Global Ocean Cooling in September 2017
Written by Ron Clutz
September Sea Surface Temperatures (SSTs) are now available, and we see downward spikes in ocean temps everywhere, led by sharp decreases in the Tropics and SH, reversing the bump upward last month. The Tropical cooling in particular factors into forecasters favoring an unusually late La Nina appearance in coming months.
HadSST is generally regarded as the best of the global SST data sets, and so the temperature story here comes from that source, the latest version being HadSST3.
The chart below shows SST monthly anomalies as reported in HadSST3 starting in 2015 through September 2017.
The August bump upward was overcome with the Global average matching the lowest level in the chart at February 2015. September NH temps almost erased a three-month climb; even so 9/2017 is well below the previous two years. Meanwhile SH and the Tropics are setting new lows for this period. With current reports from the El Nino 3.4 grid sector, it seems likely October will go even lower, with downward moves across all oceans.
Note that higher temps in 2015 and 2016 were first of all due to a sharp rise in Tropical SST, beginning in March 2015, peaking in January 2016, and steadily declining back to its beginning level. Secondly, the Northern Hemisphere added two bumps on the shoulders of Tropical warming, with peaks in August of each year. Also, note that the global release of heat was not dramatic, due to the Southern Hemisphere offsetting the Northern one.
Note: Last month someone asked about HadSST calculations, especially as the Global appeared to be a simple average of NH and SH, which would be misleading. My queries to Met Office received these clarifying responses:
My colleague in the Climate Monitoring and Research team has advised the following:
For HadSST3, we take an area-weighted average of all the grid boxes with data in to calculate the global average. We don’t calculate the two hemispheric series and then average them. In the case of SST, this wouldn’t work because the southern hemisphere ocean area is larger than the northern hemisphere.
The uncertainty that arises from incomplete sampling is estimated and incorporated into the global average SST files. Coverage varies throughout the record with the northern hemisphere being generally better observed, but at other times, coverage is concentrated other places, dictated by where shipping happened to be at those times. Since the mid 2000s drifting buoys have provided a more uniform sampling of the world’s oceans. When we compare to other data sets, we typically compare where both data sets have data which minimizes the coverage problems.
Kind regards, Misha, Weather Desk Climate Advisor
We have seen lots of claims about the temperature records for 2016 and 2015 proving dangerous man-made warming. At least one senator stated that in a confirmation hearing. Yet HadSST3 data for the last two years shows how obvious is the ocean’s governing of global average temperatures.
The best context for understanding these two years comes from the world’s sea surface temperatures (SST), for several reasons:
- The ocean covers 71% of the globe and drives average temperatures;
- SSTs have a constant water content, (unlike air temperatures), so give a better reading of heat content variations;
- A major El Nino was the dominant climate feature these years.