The world is finally in the grip of a long-awaited El Niño event that could alter global weather patterns - but the phenomenon is far weaker than expected. Scientists have announced that sea surface water temperatures have been warmer than average in the western and central Pacific Ocean and have started to alter wind patterns. The US National Oceanic and Atmospheric Administration now predicts that there is now a 50 to 60 per cent chance El Niño will continue through the summer. The US National Oceanic and Atmosopheric Administration has detected warmer than average water surface temperatures around the equator of the Pacific Ocean (shown above) which indicates the arrival of El Niño . However, they say that the El Niño conditions are currently 'extremely weak' and come at a time of the year when the influence of El Niño on weather patterns usually declines. This means that while El Niño conditions normally brings wetter weather to the northern hemisphere in the summer, it is unlikely to be bring substantially more rainfall than this spring. El Niño is caused by a shift in the distribution of warm water in the Pacific Ocean around the equator. Usually the wind blows strongly from east to west, due to the rotation of the Earth, causing water to pile up in the western part of the Pacific. This pulls up colder water from the deep ocean in the eastern Pacific. However, in an El Niño, the winds pushing the water get weaker and cause the warmer water to shift back towards the east. This causes the eastern Pacific to get warmer. But as the ocean temperature is linked to the wind currents, this causes the winds to grow weaker still and so the ocean grows warmer, meaning the El Niño grows. This change in air and ocean currents around the equator can have a major impact on the weather patterns around the globe by creating pressure anomalies in the atmosphere. The NOAA said that it was unlikely the phenomenon would bring enough rain this spring to break the three-year record-breaking drought that is currently affecting California. Some meteorologists, however, have warned that this El Niño could combine with another slow movement of water temperature in the Pacific Ocean called the Pacific Decadal Osciallation (PDO). The PDO reached record levels in December last year and January this year, and should it stay strong, it may combine with El Niño to increase global temperatures. Mike Halpert, deputy director of NOAA's Climate Prediction Centre, said: 'Based on the persistent observations of above-average sea surface temperatures across the western and central equatorial Pacific Ocean and consistent pattern of sea level pressure, we can now say that El Niño is here. 'Many climate prediction models show this weak El Niño continuing into summer.' The arrival of this El Niño is considered to be late after scientists last year predicted they were expecting one last year, but the conditions failed to materialise. According to the Met Office El Niño conditions generally bring warmer temperatures to Europe during the Autumn and colder winter temperatures. The above diagram shows the sea surface temperature (SST) in the Pacific Ocean on 25 Feburary 2015 and it can be seen that around the equator the temperature was above average (marked by red) on that date . It can also make sourthern Europe wetter during the late summer and autumn. Perhaps the greatest impact is felt in the monsoon regions of south east Asia where the climate is usually drier than normal in El Niño years. El Niño is one of the most powerful climate altering phenonoma on the planet. It is the result of the El Niño Southern Oscillation, which is the cycle of warm and cold water across the Pacific Ocean. This cycle occurs at intervals of between two to seven years and can last for nine months to two years. When the ocean is warmer than average for seven to nine months, the NOAA officially classes it as El Niño conditions. California is in the midst of the worst drought in 1,200 years and has caused severe drying of the Stevens Creek Reservoir (shown above). It is unlikely the El Niño will bring enough spring rain to ease the drought . The last El Niño, in 2009-2010, was a moderate to strong event. This brought freezing  weather to northern Europe in the winter of 2009/2010 and the UK was exceptionally cold. The Vancouver Winter Olympics also suffered a far warmer than average winter during the same event. Other recent El Niño’s  in 2002-2003,  2004-2005 and 2006-2007 have been weak to moderate in strength. The last very strong El Nino was 1997-1998 and was known for providing heavy rainfall in the west of the United States, especially California. The NOAA said that last week the sea temperature across much of the equatorial Pacific Ocean has been above average. They also found that unusually weak easterly winds have persisted across the eastern Pacific ocean. In Late January and February there were a couple of westerly wind bursts of over the central tropical Pacific and forecasts for March suggest these will continue. The NOAA said that this mean the Equatorial Southern Oscillation Index, which is uses to monitor the El Niño, has been a negative value for the second month in a row. The above diagram shows the impacts of a typical El Niño on seasonal temperatures around the world . The above diagram shows how a typical El Niño is expected to change seasonal rainfall around the world . In a blog post on the onset of El Niño, the NOAA said: 'At this time, these are extremely weak El Niño conditions, during a time of year when the influence of El Niño on weather patterns in North America or other locations outside of the Tropics is weakening. 'From an impacts perspective, this is not particularly momentous, as El Niño impacts are weak in the spring and summer. 'Still, after months of hovering under the threshold, we can now say that El Niño conditions have arrived.' NOAA scientists are to continue monitoring the situation and will issue an update in April. However, Eric Holthaus, a meteorologist writing for New Scientist, warned that the arrival of El Niño could result in this year being especially warm. He said: 'El Niño transfers huge amounts of heat from the oceans to the atmosphere, and there are hints that this El Niño, combined with the already very warm global oceans, could bring about a new phase in global warming. 'An associated slow-moving indicator of Pacific Ocean temperatures, called the Pacific Decadal Oscillation (PDO), reached record levels in December and January. 'A persistently strong PDO is associated with cold winters in the eastern US and drought in California – we've had both in abundance this year. 'Should the PDO stay strong, it will essentially join forces with El Niño and increase the odds that 2015 will rank as the warmest year on record globally.'
US National Oceanic and Atmospheric Administration declares warmer than average water in Pacific Ocean signal the arrival of El Niño conditions . They predict there is a 50%-60% chance El Niño will continue this summer . However this year's El Niño is currently far weaker than in previous events . Some meteorologists warn El Niño could combine with another 10 year ocean warming cycle to produce record global temperatures this year . Experts had expected El Niño to start last year but it did not materialise .