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A range of diagnostics from two GCM simulations, one of the present-day climate and one of the last glacial maximum (LGM)
is used to gain insight into their different temperature structures and eddy dynamics. There are large local increases in
baroclinicity at the LGM, especially in the Atlantic storm track, with large accompanying increases in the low level transient
eddy heat flux. However, the differences in the zonal mean are much smaller, and the increases in both baroclinicity and heat
flux are confined to low levels. Supplementary experiments with baroclinic wave lifecycles confirm the marked contrast between
local and zonal mean behaviour, but do not adequately explain the differences between the zonal mean climates. The total flux
of energy across latitude circles in the Northern Hemisphere does not change much during DJF, although its transient component
is actually reduced at the LGM (during JJA the transient component is increased). Calculations of total linear eddy diffusivity
reveal that changes in the time mean stationary waves are chiefly responsible for the seasonal range of this quantity at the
LGM, while they only account for half the seasonal range at the present-day. 相似文献
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Two methods for identifying mid-latitude synoptic time scale variability have been applied to data from the first United
Kingdom Meteorological Office (UKMO) coupled ocean-atmosphere model experiments with present day and gradually increasing
CO2 concentrations. In the first the standard deviation of the time filtered mean sea level pressure field is taken to identify
the location of the storm track and in the second individual cyclones are identified using synoptic criteria. The results
have been compared with data from a 10 year archive of UKMO analysis. In the enhanced CO2 experiment the changes in storminess identified by the two methods have been compared with changes in mean and maximum winds
with special emphasis on the North Atlantic. The relative utility of the different measures for predicting potentially damaging
synoptic events is discussed.
Received: 3 May 1995 / Accepted: 7 November 1995 相似文献
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Changes in mid-latitude variability due to increasing greenhouse gases and sulphate aerosols 总被引：1，自引：0，他引：1
Intra-seasonal variability in the Northern Hemisphere winter is investigated in ensembles of experiments using the Hadley
Centre coupled ocean-atmosphere model. Synoptic techniques which identify low centres and blocking anticyclones have been
used in preference to time-filtered variances of geopotential height as these may not uniquely identify true synoptic systems.
Changes in mid-latitude variability are qualitatively similar in experiments that include changes in greenhouse gases only
and those that also include the direct effects of sulphate aerosols. The presence of aerosols reduces the warming of the northern
continents but the relatively larger warming of the land compared to the oceans remains in both experiments and at higher
latitudes this leads to a reduced poleward transient flux of energy. There is an increase in the transient flux of energy
at most latitudes, due to a greater transport of latent heat arising from the greater moisture availability. The total number
of Northern Hemisphere storms decreases but there is a tendency towards deeper low centres. There are fewer storms in the
North Pacific and North Atlantic source regions where the local baroclinicity is reduced. The climatological tracks from these
regions are shorter with decreases at the northeastern ends of the tracks and increases in the regions with maximum counts
towards the centre of the ocean basins. These changes are not generally statistically significant. The greatest changes in
blocking anticyclones occur in the North Pacific where there is a downstream shift in the region of maximum activity. Changes
in stationary waves show a strong increase in the PNA oscillation which may be influencing the changes in blocking as has
been found by other authors. Possible mechanisms for forcing this response are speculated but it will need further experiments
to unravel them.
Received: 10 September 1997/Accepted: 2 December 1997 相似文献
5.
Two methods for identifying mid-latitude synoptic time scale variability have been applied to data from the first United Kingdom
Meteorological Office (UKMO) coupled ocean-atmosphere model experiments with present day and gradually increasing CO2 concentrations. In the first the standard deviation of the time filtered mean sea level pressure field is taken to identify
the location of the storm track and in the second individual cyclones are identified using synoptic criteria. The results
have been compared with data from a 10 year archive of UKMO analysis. In the enhanced CO2 experiment the changes in storminess identified by the two methods have been compared with changes in mean and maximum winds
with special emphasis on the North Atlantic. The relative utility of the different measures for predicting potentially damaging
synoptic events is discussed. 相似文献
6.
When greenhouse gases are increased in coupled GCM experiments there is both a direct effect and an indirect effect due to changes in the surface conditions. In this study we carry out experiments with a perpetual winter atmosphere only model in order to investigate the influence of changes to the surface conditions (sea surface temperatures, sea-ice and snow amount) on the Northern Hemisphere winter mid-latitude mean sea level pressure response. The surface conditions for the perpetual winter model experiments are prescribed from time averages of the HadCM2 control and greenhouse gas experiments. Forcing the perpetual winter model with the HadCM2 greenhouse gas surface conditions produces a negative mean sea level pressure (MSLP) response across both Northern Hemisphere ocean basins, as was found in the coupled model HadCM2 experiment. Additional PW model experiments show that the sea surface temperature forcing from the HadCM2 greenhouse gas experiment dominates the snow and soil moisture content forcings. The sea-ice forcing from the HadCM2 greenhouse gas experiment reduces MSLP at high latitudes. In the north Pacific region MSLP decreases when the global mean warming is applied to the sea surface temperature forcing field at all open sea points. In the north Atlantic region the increased tropics to mid-latitude meridional sea surface temperature gradient is required for MSLP to decrease. These experiments show that the MSLP response in the Northern Hemisphere mid-latitude storm track regions is sensitive to the non-local sea surface temperature anomaly pattern. 相似文献
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