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Jill Williams

Abstract

Zonal harmonic analysis has been carried out on 30-day mean pressure fields of several runs of the NCAR global circulation model (GCM): A January 6-layer case with the earth's orography; a January 6-layer case without orography; a January 12-layer case with orography; and a January 12-layer case without orography. Zonal harmonic mean waves 1, 2, and 3 from each experiment were compared between cases and with observed data.

In the Northern Hemisphere below 18 km, neither the mountain (M) nor the no-mountain (NM) versions of the 6-layer January model can simulate the amplitude of waves 1 and 3 correctly when compared with observed data. The 6-layer NM case does better at simulating the amplitude of wave 2 than does the 6-layer M case. For the 12-layer version of the model, on the other hand, the M case simulated the amplitudes of waves 1 and 2 quite well in the stratosphere, while the NM case only does well with wave 2. In the Southern Hemisphere, neither the M nor NM waves in general resemble those observed.

The results suggest that the rigid top boundary of the NCAR GCM does influence the structure of the zonal harmonic mean waves by preventing vertical propagation of wave energy through that level.

Comparison of amplitudes and phases of zonal harmonic waves in two 30-day periods in the same case shows that in general the waves are in the same positions and have the same amplitudes. The amplitudes and phases of waves 1, 2, and 3 in two January 6-layer cases with slightly different initial conditions were compared and the waves were not significantly different in the two cases.

The maximum amplitude of wave 1 in the model stratosphere occurs at about 60'N while the westerly jet has a maximum at 30°N. In this respect the NCAR model apparently is inconsistent with observations or linear theories.

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Jill Williams

Abstract

The NCAR global circulation model has been used with boundary conditions representing those of a glacial period July, except that areas in North America, Europe, Asia and South America, which formerly were glaciated, have no orography change but have an albedo change equivalent to covering the areas with snow. The results of the snowcover experiment are compared with those of a July control and a July ice-age case, described previously. Reduction of global mean eddy kinetic energy, elimination of the tropical easterly jet, and changes in other climatic variables suggest a weakening of the Northern Hemisphere summer monsoon in the snowcover case compared with the control case. Over North America is a ridge of high mean sea level pressure giving northerly flow over the Hudson Bay-Great Lakes area. At 6 km is a trough of low pressure over eastern North America and a somewhat weaker trough over the British Isles and western Europe. The pole-to-equator temperature gradient increased in the snowcover case when compared with the control case. The Northern Hemisphere jet stream, in zonal average, is stronger and farther south. Cloudiness increased over the snowcover areas compared with the control case; precipitation decreased but not as much as in the ice age case. Results suggest that changes in albedo and ocean surface temperature influence the simulated atmospheric circulation more than changes in orography caused by glaciation.

The results of the July snowcover experiment are analyzed to see what support they give to hypotheses regarding the inception of glacierization and to see how the resulting atmospheric circulation compares with that proposed for the Little Ice Age (1550–1850 AD).

The July snowcover experiment has reproduced many of the atmospheric circulation features in the Northern Hemisphere associated with extensive snowcover in summer months, hypotheses regarding the inception of glacierization, and the reconstructed climate of the Little Ice Age.

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Jill Williams

Abstract

Spectral analysis of 64-year time series (1897–1960) of seasonal precipitation data from 71 North American and 27 European meteorological stations has been performed. There is little evidence for statistically significant differences from the white noise spectrum.

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Harry Van Loon and Jill Williams

Abstract

This paper continues the description of the connection between transport of sensible heat by eddies in winter and temperature, wind and pressure. The emphasis is on the transport by the quasi-stationary eddies, and we demonstrate that when this transport is strong in the latitudes near 50°N, the west wind tends to be strong in the subtropics at all levels in the troposphere and to he weak at middle and high latitudes, and vice versa when the stationary eddy transport is weak. These associations stem principally from the regions of the two major troughs. We show, in addition, that the stationary-eddy transport in the two troughs is negatively correlated; and we outline the teleconnections between the stationary-eddy flux over eastern Asia and pressure and temperature elsewhere.

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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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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 Jill Williams

Abstract

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

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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Harry van Loon and Jill Williams

Abstract

There was no single relationship between mean temperature and the variability of temperature in the years 1876–1975 in Europe and North America, nor did the variability of precipitation in North America necessarily increase when the temperature decreased

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