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Warm seas protect mid-latitude islands from frigid Arctic cold in a variety of ways


iapennell

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Posted
  • Location: Alston, Cumbria
  • Weather Preferences: Proper Seasons,lots of frost and snow October to April, hot summers!
  • Location: Alston, Cumbria

Dear fellow Weather Observers 

In middle latidude islands and some continental areas in various parts of the World hundreds of miles of warm seas with average temperatures well above 0C in the coldest months separate the region from any source regions from which extremely cold airmasses (with a mean temperature well below 0C at sea-level) may originate. Such areas of the World include the UK, but also Ireland, various Mediterranean islands, southern Japan, Bermuda, New Zealand and Tasmania. Also included are southern Argentina/ Chile as well as southern Australia and South Africa because the continents to which they are attached are only large in latitudes too low to support serious continental cooling and so Antarctica is the only real source region for frigid air- meaning advection across the Southern Ocean is required for this air to reach these continental localtions- so for the purposes of the dynamics of very cold airstreams the southern tips of the Southern Hemisphere continents are included. 

Quite apart from the modifying influence of much warmer sea-surfaces in warming very cold airmasses passing over them: An airmass of 3 km depth in the lower atmosphere is warmed by roughly 10C in 24 hours if the temperature differential between it and the ocean surface is 20C to begin with because a temperatre differential of 15C leads to a 300 Watts per square metre net heating of the air by the warmer ocean (and the warming will be more than that to begin with), thouh in winter this is offset (to a rather smaller extent) by net radiative heating of the lower atmosphere.  In addition, there will be a significant amount of heat (latent heat of condensation) as the water vapour pressure from the warm sea surface greatly exceeds that from the cold atmosphere above- with the result that moisture enters the atmosphere and vigorous convection soon results in cumulonimbus and showers of rain or snow. Thus, you end up with a situation whereby an airmass that starts off at -10C and spends 24 hours over a sea-surface of +10C is likely to be about 2C in the lowest layers after 24 hours. This is why a frigid Siberian airmass, crossing Scandinavia at a temperature of -20C in the lowest layers in winter, is hardly ever likely to be below -5C at sea-level by the time it reaches the coast of eastern Scotland and North East England after its passage across the northern North Sea, the surface of which in January is typically about 8C.

But this obvious warming is not the only way warm, ice-free high-latitude seas protect middle-latitude regions from the fiercest onslaughts of Arctic, Siberian or Antarcic cold: It is very clear that the vapour-pressure of warm sea-surfaces compared to a very cold airmass over-running it pumps large amounts of moisture into the air. A strong dry and icy wind at -10C over a sea-surface of 10C can extract over 1cm of water-equivalent, which is more than sufficient to result in deep cloud-cover in an airmass at (say) -5C in the lowest layers and -30C at 3,000 metres (the air would warm more near the surface though less quickly aloft and that would encourage strong convection currents in the still-frigid air). Much of the moisture would freeze out in cloudy convection resulting in sharp snow-showers, which are a feature of very cold airsreams over warm seas, but not all of it. With the moisture chiefly freezing out at higher levels the higher-level air (at 2 to 3 km) warms quite rapidly so after 24 hours this reduces the atmospheric temperature gradient to the extent that convection then proceeds at a lower rate. However, cloud-cover at 2-3 km would act as a stronger surface from which radiative heat-loss maintains the middle-level air at a low-enough temperature to ensure convection continues.

Continued below          

Edited by iapennell
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Posted
  • Location: Alston, Cumbria
  • Weather Preferences: Proper Seasons,lots of frost and snow October to April, hot summers!
  • Location: Alston, Cumbria

Continued

Once the cold Arctic (or Antarctic) airmass reaches the island mid-latitude region it is likely to be infused with considerable cloud-cover, with cloud of considerable depth about 2 km above the surface. This cloud-cover, in winter, ameliorates the very cold conditions likely to be experienced at the surface because cloud-cover acts as a blanket, greatly reducing night-time radiative heat loss. This means that air-temperatures seldom drop more than a few degrees below the generl temperature of the cold airmass a few hundred metres above the surface. Even if the cold-air advection from high latitudes is accompanied by high-pressure moving in and subsidence up to 500 metres a day, with moisture entering the air from the warm sea-surface at a rate of 1 cm a day (especially likely if the cold airstream is accompanied by strong wind), the replacement of the lowest 2 to 3 km of the atmosphere with dry air at a rate of 20% a day is hardly likely to ensure the cloud-cover disperses. Thus, only very strong subsidence (with some warming of the upper-air to bring the inversion nearer the ground) will bring the clear skies needed for radiation cooling at the surface- a situation that must involve amelioration of the cold temperature of the air some 2 km above the surface so that the airmass is alittle less cold anyway. So, in this second way, warm seas protect mid-latitude islands from fierce high-latitude cold. 

The third significant manner in which warm seas in higher latitudes protect mid-latitude islands from great cold is by modifying surface atmospheric-pressure polewards of the locations to be affected in a manner that makes those very cold Arctic (or Antarctic) outbreaks less likely in the first place. Upper-air (above 3,000 metres) is invariably extremely cold polwards of 50N (or 50S) in winter, partly as a consequence of Westerly Atmospheric Angular Momentum (created by easterly Trade Winds' interaction with the surface in the tropics and sub-tropics) leading to strong Westerlies- and a resultant limitation in the penetration polewards of warmer air- aloft. Very cold air aloft and warmer air below (as it invariably is over warm sea-surfaces) results in rather lower atmospheric-pressure near the surface. This entire process limits the potential of high-pressure to form and persist poleward of the middle-latitude islands where it could direct frigid high-latitude air towards them.

Of course, with sea-surface temperatures in higher latitudes warmer as a result of global warming, this rather limits the scope for frigid high-latitude airstreams to even reach the UK, or to reach Ireland or New Zealand. Warmer seas also limit the scope for these very cold airstreams to then bring clear night skies and extremely low temperatures in winter.      

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Posted
  • Location: Alston, Cumbria
  • Weather Preferences: Proper Seasons,lots of frost and snow October to April, hot summers!
  • Location: Alston, Cumbria

Just to add that sea surface temperatures in higher latitudes are modified- over time- by the prevailing weather-patterns over them. If the warmer-than-usual seas cause low-pressure polewards and prevailing mild Westerlies that lock out very cold Arctic (or Antarctic) airmasses and these warm seas cloudy up frigid Arctic airstreams flowing over them minimising surface radiative heat loss, all this in turn helps to keep sea-surface temperatures from dropping, though a month of 100 Watt per square metre cooling of a 400 metre deep layer of ocean leads to just 0.13C of cooling (below 400 metres almost all deep water in high latitudes is near or slightly below 4C) ! So, cold-air advection be needed too, and it would need to be persistent, with winds pushing back any warm ocean-currents from lower latitudes so the ocean cools over a season for a significant impact such that further cold incursions are encouraged (with higher pressure to the poleward encouraged) and are severe and without blanketing cloudy convection preventing night-time radiative cooling over affected mid-latitude islands.

And, of course, the entire process can be undone- and more- by the Summer Sun, with clear skies (and high-pressure) more likely over cooler seas. Ocean surfaces have a low albedo when the Sun is high. Thus, unless something major climatic happens (like a quiet Sun with a Grand Solar Minima, or a massive volcano reducing Solar input into the oceans), it means that under current climatic conditions with rising CO2 levels higher mid-latitude oceans will stay well above freezing. This means that middle-latitude islands will continue to be protected from fierce Arctic (or Antarctic) cold in the coming winters- that is unless some pretty extreme set of high-latitude blocking weather-patterns occurs and persists over two or three winters overcoming the ability of the Summer Sun to warm the oceans!   

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