The periodic switch between warm and cool surface ocean temperatures over the eastern equatorial Pacific has long been a powerful driver of global weather. The reason is that such a large body of water can do major work moving weather patterns depending on its relative cool or warm state. This area of the Pacific is very large — about 9 million square kilometers and it sits directly below the equatorial wind flows. So warming and cooling in this region pushes weather governing winds, changing the direction of storms and the location of droughts, serving as a powerful governor of world weather and climate.
But human caused global warming may be shifting the role of ENSO into a secondary governor of climate. The reason? Large areas of open water in the Arctic and northern oceans are now subject to excess heating.
Take a look at the map above and you will notice a large region of much warmer than average water located in the polar, Arctic, and high temperate zones. Almost the entire Arctic Ocean, the region of Hudson Bay, the Canadian Archipelago, the sea directly west of Greenland and the North Atlantic are all experiencing sea surface temperatures far exceeding the average range. A large area of the northern Pacific is also experiencing abnormally hot conditions. The abnormally high temperatures in these regions cover ocean areas as much as twice as large as the region typically affected by El Nino.
The change in heat distribution of these waters alters the circumpolar jet stream. It changes the path, location, duration and intensity of storms. It can make cool and wet periods last longer. It can make dry and hot periods also last longer. It is the main element influencing the blocking patterns that have been so prominent in weather events over the past decade and a half. Finally, it may eventually alter the turn of the northern hemisphere seasons. For as it intensifies, it has the ability to change winter into something that may be well unrecognizable from the seasons as they’ve existed over the 20th century.
This heating of the northern hemisphere land and water becomes a periodic event intensifying toward the end of summer. But it is the ability of this latent heat to melt ice, move air masses, cause huge swoops in the jet stream, alter the seasons, and extend the duration of weather events that may result in its over-shadowing the signal coming from El Nino and La Nina.
It is particularly worth noting that, despite La Nina being very strong over the end of 2010 and throughout 2011, 2010 was the hottest year on record and 2011 was the 9th hottest year on record and the hottest La Nina year on record. Usually, a powerful La Nina like the one occurring in 2011 would have pushed world temperatures dramatically down. But we still experienced a record year. Also, strong El Nino years have typically resulted in the world reaching new record high temperatures under global warming. Not so with 2010. That year was a year that transitioned from weak El Nino to strong La Nina. Yet temperatures were still high enough to break a new record.
What appears to be happening is that the global warming signal is becoming large enough to over-ride the signal coming from El Nino and La Nina. If we think of these two forces as wave patterns, the size of the human caused global warming wave is now large enough to confuse and dilute the, now smaller, El Nino, La Nina wave signal. This seems to be true, again, for 2012 where a strong La Nina is transitioning into a very weak El Nino. Yet all the recent months have been much hotter than the climatological average with June and July both being the 4th hottest on record.
And though it is likely that the strong La Nina influenced the Texas drought of 2011 and the 50 year drought of 2012, the fact that La Nina has faded while drought conditions have persisted should be a clear indicator that La Nina is no longer sitting in the driver’s seat.
The fact that human caused global warming is now the primary driver of extreme weather events has now been validated by many scientists. The IPCC, NOAA, and NASA have now all linked extreme weather to human caused global warming. In addition, Jennifer Francis and colleagues have also produced a report showing how loss of sea ice is driving the powerful blocking patterns we’ve seen linked to extreme weather events over the past two decades. So, moving forward, it will be important for weather forecasters to identify atmospheric changes due to human global warming if they are to accurately predict weather over extended periods in the future.