conditions along the shipping route between Pt. Barrow and Prudhoe Bay was investigated using data available from 1953–1975. Light-ice summers are associated with higher than normal sea level pressure (SLP) northeast of the Beaufort Sea (centered on 80°N, 120°W), lower than normal SLP over the east Siberian Sea and with more frequent surface winds primarily from the directions 135–195°. A reversal in this pressure and wind direction pattern occurs during heavy-ice summers.

Air temperature, in the form of thawing degree days (TDD's), is the parameter most highly correlated with the summertime ice margin distance (r = 0.815) and it is highly correlated with SLP and wind direction. Correlation coefficients between these meteorological factors and ice margin distance increase during the summer suggesting their increasing importance to melt processes with time. The amount of open water by late summer and during autumn in the Beaufort Sea influences subsequent air temperatures, but has little or no influence upon subsequent local surface winds or the SLP distribution over the Northern Hemisphere. This interaction between ocean and overlying air, which encourages mild summers to be followed by mild autumns, accounts for the bimodal distribution in maximum accumulated TDD's noted at Borrow.

Analysis of Landsat images of sea ice conditions and concurrent TDD's between 1972 and 1976 showed that at least 400 TDD's are needed to assure favorable ice conditions. Trends in SLP and TDD's since 1939 and 1921, respectively, suggest increasing severe ice-conditions as only 13 of 24 summers have accumulated 400 TDD's since 1953, while 28 of 32 had done so between 1921 and 1952. This was associated with a decline in Barrow mean summer temperature of 0.4°C from 1921–52 to 1963–75.

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Jeffery C. Rogers

Abstract

Data on monthly sea surface temperatures (SST) over the eastern North Pacific as well as surface pressure and 1000–500 mb layer thickness over North America during the period 1960–73 were analyzed. Factor analysis of the SST data, used to find areal patterns of anomalous SST in the ocean, revealed that while three large regions dominated the eastern North Pacific from 1960 to 1970 there was a change, possibly during 1971, resulting in the predominance of a new region called the southwestern oceanic region. At nearly the same time there was noted a reversal in the tendency toward abnormally cold winters throughout the eastern United States.

Fluctuations in pressure and thickness over North America associated with anomalous periods of warm and cold water in the original three SST cells were then analyzed. The east-central North Pacific and Gulf of Alaska regions were found to be associated with statistically significant fluctuations in pressure near the Gulf of Alaska and in thickness over west-central Canada. The southeastern oceanic region was associated with statistically significant fluctuations in pressure near the Pacific anticyclone and in thickness over a large area centered on the Arctic archipelago.

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Harry van Loon
and
Jeffery C. Rogers

Abstract

We have investigated the relationship between the extremes of the Southern Oscillation and the following quantities at 700 mb in winter, 1948/1949 to 1978/1979: eddy transfer of sensible heat, temperature, geopotential height and geostrophic wind. In the phase of the Southern Oscillation when pressures are high over the tropical South Indian Ocean and low over the tropical South Pacific Ocean, in contrast with the opposite pressure distribution, the zonal mean poleward flux of sensible heat in the quasistationary waves tends to be higher in middle latitudes; the temperatures and heights tend to be lower between 30 and 60°N with the maximum difference at 45°N; the geostrophic wind tends to be stronger south of 45°N and weaker to the north; and the transfer of sensible heat by the transient waves tends to be stronger south of 45°S, and weaker to the north.

In this extreme of the Southern Oscillation the zonal mean geostrophic wind on both hemispheres is stronger in the subtropics and weaker at higher latitudes than in the other extreme when pressures are high over the tropical South Pacific and low in the tropical South Indian Ocean.

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Jeffery C. Rogers
and
Harry van Loon

Abstract

The spatial variability of mean sea level pressure (SLP) and 500 mb height anomalies over the Southern Hemisphere during summer (DJF) and winter (JJA) is determined using eigenvector analysis based on daily synoptic maps from 1972 to 1979. The patterns of spatial distribution of pressure and height anomalies are further verified and examined by means of station data, and the eigenvectors are compared between the seasons and to those found for the Northern Hemisphere.

The first eigenvector shows that midlatitude anomalies of SLP and 500 mb height are of an opposite sign to those found over and around Antarctica. The pattern is highly barotropic and suggests strengthening and weakening of the zonal wind in alternating latitude belts. The 500 mb height differences are calculated for five midlatitude to Antarctic station pairs using data from the late 1950's onward. These latitudinal height differences are also used to describe the association between the hemispheric westerlies and the Southern Oscillation. They also suggest that the summertime westerlies strengthen and weaken simultaneously around the hemisphere, whereas in winter this zonal symmetry is interrupted in the South America–Antarctic Peninsula region. In this region, wintertime variations in the westerlies are associated with the second eigenvector, and large interannual variations in the SLP and latitudinal variations in storm tracks occur over the Drake Passage. The second eigenvector is shown to be associated with the large standard deviations of winter mean temperature along the Antarctic Peninsula, to variations in sea ice duration at Laurie Island in the South Orkneys, and to the Trans-Polar Index of Pittock, which describes the tendency for the surface polar vortex to be displaced either toward Tasmania or the Falkland-South Sandwich Islands region.

It is concluded that temperature and circulation teleconnections, similar to those of the Northern Hemisphere, also occur in the Southern Hemisphere, and are associated with standing waves in the atmosphere The small amount of land south of 35°S probably accounts for the uniform variations in the westerlies across the Southern Hemisphere, whereas, in the Northern Hemisphere, variations in the westerlies shown by the eigenvectors are largely confined to the oceans.

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Harry van Loon
and
Jeffery C. Rogers

Abstract

The circulation anomalies over the whole Southern Hemisphere in the First GARP Global Experiment (FGGE) were not those that one would expect in either extreme of the Southern Oscillation; examples of the anomalies in such extremes are given. The zonally averaged pressure gradients between 50 and 65°S in FGGE when compared with those of several other years turned out to be abnormally strong in winter (2.5σ above the mean), and moderately strong in summer (1.4σ. above the mean). The 500 mb heights were above normal in middle latitudes and below normal at high latitudes when compared with station data from series 16–29 years long. As the computations are based on operational analyses they are not final, although the conclusions are unlikely to be changed by the use of the complete FGGE data set.

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Harry van Loon
and
Jeffery C. Rogers

Abstract

We have investigated the well-known tendency for winter temperatures to be low over northern Europe when they are high over Greenland and the Canadian Arctic, and conversely. Well-defined pressure anomalies over most of the Northern Hemisphere are associated with this regional seesaw in temperature, and these pressure anomalies are so distributed that the pressure in the region of the Icelandic low is negatively correlated with the pressure over the North Pacific Ocean and over the area south of 50°N in the North Atlantic Ocean, Mediterranean and Middle East, but positively correlated with the pressure over the Rocky Mountains. The composite patterns of pressure anomalies in the seesaw are almost identical to the fist eigenvector in the monthly mean pressure, but the standard deviations of pressure anomalies in seesaw mouths are as large as the standard deviations of monthly means in general. Since 1840 the seesaw, as defined by temperatures in Scandinavia and Greenland, occurred in more than 40% of the winter months and the occurrences are seemingly not randomly distributed in time as one anomaly pattern would be more frequent than the other for several decades. For this reason the circulation anomalies in the seesaw come to play an important part in deciding the level of regional mean temperatures in winter and thus in deciding the long-term temperature trends. These regional temperature trends are then closely associated with change in frequency of atmospheric circulation types, and it is therefore unlikely that the trends are caused directly by changes in insolation or in atmospheric constituents and aerosols.

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