What caused the past winter of 2013-14 to become so frequently stormy in Ireland and Britain?
Of course, you’ll hear that the reason is that the jet stream became very strong and fixed in one general location running from north of Bermuda and the Azores towards Ireland and then on into south-central Scandinavia. This is quite true but it just shifts what we are trying to explain from the stormy winter to the jet stream’s location. However, before we tackle that related question, some might wonder at this point, why would that particular jet stream path lead to a stormy winter. This part is basic meteorology. As long as the jet stream is of at least average intensity (and this winter it seemed 20-30 knots stronger than average much of the time) the storm track will tend to follow along. This particular path is very conducive to strong storm formation. It draws in very cold air from eastern Canada and Greenland, and very mild to warm air from the subtropical regions. Several of our major storms had warm sectors in the 12-14 C range when they approached Ireland — not every one of them delivered that warm sector to any part of land, but usually temperatures reached at least 10-12 C and the winter was also about 2 deg milder than average in many places.
In fact, I do extensive research on the 242-year data base of daily temperatures available from the U.K. Metoffice (known as the CET data set, Central England Temperatures) and this winter half-year thus far has the highest minimum daily value of any winter, namely 2.5 C which was achieved back on 23 November. The existing record is 1.7 C in the mild winter of 1974-75. There may not have been record warmth but there has been a near total absence of actual cold weather except for some slightly cooler than average days when the flow gets a bit more north of due west in its source.
Anyway, you may ask, what put the jet stream into this locked position and when will it unlock? We think that events in the Pacific Ocean region may hold the key to this. A very strong ridge has been present in the atmosphere most of this winter over the eastern Pacific. Some relate this to cold ocean temperatures near the equator west of South America. Whether that’s the case or not, this persistent ridge has created a very stable supply of bitterly cold arctic air for central and eastern North America. A lack of upper level (stratosphere) warming has contributed to locking in this pattern and keeping features from being disrupted by retrograde (westward moving) blocking patterns. So we’ve seen this storm-producing pattern reinforced every three to five days since mid-December, with one or two exceptions, by remnants of storms moving out of the northeastern U.S.
The models have been doing quite an impressive job on the “macro” scale this winter, showing us development scenarios sometimes a week to ten days in advance. There was one occasion when I could see modelled forecasts of one of the more intense storms in early January some time around Christmas Day here in your forecast office in western Canada, and was aware that the rather minor frontal system dropping rain outside my window was likely to evolve over five or six days into that particular storm. Some of your more intense storms were rather briefly active as secondary lows that formed in the wake of other more northward-moving storms. Those usually form about 48 hours before hitting Ireland somewhere south or east of Nova Scotia along a stalled polar front. Other cases form in the central plains states of the U.S. and can be tracked from there across the Great Lakes or Ohio valley to the Atlantic. The 26-27 Dec storm was of that sort, in fact, it was already a major storm for the northeastern U.S. and southern Ontario, Canada, where it dropped 50 mm of freezing rain on Toronto around 21-22 Dec.
Will the storm parade end soon? We expect it will. Models are showing signs of a considerable pattern change in North America. This may shift the storm track to latitudes closer to Labrador and Greenland, which in turn would allow some other, less energetic pattern to replace the current one over Europe. The best guess as to what that might be is some kind of split flow scenario with weaker systems taking a variety of paths into Europe at all latitudes, mixing up the menu of weather that you get in Ireland and the UK … in fact, there could even be some minor blocking episodes and cool, dry weather which could then morph into milder or warmer dry patterns at times. Some northerly flow is inevitable too, and there could be a touch of unseasonable snow in April for some.
In any case, this winter will be long remembered for the frequency of significant storms. The worst of them was probably “Darwin” of the 12th of February in Ireland, with “Erich” on 26-27 December often mentioned second, but frankly, there have been so many wind alerts issued that even the most enthusiastic weather “weenies” probably can’t recall every single one of these storms. I think there was a couple before Christmas, two right after, three in early January, a bit of a break mid-month, some more action in late January into early February, then the “biggie” on 12th of February, and now we’re looking at another relatively minor storm that could still bring its own little contribution tonight. Rainfall with all of these storms, which you may recall was sitting at almost zero on 12 December after a dry November, has since then exploded to reach just about twice normal values for many places in Ireland and three to four times normal in parts of southern England.
Some may ask if the Sun is responsible, or human activity and “climate change.” My assessment is that these are only background causes, after all, solar variation has not changed much from last winter to this one (we’re still near the peak of a relatively weak solar cycle) and human activity in the form of greenhouse gas increases can only be a small part of any given year’s variation especially if there is no continuing trend in one direction. I can’t say that these factors are zero or even negligible, but they aren’t the main story. Some natural causes in a complex chain of events must be responsible. We had some success in predicting this if you check our long-range outlook. However, this is based on analogues or pattern matching, and as to what actually causes these patterns, that would need a much longer post and I don’t claim to have a lot of the answers worked out (some, perhaps).
— Peter O’Donnell