Mood swings of an Atlantic weather pattern

ANOTHER LIFE:THE SPRING'S first swallows crossed the Irish coast some three weeks ago, but goodness knows what they've found to eat since then. Apart from the odd bumblebee, nuzzling the same primroses over and over, there have been virtually no insects on the wing in my garden. Our horse-chestnut tree thrust out its first two leaves on early cue, still hankering after Albania, only to clench them tight again in the late blast from the Arctic.

Does this mean it’s all up with global warming, and the nutters on the web are right – it’s all a hoax? Definitely not. But it is long past time that more people knew about the workings of the North Atlantic Oscillation (NAO), a phenomenon Irish weatherpersons seem to shrink from mentioning on telly, or even on websites.

Last year may have been the fifth warmest on global record, but global isn’t western European, as the NAO persists in showing. And when it’s making itself felt, perhaps we should be told.

The NAO is one of the oldest weather patterns on record – even the Vikings knew about it. And while it may not have quite the tropospheric reach of the Pacific’s El Niño, it produces swings in climate right across the Atlantic basin, from the eastern seaboard of the US to Europe and even Siberia, and from the Arctic to the Azores.

Its effects show up mostly in winter, helping to give us the bitter one now ended, just as it provided some 20 years of wet and warm ones after 1989.

The NAO isn’t the simplest thing to pin down and still defies prediction, which may explain its general lack of notoriety. What oscillates, or alternates, in the NAO is sea-level pressure of the atmospheric mass between Iceland and the Azores – islands on the same line of longitude but with two very different climates. In winter, the mass of air swings between Iceland’s low-pressure centre that generates so many Atlantic storms, and high pressure over warm water between the Azores and Gibraltar.

Swings from one phase to another produce big changes in wind speed and direction over the Atlantic, in the warmth and moisture carried to its neighbouring continents, and in the number, paths and intensity of storms. When there’s a large and steady difference between pressure in the Azores and Iceland, the NAO is said to be in a “positive” phase, with a “high seasonal index”. This brings warm, wet air to Ireland with winds and higher waves, but cool, dry winters to the Mediterranean.

In a “negative” NAO winter, the difference in pressure is small, and the Med gets the warm rain and storms while cold polar air from the north and east moves into northern Europe.

The NAO plunged into negativity this winter, giving France a February hurricane that killed 40 people and Ireland long weeks of frost and snow. You can see the graph at cpc.noaa.gov.

The phenomenal snowfalls in the eastern US, Scotland and northern Europe could all speak for the extra load of moisture that global warming has raised into the atmosphere.

And the degree to which climate change is affecting the NAO, and exactly what mechanisms drive such a powerful system, are under urgent scrutiny.

Meanwhile, its phases have to be sorted out from the wider global warming when it comes to studying the impacts of climate change on nature.

NAO effects have been linked to population changes in species from algae to whales, to the growth of plants, and events in the lives of animals, butterflies and birds, including habits of migration shaped over thousands of years.

Many long-distance, trans-Saharan migrants are arriving in western Europe several days earlier in spring. Ecologists at Trinity College, led by Dr Alison Donnelly, studied 31 years of Irish records and found that swallows and house martins, for example, can be expected to arrive more than a week earlier with every one-degree rise in spring air temperatures. Those of March, it seems, matter most.

The team also discovered the powerful impact of a “step change” in the NAO. In 1987 it switched to the extended positive phase that brought westerly winds, wet and warmth to most Irish winters. There was a sudden increase in the number of rare ospreys and hobbies in Ireland and records of new nesting species – even the colonisation of Ireland by the little egret, the small white Mediterranean heron.

The winter just ended, however, was one that massacred tiny birds in Britain and littered our western seaboard with the corpses of European redwings. Could the NAO get stuck in the negative for a few winters, as it did in the mid-1960s?

The birds, at least, must hope not.

EYE ON NATURE

We have many birds, including a pair of reed buntings, coming to a feeding tray on top of a hedge. But the chaffinch sidles over to the tray, doesn’t feed, then flies away.

He repeats this twice before he finally stays to eat. What does this behaviour mean?

Niamh Neumann, Newcastle, Co Wicklow

Because the feeding tray is exposed, and the chaffinch feeds mainly on the ground, your bird makes sure that there are no predators around.

I have seen thale cress all over the place, although you said it was only occasional in Ireland (Another Life, Mar 27). The weed I have in mind has seed pods about 2cm long which, when triggered, fling the seeds up to one metre in all directions.

Milo Doyle, Malahide, Co Dublin

Hairy bittercress (Cardamine hirsuta), which is common, disperses its seeds in the same way as thale cress and looks similar. You would need to examine the plant closely with a good botanical description at hand to decide which was which.

I saw a large, pigeon-like bird in woodland. It was round, slate-grey/blue with a white collar, petrol-blue head, narrow yellow beak and pigeon-like eyes.

Francis Roberts, Manorhamilton, Co Leitrim

It was a wood pigeon.

Michael Viney welcomes observations at Thallabawn, Carrowniskey PO, Westport, Co Mayo. E-mail: viney@anu.ie. Include a postal address.