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  • Location: The Netherlands
  • Location: The Netherlands
Posted (edited)

Positive temperature anomalies

 

The year 2014 has been an above-average year of temperature throughout most of the year (excluding August). In october, this positive anomaly is most likely to continue. For example, the provisional average temperature for october up-to-date in central England is 12.68*C. However, the normally experienced temperature during October (using the 1971-2000 average) is only 10.4*C1. The average temperature for October during 2014 will be dropping slightly (given the fact that there are still about 8 days to go), but all 8 days from now are expected to be above average temperature-wise. Therefore, a significant positive anomaly in temperature compared to climatology will remain.

 

Looking at the anomalies throughout the UK, the temperature anomalies are varying between 0.5 and 2.5*C up to now in October. This can be seen in the chart from MetOffice below:

 

2014_10_MaxTemp_Anomaly_1961-1990.gif

Temperature anomalies comparing current October temperatures with average temperatures during the 1961-1990 period. Courtesy: MetOffice.

 

Note that a different climatology period is taken in comparison to the climatology period given by NetWeather. 

 

Once again, given that we are not yet fully through october, the final temperature anomalies of October will be slightly lower than indicated in the image.

 

Comparing this chart with the NetWeather data for central England (which is not really objective given the difference in climatology periods used), shows that the anomalies given by the MetOffice are about 1 degree lower than the anomalies given by the NetWeather data. Still, the anomalies are largely positive (in excess of 1 degree in the plus).

 

Wind direction analysis

 

The main cause for the anomalously high temperatures throughout October (and probably throughout the year as well) is the abundance of southerly winds. To show this, below is the wind direction configuration of De Bilt, the Netherlands, in October:

 

b72de7638b.png

Wind direction and -force analysis of De Bilt, the Netherlands in October. The circle consists of 12 pieces, each having their own wind direction. The circles indicate the percentage of occurrences (from 10 up to 30%). 

 

What can be seen is that a very significant dominance of southerly winds has been present during October. In fact, if one takes all southerly components together (from WSW to ESE), the total comprises about 70 %. That means that in about 70% of the days so far in October, the wind has had at least some kind of a southern component. Also note the absence of purely westerly winds, given that only about 3% of the total time, the wind blew from the west.

 

The pressure charts of October can explain why the winds blew mainly from the south. This has to do with the fact that persistent troughing to the west of the UK has been in place throughout October. For example, look at the pressure chart of 7 October below:

 

post-20885-0-35417300-1414152675_thumb.j

GFS analysis of surface pressure + 500 hPa heights of 7 October (T0).

 

For a better view, it is advisable to open the image itself individually.

 

What can be seen is that there is a low pressure area, along with a deep trough (indicated by the blue colours) over and to the west of the UK. If one follows the isobars (white), one can clearly identify the southerly component of the wind (as shown by the black arrow).

 

Reasoning

 

The persistence of a trough to the west of the UK has been remarkable at least. An explanation (given in the Dutch article given in the Sources section) might be that anomalously warm waters over the Atlantic and Western Europe might be causing a sharper temperature gradient between the lower- and upper atmosphere, aiding in the formation and strength of low pressure areas west and over the UK.

 

The anomalously warm SSTS (sea surface temperatures) are visible on the global scale:

 

wksst.20141015.gif

 

Sea surface temperatures anomalies as analysed by NOAA at 15 October 2014.

 

If one looks closely at Europe (eastern edge of the map), one can indeed see that (as of 15 October 2014) sea surface temperatures have been about 1.5-2.5*C higher than average in the North Sea, in the Bay of Biscay and to the west of the UK/France.

 

However, this does not explain the full event. A more pronounced temperature difference between the upper- and lower atmosphere could indeed partially be explained by the anomalously warm SSTS looking from the ground level. But this cannot explain the presence of anomalously cold air aloft being advected southward over the ocean just west of the UK. Probably a more complex mechanism is at play here. 

 

Open questions

 

Does one have any ideas about what may be causing the continuous southward advection of cold air over the Atlantic to the west of the UK? The answer might be rather complex, but it would be nice to find some kind of an explanation (probably related to tropical convective activity of some sort, like MJO etc.?)

 

Sources:

1http://www.netweather.tv/index.cgi?action=cet;sess=

http://www.metoffice.gov.uk/public/weather/climate-anomalies/#?tab=climateAnomalies

http://www.weer.nl/weer-in-het-nieuws/weernieuws/ch/deae3bbe84de36d7985107d77fe9f13b/article/zuidelijke_wind.html (Dutch article, including reasoning)

http://www.wetterzentrale.de/topkarten/fsavneur.html 

http://www.ncdc.noaa.gov/teleconnections/enso/indicators/sea-temp-anom.php?begmonth=10&begday=15&begyear=2014&endmonth=10&endday=15&endyear=2014

Edited by Vorticity0123
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