The El Nino phenomena is among the most fascinating yet least understood topics of climate science. Every few years a wave of heat and surface pressure changes sweeps across regions of the Pacific. It lasts nine months to two years and has a worldwide effect.
Some believe that the frequency of El Ninos is changing as a result of climate change. I have recently heard it said many times that it is the increased frequency of El Nino and the associated La Nina that has resulted in the flatness of the global annual average temperature in the past decade or so. If true it’s an important observation.
Trenberth and Hoar in 1996 said there is a tendency for more frequent El Nino and fewer La Nina since the late 1970’s which they think is linked to decadal changes throughout the Pacific basin due to global warming. They note a non-El Nino warming between 1990-95 “connected to” the El Nino, which they say is unprecedented in the previous 113 years, and suggest it is linked to increases in greenhouse gasses.
Following criticism of their paper Trenberth and Hoar 1997 restated their assertions saying the El Nino from 1990-95 was unusual and that the pattern seen since late 1970s was unlikely to be due to natural variability. However, Solow 2006 pointed out statistical shortcomings in Trenberth and Hoar and questioned their conclusions. Rajagopalan et al also disputed Trenberth and Hoar and using a more sophisticated statistical analysis said that any changes seen in El Nino is due to natural variability. Subsequently, Solow and Huppert 2003 did not consider the 1990-95 ‘event’ significant. They also don’t see a change in ENSO frequencies. Indeed, reversing the philosophy of Trenberth and Hoar, Soon-Il An et al 2003 say that it is not background global warming that is driving changes in El Nino but the other way around; the strong El Nino of 1998 had a strong influence on the Earth’s climate.
In 2001 the IPCC could find no link between global warming and El Nino but the position the IPCC took in 2007 was somewhat more nuanced. The IPCC was not quite so sure even though it couldn’t really find any evidence.
Consider Figure 1 which is from Cane 2005. Can you see any changes between the late 19th and 20th centuries? Click on image to enlarge.
One can crudely examine a list of these events obtained from NOAA which shows that over the past 60 years there has not been an increase in the intensity on the frequency of El Ninos or La Ninas.
El Nino Events
La Nina Events
The “Modoki Industry”
Some believe there has been a change in the El Nino over the past few decades, an important one. Recent observations are showing them to be increasingly complex. In the 1990s scientists noticed a new kind of El Nino. They were not the usual Eastern Pacific El Nino but rather ones where the warm water occurred more centrally in the Pacific. They were termed the Central Pacific El Nino or Modoki after the Japanese word for “same but different.” Some maintain CP El Nino have increased in frequency over the past 30 years and might even become the dominant form of El Nino events in the future. The 1998 Super El Nino was an Eastern Pacific one whilst the 2009-10 El Nino was of the Central Pacific type.
Some commentators have said that the increased prevalence of Modoki El Ninos is a direct result of global warming. Personally I find this assertion difficult to believe. The first recorded Modoki El Nino was in 1986 and I am hesitant to believe that El Ninos were changing given the global temperatures of the mid-1980s.
Jin Soo-Kim et al 2012 have looked at the sea surface temperature between 1951 -2010. They notice that the two modes have been in phase since the late 1990s and their superposition may explain the rise of the Modoki El Ninos. In a somewhat curious phrasing they state that it “cannot be ruled out” that the new type of El Ninos are a natural phenomenon. Takahshi et al think the two kinds of El Nino are not really all that different and not different enough to make much of a distinction. Ashok et al 2007 compile charts for the two periods of 1979–2004 and 1958–1978 for various oceanic/atmospheric variables suggest that the recent weakening of equatorial easterlies related to weakened zonal sea surface temperature gradient led to more flattening of the thermocline. This appears to be a cause of more frequent and persistent occurrence of the ENSO Modoki event during recent decades. Hans-F Graf et al states that Modoki El Ninos might even explain the recent spate of very cold European winters we have experienced.
Note however that there are scientists who don’t go along with this. For example Takahashi et al do not think the CP El Nino is a seperate thing at all. Indeed one researcher I spoke to referred to the “Modoki Industry.”
Another interesting point about the El Nino is the very strong ENSO of 1998 which has drawn much comment over the years. Some see it as an event that marks a transition between two modes of behavior of global temperatures. This can be seen in atmospheric temperature data. Looking at the lower atmosphere temperature one can see where this impression comes from, although I have seen numerical analysis that supports and refutes the idea. Such strong events happen from time to time, such as in 1982, 1888 and 1877.
The Next El Nino
NOAA says that there is a 50% chance that an EL Nino could begin sometime during the remainder of this year. If it is a strong one then it could raise 2013 to be among the warmest years of the past decade. Given the cool start to the year we have had in 2012 it seems unlikely, though not impossible, that an EL Nino in the next six months will do the same for this year.
If there is a record temperature set, even if it is only one thousandth of a degree, no doubt the blogosphere and the media will make a big deal of it. Conveniently forgotten will be the basic statistical fact that if the temperature of the past 15 years is a constant straight line, as the numbers indicate, then a new record is bound to come along from time to time just due to the spread of the individual years about the mean.
An El Nino will increase the global temperature a bit but that is not global warming. The 2010 ENSO did push temperatures up giving 2010 an almost record status – but by hundredths of a degree – statistically well within the noise of inter-annual variation.
In summary, no two El Ninos are alike, and the case for a change in their intensity or frequency in recent years has not been proven.