søndag, januar 10, 2010

Oceanic oscillation causality

This posting is a response to a post on another blog.

Great article, Scot!

I've just come to realize the relationship between the Pacific and Atlantic oscillations myself – which let me to your posting. The rarity of your insight prompted this comment:

There seems to be little to no significant insight into the causality of the major ocean oscillations, which strikes me as odd – to say the least. Indeed I often hear a reference to the jet stream phenomena or polar vortex, as if it were complicit in the rise and fall of the AO regime. I’ve seen the method of forecasting the AO, and I’m not impressed:

The daily Arctic Oscillation (AO) index is constructed by projecting the daily (00Z) 1000mb height anomalies poleward of 20N onto the loading pattern of the AO.

The daily AO index and its forecasts using MRF and Ensemble mean forecast data are available for the previous 120 days and they are normalized by standard deviation of the monthly AO index from 1979 to 2000. A 3-day running mean is applied for the forecast indices.

We recognize causality in the pressure differences between the subtropical and subpolar fronts, but the application of statistics for prediction implies little or no understanding as to the origins of these pressure variations. This is a mystery to me!

I propose the following idealised causality:

At the ITCZ (Intertropical convergence zone) air is lifted towards the tropopause and drained for its heat and vapour contents. Drifting towards the poles by way of expansion it is then sunk back towards the surface by way of contraction at about 30 degrees north and south creating broad high pressure belts to the north and south of the ITCZ. Caused by the low pressure conditions in the convective areas parts of this higher pressured air mass is then returned to the ITCZ creating surface winds that are directed on a westward track by means of the Earths rotation – the so called trades. In the process of their return they push surface water in the high pressured area towards the west of the ocean basin causing an imbalance in the water distribution. In the case of both the Atlantic and the Pacific oceans and in contrast to the Indian ocean the water mass displaced to the west of the basin must be replaced in the east basin by the upwelling of colder bottom waters off the west coasts of Africa and Europe and south and north America respective to each distinct basin. Consequently cooler waters replace warmer surface waters which are now pushed north and south towards the subpolar regions following the continental plates of north and south America and Asia, Philippines (Indonesia) and Australia respectively. Warm north and south going ocean currents are thus directed towards the pole areas where they cause energy/mass exchange with dry high pressured polar air by means of convection in the areas of 50-60 degrees north/south latitude causing extensive cyclonic activity. Depending on the scale of convective mass exchange the resulting Ferrell cell contribution to the high pressure zones derived from convection at the ITCZ causes them to strengthen at 40-45 degrees north and south, thus lending further momentum to the trade winds driving more of the warm water masses at the equator towards the Polar Regions upping the scale of convection in those areas further driving the engine into high gear.

However, this is strength through exhaustion and the upwelling cooler bottom waters from the eastern continental shelves of the respective ocean basins gradually cools the ITCZ surface waters, thereby severely reducing the convective processes on which the motor runs. At some point convection in the tropics is brought almost to a standstill. As we have seen, this must cause a weakening of the Hadley circulation in the tropics and the transfer of energy into the sub polar regions resulting in a weakening of the convection there, further reducing the pressure areas - causing them to retreat towards the ITCZ. Consequently, the winds that transferred the basins hot waters to the west relax even further, and the bulk of hot water in the west starts to slide back towards the central and eastern basin to equate the pressure differential previously upheld by the now decaying convective processes.

While the Tropical convective regime is strong it causes cyclonic activity due to ocean currents all the way to the Arctic basin. Cyclones climb a ladder which starts in the North Atlantic off the North American east coast of New Foundland in the weak tropical phase, which can then help push warm subtropic water masses by way of mixing and salt sinking further northeast to develop a cyclonic regime in the Norwegian Sea and from there by the same means go on to form cyclones in the Barents Sea and the Kara Sea. The mass energy transfer causes the arctic troposphere to warm and expand, weakens the polar anticyclonic regime and the sinking of (relatively) warm salty waters under the arctic sea ice causes severe melting leaving a lid of fresh water floating in the arctic basin where the ice melted. Due to the peculiar geology of the Arctic however, this fresh water is contained within the basin by the cyclonic regime. When this regime collapses by way of the above described processes the anticyclonic regime returns and changes the winds and currents in the area causing the fresh water pool to expand into the neighbouring Barents, Kara and Norwegian seas and possibly even the Baffin bay area.

In any case, the point is this: a particularly strong and long lasting Arctic cyclonic regime could leave behind vast amounts of melt water in the Arctic basin, which can then be dispersed throughout the regional seas during autumn, if the anticyclonic regime is reinstated at this time. This will hamper cyclone activity, when convection in the tropics return, because the cyclonic regime is usually aided and abetted by the salt sinking mechanics of the north Atlantic and Arctic Seas. Freshwater lids in these seas will reduce both the salt sinking mechanism which contributes to the momentum of the Gulf Stream, as well as the convection on which the cyclones feed. In the extremity of those conditions it is conceivable that the anticyclonic front can hold off the next pulse of seawater heat generated in the subtropic regions, especially if El Nino conditions persist in the Pacific and mass energy transfer of that vast ocean to the sub polar front is in the weak mode. In that case the Arctic front is pushed south by as much as 10 degrees over the whole of the hemisphere, which is unusual to say the least.

It may be that it requires melting over decades for the arctic ice sheet to produce the conditions necessary for a really severe cold shock to be delivered. The reasoning is as follows: Young sea ice is more prone to melting in the summer; the arctic winter will never the less ensure that most of the seawater in the arctic region produces ice - excreting salt to the deep currents. This produces a ready freshwater reservoir within the arctic basin by end of September. If the melting the previous decade has been severe, there is a lot of young ice being melted during the summer. A reinstated anticyclonic regime during autumn will deliver a significant freshwater pulse to the arctic seas effectively blocking the next energy pulse emanating from the tropics and hampering the salt driven subduction, further weakening the Gulf Stream. This regime could persist for almost a decade.

These are the most severe winters in Denmark, Scandinavia dating back almost a century:

1986-87 - 1985-86 - 1984-85 - 1981-82
1969-70 - 1962-63
1946-47 - 1941-42 - 1940-41
1928-29 - 1923-24

Notice the clusters around the forties and the eighties? There seem to be decade long lulls followed by a series of truly severe winters 4-5 within a decade or less. When Scandinavia has cold winters Greenland on the other hand has warm winters, and melting of that ice sheet can contribute to freshwater conditions in the north Atlantic and Labrador Sea. September of 2007 saw the lowest levels of Arctic sea ice in a very long time. Maybe we’re in for the chill of a lifetime. That however – in my thinking at least - depends on the Pacific Ocean.

That was my two cents worth, anyways…