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Sunday, April 14, 2013

Readers, a must-read: The Antarctic Half of the Global Thermohaline Circulation Is Faltering

Anvers Island, Antarctica moon rise over sea ice
The sudden cooling of Europe, triggered by collapse of the global thermohaline circulation in the north Atlantic and the slowing of the Gulf Stream has been popularized by the movies and the media. The southern half of the global thermohaline circulation is as important to global climate but has not been popularized. The global oceans' coldest water, Antarctic bottom water forms in several key spots around Antarctica. The water is so cold and dense that it spreads out along the bottom all of the major ocean basins except the north Atlantic and Arctic. Multiple recent reports provide strong evidence that the formation of Antarctic bottom water has slowed dramatically in response to massive subsurface melting of ice shelves and glaciers. The meltwater is freshening a layer of water found between depths of 50 and 150 meters. This lightened layer is impeding the formation of Antarctic bottom water, causing the Antarctic half of the global thermohaline circulation to falter.
Update from the comments
I have been asked what's going to happen in response to the faltering of the thermohaline circulation around Antarctica. This post is based on a synthesis of very recent research reports. The key report, that found the layer of fresh water between 50 and 150 meters deep, was just published. Deward Hastings explained, in a comment, how disruptive this lens of freshened water could be to the earth's climate system and our models of it:
it IS complicated, and confusing
That lens of (relatively) fresh water that is forming around Antarctica is challenging, and changing, almost everything in global circulation patterns.  It freezes sooner (and at a higher temperature).  That shields the water from the wind, and reduces wind-driven mixing.  It reduces, perhaps to the point of stopping altogether, the present global ocean circulation patterns.  That in turn will change global atmospheric weather.
Nobody knows exactly what comes next.  We've never seen it happen, and our models, not terribly accurate in describing the world we know, are completely untested in the coming world that we don't know.
Without a constant flow of cold water from the poles the Abyss will warm . . . and without cold slowly rising from the Abyss the mid-ocean and ocean surface will warm (already happening).  That will lead to more evaporation (driving a different haline circulation in the tropics) and stronger tropical winds driving different surface currents and greater mixing.
Pretty much everything changes as a result . . . pretty much everywhere.  After it's all over some places will have it better and some worse.  While it's changing everywhere will be worse, because there is no way to know what to expect (except that it won't be what you've prepared for).
The best guesses we can make now about the effects of this melt layer are based on paleoclimatology research. Possible effects, based on paleoclimatology studies, are presented in the last few paragraphs. The results of these new studies will be challenging climate modelers for many years.
Sea ice extent has been increasing around Antarctica. In September 2012, while Arctic sea ice was at record low levels, Antarctic sea ice extent hit a record high. Climate skeptics jumped on the Antarctic record as evidence of cooling, while sea ice researchers blamed it on the wind.
Since the start of the satellite record, total Antarctic sea ice has increased by about 1% per decade. Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent. Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.
NSIDC scientist Ted Scambos said, "Antarctica's changes—in winter, in the sea ice—are due more to wind than to warmth, because the warming does not take much of the sea ice area above the freezing point during winter. Instead, the winds that blow around the continent, the "westerlies," have gotten stronger in response to a stubbornly cold continent, and the warming ocean and land to the north."
Several recent reports, however, paint a more complex and disturbing picture where the intensifying winds are speeding up below surface currents bringing more above freezing water in contact with deep ice around Antarctica. Twenty of the ice shelves and many of the glaciers that feed them are melting from below.
Researchers used 4.5 million measurements made by a laser instrument mounted on NASA’s ICESat satellite to map the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt across the continent. Of the 54 ice shelves mapped, 20 are being melted by warm ocean currents, most of which are in West Antarctica.
Antarctic Ice Melting from below.
Figure 2 | Antarctic ice-shelf ice-thickness change rate DT/Dt, 2003–2008.
Seaward of the ice shelves, estimated average sea-floor potential temperatures (in uC) from the World Ocean Circulation Experiment Southern Ocean Atlas (pink to blue) are overlaid on continental-shelf bathymetry (in metres)30 (greyscale, landward of the continental-shelf break, CSB) Grey circles show relative ice losses for ice-sheet drainage basins (outlined in grey) that lost mass between 1992 and 2006 (after ref. 2).
The melting from below is creating a layer of relatively fresh water 50-150 meters below the surface around Antarctica. This layer of light fresh water is floating above a  salty layer below. When ice forms at the surface in the Antarctic winter, it creates cold dense salty water that tends to sink to the bottom, forming bottom water. However, this layer of light melt water is tending to block the water in the top 50 meters from sinking. The area of Antarctic sea ice has expanded because the layer of cold water has stayed on top and expanded outwards instead of sinking. Melting from below has created 2 stratified cold layers in the top 150 meters.
Note the bright pink area in the top 25 meters between 65° and 70° S. This top layer is becoming more saline. Brine is rejected from ice when sea ice forms. It isn't sinking because it is ponding above a freshening layer located at depths between 50 and 150 meters.
The freshened water column around Antarctica has become more stable between depths of 100 and 150 meters. This increasing stability is impeding the formation of Antarctic bottom water. Water that does sink is freshened through incorporation of glacial melt water.
Figure 3.  Austral winter half-year (April–September) zonal mean trends (1985–2010) of observed salinity, vertical density gradient and potential temperature, in the Southern Ocean. a, Salinity. b, Vertical density gradient. c, Potential temperature. Contours indicate the 1985–2010 mean state (psu; kg m-4, °C). Colouring (bright or faint) indicates whether the trend is significant (yes or no) at p<0:1 65="" 70="" a="" according="" analysis="" and="" based="" between="" brine="" due="" en3="" font="" forms.="" from="" ice="" in="" increase="" is="" likely="" met="" most="" near-surface="" observations.="" observations="" ocean="" office="" on="" rejection="" salinity="" sea="" situ="" sub-surface="" t-test.="" taken="" the="" to="" two-sided="" were="" when="" which="">
Analysis of potential temperatures, which are temperatures adjusted for the effects of increasing pressure with depth, shows the surface water in the top hundred meters is cooling over a vast area from 40°-80° S, while the water in that vast area below 150 meters is warming.
These results show a trend towards reversal of vertical motions around Antarctica. Intermediate water is welling up around Antarctic melting ice from below, creating a freshened layer. Strengthening winds are blowing the cold surface water away from Antarctica. Bottom water formation, caused by the sinking of cold salty water formed by brine rejection, is declining.
The results of this study are confirmed by a detailed study of anthropogenic tracers in the Weddell sea.   Chlorofluorocarbon (CFC) observations showed increasing average ages of the deep water in the sea from 1984–2010. The average age increased because because bottom water formation, and outflow from the Weddell sea, declined.
...we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the anthropogenic Carbon increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations.
The decline in Antarctic bottom water formation, combined with the southward expansion of warm subtropical water in the south Pacific and south Indian oceans has led to the rapid heating of intermediate and deep ocean water in the southern hemisphere.
Ocean heat content vs time. The deep ocean is heating up.
Figure: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.
A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth, and Källén (2013).  There are several important conclusions which can be drawn from this paper.
• Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.
• As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.
As the earth has warmed in response to the effects of increasing levels of greenhouse gases the southern subtropical belt in the oceans and atmosphere has expanded, tightening the rings of winds and ocean currents around Antarctica. Enormous volumes of warm subtropical water have been added to the southern ocean at depths greater than 300 meters (greater than approximately 1000 feet).
Observed temperature trends in the Indian Ocean present complex patterns that cannot be explained by surface heating alone. The heat storage has apparently increased more in the southern part than in the northern part of the Indian Ocean (Levitus et al. 2005), although this result may be biased by the sparse data coverage, particularly in the south (Harrison & Carson 2007). The strongest warming is found near the subtropical front and extends as deep as 800 m; it is not directly linked to surface heating but rather due to a southward shift of the oceanic gyre circulation and associated thermal structure (Alory et al. 2007).
Another recent detailed study of the water properties of the southern ocean has independently determined that the southern branch of the global thermohaline circulation has slowed dramatically, contributing to a large uptake of heat by the deep southern ocean.
A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the Meridional Overturning Circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW.
Rates of change in AABW-related circulation are estimated in most of the world’s deep
ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The
Southern Ocean is losing water below θ = 0 °C at a rate of -8.2 (±2.6) × 106 m3 s-1.
The budget calculations and global contraction pattern are consistent with a global scale slowdown of the bottom, southern limb of the MOC.
The slowdown of the southern branch of the thermohaline circulation and the cooling of the surface waters close to Antarctica are enhancing the thermal gradient from the tropics to the pole, speeding up the winds in the Southern Hemisphere. These increases in wind speeds are likely increasing the flow of water from the Pacific to the Atlantic ocean, enhancing the northward flow of water, salt and heat from the south to the north Atlantic. Moreover, the southward movement of the subtropical front allows more flow of the Agulhas current around the south African capes from the Indian ocean to the south Atlantic.
Thus, increased melting of Arctic sea ice may be related to declines in Antarctic bottom water formation. Likewise, the cool Pacific, warm Atlantic pattern causing increased U.S. droughts and storminess in the north Atlantic may be tied to these changes in ocean circulation patterns. Paleoclimate studies have consistently shown oscillations between Antarctic and north Atlantic bottom water formation and between relative coolness around Antarctica and north Atlantic warmth.
The Arctic melt down that is far exceeding model predictions is connected to the slow down in Antarctic bottom water formation. Climate modelers will be challenged to model the connections and the details. The cooling waters around Antarctica, while apparently good news, are not. The rapid melting of the Arctic will be enhanced.

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