Floods & La Niña: direct, indirect and other causes to heavy rains around the world

Extreme rainfalls and floods across different continents dominate the global news. A connection to La Niña has been suggested.

But to what extent can the floods in Colombia, Venezuela, Brazil, Pakistan, Sri Lanka, Australia, the Philippines and north-western Europe be attributed to the Pacific climate phenomenon? Here’s an extensive summary of the meteorological situation for each of the affected areas and an attempt to show the entire climate system is connected – although we can’t call all of that La Niña.

Pakistan: La Niña added moisture to monsoon

There were a multitude of causes behind the floods in Pakistan. They were however exacerbated by above average temperatures in the Indian Ocean as a consequence of La Niña, adding extra moisture to the Intertropical Convergence Zone (ITCZ), locally referred to as the monsoon, that is drawn towards the Karakoram and Himalayas during northern hemisphere summer (perhaps with unusual force due to the Russian heat waves) and then feeds the Indus catchment. So, a connection between the 2010 Pakistan floods and the current La Niña episode does exist.

Australia: La Niña uncorked by ITCZ

The current floods in Australia can be directly linked to La Niña. Especially to the west and east the Australian coastal waters have a rather large (1-2C) positive temperature anomaly, favouring the formation of depressions that recently even brought extreme rainfalls in the bone dry inland deserts of the Northern Territory and South Australia.

The reason however that for instance Queensland hasn’t really been hindered by the current La Niña until recent weeks is, just like in Pakistan several months earlier, much of the energetic build-up in the ocean waters had been masked by relatively high pressure air systems – as long as the ITCZ lay further to the north.

During northern hemisphere winter the northern margins of Australia have their own rainy season with the La Niña now adding fuel to the formation of tropical storms, that from time escape the ITCZ to travel southwards across the continent or its shores.

La Niña may (also) cause Australian sea level rise

In Queensland and New South Wales yet another direct La Niña effect may increase the damage of extreme rainfall and river discharge.

A La Niña is created by above average wind speeds in the (westbound) trade winds across the equator. All the sea surface water, warmed by the tropical sun, is blown to the west of the Pacific and, to compensate part of the imbalance, cooler deep ocean waters well up on the western shores of Latin America (and spread all the way up to the Solomon Islands).

With warm waters in the west and cool waters in the east there is an upward air movement (low pressure systems) in the west and a downward movement (high pressure systems) in the east, leading to a lack of air molecules over the sea surface in the west and an overabundance in the east. A positive feedback is created: to compensate the pressure differences the stronger trade winds become permanent and La Niña keeps itself firmly in place, for months or longer.

The permanent strong winds blowing form east to west can then actually lead to differences in the Pacific sea level, that may drop in the east and become elevated in the west – possibly, locally, up to a meter above average. This can hinder the normal shallow river gradient in coastal areas, blocking the flow and forcing rivers and streams to breach their levees.

Philippines: purely La Niña – more to come?

The Philippines too are currently being plagued by heavy rainfall and floods. This is actually the only region where flooding can be seen as purely being a consequence of La Niña. The positive temperature anomaly is clearly visible among the isles, leading to the development of storms. The ITCZ is still far to the south, so the Philippines are still in region of seasonal (relative) high air pressures. If the current La Niña is to last into the northern hemisphere spring or early summer of 2011, it is likely conditions in the Philippines will get worse.

Sri Lanka: a different story – but La Niña isn’t over yet

Sri Lanka too has been severely hit by excessive rain and devastating floods. The connection to La Niña is however very weak. Only in the Gulf of Bengal can we still witness above average water temperatures, but to the south and west of the island there is at present a negative (0.5-1.5C) temperature anomaly, with waters being a bit ‘too cold’ – without an clear-cut connection to the Pacific climate phenomenon.

The ITCZ is presently very active above the Indian Ocean, but quite a bit further to the south, across the equator and well out of (direct) reach of Sri Lanka.

The true cause of the extreme rains lies in the outflow of relatively cold air from the Indian subcontinent meeting with a warmer air mass to south, and Sri Lanka in the battle zone for several days. The warm air is pushed upwards, forced to cool with water vapour condensing and precipitating.

This does not exclude Sri Lanka as a risk area for La Niña effects should these persist, possibly well into the northern spring of 2011.

Brazil and South America: La Niña may cause slight drop in air pressures east of the Andes

La Niña clearly influences weather patterns over South America. West of the Andes mountains (and Sierra Nevada, Rocky Mountains) the coastal areas become dominated by large (relative) high pressure systems. In countries like Peru and Ecuador the weather is remarkably dry during La Niña phases.

East of the Andes the connections are different – and especially a lot weaker. The air pressure over the Amazon basin may be inclined to be relatively low during La Niñas, possibly drawing in more Atlantic air – and moist, leading to increased precipitation.

The current floods however can be explained in a different manner, requiring geographical differentiation of the current hotspots.

Rio de Janeiro: big warm front – no clear connection to La Niña

The affected areas around Rio de Janeiro lie well south of the zone of active tropical rainfall of the ITCZ. It was caused by a weather front with a warm tropical air mass (it is currently high summer in South America) moving southward and meeting a somewhat cooler air mass. The warmer air won the battle and the weather front is now active well to the south of Rio, even south of Buenos Aires, halfway over Argentina. The movement of air can in part be attributed to a high pressure system somewhere to the south over the South Atlantic – not a La Niña phenomenon at all.

Venezuela and Colombia: ITCZ is always rainy, but perhaps a little more so during La Niña

Also in Venezuela, Colombia and elsewhere across the northern margins of South America there is a lot of rainfall at present. From a distance it’s easy to pile all the weather on a continent, overlooking the 4.500 kilometre distance between Rio de Janeiro and Bogotá. Excessive rainfall in the north of South America can be attributed to the rainy season. (The ITCZ lies a bit further to the north than what one would expect.)

It is worth mentioning that the southern part of the Atlantic Ocean has its water temperatures slightly above average. This promotes evaporation and a higher water content of the local atmosphere. In the tropical regions the trade winds can add the Atlantic moisture to the ITCZ. The warmer ocean waters however appear to be an independent phenomenon – and are not to be directly linked to La Niña.

North-western Europe: persistent positive NAO due to geographical gradient in Atlantic temperature anomaly

The same holds true for a remarkable positive temperature anomaly in the northern part of the Atlantic Ocean. For months (and years in fact) the North Atlantic waters have been too warm. Recently however the temperature extreme has become divided – creating sharp geographical gradients and thereby greatly increasing the chances of truly influencing European weather through the North Atlantic Oscillation (NAO).

Relatively (it’s always relative changes that are most relevant to breaking the climate averages) cool waters from the Caribbean have over recent weeks and months increasingly spread to the northeast, across the Atlantic Gulf Stream, creating a negative temperature anomaly around the islands of the Azores and reaching further to the British Isles and the North Sea, where sea water is low due to the very cold December.

Relatively warm waters are now being concentrated (locally the temperature anomaly is as high as +4C) in the Labrador Sea and adjoining Atlantic waters, between Canada and Greenland, up to Iceland.

This area has now become the cradle of Atlantic depressions, which over the past (and coming) weeks formed the one after the other, coursing for North Europe, bringing much rain, leading to a very mild month of January, melting snows and high river discharges and flooding in parts of Germany, Belgium and the Netherlands.

(Another high positive temperature anomaly can now be witnessed along the western coasts of West Africa – it is in fact a relatively cool Gulf Stream cutting through an remarkably warm North Atlantic.)

Due to the relatively cool waters around the Azores the classical ‘Azores high’ had a good chance to develop and settle, together with low pressure systems around Iceland leading to a strong west circulation (a positive NAO) over Europe. The Atlantic depressions are now brought over the northern mainland the one after the other – instead of being blocked by high pressure. Both precipitation and temperature are high during a west circulation – and increase with its strength.

Indirect connections between La Niña and European winter weather

There are however two indirect connections between ‘autumn weather’ in north-western Europe and La Niña that one could theorise:

Firstly the relatively cool Gulf Stream waters can be attributed to the early onset of La Niña, as a witnessed statistical connection between Pacific and Caribbean coastal waters, called Western Hemisphere Warm Pool (WHWP). (As mentioned above, months later, this can help establish a positive phase in the NAO, by encouraging the Azores high.)

La Niña also has a quite prominent effect on North American weather. Due to high pressure systems along the Pacific coast Alaskan cold snaps can reach the east coast of Canada and the US (as is currently the case).

Offshore winds from Canada and Greenland (with temperatures around -20C) carry across the warm waters of the Labrador Sea, creating a very unstable atmosphere and immediately leading to the formation of depressions (like polar lows). These too then feed the positive NAO, increasing rainfall over north-western Europe.

© Rolf Schuttenhelm | www.bitsofscience.org

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