Jump in NW Atlantic Sea Level Driving Gulf Stream Water into Arctic, Sea Ice Collapsing

by FishOutofWater, DailyKos, March 20, 2015

Sea level has jumped off the east coast of north America since March 2013. The warmest and saltiest water ever seen in the northwest Atlantic is mixing with icy water drained from the Arctic ocean and sinking to the bottom of the Labrador Sea.

AVISO's global map of sea-surface height departure from normal for March 14, 2015, shows a huge rise in sea level off the east coast of north America. This water has the highest levels of heat and salt ever measured in this region.

Sea-surface heights were relatively low off the east coast on March 14, 2013, when the polar vortex split and deep water formation in the Labrador Sea collapsed. Arctic sea ice staged a major recovery because cold fresh water stayed in the Arctic and the Gulf Steam and Norwegian currents weakened.

March 1995 had the most similar sea-surface-height pattern to 2015 in AVISO's 22-year, on-line record. March 1995 was when deep water formation in the Labrador sea was at a 50-year maximum

Sea-surface heights offshore of North America's east coast have risen to the highest levels ever measured in March, when it is usually at an annual minimum because the water column is at its coldest.  The saltiest and warmest waters ever found in this region have spawned a series of extreme storms which have spun up both the polar vortex and the North Atlantic Ocean's currents and deep-water formation. Cold, relatively fresh water is draining from the Canadian side of the Arctic and sinking in the Labrador Sea as it mixes in the stormy waters with dense salty Gulf Stream water cooled by Arctic air.

The sudden return to active deep-water formation in the Labrador Sea has profound implications for Northern Hemispheric weather and Arctic sea ice. The strong deep convection in the 1990s led to the rapid North Atlantic and Arctic Ocean warming that occurred in the following decade. Moreover, strong Labrador Sea deep convection brings cold air down from the Arctic into eastern North America and brings warm Atlantic Ocean air to the coastal areas of Europe. And it brings very intense storms to the east coasts of North America and Greenland. This pattern explains the very severe weather the U.S. east coast went through in February 2015. The east coast of North America was the only continental area on earth colder than normal in February. Western Europe and western North America were very warm.

After the very weak polar vortex year of 2013, when a sudden stratospheric warming split the polar vortex in two and deep convection failed to start in the Labrador Sea, the polar vortex strengthened and storminess in the western North Atlantic intensified. Cooling began in 2014 in the Labrador Sea, while water temperatures rose to record levels in the Gulf of Maine and adjacent North American waters. The polar vortex and Labrador Sea deep convection grew stronger in late winter 2015 after an incipient stratospheric warming formed an anticyclone high over the subtropical North Atlantic. This anticyclone  in the stratosphere disrupted the westward movement of atmospheric waves across the Atlantic, directing them northeastwards towards Norway and the Arctic Ocean. This block in the flow intensified the polar vortex and strengthened storms over the North Atlantic.

Strong polar vortex and Labrador Sea deep convection pattern. This strong polar vortex, strong deep convection pattern described by Reichler and others in Nature Geoscience fits this March's sea-surface height pattern exactly. "Shown are composite anomalies averaged from day 0 to 60 following the strong vortex events of Fig. 2. Sea-level pressure anomalies are contoured at positive and negative 0.5, 1, 2, 3, 4 hPa; red and blue lines indicate positive and negative values, respectively. Shading shows the sum of latent and sensible heat flux anomalies (in Wm^-2), with positive and negative anomalies indicating oceanic heat gain and loss, respectively. Vectors represent the magnitude and direction of surface wind stress anomalies."

The flow of warm salty water from the Gulf Stream has sped up off the coast of Norway, driven by strong southwesterly winds. The extent of sea ice on the European side of the Arctic began to drop in mid-February, a month early in response to the intrusion of warm air and water. The National Snow and Ice Data Center has announced that a very early record-low winter-maximum sea-ice extent apparently occurred on February 25. Japanese high-precision measurements of sea ice extent apparently reached a record early and record-low maximum on February 15, 2015.

Preliminary NSIDC figures show that a record minimum winter sea ice maximum extent was reached on February 25, 2015. "Figure 1. Arctic sea ice extent for February 25, 2015, was 14.54 million square kilometers (5.61 million square miles). The orange line shows the 1981 to 2010 median extent for that day."

On February 25, 2015, Arctic sea ice likely reached its maximum extent for the year, at 14.54 million square kilometers (5.61 million square miles). This year’s maximum ice extent was the lowest in the satellite record, with below-average ice conditions everywhere except in the Labrador Sea and Davis Strait. The maximum extent is 1.10 million square kilometers (425,000 square miles) below the 1981 to 2010 average of 15.64 million square kilometers (6.04 million square miles) and 130,000 square kilometers (50,200 square miles) below the previous lowest maximum that occurred in 2011. This year’s maximum occurred 15 days earlier than the 1981 to 2010 average date of March 12. The date of the maximum has varied considerably over the years, occurring as early as February 24 in 1996 and as late as April 2 in 2010.

The outlook for Arctic sea ice this summer is poor because warmer than normal water is surging up the coast of Norway into the Arctic, while cold water which has been stored for years in the Arctic ocean is draining through the Canadian passages into the Labrador sea. Moreover, stratospheric and Arctic patterns can reverse with the seasons, so a strong winter polar vortex may be followed by high pressure and warm sunny weather in the late spring. The Arctic sea ice recovery of 2013 is very likely over. Summer sea ice appears to be back on the long downward trend towards zero.

NOAA's Ocean Prediction Center made a short silent movie of two hurricane-force storms in early March this year. These storms were two of the most intense lows to develop in this extremely stormy winter in the North Atlantic. The extraordinarily stormy weather in February and March has spun up the thermohaline circulation, draining icy, relatively fresh water from the Canadian side of the Arctic Ocean and driving warm salty water that originated in the Gulf Stream up the coast of Norway, into the European side of the Arctic Ocean. The influx of warm air and water from these storms caused Arctic sea ice to retreat and set a provisional record-low maximum on February 25, 2015.

Supplementary figure showing the intense polar vortex event associated with the extreme winter storms and Labrador sea deep convection in February and March, 2015

The polar vortex was stronger than normal in February and the first half of March 2015. Blue from the surface to the stratosphere shows a strong polar vortex. The polar vortex strength for winter 2014-2015 is displayed as the geopotential height anomaly versus time for 65-90 degrees N.

Supplementary figures showing the surge of warm Atlantic water into the Arctic.

Warmer than normal water is flowing up the coast of Norway into the Arctic.

Warmer than normal water is flowing up the coast of Norway into the Arctic.

These two figures for March 20, 2014, and March 20, 2015, taken from the Mercator Ocean analysis of water temperatures at a depth of 1,000 feet (318 meters), show a surge of warm Atlantic water deep into the European basin of the Arctic Ocean and show the expansion of the area of cold sinking water in the Labrador Sea and North Atlantic waters.

Water temperatures at a depth of 1,000 feet (318 m) March 20, 2014, Mercator Ocean analysis, in and around the Arctic.

attribution: Mercator Ocean

Map of water temperature at a depth of 1,000 feet for March 20, 2015. Warm Atlantic water has surged into the Arctic and cold Arctic water has drained into the Labrador sea since the same date a year ago.

http://www.dailykos.com/story/2015/03/20/1372161/-Jump-in-NW-Atlantic-Sea-Level-Driving-Gulf-Stream-Water-into-Arctic-Sea-Ice-Collapsing

Hurricanes May Threaten Northern Cities Like Never Before as Tropics Expand, Study Finds

by Andrew Freedman, Mashable, May 15, 2014

In this December 5, 2012, file photo, men walk past a house damaged during Superstorm Sandy in the Belle Harbor section of the Queens borough of New York. IMAGE: KATHY WILLENS/ASSOCIATED PRESS

BY ANDREW FREEDMAN1 DAY AGO

Hurricanes and tropical storms are reaching their peak intensity closer to the poles, migrating at about 30 miles per decade, according to a new study published Wednesday. If this shift continues, it could have major consequences for places like New York City, Tokyo, Japan and Brisbane, Australia, as well as other high latitude areas that don't normally see intense hurricanes.

The study, published in the journal Nature, is the first to detect this trend, and in doing so it reveals a somewhat indirect but tangible link between human activities and Mother Nature's most powerful storms.

SEE ALSO: What Irreversible Antarctic Ice Melt Could Mean for Some U.S. Cities

In order to reach their conclusions, the researchers overcame a formidable obstacle that has long hindered research on the links between hurricanes and global warming. Changes in how we detect and measure the intensity of tropical cyclones over the years makes finding statistically significant trends difficult to near impossible.

Typhoon Francisco and Super Typhoon Lekima seen from a satellite on October 23, 2013, as they tracked northwestward toward China and Japan. IMAGE: NOAA

The new study examines a metric known as a storm's "lifetime-maximum intensity" during the period from 1982 to 2012, which is a time frame that is not complicated by changes in storm observations. The metric refers to the point where storms max out in strength.

By examining storm data using this metric, the researchers found a strikingly apparent poleward shift in the locations where storms are reaching their peak intensity. Although the changes varied from ocean basin to ocean basin, with the greatest migration seen in the western North Pacific Ocean, which is the most active area for tropical cyclones, the shift was found in both the Northern and Southern Hemispheres.

In the Northern Hemisphere, the lifetime-maximum intensity point is moving north at 33 miles per decade, whereas in the Southern Hemisphere, that point is moving south at 39 miles per decade.

Such a change is enough to alter the risk that a coastal location will be hit by a major storm, said lead author James Kossin of the National Climatic Data Center.

"There’s no doubt that a signal like this introduces the potential for a change in the risk,” Kossin said in an interview with Mashable. There's certainly a potential for decreased risk in some areas and increased risk in others at higher latitudes.”

The study itself says that if the observed trends continue, there could be "potentially profound consequences for life and property."

In other words, the risk of damage and fatalities from tropical storms and hurricanes could increase in northern latitudes, while declining somewhat at lower latitudes.

Color-enhanced infrared satellite image of Typhoon Usagi as it moved northwestward toward Hong Kong while explosively intensifying to a Category-5 storm. IMAGE: NOAA/COOPERATIVE INSTITUTE FOR METEOROLOGICAL SATELLITE STUDIES, UNIVERSITY OF WISCONSIN-MADISON.

In addition, parts of the tropics that depend on rainfall from tropical storms and hurricanes (collectively referred to as tropical cyclones) to provide water resources may be at risk for lower water availability as the storms move away from them.

One caveat from the study is that there hasn't been a detectable poleward shift in the maximum intensity point of Atlantic tropical cyclones. However, this may be due to other factors that are masking this movement.

"The Atlantic is fairly unique in the last 30 years in how it’s been behaving,” Kossin says. "We kind of suspect that this global signal that we uncovered is just kind of getting muddled up with strong regional effects.

“The takeaway from that is just because we have not seen a trend there in the last 30 years doesn’t mean this effect is not present there, it’s just being masked by other things and it won’t necessarily be masked forever.”

Tracks of all tropical cyclones which formed worldwide from 1985 to 2005. IMAGE: WIKIMEDIA COMMONS

Chris Landsea, a meteorologist at the National Hurricane Center in Miami, Florida, who was not involved in the study, told Mashable: "This is an important, very well researched paper that uncovers something that was unknown previously."

He continued, "Such changes in where storms are peaking are somewhat unexpected (at least to me) but are apparently due to global scale changes in the atmosphere, perhaps tied to anthropogenic [man-made] global warming."

Expanding tropics

The study ties the migration of storms' peak intensity points to a gradual expansion of the tropics, which in turn has been linked to manmade factors. The expansion has to do with the widening of what is known as the Hadley Cell, which is a pattern of air circulation that causes air to converge near the Equator, sparking thunderstorms with heavy rain throughout the tropics. (This helps explain the typical location of rain forests worldwide.)

As air spills out of the Hadley cell and descends, it dries and warms, which is why the subtropics at the edges of the Hadley Cell are often home to deserts.

Studies of how emissions of planet-warming greenhouse gases will affect the tropics have consistently shown that the tropics will expand poleward in both hemispheres, which will push the dry subtropics poleward as well. This is part of the reason why the Southwest U.S. is poised to become drier as the climate warms, since the subtropics are starting to encroach on that area.

Diagram of tropical air circulation, including the Hadley Cell.

According to Kossin, the expansion of the tropics has also been tied to emissions of aerosols, which are small particles in the atmosphere, from factories and natural sources like desert dust, as well as manmade depletion of the stratospheric ozone layer. It is not yet clear which factor is the biggest contributor to the expansion of the tropics, however, Kossin said.

If greenhouse gases are driving the expansion of tropical atmospheric circulation, "then we won’t be seeing any stop to this in the near future," Kossin says.

However, if the main driver is the depletion of the ozone layer, the phaseout of ozone-depleting pollution should slow this expansion over time.

In addition to the growing tropics, two other environmental changes have taken place that affect storm intensity. The first concerns vertical wind shear. This refers to the change in wind speed or direction (or both) with height. Vertical wind shear has increased in the tropics, but decreased farther away from the tropics, closer to the poles. Vertical wind shear can make it difficult for tropical storms and hurricanes to form and intensify, since they tear apart the thunderstorms that comprise the inner core of such storms.

The second is that potential intensity, which is how strong a storm can get given sea surface temperatures and the temperature and moisture content of the upper atmosphere, is also changing. “In the tropics the potential intensity is decreasing, and it's increasing outside of the tropics,” Kossin said.

Hugh Willoughby, a hurricane researcher at Florida State University, told Mashable in an email that the study's results match his understanding of how air circulation is changing in and around the tropics. "My take on the situation is that shear attributable to the subtropical jet will increase as the planet warms; whereas the shear attributable to the middle latitude jet will decrease as high latitudes warm more than the tropics," he said. Willoughby was not involved in the new study.

The link between global warming and tropical cyclone behavior has been a source of controversy in the past, but this study appears to present clear, and unexpected, evidence that storms are already responding to the changing climate.

The big question is what it will mean for coastal residents around the world, but early hints are not encouraging for higher latitude cities.

http://mashable.com/2014/05/14/migrating-hurricane-intensity-tropics-expand-global-warming/

"The poleward migration of the location of tropical cyclone maximum intensity," by James P. Kossin, Kerry A. Emanuel & Gabriel A. Vecchi, Nature (2014); doi:

Nature, 509 (15 May 2014) 349–352; doi: 10.1038/nature13278

The poleward migration of the location of tropical cyclone maximum intensity

• James P. Kossin,
• Kerry A. Emanuel and
•  Gabriel A. Vecchi

Abstract

Temporally inconsistent and potentially unreliable global historical data hinder the detection of trends in tropical cyclone activity^1,2,3. This limits our confidence in evaluating proposed linkages between observed trends in tropical cyclones and in the environment^4,5. Here we mitigate this difficulty by focusing on a metric that is comparatively insensitive to past data uncertainty, and identify a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years. The poleward trends are evident in the global historical data in both the Northern and the Southern Hemisphere, with rates of 53 and 62 kilometres per decade, respectively, and are statistically significant. When considered together, the trends in each hemisphere depict a global-average migration of tropical cyclone activity away from the tropics at a rate of about one degree of latitude per decade, which lies within the range of estimates of the observed expansion of the tropics over the same period⁶. The global migration remains evident and statistically significant under a formal data homogenization procedure³, and is unlikely to be a data artefact. The migration away from the tropics is apparently linked to marked changes in the mean meridional structure of environmental vertical wind shear and potential intensity, and can plausibly be linked to tropical expansion, which is thought to have anthropogenic contributions⁶.

http://www.nature.com/nature/journal/v509/n7500/full/nature13278.html

Australia: Wilder winds, less rain, as Roaring Forties become Furious Fifties

by Peter Hannam, Environment Editor, The Sydney Morning Herald, May 11, 2014

Photo: Fairfax Graphics

The Roaring Forties, the Southern Ocean winds which once bore European sailors to Australia and the East Indies, are becoming more like the Furious Fifties as climate change triggers a shift in key weather patterns poleward, an Australian-led team of scientists has found.

Using data derived from Antarctic ice cores and other sources, the researchers found Southern Ocean winds are now stronger than at any time in the past 1,000 years.

Greenhouse gases are what are causing the winds to intensify now and that’s really moving the system beyond the natural range,” said Nerilie Abram of the Australian National University’s Research School of Earth Sciences and lead author of the research, published on Monday in Nature Climate Change.

In the past half century, the westerly winds have quickened 10-15% and moved 2-5 degrees closer to the South Pole – meaning fewer storms are reaching as far north as Australia.

“That isn’t good news for farmers in the southern parts of Australia who are reliant on the winter winds that come out of the Southern Ocean,” Dr Abram said. Winter rainfall has dropped 20% in southwest Western Australia since the 1960s, with cool-season rain tallies also lower in Australia’s southeast.

The stronger winds also help resolve a climate-change conundrum – why Antarctica is not warming as fast as other continents and the Arctic. “Over a large part of Antarctica we don’t get much warming at all,” Dr Abram said. [This may be changing - temperature anomalies over Antarctica have become very significant this year.]

The reason for the discrepancy is that cool air is being trapped over Antarctica, resulting in increased snowfall for some regions. However, areas exposed to stronger winds and warming seas, such as the Antarctic Peninsula, are heating up faster than anywhere else in the Southern Hemisphere.

“The West Antarctic Ice Sheet [adjacent  to the peninsula] is probably the bit of the Antarctic ice mass that we’ve been most concerned about for the longest time,” said Matthew England, from the University of NSW’s Climate Change Research Centre, and a co-author of the paper. If it all melted, that ice sheet could lift global sea levels by 4-5 metres, he said.

Professor England said the changes to atmospheric variability that see the band of westerly winds oscillate north or south – known in the Southern Hemisphere as the Southern Annular Mode – are driven roughly equally by the effect of rising greenhouse gases and the ozone hole.

The relative contribution, though, should alter as internationally agreed constraints on the use of chemicals that destroy the protective ozone layer take effect, potentially slowing the pick-up in wind speeds.

“Going forward, the greenhouse aspect will dominate as the ozone hole starts to repair and, of course, greenhouse gases are going terrifyingly upwards in their concentration,” he said.

Wenju Cai, an atmospheric scientist at the CSIRO who was not part of the research team, said the findings would assist the study of other key processes, such as whether the rate at which the Southern Ocean absorbs heat and carbon dioxide is changing.

The faster winds “may have a lot of influences that we do not know now,” Dr Cai said. “We may even solve some of the big issues that have been puzzling scientists for many, many years.”

http://www.smh.com.au/environment/weather/wilder-winds-less-rain-as-roaring-forties-become-furious-fifties-20140511-zr9b1.html

South Africa could face tropical storm threat by 2050

from environmentalresearchweb, April 24, 2014

South Africa could be at risk of direct impact from tropical cyclones within the next 40 years. A recent study found that the impression that tropical storms are increasing as a result of climate change is incorrect, rather the storms are shifting south, towards South Africa.

Jennifer Fitchett and Stefan Grab from the University of the Witwatersrand, South Africa, analysed storm data for the south-west Indian Ocean over the past 161 years. Their results are in line with previous studies that found there has been no increase in the number of tropical cyclones.

Much of the perceived increase in storm frequency could be due to better observations, the researchers believe. From 1850 to 1940, storms were often observed from ships, which would try to avoid regions of bad weather, leading to a significant undercounting of storm numbers.

Since the 1940s, aerial reconnaissance followed by satellite imagery have improved observations – researchers can now detect every single tropical cyclone that forms, moves and makes landfall on a coastline, Fitchett said.

Two other factors could have triggered perceptions of increasing numbers of tropical cyclones, the team believes. The era of global Internet connectivity has made people more aware of each storm that occurs as and when it happens, rather than just those in their immediate vicinity, or those that are severe enough to warrant global news coverage. "So the increase in reporting suggests an increase in numbers to the lay-person," said Fitchett.

And as the world population grows, more people are occupying coastal regions, and coastal towns and cities are getting bigger. "Consequently, when tropical cyclones make landfall in an area, they are now more likely to affect a town or city, and hence are worth writing about," said Fitchett.

The study found no statistically significant trends in the frequency of tropical cyclone landfalls over Madagascar and Mozambique over the past 60 years.

"Recent trends indicate an increasing number of tropical cyclones tracking to the south of Madagascar, potentially associated with the southward shift of the 26 °C isotherm, combined with a decrease in the steering flow during La Niña years," the researchers wrote in the International Journal of Climatology.

The 26.5 °C isotherm has moved south at a rate of 0.6° latitude per decade since 1850. "At current rates we could see frequent serious damage in South Africa by 2050," said Fitchett. "Our findings of a southward trend in storm trajectories will hopefully allow policymakers in South Africa and Mozambique to ensure that infrastructure is sufficiently sound to withstand the storms before they become a very regular occurrence. It allows for a certain amount of forewarning, which they previously did not have."

Related links

• A 66-year tropical cyclone record for south-east Africa: temporal trends in a global context, Jennifer M. Fitchett and Stefan W. Grab, International Journal of Climatology Early View

http://environmentalresearchweb.org/cws/article/news/57006

Alun Hubbard on the 50- to 150-year storms hitting Aberystwyth, Wales

by Peter Sinclair, "This Is Not Cool," Yale Forum, February 25, 2014

I first interviewed Dr. Alun Hubbard on the edge of the Watson River in Kangerlussuaq, Greenland last summer.  His vivid language and lucid storytelling made that video on of the most popular in the Yale Series (see below).

Both Dr. Hubbard, and my Dark Snow Project cohort, Sara Penrhyn Jones, live in the tiny village of Aberystwyth, on the coast of Wales, and teach at the local university. I skyped with Alun a week or so ago in the midst of the storms hammering the area. Shortly after that he wrote me to explain that his roof had just blown off in hurricane force winds.

Sara was kind enough to shoot some video of the surf pounding the area, although not at the height of the storms, and she caught up with Hubbard long enough for a colorful and well-informed take on a seminal weather event.

I’ll cut together some of Alun's further remarks that did not make this video in the next day or so – see Hubbard’s Greenland interview below:

http://climatecrocks.com/2014/02/25/new-video-uk-hammered-by-climate-fueled-storms/

David Spratt, Climate Code Red: With Arctic freezer door open, frigid air drains into USA and Eurasia, with Arctic unusually mild

by David Spratt, Climate Code Red, January 28, 2014

It's a cliche that a picture tells a story better than a thousand words, and it's really true in the case of this extraordinary map of weather modelling of northern hemisphere temperature anomalies (variations from the expected values based on climate records) for 29 January 2014:

It shows swathes of North America and northern Eurasia with winter temperatures up to 20 degrees Celcius (20 C) below the average for this time of the year (deep purple), whilst much of the Arctic is up to 20 C warmer than usual (bright red).

If the Arctic is the Northern Hemisphere's refrigerator, then the freezer door has been opened wide, with frigid air draining into USA/Canada/Eurasia, and unusually warm air rushing into the Arctic. 

The reason is well understood, and it's climate-change-driven destabilisation of the jet stream. The jet stream is the river of high altitude air that works to separates Arctic weather from that of northern Europe, Russia and Canada, and which governs much Northern Hemisphere weather. Arctic summer sea-ice loss ice loss has added to ocean and atmospheric heat, pushing the jet stream into a more meandering, S-shape pattern, dragging down and stalling cold and wet conditions over Europe, and bringing extreme weather in its wake. 

There is evidence connecting sea-ice loss to the more severe and extreme weather patterns in Europe and North America, consistent with research from the US National Oceanic and Atmospheric Administration (NOAA). As Arctic melting and warming destabilize the jet stream and making it more "wavy," it allows frigid air to plunge farther south. As the jet stream's waves become larger, they slow down or even stall at times, leading to a significant increase in so-called "blocking events," such as the current stalled cold front. These cause extreme weather simply because they lead to unusually prolonged conditions of one type or another.

The consequences are as diverse as the prolonged 2013 summer heatwave, drought and wildfires in the USA; the cool, dull and extremely wet first half of summer 2012  and subsequent extended winter in the UK and other parts of Eurasia; the current extreme cold in much of North America and Eurasia; record heat to the Arctic, as was dramatically experienced in Alaska; and unusual cold, heavy snowfall, record rain and hot spells to much of northern Europe and North America. Following Superstorm Sandy’s battering of the US northeast coast in 2012, flooding in June across central Europe was the worst in 400 years. It's not hard to connect the dots between Sandy and global warming as extreme weather becomes the new norm

Professor Jennifer Francis, of the Rutgers Institute of Coastal and Marine Science, says the Arctic-driven changes to the jet stream allow “the cold air from the Arctic to plunge much further south. The pattern can be slow to change because the [southern] wave of the jet stream is getting bigger…  so whatever weather you have now is going to stick around.”

In March, new research found that “the severe loss of summertime Arctic sea ice — attributed to greenhouse warming — appears to enhance Northern Hemisphere jet stream meandering, intensify Arctic air mass invasions toward middle latitudes, and increase the frequency of atmospheric blocking events like the one that steered Hurricane Sandy west into the densely populated New York City area.”

A recent study by Liu et al. found that “the recent decline of Arctic sea ice has played a critical role in the recent cold and snowy winters” across the Northern Hemisphere, and Qiuhong Tang of the Chinese Academy of Sciences and colleagues from Beijing and the US report in Nature Climate Change that they have identified a link between declining snow and ice in the polar north, and catastrophic heat waves, droughts and floods in the mid-latitudes. They found a distinct set of patterns of circulation associated with the loss of snow and ice: the upper atmospheric winds in the north become weaker, and the jet stream shifts northwards, which means that weather systems become more stable. The longer a weather system stays in one location, the greater the probability that the conditions will become extreme.

http://www.climatecodered.org/2014/01/with-arctic-freezer-door-open-frigid.html

Jennifer Francis: The where and when of wetter and drier: disappearing Arctic sea ice plays a role

Environmental Research Letter, 8 (2013) 041002; doi:10.1088/1748-9326/8/4/041002

The where and when of wetter and drier: disappearing Arctic sea ice plays a role

Abstract

Summer precipitation in northern Europe has been above average for each of the past six years (2007–2012), a pattern that is unprecedented in over a century. During these same years, the summer Arctic sea-ice cover has averaged about 40% below its typical extent prior to the 1950s and set two new record minima. Could there be a connection? This is the question that motivated the new study by Dr. James Screen, a Research Fellow at the University of Exeter, UK, that appears in this issue of ERL (2013 Environ. Res. Lett. 8 044015). Adding to the growing body of evidence linking rapid Arctic warming to changing weather patterns in the northern hemisphere mid-latitudes, he concludes that sea-ice loss and associated surface warming lead to large-scale circulation patterns that favor wet summers in northern Europe and dry summers along the northern Mediterranean.

by Jennifer Francis

Dr. Screen used a state-of-the-art atmospheric model to compare the mid-latitude circulation during conditions of extensive sea ice (representative of the late 1970s) to that with much reduced sea ice (representative of present day). Except for the sea-ice extent and ocean temperatures where ice was lost, all surface conditions were fixed at climatological values in the model, thereby isolating the influence of sea-ice loss. Observation-constrained reanalysis fields were composited for the wettest and driest summers in northern Europe and used to verify the model simulations.

The atmospheric responses to reduced sea ice in both real and modeled worlds show not only precipitation patterns in Europe similar to those observed during the past six abnormally wet summers, but they also reveal features in the large-scale circulation that appear coincident with unusual weather patterns experienced elsewhere around the Northern Hemisphere in recent years. For example, the meridional wind anomalies near the jet-stream level (Figure 6a in Screen 2013) suggest that reduced sea ice favors enhanced ridging over the western North Atlantic, which is consistent with increased high-pressure blocking observed during June (Hanna et al. 2013) and expanded melting of the Greenland ice surface (Nghiem et al. 2012). The increased ridging implied by the pair of positive/negative anomalies over North America is consistent with recent drought and heat waves in the western part of the continent. Enhanced troughing in the eastern North Pacific agrees with an observed poleward shift in the Aleutian storm track (Bender et al. 2012). More generally, the patterns of response to sea-ice loss revealed in this investigation further support previous studies linking Arctic warming and ice loss with changes in the large-scale circulation (e.g., Overland et al. 2012, Screen and Simmonds 2013, Francis and Vavrus 2012, Petoukhov et al. 2013 and Cassano et al. 2013 and references therein).

This new study by Dr. Screen contributes additional evidence that Arctic sea-ice loss is partly responsible for shifting weather patterns, and provides new detail about the timing, location, and types of patterns that are expected to emerge as Arctic and global warming continue unabated.

http://iopscience.iop.org/1748-9326/8/4/041002/article

"Identifying external influences on global precipitation," by Kate Marvel & Celine Bonfils, PNAS (November 2013); doi: 10.1073/pnas.1314382110

Proceedings of the National Academy of Sciences, (November 11, 2013); doi:10.1073/pnas.1314382110

Identifying external influences on global precipitation

1. Kate Marvel1 and 
2. Céline Bonfils

1. Edited by Kerry A. Emanuel, Massachusetts Institute of Technology, Cambridge, MA, and approved October 18, 2013 (received for review July 30, 2013)

Abstract

Changes in global (ocean and land) precipitation are among the most important and least well-understood consequences of climate change. Increasing greenhouse gas concentrations are thought to affect the zonal-mean distribution of precipitation through two basic mechanisms. First, increasing temperatures will lead to an intensification of the hydrological cycle (“thermodynamic” changes). Second, changes in atmospheric circulation patterns will lead to poleward displacement of the storm tracks and subtropical dry zones and to a widening of the tropical belt (“dynamic” changes). We demonstrate that both these changes are occurring simultaneously in global precipitation, that this behavior cannot be explained by internal variability alone, and that external influences are responsible for the observed precipitation changes. Whereas existing model experiments are not of sufficient length to differentiate between natural and anthropogenic forcing terms at the 95% confidence level, we present evidence that the observed trends result from human activities.

Significance

This study provides evidence that human activities are affecting precipitation over land and oceans. Anthropogenic increases in greenhouse gases and stratospheric ozone depletion are expected to lead to a latitudinal intensification and redistribution of global precipitation. However, detecting these mechanisms in the observational record is complicated by strong climate noise and model errors. We establish that the changes in land and ocean precipitation predicted by theory are indeed present in the observational record, that these changes are unlikely to arise purely due to natural climate variability, and that external influences, probably anthropogenic in origin, are responsible.

http://www.pnas.org/content/early/2013/11/05/1314382110