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Harry Van Loon and Jill Williams

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 averaged temperature.

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Jill Williams and Harry Van Loon

Abstract

The sign of the weighted temperature change over two periods (1950–64, 1942–72) for 15°–80°N was determined by the change in the polar region in spring and autumn, as it was in winter but not in summer. Each of the four seasons shows a different distribution of zonally averaged temperature changes.

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Karin Labitzke and Harry Van Loon

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Jill Williams and Harry Van Loon

Abstract

For each grid point (5° latitude by 5° longitude) and each season, the long-term mean sea-level pressure (1899–1972) and its standard deviation were found, using a data set compiled by NCAR. Individual deviations from the mean greater than three standard deviations were compared with nearby station data from World Whether Records. Some deviations were found in the sea-level pressure data and not in the station pressure data. Comparison was made between the NCAR sea-level pressure data set and the United Kingdom Meteorological Office data set; large differences are found since 1940 when the data acts started using different sources.

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Jill Williams and Harry Van Loon

Abstract

No abstract available.

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Karin Labitzke and Harry Van Loon

Abstract

The probable association in the northern winter between the atmosphere and the 11-yr solar cycle extends to the frequency of lows in the North American east coast trough and thus adds a synoptic aspect to the previously described atmospheric variability on the 11-yr time scale. Statistically significant correlations of sea level pressure, 700-mb height, and surface air temperature on the Northern Hemisphere in July–August with the 11-yr solar cycle are found primarily over the oceans. The few years for which data of sea level pressure at grid points are available an the Southern Hemisphere yield coherent correlation patterns in summer and winter which are especially marked in the East years of the QBO. The temperature in the lower stratosphere over the South Pole in spring is well correlated with the solar activity in the East and hardly at all in the West years of the QBO. On the Northern Hemisphere the West years in spring are as strongly correlated with the solar cycle in the stratosphere as they are in winter. The pattern of positive and negative correlations is, however, the opposite of that in winter, which we interpret as being related to the different time of occurrence of the final warming in years with or without major midwinter warmings.

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Harry van Loon and Jill Williams

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During 24 years when the 700 mb winter mean temperature dropped over most of the hemisphere north of 20°N, the biggest falls were in the belt of strongest baroclinity and were simultaneous with a southward movement and strengthening of the peak in total meridional eddy transport of sensible heat. These changes were accompanied by a southward displacement of the region of most frequent storm tracks at the surface and by compatible trends in surface mean temperature and sea level pressure. At middle latitudes the layer between surface and 700 mb destabilized, while in the arctic it stabilized as the surface temperature over a large part of the polar cap fell more than the 700 mb temperature.

A comparison with the Southern Hemisphere showed that local temperature trends in the antarctic also take place on the scale of long waves, that they are as large as those in the Northern Hemisphere and that a zonally averaged trend is not necessary the same in summer and winter. The net transport of sensible heat by stationary waves is much smaller in the Southern than in the Northern Hemisphere, and changes in stationary wave transport in the Southern Hemisphere are therefore not likely to contribute much to large changes in the net poleward transport of sensible heat by waves. This is connected with the observation that the stationary waves in temperature and pressure are nearly in phase over the almost continuous water surface in southern temperate latitudes.

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Harry Van Loon and Jill Williams

Abstract

We have demonstrated that regional temperature trends at the surface in winter are connected with circulation changes on the scale of long waves, and that within a given period the trends change sign both with longitude and with latitude. Since the biggest zonally averaged temperature trends north of about 50°N in our samples exceed the biggest zonally averaged trends over the rest of the Northern Hemisphere by a factor of seven to eight, and since the sign of the zonally averaged trends is not uniform, the sign of the average trend over the subpolar and polar regions in winter becomes decisive for the sign of the average temperature trend of the hemisphere.

An important difference between a period when the average temperature of the Northern Hemisphere increased (1900–1941) and one when it decreased (1942–1972), was in the amount of sensible heat transported poleward by the, mean eddies north of the latitude of maximum transport (based on maps of sea level pressure). While the higher latitudes warmed, the poleward transport north of about 55°N (and thus the convergence of heat over the polar cap) was larger than during the period of cooling. This difference was associated with a stronger meridional circulation around the Icelandic low and on the east side of the Siberian high during the warming than during the cooling

The trend of the zonally averaged poleward transport by the mean eddies at sea level, which was positive during the warming and negative during the cooling, amounted at each latitude to a very small fraction of the quantity transported across that latitude.

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Harry Van Loon and Karin Labitzke

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 property of the atmosphere's internal dynamics, a favored resonance evoked within the atmosphere itself or by extraneous effects.

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Harry van Loon and Roy L. Jenne

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