Harry van Loon

Decadal Variations in Climate Associated with the North Atlantic Oscillation

Large changes in the wintertime atmospheric circulation have occurred over the past two decades over the ocean basins of the Northern Hemisphere, and these changes have had a profound effect on regional distributions of surface temperature and precipitation. The changes over the North Pacific have been well documented and have contributed to increases in temperatures across Alaska and much of western North America and to decreases in sea surface temperatures over the central North Pacific. The variations over the North Atlantic are related to changes in the North Atlantic Oscillation (NAO). Over the past 130 years, the NAO has exhibited considerable variability at quasi-biennial and quasi-decadal time scales, and the latter have become especially pronounced the second half of this century. Since 1980, the NAO has tended to remain in one extreme phase and has accounted for a substantial part of the observed wintertime surface warming over Europe and downstream over Eurasia and cooling in the northwest Atlantic. Anomalies in precipitation, including dry wintertime conditions over southern Europe and the Mediterranean and wetter-than-normal conditions over northern Europe and Scandinavia since 1980, are also linked to the behavior of the NAO. Changes in the monthly mean flow over the Atlantic are accompanied by a northward shift in the storm tracks and associated synoptic eddy activity, and these changes help to reinforce and maintain the anomalous mean circulation in the upper troposphere. It is important that studies of trends in local climate records, such as those from high elevation sites, recognize the presence of strong regional patterns of change associated with phenomena like the NAO.

The Seesaw in Winter Temperatures between Greenland and Northern Europe. Part I: General Description

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%...

Associations between the 11-year solar cycle, the QBO and the atmosphere. Part I: the troposphere and stratosphere in the northern hemisphere in winter

Abstract Linear correlations between the three solar cycles in the period 1956–1987 and high-latitude stratospheric temperatures and geopotential heights show no associations. However, when the data are stratified according to the east or west phase of the quasi-biennial-oscillation (QBO) in the equatorial stratosphere significant correlations result: when the QBO was in its west phase the polar data were positively correlated with the solar cycle while those in middle and low latitudes were negatively correlated. The converse holds for the east phase of the QBO. Marked relationships existed throughout the troposphere too. No major mid-winter warming occurred in the west phase of the QBO during a minimum in the three solar cycles. In the east phase major warmings tended to take place in the minima of the cycle. Thus the signal of the quasi-biennial-oscillation in the extratropical stratosphere tends to be strengthened in solar minima, and weakened in solar maxima.

The Southern Oscillation. Part I: Global Associations with Pressure and Temperature in Northern Winter

Abstract We describe the global correlations between a measure of the Southern Oscillation and sea level pressure and surface air temperature in the northern winter. The stability of these correlations were tested on the Northern Hemisphere for an 80-year period, and it turned out that most stable correlation coefficients were found over India, the North Pacific Ocean, the Rocky Mountains, and the central and western North Atlantic Ocean. On the Southern Hemisphere most records are too short for a similar test, but the following may tentatively be said about the Southern Oscillation in middle and high southern latitudes: when pressure is low in lower latitudes over the South Pacific Ocean it tends to be high at higher latitudes of that ocean, high over East Antarctica and low in the belt of westerlies in the Indian and South Atlantic oceans. In the zonal average on both hemispheres the pressure gradients in this extreme of the oscillation tend to be steeper at lower latitudes and flatter at higher latitud...

The Seesaw in Winter Temperatures between Greenland and Northern Europe. Part II: Some Oceanic and Atmospheric Effects in Middle and High Latitudes

Abstract Description of the seesaw in wintertime climate between Greenland and northern Europe is continued in terms of variations in long waves, frequencies of highs and lows, zonal geostrophic winds, precipitation, sea ice and sea surface temperatures. The monthly variations in four circulation modes are described. Significant spatial correlations exist between the zonal. component of the geostrophic wind in the area of the strongest westerlies in the North Atlantic Ocean and the zonal geostrophic wind elsewhere north of 20°N in winter. Long waves 1 and 2 change substantially from one phase of the seesaw to the other at 63°N. At 45°N only wave 2 changes appreciably between phases. Large, statisticaly significant anomalies occur during and after seesaw winters in the atmosphere-ocean-ice system of the North Atlantic Ocean and its periphery, as well as in the North Pacific Ocean. Anomalies of sea surface temperature which develop during seesaw winters tend to persist through the subsequent spring and summ...

Spatial Variability of Sea Level Pressure and 500 mb Height Anomalies over the Southern Hemisphere

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 1950s onward. These latitudinal height differences are also used to describe the association between the hemispheric westerlies and the Southern Oscil...

The Connection Between Trends of Mean Temperature and Circulation at the Surface: Part I. Winter

Abstract The local temperature trends in summer are not so obviously associated with advection changes as are those in winter. This appears to be due to weaker temperature contrasts at middle and high latitudes in summer combined with a smaller amplitude of the mean waves. A larger share of the total variance in the trend of sea level pressure is accounted for by the shorter waves than in winter. Local temperature changes are as big in summer as in winter in many places at middle latitudes, whereas in the arctic they are appreciably smaller. The zonally averaged trends in summer are larger at middle than at high latitudes, which is the reverse of winter. The sign of the zonally averaged temperature changes differs from one latitude belt to another as in winter, and the sign at a given latitude is not necessarily the same in both seasons. In contrast with winter, the sensible heat transport by mean waves in the sea level pressure in summer plays an insignificant part in causing trends in the zonally averag...

Association between the 11-Year Solar Cycle, the QBO, and the Atmosphere. Part II: Surface and 700 mb in the Northern Hemisphere in Winter

Abstract Sea level pressure, surface air temperature, and 700-mb temperature and geopotential height show a probable association with the 11-year solar cycle which can be observed only if the data are divided according to the phase of the Quasi-Biennial Oscillation. The range of the response is as large as the interannual variability of the given element, and the correlations prove statistically meaningful when tested by Monte Carlo techniques. The sign of the correlations changes over the hemisphere on the spatial scale of extensive teleconnections. The correlations at 700 mb tend to be of opposite sign in the east and west years of the QBO, a result which Labitzke and van Loon also found in an analysis of the stratosphere. The pattern of correlation between the 700-mb heights on the Northern Hemisphere and the solar flux is the same as that of point-to-point correlations (teleconnections) between the 700-mb height at selected points and the heights at all other points. We interpret this similarity as a ...

The Southern Oscillation. Part VII: Meteorological Anomalies over the Indian and Pacific Sectors Associated with the Extremes of the Oscillation

Abstract Composite surface pressure, temperature, and precipitation anomalies are mapped over the Indian and Pacific sectors during the various stages of Warm and Cold Events in the Southern Oscillation. In the year before the development of positive sea surface temperature anomalies in the central and eastern equatorial Pacific (Year–1 of a Warm Event), a strong South Pacific High is associated with below normal surface pressure over Australia and the Indian Ocean. This occurs concurrently with a poleward displacement of the Pacific convergence zones, with above normal air temperature and precipitation over the subtropical Pacific, and opposite conditions along the equator. By the next year (Year 0) of the Warm Event, thew anomalies have the opposite sign. The sequence of anomalies during a Cold Event is inverse to that during a Warm Event but otherwise the anomaly patterns are remarkably similar. It appears that enhanced convection and low surface pressure within the Pacific convergence zones contribute...

The Southern Oscillation. Part II: Associations with Changes in the Middle Troposphere in the Northern Winter

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 high...