Posted by: mynaturaldiary | December 21, 2015

Whither the weather

We’ve had a very mild November and December, beset by storms. This is probably due to the vagaries of the Jetstream, steered by the North Atlantic Oscillation, which has been strongly positive since early October.

nao.mrf.obs151220

This is in turn part of the Arctic Oscillation.

ao.obs151220

These two measures of air pressure over the Atlantic drive our weather on a daily basis. Thus

“The degree to which Arctic air penetrates into middle latitudes is related to the AO index, which is defined by surface atmospheric pressure patterns. When the AO index is positive, surface pressure is low in the polar region. This helps the middle latitude jet stream to blow strongly and consistently from west to east, thus keeping cold Arctic air locked in the polar region. When the AO index is negative, there tends to be high pressure in the polar region, weaker zonal winds, and greater movement of frigid polar air into middle latitudes.”

CRHQCB4W8AAyCJI.png:large

They are chaotic in behaviour over a short time range (hence the British phrase about ‘the bloody weather’) and beyond prediction over a few weeks.

Today we have a series of strong westerlies bringing in Atlantic warm moist weather.

bracka20151221

What of the longer march of weather, which we might call climate? We have the annual Central England Temperature record (CET), running back to 1659 for this. A time series analysis of the data, using exponentially weighted moving averages (EWMA)  for 10 (red line) and 30 (blue line) years is revealing.

CET 10 & 30 year trends

One can see that the annual Central England Temperature fluctuates from year to year. The yearly difference has a mean of 0 degC and a standard deviation of 0.8 degC. It looks gaussian, which is a requirement for random behaviour.

Delta Annual CET

However the EWMA for 10 and 30 years reveal decadal trends; sometimes up and sometimes down. The main turning points for the 30 year trends  are highlighted below.

CET Trends

The year where the trend changes is in the first column; the 30 year average temperature for this year is in the second, the number of years the trend lasted is in the third, and the ‘slope’ (difference in column 2 divided by column 3, multiplied by 100) gives the ‘100 year’ trend during each phase. Doing this gives a measure of the ‘strength’ of the change. You can see that the trends last between 13 to 77 years, with the current trend 5 years long.

Perhaps a clear marker for when the trend has changed is when the red line 10 year EWMA cuts through the blue line 30 year EWMA. When the red line is above the blue line, Central England is in a warming trend; when the red line is below the blue line, Central England is in a cooling trend. Right now, we are in a warming phase. This may be due in part to sea temperatures in the northern hemisphere

figure-101

which shows a similar pattern of falling, rising, plateauing and rising temperatures at roughly the same dates as the recent CET trends.

CET Trends 1860 2020

Superposed on this will be the steadily rising contribution from CO2, which shows continuous growth.

Our current positive phases of the NAO/AO is keeping the weather warm for this time of year, which may drag the yearly CET to a new all time high. This may be due in part to the El Niño event happening in the Pacific this year.  The red line 10 year trend in the CET points downwards. I guess we will have to wait a few more years to see how the current trend turns out, and whether it falls below the blue line 30 year trend, at which point we can say we are in a cooling phase and identify clearly when it begun.

 

 


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