Shortly after an El Niño event there is elevated heat exchange from the upper ocean layers to the cosmos over the tropical Pacific Ocean. In the North Atlantic Ocean, variations in the ocean circulation affect the heat exchange to the deeper waters of the ocean.
Together, these effects explain a measured decline in the upper ocean warming of 0.02 degrees Celsius since 2003, say climate researchers of the Royal Netherlands Meteorological Institute KNMI.
For their research climatologists Caroline Katsman and Geert Jan van Oldenborgh have used a special climate model that was newly developed by the ESSENCE project (that also involves Utrecht University).
The model shows how heat can be transported from the upper layer to a depth of 1 to 2 kilometers, in particular in parts of the North Atlantic Ocean, notably to the south of Greenland.
Heat transfer to the cosmos is in fact partially an increase in solar radiation reflection due to an El Niño cloud feedback [although we don't quite get (yet) how this time reflection seems to outweigh the El Niño extra clouds' absorption].
According to KNMI the model results are comparable to other observations of the El Niño Southern Oscillation (ENSO) and ocean layer mixing over the past decade.
Ocean warming resumption ahead
After a La Niña event the upper ocean is relatively cool, so it would absorb heat from the atmosphere, rather than emitting it to it. A period of 8 years is too short to encompass a balanced representation of the full ENSO cycle, which would demonstrate itself in periods of slower and faster warming – as the driving force of the rising temperatures, elevation of the level of atmospheric greenhouse gases, continues.
KNMI therefore expects “an upcoming resumption of the heating of the upper ocean,” as the below figure shows:
Ocean temperature graph showing temperature measurements in blue. KNMI states data before 1970 ‘are not very reliable.’ In grey 17 different runs of the ECHAM5/MPI-OM model. [Source: KNMI - Tracing the upper ocean's 'missing heat']
The oscillating trend: how the old hot becomes the new cold
ENSO is of course very important in explaining the sinus curve that is superimposed on the rising trend line of global mean temperature. Notable on the temperature graph are peaks, like the strong 1998 El Niño – and incursions, like the 2008 La Niña.
Some – instead of calculating the trend – simply draw a line between these two years*, and for unapparent** reasons that is not just opportunistic climate skeptics, but also real climate scientists with a funny definition of decades.
[*) The temperature plateau between 1998 and 2008 would not just be ENSO, but also due to increased sulphur aerosol pollution in China.]
[**) We know that we should, but we can't resist to speculate - if someone who fell victim to a hockeystick manhunt may now be overanxious to show just how unbiased he is, by drawing temperature graphs in a cherry-picking way no single coal industry lobbyist could oppose.]
Anyway, keeping in mind how important ENSO is, the trend line should be clear. Last year we had the strongest La Niña on record (after a first half year of El Niño) and here is how the associated cooling failed to prevent* a whole range of new climate records.
[*) And just as ENSO is incapable of stopping the Earth's warming trend, in turn this warming won't be able to silence the ENSO variability, a recent paleo comparison to the Pliocene indicates.]
© Rolf Schuttenhelm | www.bitsofscience.org