by Professor Marcel Leroux
Laboratoire de Climatologie, Risques, Environnement
(LCRE : http://lcre.univ-lyon3.fr)
266 CD 10 - 13 126 Vauvenargues, France
The rapid mode of general circulation (cold scenario)
The major traits of this rapid mode of general circulation are:
MPHs are powerful, deep and of vast dimensions, and they maintain considerable coherence, retaining their original low temperatures across longer distances. They transport a larger amount of cold polar air, they move more rapidly, and their trajectories are more meridional, so that they drive more deeply into tropical margins.
The greater dynamism of MPHs causes an intensification of the cyclonic circulation at their lea-ding edges, and as a result there is a much-enhanced transfer of energy from the tropics towards the poles. This transfer involves perceptible heat and latent heat through the intermediary of water vapour, vigorously diverted towards higher latitudes. Remember that the more meridional trajectory of the MPHs means that they are able to both reach and divert warmer tropical or subtropical air, with its supply, over the oceans, of a richer precipitable water potential.
This transfer towards the poles may also involve continental air, and therefore much drier condi-tions are favoured as Anticyclonic Agglutinations (AAs) are extended and the unproductive character of the Trade Inversion (TI) is reinforced. This continental air can carry much greater quantities of dust. The greater densities of dust in ice-core samples for colder periods from Greenland and the Antarctic testify to the more vigorous nature of this transportation, typical of greatly accelerated fluxes in the lower layers and at altitude.
Disturbances in middle latitudes are more violent, because of accentuated thermal contrasts and the greater vigour of the MPHs, which leads to more powerful updrafts (with deeper and wider depressions). Westerly jets, abundantly supplied by these enhanced updrafts, are in their turn accelerated and displaced in the direction of the tropics.
Anticyclonic Agglutinations (AAs), fed by strengthened MPHs, become in their turn more vigorous and spread further northwards and southwards over both oceans and land masses. Over the land, continental AAs are much stronger in winter, bringing about stable anticy-clonic conditions which last for a long time. These continental anticyclonic situations are just as frequent in the summer, and are accompanied by dry conditions (cf. chapter 11). Generally speaking, in winter as in summer, AAs form at more tropical latitudes, their stability is reinforced and their unproductive character (or inversion) covers a wider area, especially in the tropical zone where the desert extends towards the Equator.
Tropical circulation, which is powerfully supplied by MPHs, is much accelerated, but the trades cover a relatively smaller area; their energy may be handed on to monsoon fluxes extending from them, and these too will be more limited in their range.
At the heart of the tropical zone a paradoxical situation occurs: fluxes are more vigorous and faster, but the area across which they sweep is reduced because of the northward and south-ward rapprochement of MPHs and the resulting contraction of the tropical zone. Another factor is that of the strong opposition set up by contrary fluxes from the other hemisphere, which hinder the migration of the Meteorological Equator (Fig. 34a), less strongly drawn, at the surface, by less deepened thermal depressions.
The structure of the Meteorological Equator (ME), with its limited range of migration, becomes closer to the vertical (VME). The likelihood of any extension of its inclined structure (IME) is reduced by the dynamic nature of the opposing flux and the weaker continental thermal depres-sions. The monsoon circulation which is characteristic of the IME's structure is also considerably reduced in its amplitude, not straying far from the Equator, especially in Africa and Asia (Fig. 34a).
The migration of tropical pluviogenic structures is limited to a narrow zonal belt, notably in the case of the VME, which remains relatively close to the Equator and is associated with regular and abundant rainfall. Within the structure of the IME, now moving across a much more limited area, the same narrow range applies to squall lines, beneath which rain is light and uncertain.
The water vapour of the tropics is normally exported in two ways: either to the fore of MPHs, or following the updrafts of the Meteorological Equator (chapter 8). The greater power of MPHs progressively encourages more intense transfers towards the poles, at the cost of diminished precipitable water potential over the tropical zone, where rainfall is now generally in short supply.
The polar thermal deficit is further aggravated in the Northern Hemisphere by the extent of the land masses, sea ice and the inlandsis. The relative strengthening of the northern meteorological hemisphere means that it trespasses upon its southern counterpart, and the Meteorological Equa-tor is therefore shifted southwards all along its line (Fig. 34a).
Oceanic surface circulation (cf. chapter 14) is in its turn everywhere accelerated, impelled by colder MPHs, which exert greater pressure upon the water, and by trade winds which are now, like all the other fluxes, more rapid. The great gyres are now closer to the geographical equator, and ocean currents are accelerated, in particular those cold currents which flow towards the heart of the tropical zone along the eastern edges of oceans, where upwellings are also more vigorous. Currents of density at high latitudes are reinforced by lower temperatures and by the growth in area of icefields, and more CO2 is absorbed and stored, meaning that there is a lesser concentration of CO2 in the atmosphere. With this generally increased circulation in the oceans, there is a much greater intensity of thermal transfers in the water, as well as in the atmosphere.
• The slow mode of general circulation (warm scenario)In either of the meteorological hemispheres in summer, phenomena are less intense as a result of the diminution of the polar thermal deficit (Fig. 30). A slow mode of general circulation there-fore combines, schematically, two meteorological hemispheres in summer conditions, i.e. there is a global situation corresponding to a moderated thermal deficit, all the year round and simulta-neously (with more or less marked deviations), at high latitudes in both the north and the south (Fig. 35). The general tone of the climate is a warm one, with mild winters, and seasonal ther-mal contrasts are wider as meteorological phenomena have greater freedom of movement.
The major traits of this slow mode of general circulation are:MPHs are less powerful, more shallow and of smaller dimensions, and they are less coherent as they move along their trajectories, warming more rapidly. They have a smaller amount of cold polar air to transport, and their trajectories are less meridional, so they do not go beyond tropical margins.
The reduced dynamism of MPHs causes an attenuation of the cyclonic circulation at their leading edges, and as a result there is a reduced transfer of energy from the tropics towards the poles. The supply of perceptible heat and latent heat is favoured, but the intensity of the transfer is slowed. The less meridional trajectory of the MPHs means that they are unable either to reach or divert the maximum possible precipitable water potential.
When transfer towards the poles involves continental air, less dust is transported because of the deceleration in fluxes, both in the lower layers and at altitude, and because of the reduced extent of regions dried out by winds now reduced in strength.
Disturbances in middle latitudes are more clement, because of attenuated thermal contrasts, and the reduced vigour of the MPHs is fuelling less powerful updrafts (with shallower depressions). However, a stormy character may be more prevalent as a result of thermal convection and the greater availability of energy. Westerly jets, less well supplied by reduced updrafts, are less rapid, and displaced away from the tropics.
Anticyclonic Agglutinations (AAs), fed by the weakened MPHs, exhibit reduced pressures and have smaller areas over both oceans and land masses. Over the land, continental AAs may form only in winter, with stable anticyclonic conditions being less frequent. Generally speaking, these weaker AAs form at latitudes more removed from the tropics. They are less stable, and their unproductive character (beneath the inversion) covers a smaller area.
Tropical circulation, less generously supplied by MPHs, is decelerated, but the trades cover a relatively larger area, as do the monsoon fluxes extending from them.
At the heart of the tropical zone, a paradoxical situation still occurs: fluxes are slower, but the area across which they sweep is much wider because of the northward and southward move-ment of AAs away from each other, and the resulting dilation of the tropical zone. Another factor is that of the relative lack (or even the complete absence) of opposition from contrary fluxes originating in the other hemisphere (Fig. 34b).
The Meteorological Equator (ME), its range of migration having broadened, now possesses a very obvious double structure. The VME migrates beyond 12°-15°N and S, and the zonal band of abundant rainfall is thereby widened. Meanwhile, the lower layers of the IME may impose its structure beyond the Tropics, drawn mainly by much deeper continental thermal summer depre-ssions, and also because of the less dynamic nature of the opposing flux. Transequatorial monsoon circulation is considerably enlarged, being found far from the Equator, and bringing with it its load of moisture from the oceans, deep into tropical continents, especially Africa and southern Asia (Fig. 34b).
The seasonal migration of tropical pluviogenic structures covers a wider area, in the case of both the VME and the structure of the IME, which benefit from a richer precipitable water potential, delivered more evenly over a larger area (fig. 34b).
The existence of less vigorous MPHs reduces the intensity of transfers of tropical energy towards the poles, and the tropical zone is able to conserve most of its precipitable water potential, which is transported into the interiors of tropical continents by amplified monsoon airstreams.
Now it is the turn of the southern meteorological hemisphere to trespass upon its northern coun-terpart, its encroachment being more extensive over northern tropical continents, where it is drawn in by deep continental thermal summer lows in the lower layers. The Meteorological Equa-tor is shifted to the north of the geographical equator all along its line (Fig. 34b).
Oceanic surface circulation, impelled by less dense MPHs and slower trade winds, is, in its turn, slowed. The great oceanic gyres are now further from the geographical equator, and ocean currents driving them are decelerated, especially along the eastern edges of oceans, where upwellings are also less vigorous. Currents of density at high latitudes are less marked, absorbing and storing less CO2 and thereby increasing the concentration of this gas in the atmosphere. The whole of the circulation in the oceans is slowed, and thermal transfers are diminished.
Remember that the repercussions of the modifications dealt with in Figs. 33, 34 and 35 take pla-ce principally within the lower layers of the troposphere. It will therefore be useful to comple-ment them with (horizontal) schemas of circulation for the six aerological units defined in Fig. 26 (chapter 8). This will not be done now, but later when we come to examine recent climatic evolution in certain sectors such as the North Atlantic and the North Pacific (chapters 12 and 13). As an aid to comprehension, it will now be possible to refer if need be to these complementary schemas of slow and rapid circulation in the lower layers.
Marcel Leroux,