When you look at a map of global surface temperatures for 2015, the first impression you might get is a planet with a bad sunburn. Almost every part of the globe saw above-average temperatures during Earth’s warmest year on record, and there was unprecedented warmth across many parts of the tropical and subtropical oceans (Figure 1). The next thing you’d probably notice is a blue blob in the North Atlantic, sticking out like a frostbitten thumb. No one knows exactly why, but this blob of unusually chilly water, roughly half the size of the United States, has taken up what seems like semi-permanent residence in the North Atlantic Ocean.
It’s normal for ocean temperatures to wax and wane on all kinds of time scales. What’s more uncommon is for a cold anomaly this large and strong to persist for so long, especially when the rest of the planet is trending ever warmer. Figure 1.
Surface temperatures for 2015 were at record cold values for part of the far North Atlantic, even as most of the globe was unusually warm. Image credit: NOAA/NCEI
.Heat waves and cold waves at sea
The North Atlantic’s cold blob once had a hot-headed cousin. Thousands of miles away, on the other side of North America, a zone of above-average sea surface temperatures (SSTs) in the northeast Pacific gained fame as “The Blob.”
While it was in place, from about 2013 through most of 2015, The Blob was closely linked with intense upper-level ridging over and near it. The Pacific jet stream arced northward, away from California, which helped strengthen the fierce multiyear drought still plaguing much of the state. Once it became clear last autumn that a strong El Niño was on its way, experts predicted
that a juiced-up storm track in the Northeast Pacific would churn up the waters enough to dilute and vanquish The Blob. Sure enough, The Blob eroded to near-nothingness
in just a few weeks during late 2015, and the West Coast from San Francisco northward got drenched by wet Pacific storms throughout the subsequent winter.
One way to think of The Blob is as a “marine heat wave,” according to Hillary Scannell. Now a graduate student at the University of Washington, Scannell analyzed the full spectrum of these oceanic warm spots in a Geophysical Research Letters paper
, written with colleagues at the University of Maine, NOAA, and the Gulf of Maine Research Institute and published in March. Scannell is also coauthor on a new Progress in Oceanography paper
, led by Alistair Hobday (CSIRO), that lays out suggested metrics for defining marine heatwaves. Such events can have big impacts on ocean ecology: marine heatwaves have been implicated
in hundreds of years of coral-reef damage, and record-warm ocean temperatures have caused extensive damage this year
to the Great Barrier Reef.Figure 2
. Bleached coral at Lizard Island, north of Cooktown, Australia, captured by the XL Catlin Seaview Survey
in March 2016. The global insurance firm XL Catlin
is working with scientific institutions around the world to carry out the ongoing survey, which has collected more than 700,000 panoramic images
along nearly one million kilometers. Image credit: XL Catlin Seaview Survey, via globalcoralbleaching.org
Just like atmospheric heat waves, oceanic heat waves come in all sizes and shapes, typically defined by SST anomalies (departures from the seasonal average). But while a heat wave over land is ultimately doomed by the arrival of autumn, marine heat waves can recur for two or more years, because of what’s known as the reemergence mechanism
: warm anomalies that stay just below the surface during the placid flow of summer can return to the surface in winter by the deeper mixing brought about by strong storms. As one might expect, large, long-lived oceanic heat waves lasting a year or more are less common than briefer ones.
Using this analogy, we might picture the North Atlantic’s blob as a “marine cold wave.” Although such cold waves weren’t addressed in the GRL paper, Scannell has analyzed them. As with marine warm waves, she told me that brief, smaller, and/or weaker marine cold waves are more common than large, long-lived, stronger ones.
Once a marine heat wave or cold wave takes shape, its blob of above- or below-normal SSTs may feed back into the atmosphere, helping to intensify and reinforce the circulation patterns that brought it to life. There is one important distinction: cold surface blobs are easier than warm blobs to disrupt. This is because the ocean is more likely to mix (convect) when colder water sits astride warmer water, just as the atmosphere is more prone to intense storminess when cold upper level air moves atop warm, moist surface air. When you consider this instability, it’s even more impressive that the North Atlantic’s cold blob has outlived the Northeast Pacific’s warm blob.
“The persistence--the staying power--of this anomaly is really pretty remarkable,” noted Michael Mann (Pennsylvania State University) in an email. “It is particularly striking that during the warmest year on record globally , this region saw its coldest year on record.”Putting the brakes on Atlantic circulation
Might the cold blob be a sign of something else going on--in particular, a long-term slowdown in the Atlantic Meridional Overturning Circulation? The AMOC transports heat northward through the North Atlantic, via the Gulf Stream and related currents. Completing the loop, dense surface water--generated near the surface in the frigid Labrador Sea, southwest of Greenland--sinks and flows southward. Figure 3.
A schematic of the Atlantic ocean circulation, with surface currents in red, deep currents in blue, and winter sea-ice cover in white. NADW denotes North Atlantic Deep Water. Image credit: Stefan Rahmstorf, “Risk of sea-change in the Atlantic,” Nature 1997
Scientists have warned for years that the AMOC is likely to slow down in the coming decades due to human-induced climate change. Estimates in the latest IPCC assessment (2013)
range from an 11% to 34% slowdown during this century, depending on the pace of global greenhouse emissions. A warming planet would produce this slowdown in several ways, such as increased melting from the Greenland Ice Sheet or increased export of sea ice out of the Arctic Ocean. Either or both of these mechanisms could send additional fresh (non-salty) water around southern Greenland and into the Labrador Sea. Because fresh water is less dense than salty water, this could inhibit the formation of bottom water and slow down the AMOC. (Fortunately, atmospheric warming should more than offset AMOC-related cooling, even on a regional basis, so the shift this century is not expected to bury New York in mountains of snow as depicted in the 2004 film “The Day After Tomorrow.”
A number of leading scientists believe that meltwater from Greenland has already produced an AMOC slowdown. Mihai Dima and Gerrit Lohmann (Alfred Wegener Institute for Polar and Marine Research) argued in a 2010 paper
that the global conveyor belt has been weakening since the 1930s, with a dramatic shift around 1970. In a 2015 Nature Climate Change
paper, Stefan Rahmstorf (Potsdam Institute for Climate Impact Research) and colleagues, including Mann, made this case by using 1000 years of paleoclimate proxy data. They estimate that the AMOC actually began slowing in the 20th century--and especially since the 1970s, when the slowing has no precedent in their millennial-scale analysis. “Further melting of Greenland in the coming decades could contribute to further weakening of the AMOC,” they warned. The paper also presents NASA data for the 20th century that show a cold trend in SSTs in roughly the same location as the current North Atlantic cold blob. In a 2015 RealClimate article
, Rahmstorf noted that the location of the current cold wave “happens to be just that area for which climate models predict a cooling when the Gulf Stream System weakens.”
To gauge the strength of the AMOC more directly, a project called RAPID
has been using an array of buoys and other instruments straddling the subtropical northwest Atlantic for the last decade-plus. “RAPID does show a declining decadal trend in the AMOC since 2004, which is consistent with our data,” Rahmstorf said in an email. More data will soon arrive from a new project called OSNAP
(Figure 4), which is in the process of deploying an observational network close to Greenland to measure the undersea flow in the critical north end of the AMOC.Figure 4.
The observational array being installed by OSNAP (Overturning in the Subpolar North Atlantic Program) will monitor surface and subsurface flow at several points over the far North Atlantic. Image credit: OSNAP
.The NAO at work
On top of longer-term climate change, there is some interesting decadal-scale variability in the mix. I checked in with oceanographer Steve Yeager (National Center for Atmospheric Research), who serves on the U.S. AMOC Science Team
. Yeager agreed with Rahmstorf that the AMOC will tend to slow down over coming decades as a result of human-produced climate change. However, he added, “on the decadal time scale, you have the role of the North Atlantic Oscillation in slowing down or enhancing the AMOC.”
Although the North Atlantic Oscillation is notoriously variable from week to week, month to month, and year to year, it can lean toward one phase or the other for as long as 10 or 15 years (see Figure 5 below). When the NAO trends positive during the winter, as it did during most of the 1990s and early 2000s, it favors colder air staying cooped up in the polar and subpolar regions. This would tend to enhance the formation of bottom water in the Labrador Sea and boost the AMOC, and in fact the AMOC did strengthen in the 1990s. The NAO turned largely negative in the late 2000s and early 2010s, and this transition to more neutral/negative NAO conditions may have caused the AMOC to weaken substantially in the last decade, as suggested by RAPID data. In addition, the NAO-driven slowdown in ocean heat transport could have contributed to the extremely cold conditions observed recently in and near the North Atlantic blob, as discussed by Yeager in a recent paper in Geophysical Research Letters
. The NAO has again averaged positive in three of the last four winters. Alas, Yeager noted, “we can’t predict the NAO. That’s the missing part of our predictability.”Figure 5.
The North Atlantic Oscillation (NAO), standardized and averaged across the January-to-March period for each year from 1950 to 2015. The black line is the five-year running mean. Based on monthly data
, the 2016 value (not shown) will end up somewhere between 0.5 and 1.0. Image credit: NOAA/CPC
.What about the tropics?
Hurricane watchers may be wondering what all this means for the frequency of Atlantic tropical cyclones. In general, as demonstrated by Phil Klotzbach (Colorado State University) and colleagues, a stronger AMOC tends to lead to a more robust Atlantic hurricane season. There is typically a lag of several years before a switch in the AMOC influences sea-surface temperatures and hurricane activity. For example, the AMOC began strengthening in the early 1990s, followed by the spectacular onset of enhanced Atlantic hurricane activity in the mid-1990s. Likewise, the decline in the AMOC over the last decade has been followed by a ramp-down in Atlantic hurricane activity since 2013. In a Nature Geoscience paper
last September, Klotzbach and colleagues argued that we may have already seen the end of the active cycle that began in the mid-1990s.
The potential arrival of La Niña later this year could prove favorable for Atlantic hurricanes, but the resilient cold blob may work in the opposite direction, according to Klotzbach. “When the far North Atlantic is cold, it tends to force wind and pressure patterns that then cool the tropical Atlantic,” he told me. “We've seen a significant cooling of the eastern subtropical Atlantic in recent weeks, and there is the potential that these cold anomalies could propagate into the tropical Atlantic for the peak of the Atlantic hurricane season. If this occurs, there is the potential that the hurricane season may not be particularly active.”
We’ll take a closer look at what 2016 may have in store next Thursday, when CSU issues its outlook for Atlantic hurricane activity.