Abstract

A sharp increase in the Arctic mean winter temperature occurred around 1920. The occurrence of consistently positive temperature anomalies at Spitsbergen (starting in 1919), Iceland (1921) and Greenland (1926) subjoined the more gradual warming trend over northern Europe which had been in progress since the mid-1890s. The atmospheric circulation changes associated with the warming of the 1920s are examined using the historical surface weather maps, surface zonal and meridional wind components and station air temperature data.

From 1900 to 1925, when European winters were gradually becoming milder, winters in Greenland were generally colder than the 1851–1960 normal. The mean Icelandic Low reached maximum intensity and a northernmost position at about 65°N latitude throughout the period. After 1925, the Low shifted closer to the southern tip of Greenland; this was accompanied by a slight weakening of the westerlies and an increase in meridional flow (southerly) over Greenland and Baffin Bay. In northern Europe the westerlies maintained their pre-1926 strength. In the warmest winters of 1926–44, the westerlies advected mild air over Europe while southerly air flow prevailed over Greenland. In some winters, however, the European westerlies weakened and only Greenland was mild. After 1944, the mean position of the Icelandic Low shifted eastward and the warm period over the Atlantic arctic sector ended.

Abstract

Seasonal and annual (July through June) precipitation data for up to 197 stations are used to analyze variability in the High/Dry (H/D) and Low/Wet (L/W or warm event) modes of the Southern Oscillation (SO) over the tropical Americas and the Caribbean Islands to 30°S latitude. Precipitation is significantly higher during H/D events than during L/W in northern summer, autumn and winter at most stations in the southern Caribbean and northern South America including the northern Andes and Amazon River basin. Precipitation is, in general, significantly higher in L/W than in H/D modes at higher subtropical latitudes of both hemispheres, especially in northern autumn and winter over the southern United States, Cuba and Mexico, and in spring and autumn over southern Brazil, Paraguay and Argentina. A similar pattern of spatial precipitation variability occurs in the July–June data and significant H/D minus L/W precipitation differences are found at one-third of the stations. Although drought occurs in Northeast Brazil in some L/W years such as 1982–83, rainfall in other L/W events is above normal, and overall the role of the SO in precipitation variability is not as apparent in Northeast Brazil as it is in northern South America and the Caribbean.

Abstract

Spatial patterns of low-frequency sea level pressure (SLP) variability are identified by performing rotated principal component analysis (RPCA) on a long-term (1899–1986) Northern Hemisphere gridded dataset. The analysis is limited to the region 160°E eastward to 40°E due to missing data early in the century. The objective is to identify a comprehensive set of highly recurrent SLP teleconnection patterns; to examine some aspects of their seasonality; and to identify the associated mean winter pressure fields and cyclone frequencies occurring at times of opposite eigenvector polarity. The results are further described in the context of the Southern Oscillation and known midtropospheric teleconnection patterns.

Four low-frequency variability patterns are identified over the Atlantic-European sector, including (i) the North Atlantic 0scillation (NAO), and spatial patterns with SLP variability centers over (ii) the eastern Atlantic (EATL), (iii) southern Europe and the northern Mediterranean basin (SENA). and (iv) Scandinavia (SCAN). The Pacific sector low-frequency variability patterns include the (v) North Pacific Oscillation (NPO), and patterns with centers over (vi) the north-central Pacific (PAC) and (vii) the Bering Sea (BER). The EATL, SENA, SCAN and BER patterns have not been identified in previous SLP RPCA studies. Seasonal variations take place in the location of the primary and secondary centre of SLP variability in each teleconnection.

Each of the Atlantic teleconnections have at least one polarity mode in which cyclones migrate toward Greenland and Iceland. However, the opposite modes in the NAO and EATL are associated with distinct zonally oriented cyclone tracks along latitudes 40°–45°N. One mode of each Pacific pattern is characterized by a zonally oriented cyclone track but the opposite modes are characterized by cyclone maxima in the eastern Pacific (NPO), the western Bering Sea (PAC) and another track that turns northward toward the Bering Sea along the date line (BER). The BER and PAC patterns closely resemble the upper-air west Pacific and Pacific-North American patterns, although the correlations between the RPCA scores and the midtropospheric pattern indices can have small but significant correlations to other midtropospheric patterns. The winter BER scores am significantly lag correlated to the Southern Oscillation index.

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.

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.

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.

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.

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.

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.

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.