Rain in southern Spain: a closer look at the influences of the North-Atlantic Oscillation and El Niño.


There appears to be a subtle connection between rain in southern Spain and the North Atlantic Oscillation (NAO) and El Niño (ENSO). The NAO has a large influence in the winter, the monthly correlation is -0.5 to -0.6 in Gibraltar and Sevilla for December to February. A low NAO index (like this moment, November 1997) means that depressions tend to take a more southerly course and give rain in this area. In spring and fall the connection is weaker. Unfortunately we cannot predict the NAO index a month ahead of time.

On the other hand, an El-Niño event is associated with more rain in south-western Europe in the late summer and early fall (with a correlation of 0.4 to 0.5), and there is a hint of less rain in the spring. We can predict the NINO3.4 index 3-6 months ahead of time, so this might be useful to say something about the chance of a wetter autumn in strong El-Niño years. (See also our article on the Spring rain in Spain and El Niño)

Precipitation in the months of July, August and September has a 0.6 corealtion with the July NINO3.4 index, both in Gibraltar and Sevilla. A linear correlation coefficient is not a good measure of a relation in precipitation, the most convincing is a look at the data. Out of six very wet seasons, four were during a strong El Niño, one at a La Niño (the opposite phase of the Southern Oscillation), and one in between. Conversely, four out of the six strongest El-Niño years gave more than twice the average precipitation in July-August-September, and the other two above average.

Both large climate fluctiations seem to have an influence on the rain in southern Spain. The big storms at the beginning of November 1997 cannot be attributed to one or the other, but it is striking that bothe El Niño and the North-Atlantic Oscillation had a tendency to increase precipitation in this area at the time.

Geert Jan van Oldenborgh

The relationship between the fall rain in Spain and El Niño is also clearly visible in a plot of correlations between air pressure (SLP) and the NINO3 index. A low pressure zone between the Azores and Bermuda and a high pressure zone near the Cape Verde Islands give a south-westerly wind towards southern Spain and Portugal - with lots of rain.

The correlation between sea-level pressure and the NINO3 index in july-august-september.

Geert Jan van Oldenborgh

The connection between rainfall and the NAO can also be seen clearly on correlation maps between the Azores-Iceland NAO and ERA P-E (1979-1993), this should be compared with Hurrell's famous figure (which I am unable to reproduce).

The correlation between precipitation minus evaporation and the NAO index in December, January, February and March 1979-1993.

Using the much longer time series of the NOAA rainfall database, we get a very similar picture over land

The correlation between precipitation and the NAO index in the December-March winter.

In other words, the influence of the NAO is largest over Portugal, western Spain and Northern Marocco (negative), and Scotland and western Norway (positive). There are secondary centers over the Balkan and the Black Sea (negative) and Finland (positive).

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Geert Jan van Oldenborgh