The El Nino phenomenon has been a long-recognised warming of the waters of the East Pacific.
What appears to cause an El Nino event is that air pressure conditions change over the equatorial Pacific, affecting the trade winds, which usually blow from east to west (South America to Australia say) and as a result of this weakening or even change of direction of the trade winds, less water moves westward and less cold water rises off the east coast of South America, and so the eastern Pacific warms up.
With warmer water than usual, warm, moist air rises from the sea surface in the eastern Pacific, altering the globe's atmospheric circulation linked to the position of the jet stream. The consequences are heavy rains and flooding in South America, and droughts in west Pacific Oceans countries such as Asia, Australia, and India, etc., and rain and cooler conditions in the southern USA.
"Super El Niño’s" occurred in 1982-83, 1997-98 and 2015-16 are some of the stronger such events and may tie up with lowered hurricane activity in the Atlantic Ocean as a result of stronger westerly winds at upper levels of the atmosphere which in turn causes increased vertical wind-shear which can act to forestall the creation of storms/hurricanes.
La Nina events are associated with more active hurricane seasons. Nothing is certain with these phenomena though. Neutral phases of the oscillation between an El Nino and a La Nina have seen severe hurricanes such as Andrew in 1992 and Katrina in 2005.
AON's latest "Weekly Cat Report" contains an article reporting that probably the current long-running La Nina phase is changing to an El Nino phase during Summer this year.
We shall keep an eye on this. Updates are available through the WMO's website available here. We are not experts on this phenomenon but hope it makes for interesting reading.
We have had three consecutive years of La Nina according to a WMO report available here
Update - 7 March
The World Meteorological Organisation ("WMO") has produced an update (available here) opining that El Nino may indeed be returning in May onwards. Forecasts for June-August suggest a 55% chance of El Nino returning. The predictions are not reliable however and it looks as if ENSO neutral conditions could apply for some months yet.