The graph [after the jump] is noisy, but the density is needed to see the seasonal patterns in the oceanic fluctuations. Previous posts focused on the rise and fall of the last El Nino starting in 2015.
This post takes a longer view, encompassing the significant 1998 El Nino and since. The color schemes are retained for Global, Tropics, NH and SH anomalies.
Despite the long time frame, I have kept the monthly data (rather than yearly averages) because of interesting shifts between January and July.
The year 1995 is a reasonable starting point prior to the first El Nino. The sharp Tropical rise peaking in 1998 is dominant in the record, starting Jan. ’97 to pull up SSTs uniformly before returning to the same level Jan. ’99. For the next 2 years, the tropics stayed down, and the world’s oceans held steady around 0.2C above 1961 to 1990 average.
Then comes a steady rise over two years to a lesser peak., but again uniformly pulling all oceans up around 0.4C. Something changes at this point, with more hemispheric divergence than before. Over the four years until Jan 2007, the tropics go through ups and downs, Northern Hemisphere (NH) a series of ups and Southern Hemisphere (SH) mostly downs. As a result, the global average fluctuates around that same 0.4C.
The year 2007 stands out with a sharp drop in temperatures so thatmatches the low in Jan. ’99, but starting from a lower high. The oceans all decline as well until temps build peaking in 2010.
Now again a different pattern appears. The tropics cool sharply to Jan 11, then rise steadily for 4 years to Jan 15, at which point the most recent major El Nino takes off. But this time in contrast to ’97-’99, the Northern Hemisphere produces peaks every summer pulling up the Global average.
In fact, these NH peaks appear every July starting in 2003, growing stronger to produce 3 massive highs in 2014, 15 and 16, with July 2017 only slightly lower. Note also that starting in 2014 SH plays a moderating role, offsetting the NH warming pulses. (Note: these are high anomalies on top of the highest absolute temps in the NH.)
What to make of all this? The patterns suggest that in addition to El Ninos in the Pacific driving the Tropic SSTs, something else is going on in the NH. IMO the culprit is the North Atlantic since I have seen this sort of pulsing before. After reading some papers by David Dilley, I confirmed his observation of Atlantic pulses into the Arctic every 8 to 10 years as shown in this graph:
The data are annual averages of absolute SSTs measured in the North Atlantic. The significance of the pulses for weather forecasting is discussed in AMO: Atlantic Climate Pulse.
But the peaks coming nearly every July in HadSST require a different picture. Let’s look at August, the hottest month in the North Atlantic from the Kaplan dataset.
Now the regime shift appears clearly. Starting with 2003, seven times the August average has exceeded 23.6C, a level that prior to ’98 registered only once before, in 1937. And other recent years were all greater than 23.4C.
The oceans are driving the warming this century. SSTs took a step up with the 1998 El Nino and have stayed there with help from the North Atlantic, and more recently the Pacific northern “Blob.” The ocean surfaces are releasing a lot of energy, warming the air, but eventually will have a cooling effect. The decline after 1937 was rapid by comparison, so one wonders: How long can the oceans keep this up?
Read more at Science Matters
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