[The fundamentals of Atmospheric Angular Momentum]

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Even in the absence of mountains in the Northern Hemisphere, if all surfaces and land areas north of 25N were frozen, then the strong North-East Trade winds in the tropics that would result would still pump lots of Westerly AAM into the atmosphere, yet this would still be unlikely to lead to depressions forming in higher latitudes. This is most certain to be the case in winter when extremely frigid conditions with high-pressure would be liable to dominate and prevent any depressions or Westerlies touching the surface. Instead, it is likely that the Westerly AAM would be forced to accumulate in the upper-air until early Spring when conditions become favourable in low latitudes (under the rapidly- descending part of the Hadley Cell) for daytime surface heating over land to permit some of the fast-moving air to reach the surface- bringing very strong Westerly winds (or likely north-westerly winds as the Hadley Circulation returns to the Equator at low- levels). Such very strong daytime surface Westerlies, between (say) 20N and 30N during Spring could cancel out north-easterlies in remaining latitudes over the rest of the year. Strongly- accumulated Westerly AAM in the upper- atmosphere is liable to be "thrown back" towards low latitudes to make contact with the surface there- if surface and low- atmosphere conditions were very much colder in middle and higher latitudes. 

There is nothing in the Laws of Physics to preclude the global climate from becoming like this with the right climatic conditions: Indeed, climatologists have established that there was  extensive dryness across large parts of the world, prevailing strong Easterlies in both higher and low latitudes (at least in the low atmosphere), during the coldest phase of the last Ice Age. This has been observed in the Southern, as well as the Northern Hemisphere. The extensive Westerly AAM being put into the atmosphere by stronger North Easterly (and in the Southern Hemisphere, South Easterly) Trade Winds clearly did not lead to stronger (and more extensive) Westerlies in higher latitudes then. There is nothing in the Law of Conservation of Angular Momentum to suggest that more extensive Easterlies below cannot be compensated by much stronger Westerlies aloft (rather than in higher latitudes) with the high mountain areas like the Himalayas and the Bolivian Altiplano/ North Chilean Andes being the sinks for Westerly AAM rather than the higher latitudes. Icy oceans and frozen lands at higher latitudes are poor sources of energy for deep depressions to form and bring moisture- laden Westerlies.

However, for sure, in the absence of significant amounts of Westerly AAM being lost to the Earth- Atmosphere system (and it is invariably insignificant- except were a large meteorite to careen through the upper atmosphere), then the manner of the exchanges of Westerly AAM between the surface and atmosphere vary considerably with changing climatic conditions, with the seasons and within seasons. The little illustrations above- showing Westerly AAM being transferred to higher latitudes (and especially in Winter) are not always a given- and they probably do not represent what happens during the coldest  winter in an Ice Age (for example). Under extreme conditions, like those I have illustrated, the tropical and sub-tropical  mountains (i.e. those between 30N and 30S) can become net sinks for Westerly AAM before the Westerly AAM can even reach higher latitudes.               

[The fundamentals of Atmospheric Angular Momentum]

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However, the manner in which Westerly AAM is returned to the Earth's surface varies with the seasons and is also dependent on whether conditions are such at higher latitudes to favour the formation of depressions, upon which strong Westerly winds are dependent. In winter the frictional impact of stronger North East Trade Winds between the cooler sub-tropics and hot, steamy Equatorial zone add Westerly AAM to the atmosphere at a higher rate as these blow over land and oceans, than in summer. However it does not follow that higher latitudes are always the sink for such Westerly AAM: Indeed the atmosphere can merely store this Westerly AAM leading to much stronger Westerlies in the upper troposphere/ Stratosphere whilst easterlies persist below and the Earth's rotation slows down by a millisecond or two- that is, until there is somewhere at the Earth's surface where conditions become suitable to act as a sink for Westerly AAM. The overall axial Angular Momentum of the Earth-Atmosphere System can still remain constant and the Law of Conservation of Angular Momentum is not violated by the Stratosphere having Westerlies blowing at hundreds of miles per hour whilst there are more Easterlies below and the Length of Day increases by one millisecond.

There are locations in higher and middle latitudes in the Northern Hemisphere that do not get Westerly winds in the winter months, which normally get winds from north or east. Eastern Siberia and Mongolia are amongst such locations and, of course almost all of Russia and continental Europe come under the influence of bitter easterly winds at times. Westerly AAM is still being pumped into the troposphere by the frictional influence of North East Trade Winds at those times and also, one must assume, by these winter-time easterlies at higher latitudes too. So what happens to the Westerly AAM pushed into the atmosphere?

A useful little weather website called Windy (see here: https://www.windy.com/ ) can provide fascinating insights at such times, as I observed in January/ February 2021 when all of northern Eurasia was under the influence of intense cold associated with Arctic and Siberian blocking highs: The Himalayas and High Tibet become a major sink for Westerly AAM when severe weather pushes south and west across the Euro-asiatic continent in winter as Westerly wind speeds high over the subtropics increase sharply. Very cold conditions with extensive easterlies and northerlies in higher latitudes can also lead to an increase in the strength of the North East Trades, both of which would add Westerly AAM until another sink for Westerly AAM arises just south of the Equator when the Intertropical Convergence Zone (ITCZ) is pushed well south of the Equator: Where and when the surface North East Trades overshoot the Equator moving south, the changed influence of the Earth's rotation on the movement of air causes these winds to become north-westerly winds (these remove Westerly AAM through friction with the underlying surface).

Westerly AAM being pumped into the atmosphere by the frictional impact of tropical Easterlies is not the only prerequisite for wet, windy Westerlies to affect higher latitudes: It is one of them. The other prerequisites are for suitable temperature and pressure contrasts in the atmosphere that favour the formation of depressions- and without depressions (or at least a sustained fall in surface pressure with latitude) there can be no Westerlies, and also for a source of energy: Depressions need energy- warmth from an ice- free water surface for latent heat (or land warmed by the Sun).

If there was just one big continent north of 25N- or the Atlantic and Pacific oceans north of these latitudes were frozen over- no depressions would form in higher latitudes in winter. Instead the low-atmosphere over all middle and high latitude areas would become extremely cold under one large area of intense high-pressure (greatest near the North Pole): This would lead to very cold surface north- east winds  which would also have the effect of strengthening the North East Trade Winds through the injection of colder air from high latitudes. Would would happen to all the Westerly AAM put into the atmosphere? 

In this scenario, with all middle and high latitudes frozen, the Hadley Cell would be associated with stronger Westerlies aloft, particularly where the upper-air approaches the sub-tropics. The troposphere over the frozen middle and high latitudes would be shallower and the sub-tropical jet-stream would be stronger (and a bit lower) as a result. The descending air over the sub-tropics would (initially) be moving from west to east very rapidly (probably at over 200 mph) but it would not be slowed by air moving equator-wards from higher latitudes aloft (a feature of warmer conditions with depressions at higher latitudes), the subsidence would also be much stronger than nowadays and the very strong subsiding Westerlies would not really start to lose their speed until they reached the zone where the atmospheric pressure gradient was such that it no longer supported them. So, 200 mph Westerly winds over the Himalayas and Karakoram mountains, with 100 Westerlies during the day affecting High Tibet (even in January, the Sun has sufficient strength at 30N - even over an icy surface to heat the surface and lower air and remove any temperature inversions that could keep such winds from directly impacting the surface!). These winds, consisted of subsided air would be dry and (at this elevation) very cold and, since the frictional impact of the wind increases with the square of their speed, then even though the Tibetan plateau and Himalayas covers just 0.5% of the Earth's surface and the air density just half of that at sea- level at 6,000 metres, such Westerlies would be strong enough to counterract low-level north-easterlies elsewhere in the Northern Hemisphere- even if (as likely) the North East Trade Winds would have a mean speed of 30 mph between 30N and the Equator. 

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[The fundamentals of Atmospheric Angular Momentum]

Global Atmospheric Angular Momentum (AAM) is a fascinating subject because it is very central to the type of weather that is experienced over much of the Earth's surface, particularly at higher latitudes. In essence, what goes up must come down, so if the atmosphere gains Westerly AAM with respect to the Earth's surface in part of the World, it must lose it elsewhere.

Much is made of the fact that the Earth- Atmosphere system as  whole must maintain the same level of axial Angular Momentum overall- through rotation of the Earth and atmosphere from west to east- for the Law Conservation of Angular Momentum to be observed: However that is not strictly true. Total Angular Momentum must be conserved only  in the absence of outside forces acting on the Earth- Atmosphere System. There are outside forces, chiefly the gravitational effects caused by the Moon (and to a lesser extent the Sun) as the Earth rotates: The Earth (and atmosphere) has to rotate through "tidal bulges" caused to oceans and atmosphere by the Moon and Sun, which leads to a mean increase in the Length of Day by about one millisecond every fifty years. Additionally, meteorites reach the Earth from outer space and can bring small changes in the total axial Angular Momentum of the Earth-Atmosphere system, as can the Solar Wind. 

Atmospheric out-gassing to space involves the loss of one ten-billionth of the mass of the atmosphere each year, so you are dealing with the total axial Angular Momentum of this in addition to the remainder of the Earth-Atmosphere System remaining constant. The impact of all these influences however, are miniscule compared to the mass of the atmosphere, at least under current global climatic conditions.

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