11 July 2009

To Quote Randy Quaid in Indep. Day- I'm Back! El Nino Returns

Posted by Dan Satterfield

El Nino has returned.

The NOAA scientists, who monitor the tropical Pacific, sent out the notice this week. Temperatures in what is called the NINO 3.4 region of the Equatorial tropical Pacific, have increased to the point that we can say an El Nino has begun. (The threshold is an anomaly of .5 C) The official definition is a bit more technical and you can find it on this more technical summary for Meteorologists put put by the Climate Prediction Center. If you want to really understand how El Nino’s begin and end, then Taichiro Sakagami has made it easy. His videos are highly recommended.

Picture 4El Nino’s CAN have a major impact on the climate around the world. Each one is different, however and we do not know yet how strong this particular event will be. Most of the effects happen in the Northern Hemisphere winter months.

One aspect of El Nino that does show up well in Summer is the effects it has on tropical cyclones in the Atlantic Basin. The increased wind shear caused by all that warm water tends to reduce the number of named storms.

It’s important to remember that El Nino is a natural oscillation. It was first noticed by a Meteorologist named Gilbert Walker. He was asked to look into why the Indian Monsoon had failed and to see of such failures were predictable. He collected weather observations, and noticed that when the average pressure at Darwin, in Australia, was close to the pressures much farther east in the Pacific, that the Monsoon would fail.  Australia would suffer severe droughts and other changes were noted as well. In normal years, the pressure would be much lower in Darwin, and higher to the East of Tahiti.

He named his affect the Southern Oscillation.

Picture 1The atmospheric affects of this were first shown by Jacob Bjerknes. He is the son of one of the greatest Meteorologists ever, William Bjerknes. (Father was the first to give a name to cold fronts). Jacob realized that the pressure oscillations were actually the sloshing of warm water in the Western Pacific back to the East. It would later be realized that this had a profound affect on the atmosphere.

In “normal” years the Northeast trade winds blow the warm surface water in the Pacific westward toward Asia. The warm water piles up and seal level there is actually about half a meter higher than it is off the coast of Central America! The depth of this warm water is nearly 300 meters in the West, but the thermocline in the Pacific is much shallower near the Americas.

All this warm water in the Pacific, adds incredible amounts of moisture to the atmosphere, and keeps Indonesia very wet. When the warmth moves eastward, the rains follow. This warm air changes the pressures in the upper atmosphere and these cause changes in the storm tracks. El Ninos usually push the jet stream to the north in the Pacific. This causes a trough to develop many times near California, that brings intense winter storms. These changes don’t just show up in the Pacific, but around the world. Think of the atmosphere as a water bed. Jump on one side and what happens on the other? Now you get the idea!


Temperature anomalies in the Nino 3.4 region over last 12 months. From NOAA NCEP

Bjerknes named this entire process of ocean atmosphere interaction the Walker Circulation in honor of Gilbert Walker who discovered it in the 1930’s, by pouring over pressure records of thousands of stations. Meteorologists tend to call it ENSO for El Nino Southern Oscillation.

The strongest known El Nino, of modern times, was in 1997. This event was a monster, with affects felt around the world. The warm waters transferred an incredible amount of heat to the atmosphere. The jet stream sped up so much that it actually slowed the Earths rotation by a measurable amount! (The jet stream flows from West to East and since every action has an EQUAL and opposite reaction, the Earth slowed a tiny bit. The days were a few thousandths of a second longer!).

Sea Surface Temps. Ctsy NOAA CPC

Sea Surface Temps. Ctsy NOAA CPC

Because of the El Nino of 1997, the global temperature was very warm. 1997 is among the warmest years ever measured. Because of differences in how the calculations are done, NASA ranks 97 and 2007 as tied for the warmest year, where the Hadley Center folks in the UK have it as 1998. The last few years have been slightly cooler because of the La Nina which brings large areas of colder than normal water across the Pacific. (The temperatures have still been well above those of the past because of increasing greenhouse gases).

The 1997 El Nino has been responsible for much of the silliness among climate skeptics. (Who claim that global warming stopped in 1998.) Now you know why this stuff is so silly. Many climate scientists believe that with greenhouse gases steadily increasing, the next strong el nino will bring yet another all time record global temperature (instrumental record dating to the 1880’s).


Image courtesy NOAA

They are very likely right, but there are other oscillations that have to be considered as well. The Pacific Decadal Oscillation is just one. Another is the North Atlantic Oscillation. I pay close attention to that in the winter, because it can be a harbinger of polar air deep into the southeast United States.

Claiming that climate change has stopped because this year is not as hot as last year is like saying that spring has stopped because the temperature on April 14th is not as warm as it was from April 10th through April 12th. If you look at the average temperature from the first week of April and compare it to the last week of may, you will see that this of course is ridiculous.

You would be shocked at how many emails and twitter messages I get from people who make this claim.  I cannot give them the science education they never had. Sad, still the same.


Note: A great fact sheet, with a much more detailed explanation, of the Walker Circulation has been written by my friend Bob Henson at the National Centers for Atmospheric Research (NCAR). He and Kevin Trenberth put together one of the best written pieces I have seen. I used it heavily in writing this post. The other information is from the NOAA- CPC El Nino page, and the COMET program modules on ENSO.