Was The Flip From Nina to Nino, Helped By Warm AMO, -NAO The Reason For Such A Wet UK, Scandinavian Summer?

Written by on July 29, 2012 in Rest of Europe, United Kingdom & Ireland with 0 Comments

Chart off Wikipedia

I guess a good reason behind the well above normal precipitation which set in back in April and continued ever since across the UK and Scandinavia is thanks, in part to the flip from back to back La Ninas to an El Nino, helped by the still warm North Atlantic Oscillation and prodominantly -NAO.

The big question is what jolted the major flip from super dry to super wet between March and April over the UK? My suspition was the warming of the equatorial Pacific which either influenced or helped flip the NAO negative. Of course the Pacific is cold overall despite the coming on of the el nino and this makes a difference from there being a warm PDO along with a nino. All these factors can alter upper air patterns, repositioning ridges and troughs as well as controls the amount of heat which gets lifted north up into the mid and even high latitudes. Of course a -NAO means Greenland/N Atlantic ridging which leads to trough development over NW Europe but the warm North Atlantic waters which have been running 4F above normal to the west of Iceland and generally warm across a broader area has likely fuelled the wet weather and lows spinning their way across the UK.

Warm Over Cold SST’s In North Atlantic Likely Energized Summer Jet Stream Which Enhanced Cylogenesis

Notice in the below chart the cooler waters surrounding Britain, caused by the lower heights supported by the trough as well as the cloud and precipitation during the past couple of months. The other areas of warmth can be seen around the Azores, west of Iberia. This abnormal warm water likely helped inject more moisture into the UK as UK-bound systems drew air up from the south. Another, probably more important pool of heat is just west of Iceland. This heat over cooler waters beneath may have played a role in enhancing the summer jet, helped support more influential summer cyclogenesis. This warm water is reflective of the long term warm AMO cycle and supports blocking (-NAO) with stronger than normal heights over the North Atlantic.

This sea surface anomaly chart is courtesy of AccuWeather Pro

Most towns and cities to the north of London have seen below normal temperatures June and so far this July and that’s thanks to lower heights aloft but of course the cloud and rain is the bigger factor because at this time of year while the sun is strong, had there been more sun, despite a trough aloft, temperatures would likely be above normal, so it’s the prodominantly cloudy skies and precipitation which has meant for many places seeing 1-2F below normal in June and in July.

One must accept the vast alteration in the global weather pattern when you take a back to back La Nina (colder than norm equatorial Pacific waters) which can lead to drier than normal conditions over the UK with the flip it to a warm, nino phase. The El Nino leads to a push of heat and energy up into the mid-latitudes of the northern hemisphere and a strenghening of the jet.

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