Tisdale on the importance of El Nino's little sister – recharging ocean heat content

La Nina – The Underappreciated Portion Of ENSO

Guest post by Bob Tisdale

Perform a Google Scholar search for documents including “El Nino” in quotes and there will be more than 200,000 results. On the other hand, “La Nina” will only raise 26,000+. Granted, the formal name of the coupled ocean-atmosphere phenomenon in the tropical Pacific is “El Nino-Southern Oscillation”, but that in quotes only returns 28,000+ results. So it appears that El Nino events do get much more “press” from the scientific community than La Nina events.

Figure 1 is a time-series graph of NINO3.4 SST anomalies from January 1979 to January 2010. El Nino events are a warming of the central and eastern tropical Pacific so they are displayed as a Positive SST anomaly, where La Nina events are a Negative. Visually, is the eye drawn to the upward spikes more than it is to the downward troughs? El Nino events are viewed as being larger in magnitude than La Nina events. NINO3.4 SST anomalies peaked at approximately 2.8 deg C during the Super El Nino events of 1982/83 and 1997/98, while the La Nina events that followed them failed to reach -2 deg C. But the La Nina events of 1988/89 and 2007/08 were stronger than the El Nino events that preceded them. (Refer to the note about base years at the end of this post.)

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Figure 1

El Nino events release heat from the tropical Pacific, and through ocean currents and changes in atmospheric circulation, they raise surface temperatures outside of the tropical Pacific. These upward spikes in global temperatures, Figure 2, call attention to El Nino events during periods when global temperatures are rising. During La Nina events, the tropical Pacific releases less heat than normal, and global temperatures decline, which doesn’t have the same visual impact.

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Figure 2

La Nina events are a vital portion of the El Nino-Southern Oscillation coupled ocean-atmosphere process. La Nina events recharge the heat released from the tropical Pacific during the El Nino. Figure 3 is a graph of Tropical Pacific Ocean Heat Content compared to scaled NINO3.4 SST anomalies. Note that most La Nina events do not fully recharge the heat released by the El Nino events. From 1976 to 1994, tropical Pacific Ocean Heat Content dropped almost continuously, with occasional major dips and rebounds as an El Nino discharged heat and the subsequent La Nina partially recharged it. Then, the 1995/96 La Nina event, one that was not particularly strong, replaced all of the heat that had been released (plus some) over that 18-year stretch.

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Figure 3

THE 1995/96 LA NINA PROVIDED THE FUEL FOR THE NEXT EL NINO

During a La Nina event, tropical Pacific trade winds rise above normal levels. The increase in trade winds reduces cloud cover. Reduced cloud cover allows more Downward Shortwave Radiation (visible light) to warm the tropical Pacific. These coupled ocean-atmosphere processes associated with La Nina events were discussed in the post More Detail On The Multiyear Aftereffects Of ENSO – Part 2 – La Nina Events Recharge The Heat Released By El Nino Events AND…During Major Traditional ENSO Events, Warm Water Is Redistributed Via Ocean Currents”.

As noted above, the 1995/96 La Nina was not a strong event, yet it recharged all of the ocean heat that had been released in almost two decades of El Nino events. In “Genesis and Evolution of the 1997-98 El Niño” [ Science 12 February 1999: Vol. 283. no. 5404, pp. 950 – 954, DOI:10.1126/science.283.5404.950], Michael McPhaden explains, “For at least a year before the onset of the 1997–98 El Niño, there was a buildup of heat content in the western equatorial Pacific due to stronger than normal trade winds associated with a weak La Niña in 1995–96.” Link to Science abstract:

http://www.sciencemag.org/cgi/content/abstract/283/5404/950

Link to NOAA copy of McPhaden (1999):

http://www.pmel.noaa.gov/pubs/outstand/mcph2029/text.shtml

So there was a short-term recharge of tropical Pacific Ocean Heat Content in 1995/96, which is very evident in Figure 3. And this short-term buildup of heat content provided the fuel for the 1997/98 El Nino. Contrary to the beliefs of anthropogenic warming proponents the 1997/98 El Nino was NOT fueled by a long-term accumulation of heat from manmade greenhouse gases.

AND THAT 1997/98 EL NINO WAS CALLED THE EL NINO OF THE CENTURY

The 1997/98 El Nino was strong enough to temporarily raise Global Lower Troposphere Temperature anomalies ~0.7 deg C, as illustrated in Figure 4. Note: The period of 1995 to present was used in the following graphs because there have been no explosive volcanic eruptions since 1995 to add unwanted noise to the data.

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Figure 4

And referring to Figure 5, Lower Troposphere Temperature anomalies of the Mid-To-High Latitudes of the Northern Hemisphere rose, but remained at elevated levels that varied well above the value in late 1996. This upward step (and a similar but smaller one caused by the 1986/87/88 El Nino) was discussed in the post “RSS MSU TLT Time-Latitude Plots…Show Climate Responses That Cannot Be Easily Illustrated With Time-Series Graphs Alone”.

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Figure 5

Sea Surface Temperature anomalies for the Mid-To-High Latitudes of the Northern Hemisphere also rose and remained at an elevated level. Refer to Figure 6, which compares that dataset to scaled NINO3.4 SST anomalies. The latitudes used for the SST anomalies in this illustration are 20N-65N, which are latitudes that have little impact from polar ice. This upward step in the Sea Surface Temperature anomalies for the Mid-To-High Latitudes of the Northern Hemisphere will be discussed in a future post. I have, however, discussed the impacts of El Nino events on the North Atlantic in the post There Are Also El Nino-Induced Step Changes In The North Atlantic. And the North Atlantic is also impacted by the Atlantic Multidecadal Oscillation, but that appears to have peaked in 2005.

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Figure 6

And for those wondering how well the SST and TLT anomalies for the Mid-To-High Latitudes of the Northern Hemisphere correlate, I’ve prepared Figure 7. The SST anomaly data were scaled by a factor of 1.8. There are divergences from year to year, but keep in mind that the coverage areas are very different; the TLT anomalies also include data over continental land masses. One thing is certain; the 1997/98 El Nino caused upward steps in both datasets.

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Figure 7

And there are the impacts of the 1997/98 El Nino on the East Indian and West Pacific Oceans (60S-65N, 80E-180), which I first discussed in a series of posts more than a year ago. The 1997/98 El Nino shifted Sea Surface Temperature anomalies upward in this area of the global oceans, too. Refer to Figure 8. The cause of this was discussed in the posts Can El Nino Events Explain All of the Global Warming Since 1976? – Part 1 and Can El Nino Events Explain All of the Global Warming Since 1976? – Part 2.

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Figure 8

Basically, the warm water that was built up during the 1995/96 La Nina collected below the surface of an area in the western tropical Pacific known as the Western Pacific Warm Pool (to depths of 300 meters). During the 1997/98 El Nino, the warm water contained in the Western Pacific Warm Pool sloshed east and spread across the surface of the central and eastern tropical Pacific. The warmer-than-normal waters raised Sea Surface Temperatures and changed atmospheric circulation. Then, as the La Nina of 1998/99/00/01 progressed, the trade winds, Pacific Equatorial Currents, and a phenomenon known as a Rossby wave returned the remaining surface and subsurface warm water to the western Pacific. Some of the warm water returned to the Pacific Warm Pool, but a major portion of it remained on the surface and was redistributed by ocean currents to western North and South Pacific, and a portion of the warm water migrated to the Eastern Indian Ocean.

BLAME THE 1995/96 LA NINA FOR THE RECORD TEMPERATURES DURING THE 2000s AND IN 2010

So, if you’re wondering why the present moderate El Nino event of 2009/10 is raising global temperatures to record levels, you have to go back in time. The 1995/96 La Nina provided the build-up of warm waters that was then discharged by the 1997/98 El Nino and redistributed by the 1998/99/00/01 La Nina. The end results were upward steps in SST anomalies and TLT anomalies for major portions of the globe.

One of the methods anthropogenic global warming advocates (scientists and bloggers) use to illustrate the assumed effects of greenhouse gases on global temperatures is to illustrate the divergence between the linear trends of global temperatures and a scaled ENSO index such as NINO3.4 SST anomalies. Refer to Figures 9 and 10. But the upward steps illustrated in Figure 5 and 6 bias global temperature data upwards.

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Figure 9

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Figure 10

And the biases created by those step changes in the SST and TLT anomalies of the Mid-To-High Latitudes of Northern Hemisphere are responsible for much of the differences between NINO3.4 SST anomalies and global temperature anomalies. We can illustrate this looking at the data for the rest of the world; that is, by comparing the linear trend of NINO3.4 SST anomalies with the linear trends the TLT and SST anomalies for the tropics and the Mid-To-High Latitudes of the Southern Hemisphere. Refer to Figures 11 and 12. As shown, the linear trends of the NINO3.4 SST anomalies are slightly negative, but the linear trends for the SST and TLT anomalies of the tropics and Mid-To-High Latitudes of the Southern Hemisphere are relatively flat–much flatter than the global datasets.

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Figure 11

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Figure 12

That would mean the ENSO-induced step increases in SST and TLT anomalies of the Mid-To-High Latitudes of the Northern Hemisphere caused the vast majority of the positive linear trends for the global SST and TLT anomaly datasets. See Figures 13 and 14, which show the strengths of the positive trends for those areas of the globe.

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Figure 13

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Figure 14

Figures 15 and 16 compare the SST and TLT anomalies for the Mid-To-High Latitudes of the Northern Hemisphere to the Global data and to the SST and TLT anomalies for the Mid-To-High Latitudes of the Southern Hemisphere. It should now be clear that the majority of the rises in Global SST and TLT anomalies since 1995 were caused by the 1997/98 El Nino-induced upward steps in the SST and TLT anomalies for the Mid-To-High Latitudes of the Northern Hemisphere.

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Figure 15

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Figure 16

In short, the effects of the La Nina- and El Nino-induced step changes in the SST and TLT anomalies of Mid-To-High Latitudes of the Northern Hemisphere are mistaken for, and misrepresented as proof of, anthropogenic global warming.

A BRIEF LOOK AT AN EARLIER LA NINA EVENT

The 1972/73 El Nino was a strong ENSO event. NINO3.4 SST anomalies, referring to Figure 17, peaked above 2 deg C. There were only two El Nino events stronger than the 1972/73 El Nino in the second half of the 20th Century, and they were the two Super El Nino events of 1982/83 and 1997/98.

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Figure 17

But the 1972/73 El Nino shares another superlative with the 1997/98 El Nino. Both El Nino events were followed by La Nina events that lasted through not one ENSO season, not two ENSO seasons—they lasted through three consecutive ENSO seasons. The La Nina event of 1998/99/00/01 recharged the heat content released by the 1997/98 El Nino and returned the tropical Pacific Ocean Heat Content to the new higher levels established during the 1995/96 La Nina. Refer to Figure 18. The La Nina event of 1973/74/75/76 recharged the heat released from the Tropical Pacific by El Nino events during the decade of the early 1960s to the early 1970s. And it also added to the Tropical Pacific Ocean Heat Content.

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Figure 18

The Pacific Climate Shift of 1976/77 is a much-studied phenomenon. Trenberth et al (2002) discussed the differences in the evolution of El Nino events before and after the shift, and Trenberth et al (2002) referenced other papers that discussed effects of the Pacific Climate Shift on ENSO. Link to Trenberth et al (2002):

http://www.cgd.ucar.edu/cas/papers/2000JD000298.pdf

El Nino events became stronger after the Pacific Climate Shift. The frequency of El Nino events and El Nino Modoki increased. As noted in an early post, The 1976 Pacific Climate Shift, there were notable shifts in the SST anomalies and linear trends of Pacific Ocean basin subsets.

But I have yet to find a paper that attributes the Pacific Climate Shift of 1976/77 to the La Nina event of 1973/74/75/76 or one that even suggests that the 3-year-long La Nina played a role. Yet through known coupled ocean-atmosphere processes, the 1973/74/75/76 La Nina increased the warm water available for the additional El Nino events after 1976 and for the significant El Nino events of 1982/83 and 1986/87/88.

The explosive volcanic eruption of El Chichon may have counteracted the Super El Nino of 1982/83, but the 1986/87/88 El Nino was strong enough to cause upward shifts in the SST and TLT anomalies of the Mid-To-High Latitudes of the Northern Hemisphere, and the SST anomalies of the East Indian and West Pacific Oceans, similar to the shifts caused by the 1997/98 El Nino illustrated in this post.

A NOTE ABOUT BASE YEARS

Note: The relative strengths of El Nino versus La Nina events discussed early in this post would of course depend on the base years chosen for anomalies. And as illustrated in Figure 17 there is a minor difference depending on whether the base years of 1950 to 1979 or 1979 to 2000 are used. The significance of the difference would depend on how the data is being used. Example: A scaled running total of NINO3.4 SST anomalies will reproduce the basic global temperature anomaly curve as illustrated in Reproducing Global Temperature Anomalies With Natural Forcings if the base years are 1950 to 1979. If the base years of 1979 to 2000 are used, the result will not be similar to the global temperature curve.

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Figure 19

CLOSING COMMENT

The La Nina event of 1973/74/75/76 provided the tropical Pacific Ocean Heat Content necessary for the increase in strength and frequency of El Nino events from 1976 to 1995. The 1995/96 La Nina furnished the Ocean Heat Content that served as fuel for the 1997/98 El Nino. And the 1998/99/00/01 La Nina recharged the tropical Pacific Ocean Heat Content after the 1997/98 El Nino, returning it to the new higher level established by the La Nina of 1995/96.

It would appear that La Nina events do all of the work, while El Nino events get all the glory—and the research.

SOURCE

All data for this post is available through the KNMI Climate Explorer:

http://climexp.knmi.nl/selectfield_obs.cgi?someone@somewhere