North Atlantic Oscillation

By Paul Homewood

 

 image

https://www.metoffice.gov.uk/hadobs/hadcet/cet_info_mean.html

 

If you had not noticed (!), it has been a mild and wet start to the year here in the UK, and also across in NW Europe.

No doubt this will be linked to global warming in due course, but in fact it is simply weather, as the CET chart below proves:

HadCET_act_graphEX

 

Since the year started, temperatures have consistently been within the normal band. In other the sort of temperatures commonly seen at this time of year.

However, they have also been consistently in the top half of that band, rather than being spread between cold and warm, as would happen most years.

The reason for the weather we have had is, of course, the jet stream, or more precisely the North Atlantic Oscillation (NAO), which has been strongly and stubbornly positive all winter.

The Norwegian Centre for Climate Research CICERO spotted this mild weather coming back in December, and commented on 6th January:

The unusual warm temperatures this winter and forecasts indicating milder winter conditions for January, February and March in Europe are partly due to an atmospheric circulation pattern called the North Atlantic Oscillation, or NAO. This atmospheric circulation pattern explains well the weather we get in Europe, especially in winter.

https://cicero.oslo.no/no/posts/nyheter/unseasonal-temperatures-for-norway#

 

 

Our own Met Office explains the NAO phenomenon:

 

The term ‘North Atlantic Oscillation’ is used by meteorologists to refer to variations in the large-scale surface pressure gradient in the North Atlantic region.

In the average state of the atmosphere, the North Atlantic surface pressure is relatively high in the subtropics at latitudes 20°N to 40°N (‘the Azores High’), and lower further North at latitudes 50°N to 70°N (the ‘Icelandic Low’). The North-South pressure difference determines the strength of the westerly winds across the Atlantic and is known as the North Atlantic Oscillation (NAO).

When the pressure difference is large, the NAO is positive and the westerly winds are strong and storms tend to be stronger, more frequent and travel across northwestern Europe. When the pressure difference is small, they travel across southern Europe. The NAO is also associated with changes in temperature and rainfall in Europe and North America.

The fluctuations in the NAO occur on a wide range of time-scales. There are day-to-day changes associated with weather systems, and slower changes associated with seasonal and longer term variability, which is predictable from November for the coming winter.

NAO impacts

Winter (December-January-February) conditions

When the NAO index is well above normal, there is an increased chance that seasonal temperatures will be higher than normal in northern Europe, northern Asia and South-East North America, and lower than normal in North Africa, North-East Canada and southern Greenland. The patterns for precipitation (rainfall, snowfall) are more localised, with an increased chance of higher rainfall in northwest Europe and lower rainfall in southern Europe. When the NAO index is well below normal, the tendencies are generally opposite. The figures below show where seasonally-averaged temperatures and rainfall are likely to be in the top or bottom one third of observed values, given that the seasonal NAO index is in the top or bottom quarter of observed values.

https://www.metoffice.gov.uk/research/climate/seasonal-to-decadal/gpc-outlooks/ens-mean/nao-description
 

There is one more part to the jigsaw, though.

As you may have noticed, Greenland should be colder than usual. And sure enough, that’s exactly what we find:

image

image

https://data.giss.nasa.gov/cgi-bin/gistemp/stdata_show_v4.cgi?id=GL000004250&ds=15&dt=1 

 

But a colder Greenland also means a drier winter, as mild winters are driven by low pressure systems bringing warm air from the south. In contrast, this winter has seen much colder, drier polar air trapped over Greenland.

And less snow means less build up on the ice cap:

SMB_curves_LA_EN_20200318

 Greenland Surface Ice Mass

http://polarportal.dk/en/greenland/surface-conditions/

 

Weather forecasts suggest the return of low pressure to Greenland in the next week or two, so snowfall should accumulate again. But it is unlikely to offset the shortfall so far this winter. In turn we will probably see less accumulation than average come the end of summer.

Now, here’s £50 that says the usual suspects will blame that on global warming!

via NOT A LOT OF PEOPLE KNOW THAT

https://ift.tt/39a2P9v

March 19, 2020 at 02:12PM

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