Atlantic Ocean's Circulation Yields Inevitable Surprises

The term “settled science” gets tossed around in the media a lot these days. Mostly by non-scientists, who know no better, and by some errant scientists, who should. In 2002, the U.S. National Research Council Committee on Abrupt Climate Change published its findings in a book entitled Abrupt Climate Change: Inevitable Surprises. A new report in Science recaps the surprising discoveries made since then, and they are big. So big that ocean circulation models, integral parts of all climate models, do not accurately predict reality. The observed change in AMOC strength was found to lie well outside the range of interannual variability predicted by coupled atmosphere-ocean climate models. Sounds like circulation in the Atlantic Ocean is not so settled.

I have reported on ocean circulation before, popularly known as the great ocean conveyor belt, and the Atlantic Meridional Overturning Circulation (AMOC) in particular. While the simplistic conveyor belt model, origonally described by Wally Broecker, has fallen out of favor in recent years, study of ocean circulation patterns is more intensive than ever. This is because ocean currents are the major movers of heat energy around the globe, and as such are a primary influence on Earth's climate. A review article published in the journal Science, titled “Observing the Atlantic Meridional Overturning Circulation yields a decade of inevitable surprises,” by oceanographers M. A. Srokosz and H. L. Bryden, reviews some surprising new findings from the past decade of observation. The importance of recent findings is revealed in the report's abstract:

The importance of the Atlantic Meridional Overturning Circulation (AMOC) heat transport for climate is well acknowledged. Climate models predict that the AMOC will slow down under global warming, with substantial impacts, but measurements of ocean circulation have been inadequate to evaluate these predictions. Observations over the past decade have changed that situation, providing a detailed picture of variations in the AMOC. These observations reveal a surprising degree of AMOC variability in terms of the interannual range, the amplitude and phase of the seasonal cycle, the interannual changes in strength affecting the ocean heat content, and the decline of the AMOC over the decade, both of the latter two exceeding the variations seen in climate models.

The major characteristics of the AMOC are a near-surface, northward flow of warm water and a colder southward return flow at depth. As the ocean loses heat to the atmosphere at high latitudes in the North Atlantic, the northward-flowing surface waters cool and become denser. These waters then sink and form the deep return flow of the overturning circulation. This is shown in the figure below, taken from the report.

Scientists are interested in the AMOC because changes in ocean circulation can have significant impact on global and regional climate. Unlike the Indian and Pacific Oceans, where the ocean transports heat away from the equator toward the poles, the AMOC transports heat northward across the equator. The maximum northward oceanic heat transport in the Atlantic is 1.3 petawatts (1 PW = 1015 watts) at 24° to 26°N, which is ~25 percent of the total heat transport toward the pole at these latitudes. Further north, at mid-latitudes, the temperate climate of northwest Europe is maintained by the strong transfer of heat from ocean to atmosphere. Even sea level changes are affected by the AMOC.

Scientists discovered that the flow of water in the Atlantic was much more complex than the old conveyor belt model when they started deploying an observing system across the Atlantic at 26.5°N in 2004. Last year that system marked a decade of measurements, the highlights of which are the subject of the M. A. Srokosz and H. L. Bryden report. The 26.5°N AMOC observations have produced a number of surprises on time scales of less than a year to several years. Here are the four major observation made by the author's (note that the standard unit for measuring ocean circulation is the Sverdrup (Sv), a million cubic meters per second):

  1. The range of AMOC variability found in the first year, 4 to 35 Sv, was larger than the 15 to 23 Sv found previously from five ship-based observations over 50 years. A similarly large range to that at 26.5°N has subsequently been observed at 34.5°S.

  2. The amplitude of the seasonal cycle, with a minimum in the spring and a maximum in the autumn, was much larger (~6.7 Sv) than anticipated, and the driving mechanism of wind stress in the eastern Atlantic was unexpected as well. The conventional wisdom was that seasonality in the AMOC would be dominated by wind-driven northward Ekman transport, but this was found to be small.

  3. The 30% decline in the AMOC during 2009–2010 was totally unexpected and exceeded the range of interannual variability found in climate models used for the IPCC assessments. This event was also captured by Argo and altimetry observations of the upper limb of the AMOC at 41°N. This dip was accompanied by significant changes in the heat content of the ocean, with potential impacts on weather that are the subject of active research.

  4. Finally, over the period of the 26.5°N observations, the AMOC has been declining at a rate of about 0.5 Sv per year, 10 times as fast as predicted by climate models.

AMOC flow reduction during 2009–2010 had a considerable impact on the heat transport into the North Atlantic. The heat transported north by the AMOC at 26.5°N in previous years was ~1.3 PW, but this transport was reduced by 0.4 PW. This resulted in cooler waters in the north Atlantic and warmer waters to the south. Observations showed that there was an abrupt and sustained cooling of the subtropical North Atlantic in the upper 2000 m between 2010 and 2012. Because the AMOC carries ~90% of the ocean heat transport at this latitude, the cooling seems primarily due to the reduction of the AMOC. This cooling has affected weather in the eastern US and the formation and paths of Atlantic hurricanes.

Indeed, the more observations reveal about the AMOC the more questions arise. Scientists worry if the AMOC will continue to decline or even stop all together. Such events are thought to have happened in the past, for example at the very start of the current interglacial period. Another possible effect of the AMOC slowdown may be the “hiatus” in global warming. In “Varying planetary heat sink led to global-warming slowdown and acceleration,” Xianyao Chen and Ka-Kit Tung conclude that the deep Atlantic and Southern Oceans, but not the Pacific, have absorbed the excess heat that would otherwise have contributed to global temperature rise. But the role of the AMOC in the hiatus remains uncertain and others have denied that there is any “missing heat” at all.

Srokosz and Bryden speculate on the implications of the past decade of observation—real science, the actual study of nature—and the impact the resulting data may have on climate models. Some of their comments focus on the possible bistability of the AMOC. They cast doubt on the predictive ability of today's crop of climate models.

On a more speculative note, one possibility for future AMOC surprises is the issue of the bistability of the AMOC noted earlier. This is related to the transport of freshwater in and out of the South Atlantic. Observations suggest that the AMOC transports freshwater southward in the South Atlantic, implying that the AMOC could be bistable with on and off modes. Most climate models exhibit northward freshwater transport, seemingly at odds with the observations, implying that the AMOC is stable. Some recent climate model results show that their freshwater transports can match the southward freshwater transport in the observations, but in such climate models the AMOC does not shut down under greenhouse gas forcing. In point of fact, most climate models do not include a dynamically interactive Greenland ice sheet, so they are unlikely to correctly account for freshwater input into the Atlantic from Greenland melting. In addition, the Arctic Ocean supplies freshwater to the North Atlantic, which would affect the stability of the AMOC. If the rate of freshwater input were to be greater than currently anticipated, that could lead to unexpected changes in the AMOC. Thus, there is a possibility that the ocean might respond in a way that most climate models cannot. This point has been made previously from a paleoclimate perspective, because paleoclimatic evidence suggests that the AMOC can undergo rapid changes that are difficult to reproduce with climate models.

This is more unsettling science than settled science. What does the future hold in store? Science in general, and climate science in particular, has a poor record when it comes to predicting the future. As much as we think we know about Earth's climate system there is much more that we do not know. Here are Srokosz and Bryden's list of immediate unknowns:

Despite the observational efforts over the past decade, many questions remain unanswered. First, the AMOC is changing, but will these changes persist or will the AMOC “bounce back” to its earlier strength? Second, are the changes being observed at 26.5°N coherent latitudinally in the Atlantic? Third, was the 2009–2010 decrease in the AMOC unusual or not? Fourth, is the AMOC bistable? Could it “flip” from one state to another? Finally, and perhaps most important, what are the effects of changes in the AMOC?

So there we have it, in the words of scientists involved with actually studying nature, not a bunch of armchair climatologists playing with computer models—models that have been based on false assumptions and data for years. There are questions galore that need to be answered before we even begin to understand the AMOC, one of the most important factors regulation our planet's climate. The lie that climate science is settled science can not be exposed more plainly than this.

Be safe, enjoy the interglacial and stay skeptical.