Wednesday, 2 March 2016

The slowing of the Gulf Stream

Mystery Deepens Around Greenland “Cold Spot”




Climate Crocks,
1 March, 2016


The Day After Tomorrow scenario, still live?

A year ago I  published this video around Oceanographer Stefan Rahmstorf’s paper (with Mike Mann and Jason Box) describing a mysterious “cold spot” in the North Atlantic.

The spot is still there, and the video continues to attract international attention.
dayafter

Danish. For the headline, Google translate gives “The nightmare scenario from the movie ‘The Day After Tomorrow’ keeps perhaps water”.

First paragraph:

“In the disaster film “The Day After Tomorrow ‘means changes in the Gulf Stream beginning of Earth’s destruction. New research confirms the specter, but downplays the consequences.”

More details from Stefan Rahmstorf’s write up of last year – below.

The North Atlantic between Newfoundland and Ireland is practically the only region of the world that has defied global warming and even cooled. Last winter there even was the coldest on record – while globally it was the hottest on record. Our recent study (Rahmstorf et al. 2015) attributes this to a weakening of the Gulf Stream System, which is apparently unique in the last thousand years.
The whole world is warming. The whole world? No! A region in the subpolar Atlantic has cooled over the past century – unique in the world for an area with reasonable data coverage (Fig. 1). So what’s so special about this region between Newfoundland and Ireland?
amoctemptrend
Fig. 1 Linear temperature trend from 1900 to 2013. The cooling in the subpolar North Atlantic is remarkable and well documented by numerous measurements – unlike the cold spot in central Africa, which on closer inspection apparently is an artifact of incomplete and inhomogeneous weather station data.
It happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens (experts speak of the Atlantic meridional overturning circulation or AMOC, as part of the global thermohaline circulation). That this might happen as a result of global warming is discussed in the scientific community since the 1980s – since Wally Broecker’s classical Nature article “Unpleasant surprises in the greenhouse?” Meanwhile evidence is mounting that the long-feared circulation decline is already well underway.
Difficult to measure
Climate models have long predicted such a slowdown – both the current 5th and the previous 4th IPCC report call a slowdown in this century “very likely”, which means at least 90% probability. When emissions continue unabated (RCP8.5 scenario), the IPCC expects 12% to 54% decline by 2100 (see also the current probabilistic projections of Schleussner et al. 2014). But the actual past evolution of the flow is difficult to reconstruct owing to the scarcity of direct measurements. Therefore, in our study we use data on sea surface temperatures in order to infer the strength of the flow: we use the temperature difference between the region most strongly influenced by the AMOC and the rest of the northern hemisphere.
northatcurrent
Fig. 2 Schematic of the Atlantic circulation. Surface currents in red, deep currents in blue, sea-ice cover in winter in white. (Source: Rahmstorf, Nature 1997)
Now we are not the first to have inferred from temperature data that the flow must have weakened. Evidence for this was already presented by Dima and Lohmann 2010 or Drijfhout et al. 2012, among others (for further references see the introduction of our paper).
What is new is that we have used proxy reconstructions of large-scale surface temperature (Mann et al, 2009) previously published by one of us (study co-author and RealClimate co-founder Mike Mann) that extend back to 900 AD (see “What we can learn from studying the last millennium (or so)”) to estimate the circulation (AMOC) intensity over the entire last 1100 years (Fig. 3). This shows that despite the substantial uncertainties in the proxy reconstruction, the weakness of the flow after 1975 is unique in more than a thousand years, with at least 99 per cent probability. This strongly suggests that the weak overturning is not due to natural variability but rather a result of global warming.
paleo500
Fig. 3 Time series of the temperature difference between the subpolar North Atlantic and the entire northern hemisphere, which can be interpreted as an indicator of the strength of the Atlantic circulation.
Also in 2014 we again find a remarkable cold bubble over the northern Atlantic – as a look at the NASA website shows. 2014 was globally the warmest year on record, 1 °C warmer than the average for 1880-1920. But the subpolar Atlantic was 1-2 °C colder than that baseline! And even more recently, NOAA last week released the stunning temperature analysis for the past winter shown in Fig. 4. That winter was globally the warmest since records began in 1880. But in the subpolar North Atlantic, it was the coldest on record! That suggests the decline of the circulation has progressed even further now than we documented in the paper.
noaa_2014
While global surface temperatures are increasingly dominated by warm anomalies, a conspicuous area of cold has persisted south of Greenland and Iceland visible at the ocean surface in sea surface temperature observations. The abnormal cold there has been more anomalous than the US northeast winter. While the most recent northern winter was the warmest on record globally, the ocean surface area south of Greenland & Iceland had the lowest temperatures in the 136 year record. How could this be?
A new study estimates the Atlantic Meridional Overturning Circulation (AMOC) using the sea surface temperature difference at that cold spot south of Greenland/Iceland with the Northern Hemispheric temperature from NASA GISS instrumental records since the 1880s (Hansen and others 1999) and from coral-based proxies after Sherwood and others (2011) that span years since 500 AD.
Based on this AMOC reconstruction, the study finds that the slowdown of the Atlantic Meridional Overturning Circulation (AMOC) after 1975.
1. appears unprecedented in the past millennium;
2. is expected to continue, even intensify through year 2100, as simulated with the MPI-ESM-MR global climate model of the Max Planck Institute in Hamburg (Jungclaus and others, 2013)
3. may result to a large degree from Greenland melting.
My contribution was my work of 6 years, a 172 year Greenland mass balance
reconstruction published in a 3 part series in the Journal of Climate (Box and others 2013; Box, 2013;
Box and Colgan, 2013), enabling Greenland melting to be brought more into context of its ocean thermohaline perturbation.
box_runoff
Mass Balance terms of the Greenland Ice Sheet. Data from Box and Colgan. Cumulative anomaly relative to the mean over 1840-1900, a pre-industrial period during which the Greenland Ice Sheet was approximately in balance.
Melt from Greenland produces water that is lighter and colder than the sea surface waters. The meltwater is light enough to float above the saltier sea surface waters. Because Atlantic surface waters flow northward (see map below), an increasing ice melt freshwater supply ( blue line in the figure above) may pile up near the sea surface, capping or backing up the Gulf Stream North Atlantic Drift current that a.) delivers warmth to northwestern Europe and b.) is part of a global ocean heat conveyer. While Bamber and others (2012) set the stage with “Recent large increases in fresh- water fluxes from Greenland into the North Atlantic”, the new study more directly quantifies the possible impact from Greenland on the ocean thermohaline circulation.
northatcurrent
Why should we care?
The North Atlantic ocean circulation is an important part of a global ocean circulation that exchanges heat from the equatorial surplus to the poles where the energy is lost by thermal radiation to space. A slowed global oceanic ‘conveyer belt’ may further destabilize our changing global climate. We expect no new ice-age – but major negative effects are possible. The effects could be on global climate, fisheries, or also for example storminess.
Influence on weather?
The study will stimulate discussion and research on how the large area of negative sea surface temperatures anomalies south of Greenland and Iceland may influence European and downstream weather. Given some heat exchange between a warm air mass with an anomalously cold North Atlantic sea surface, some strengthening of winds, lowering of central pressure of cyclonic systems, should result from increased “baroclinic instability” (see for example Holton et al. 1992) arising from an increased temperature difference between sea surface and atmosphere. As the subpolar North Atlantic cools and the atmosphere warms, the physics are set to strengthen cyclonic circulation in warm air masses. Converseley, cold air masses drifting off of N America would be less prone to baroclinic deepening. In any case, the perturbation may be felt not just in that part of the world, but downstream, and like the proverbial butterfly (seagull) flapping its wings, alters atmospheric and oceanic flow, with certain though hard to predict downstream consequences.
However, there are many other effects, ranging from dramatic impacts on fisheries to, perhaps most troubling of all, the potential for extra sea level rise in the North Atlantic region.
That may sound surprising, but here’s how it works. We’re starting out from a situation in which sea level is “anomalously low” off the U.S. east coast due to the motion of the Gulf Stream. This is for at least two reasons. First, explains Rahmstorf’s co-author Michael Mann of Penn State University, there’s the matter of temperature contrast: Waters to the right or east of the Gulf Stream, in the direction of Europe, are warmer than those on its left or west. Warm water expands and takes up more area than denser cold water, so sea level is also higher to the right side of the current, and lower off our coast.
So if you weaken the ‘Gulf Stream’ and weaken that temperature contrast…sea level off the U.S. east coast will actually rise!” explains Mann by e-mail.
But there’s another factor, too, involving what is called the “geostrophic balance of forces” in the ocean. This gets wonky, but the bottom line result is that “sea surface slope perpendicular to any current flow, like the Gulf Stream, has a higher sea level on its right hand side, and the lower sea level on the left hand slide,” says Rahmstorf.
We’re on the left hand side of the Gulf Stream. So weaken the flow, and you also raise the sea level. (For further explanation, see here and here.)
Indeed, researchers recently found a sudden, 4-inch sea level rise of the U.S. East Coast in 2009 and 2010, which they attributed to a slowdown of the Atlantic overturning circulation. Rahmstorf says that “for a big breakdown of the circulation, [sea level rise] could amount to one meter, in addition to the global sea level rise that we’re expecting from global warming.”
Below, my profile of Stefan Rahmstorf’s Art and Science. Not “just a scientist”.

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