The graph below 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 longer time frame, I have kept the monthly data (rather than yearly averages) because of interesting shifts between January and July.
Click on image to enlarge.
!995 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 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 Jan.2003., but again uniformly pulling all oceans up around 0.4. Something changes at this point, with more hemispheric divergence than before. Over the 4 years until Jan 2007, the Tropics go through ups and downs, NH a series of ups and SH mostly downs. As a result the Global average fluctuates around that same 0.4C.
2007 stands out with a sharp drop in temperatures so that Jan.08 matches the low in Jan. 99, but starting from less high. The oceans all decline as well, until the build upwards ensues 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.
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 elsewhere 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 by this graph:
The data is 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 registered only once before, in 1937. And the other years were all greater than 23.4C.
Summary
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 from after 1937 to 1976 was rapid by comparison, so one wonders: How long can the oceans keep this up?
via Science Matters
November 7, 2017 at 02:51PM
Iowa Climate Science Education 