Gulfstream may strengthen with more precipitation in the far north

The circulation in the Atlantic Ocean. In the Atlantic, water flows northward at the surface (red arrow), cools and sinks in the Nordic Seas, before flowing back south in the deep ocean (purple arrow). The surface current flowing north is the Gulf Stream and its extension into the Norwegian Sea.

Credit: Ellen 

A new study from researchers at the Bjerknes Centre for Climate Research gives less reason to fear a weakening of the Gulfstream due to climate change. One of the suggested 'tipping points' in the climate system is a substantial slow-down or even collapse of the Gulfstream due to increased freshwater input in the northern seas. In a warmer climate, the hydrological cycle of precipitation and evaporation will strengthen including more rainfall, river runoff and ice melt in the north. One can in its most extreme imagine this literally to close the large-scale ocean circulation between the Arctic and the lower latitudes.

In the article 'How northern freshwater input can stabilise thermohaline circulation', Erwin Lambert, PhD-student at UiB and the Bjerknes Centre and the University of Bergen, studies how ocean circulation is affected by increased freshwater input. Lambert and colleagues show how increased freshwater input in the north in some cases can even strengthen the Gulfstream extension into the Arctic -- just like a river in a typical Norwegian fjord is a driver for the fjord's exchange with the surrounding ocean.

In 1961, the American oceanographer Henry Stommel reduced the ocean to a few equations. With Stommel's model, the North Atlantic can be split into a warm part in the south and the cold Nordic Seas in the north -- a thought experiment of two boxes, without coastlines, islands or underwater ridges. In the North Atlantic, water flows northward at the surface, before sinking to the bottom in the Nordic Seas and flowing back southward in the deep ocean. The surface current flowing north is what we think of as the Gulf Stream. Stommel's description of how water circulates between warm and cold regions like this, was entirely theoretical, and it was the Finnish oceanographer Clas Rooth who applied it to the Atlantic in 1982.

Stommel's model is simple. It does not represent all factors in the real world, but still made it possible to answer a big question. You can neglect the wind, and there will be circulation in the North Atlantic. As long as water sinks in the north, the Gulf Stream will continue to flow north.

"The beauty of such a model is that we can understand the full behavior of its circulation," says Erwin Lambert.

Lambert is a PhD candidate at the Geophysical Institute at the University of Bergen and the Bjerknes Centre, and works with a box model that builds on Stommel's model. He remarks that both theoretical models, like his, and the large and more detailed circulation models used for weather forecasting and climate projections, only represent the real world to a limited extent.

"The benefit of a theoretical model is that we know, and actually choose, what these limitations will be."

Simple models make it easier to pin-point the effect of changes. Like Stommel, Lambert can choose to let the water in the north be less salty and calculate how the ocean current will react to more freshwater in the Nordic Seas. A fresher north is exactly what is expected with global warming.

In a warmer world, there will be more rain and snow in the northern regions, meltwater from glaciers and sea ice will pour into the ocean, and together this will make the water in the Nordic Seas less salty. The salty Atlantic water that flows in from the south will mix with water that is fresher than it used to, and the mix will be less dense. As a result, water entering the Nordic Seas will not sink as efficiently as it has done in the past. According to Stommel's model, this would reduce the circulation in the Atlantic Ocean. This is the background for theories that global warming may weaken the Gulf Stream.

Two thirds of the water that enters the Nordic Seas flow back south in the deep ocean. The remaining one third continues on the north-bound route and enters the Arctic Ocean. This water is not included in Stommel's model, and when calculating the effect of climate change, it must be. The old model consists of one box for the southern part of the North Atlantic and one for the Nordic Seas.

By adding a third box, the Arctic Ocean, the ciculation in the Atlantic Ocean is stabilized. When you include the effect of more freshwater in the Arctic Ocean, the current will be less reduced than in Stommel's model with only two boxes. This makes Erwin Lambert think that increasing precipitation in the north may be less important for the circulation in the Atlantic than previously believed.

He admits that it's still an open question of how well such simple box models represent reality. For example, wind -- which the Stommel model does not consider -- is a vital driver of the Gulf Stream near the surface. But Lambert maintains that simple models still make it possible to study major processes in the ocean.

"It's amazing how much knowledge can be gained from a model that consists of merely five equations."

Deep sea coral in North Atlantic faces threat from climate change

Changes to winter weather conditions could threaten the long-term survival of coral in the region, upsetting fragile ecosystems that support an array of marine species, researchers say.

Corals allow diverse forms of marine life to thrive by building reef structures that provide protection from predators and safe spaces to reproduce.

The team focused on a species of cold-water coral -- known as Lophelia pertusa -- which grows in deep waters, creating elaborate reefs that are hotspots of biodiversity. These populations are maintained by tiny, fragile coral larvae that drift and swim on ocean currents, travelling hundreds of miles between reefs where they attach and begin to grow.

Researchers at the University of Edinburgh used computer models to simulate the migration of larvae across vast stretches of ocean. They did so to predict the effect weather changes could have on the long-term survival of Lophelia pertusa populations in the North Atlantic.

They found that a shift in average winter conditions in western Europe -- one of the predicted impacts of climate change -- could threaten coral populations. Ocean currents -- affected by changing wind patterns -- could drive larvae away from key sites in a new network of marine areas established to help safeguard coral populations, researchers say.

The team found Scotland's network of Marine Protected Areas -- or MPAs -- appears to be weakly connected, making it vulnerable to the effects of climate change. A coral population on Rosemary Bank seamount, an undersea mountain off Scotland's west coast, is key to maintaining the network.

Corals also thrive on oil and gas platforms in the North Sea and west of Shetland, which may help to bridge a gap in the MPA network between populations in the Atlantic and along the coast of Norway, the team says.

The study is published in the journal Royal Society Open Science. It was carried out in collaboration with Heriot-Watt University through a Daphne Jackson fellowship and as part of the ATLAS project, funded by the European Union's Horizon 2020 research and innovation programme.

Dr Alan Fox, of the University of Edinburgh's School of GeoSciences, who conducted the analysis, said: "We can't track larvae in the ocean, but what we know about their behaviour allows us to simulate their epic journeys, predicting which populations are connected and which are isolated. In less well connected coral networks, populations become isolated and cannot support each other, making survival and recovery from damage more difficult."

Professor Murray Roberts, of the University of Edinburgh's School of GeoSciences and co-ordinator of the ATLAS project, said: "Scotland's seabed plays a unique role as a stepping stone for deep-sea Atlantic species. By teaming up with researchers in Canada and the US, we will expand this work right across the Atlantic Ocean."

Hurricane risk to Northeast USA coast increasing, research warns

Hurricanes have gradually moved northwards from the western Caribbean towards northern North America over the past few hundred years, the study led by Durham University, UK, found.

The researchers suggest that this change in hurricane track was caused by the expansion of atmospheric circulation belts driven by increasing carbon dioxide emissions.

New York and other major cities along the Northeast coast of the USA could come under increased threat from these severe storms and need to be better prepared for their potential impact, the researchers said.

The findings are published in the journal Scientific Reports. Researchers reconstructed hurricane rainfall for the western Caribbean dating back 450 years by analyzing the chemical composition of a stalagmite collected from a cave in southern Belize, Central America.

They found that the average number of hurricanes at the Belize site decreased over time. When the hurricane history of Belize was compared with documentary hurricane records from places such as Bermuda and Florida, this information showed that Atlantic (Cape Verde) hurricanes were moving to the north rather than decreasing in total numbers.

Although natural warming over the centuries has had some impact on shifting hurricane tracks, the researchers found a marked decrease in hurricane activity in the western Caribbean coinciding with the late 19th Century industrial boom associated with increasing carbon dioxide and sulphate aerosol emissions to the atmosphere.

The researchers said that initial regional cooling of the Northern Hemisphere due to increased industrial aerosol emissions should have pushed the hurricane tracks southward since Industrialization.

But they added that rising amounts of atmospheric carbon dioxide had overridden this effect by expanding the Hadley cell -- a pattern of circulating air in Earth's tropical belt -- pushing hurricane tracks further north, away from the western Caribbean towards the Northeastern USA.

This suggests that from the late 19th Century, humanmade emissions have become the main driver behind shifting hurricane tracks by altering the position of global weather systems, the researchers said.

If future trends in carbon dioxide and industrial aerosol emissions continue as expected, hurricanes could shift even further northward, exacerbating the risk to the Northeast coast of the USA, they added.

In 2012, Hurricane Sandy struck the Caribbean and much of the eastern seaboard of the United States, stretching as far north as Canada. At least 233 people died as a result of the storm.

A large number of US states were affected by Hurricane Sandy with New York and New Jersey suffering the greatest impacts. The estimated cost of the damage caused by Hurricane Sandy in the USA is said to have run into tens of billions of dollars.

The study's lead author, Dr Lisa Baldini, in the Department of Geography, Durham University, said: "Our research shows that the hurricane risk to the Northeastern coast of the United States is increasing as hurricanes track further north.

"Since the 19th Century this shift was largely driven by humanmade emissions and if these emissions continue as expected this will result in more frequent and powerful storms affecting the financial and population centres of the Northeastern United States.

"Given the devastation caused by Hurricane Sandy it is important that plans are put in place to protect against the effects of similarly destructive storms which could potentially occur more often in the future."

Co-author Dr Amy Frappier, of the Geosciences Department, Skidmore College, USA, said the research showed Atlantic hurricanes were responding to warming. Dr Frappier said: "Aerosols from volcanoes and industrialisation in the Northern Hemisphere have a cooling effect, which tend to shift moisture belts and hurricane tracks southward, closer to the equator.

"On the other hand, warming from more carbon dioxide in the air tends to expand Earth's tropical belt, pushing hurricane tracks further north away from the western Caribbean and towards the Northeastern US.

"This suggests that the tracks of Atlantic hurricanes have responded more to warming than to regional cooling." The researchers added that the northward shift in hurricane tracks may not reduce the risk of tropical cyclones in the Caribbean.

Co-author Dr James Baldini, in Durham University's Department of Earth Sciences, said: "Although hurricane tracks have gradually moved northwards away from the western Caribbean, rising sea surface temperatures could promote the development of cyclonic storms within the western Caribbean.

"Consequently tropical cyclone activity across the western Caribbean may remain essentially stable over the current century, which has important implications for water availability in this region.

"However, increased sea surface temperatures also provide extra energy, potentially fueling larger storms. We therefore need to prepare for the effects of more frequent landfalls of larger storms along the Northeast coast of the United States and stronger storms impacting the Caribbean."

A warm climate is more sensitive to changes in carbon dioxide

Global mean temperature anomaly with respect to preindustrial reference level.

Credit: Image courtesy of University of Hawaii at Manoa

It is well-established in the scientific community that increases in atmospheric CO2 levels result in global warming, but the magnitude of the effect may vary depending on average global temperature. A new study, published this week in Science Advances and led by Tobias Friedrich from the International Pacific Research Center (IPRC) at the University of Hawai?i at Mānoa (UHM), concludes that warm climates are more sensitive to changes in CO2 levels than cold climates.

Increasing atmospheric CO2 concentrations cause an imbalance in Earth's heat budget: more heat is retained than expelled, which in turn generates global surface warming. Climate sensitivity is a term used to describe the amount of warming expected to result after an increase in the concentration of CO2. This number is traditionally calculated using complex computer models of the climate system, but despite decades of progress, the number is still subject to uncertainty.

The new study, which included scientists from the University of Washington, the University at Albany, and the Potsdam Institute for Climate Impact Research, took a different approach in calculating climate sensitivity: using data from the history of Earth. The researchers examined various reconstructions of past temperatures and CO2 levels to determine how the climate system has responded to previous changes in its energy balance.

"The first step was to reconstruct the history of global mean temperatures for the last 784,000 years, using combined data from marine sediment cores, ice cores, and computer simulations covering the last eight glacial cycles," said Friedrich, a post-doctoral researcher at IPRC.

The second step involved calculating Earth's energy balance for this time period, using estimates of greenhouse gas concentrations extracted from air bubbles in ice cores, and incorporating astronomical factors, known as Milankovitch Cycles, that effect the planetary heat budget.

"Our results imply that Earth's sensitivity to variations in atmospheric CO2 increases as the climate warms," explained Friedrich. "Currently, our planet is in a warm phase -- an interglacial period -- and the associated increased climate sensitivity needs to be taken into account for future projections of warming induced by human activities."

Using these estimates based on Earth's paleoclimate sensitivity, the authors computed the warming over the next 85 years that could result from a human-induced, business-as-usual greenhouse gas emission scenario. The researchers project that by the year 2100, global temperatures will rise 5.9°C (~10.5°F) above pre-industrial values. This magnitude of warming overlaps with the upper range of estimates presented by the Intergovernmental Panel on Climate Change (IPCC).

"Our study also allows us to put our 21st century temperatures into the context of Earth's history. Paleoclimate data can actually teach us a lot about our future," said Axel Timmermann, co-author of the study and professor at UHM.

The results of the study demonstrate that unabated human-induced greenhouse gas emissions are likely to push Earth's climate out of the envelope of temperature conditions that have prevailed for the last 784,000 years.

"The only way out is to reduce greenhouse gas emissions as soon as possible," concluded Friedrich.

Climate change already dramatically disrupting all elements of 

Baby seal (stock image). A new study has found a staggering 80 percent of 94 ecological processes that form the foundation for healthy marine, freshwater and terrestrial ecosystems already show signs of distress and response to climate change.

Credit: © seabreezecairns / Fotolia

Global changes in temperature due to human-induced climate change have already impacted every aspect of life on Earth from genes to entire ecosystems, with increasingly unpredictable consequences for humans -- according to a new study published in the journal Science.

The study found a staggering 80 percent of 94 ecological processes that form the foundation for healthy marine, freshwater and terrestrial ecosystems already show signs of distress and response to climate change.

Impacts to humans include increased pests and disease outbreaks, reduced productivity in fisheries, and decreasing agriculture yields.

"There is now clear evidence that, with only a ~1 degree C of warming globally, very major impacts are already being felt," said study lead author Dr Brett Scheffers of the University of Florida. "Genes are changing, species' physiology and physical features such as body size are changing, species are rapidly moving to keep track of suitable climate space, and there are now signs of entire ecosystems under stress."

Said the study's senior author, Dr. James Watson from the Wildlife Conservation Society and University of Queensland: "The level of change we have observed is quite astonishing considering we have only experienced a relatively small amount of climate change to date. It is no longer sensible to consider this a concern for the future. Policy makers and politicians must accept that if we don't curb greenhouse gas emissions, an environmental catastrophe is likely."

But the study also points to hope as many of the responses observed in nature could be applied by people to address the mounting issues faced under changing climate conditions. For example, improved understanding of the adaptive capacity in wildlife can be applied to our crops, livestock and fisheries. This can be seen in crops such as wheat and barley, where domesticated crops are crossed with wild varieties to maintain the evolutionary potential of varieties under climate change.

See how Arctic sea ice is losing its bulwark against warming summers

"What we've seen over the years is that the older ice is disappearing," said Walt Meier, a sea ice researcher at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This older, thicker ice is like the bulwark of sea ice: a warm summer will melt all the young, thin ice away but it can't completely get rid of the older ice. But this older ice is becoming weaker because there's less of it and the remaining old ice is more broken up and thinner, so that bulwark is not as good as it used to be."

Direct measurements of sea ice thickness are sporadic and incomplete across the Arctic, so scientists have developed estimates of sea ice age and tracked their evolution from 1984 to the present. Now, a new NASA visualization of the age of Arctic sea ice shows how sea ice has been growing and shrinking, spinning, melting in place and drifting out of the Arctic for the past three decades.

"Ice age is a good analog for ice thickness because basically, as ice gets older it gets thicker," Meier said. "This is due to the ice generally growing more in the winter than it melts in the summer."

In the early 2000s, scientists at the University of Colorado developed a way to monitor Arctic sea ice movement and the evolution of its age by using data from a variety of sources, but primarily satellite passive microwave instruments. These instruments gauge brightness temperature: a measure of the microwave energy emitted by sea ice that is influenced by the ice's temperature, salinity, surface texture and the layer of snow on top of the sea ice. Each floe of sea ice has a characteristic brightness temperature, so the researchers developed an approach that would identify and track ice floes in successive passive microwave images as they moved across the Arctic. The system also uses information from drifting buoys as well as weather data.

"It's like bookkeeping; we're keeping track of sea ice as it moves around, up until it melts in place or leaves the Arctic," said Meier, who is a collaborator of the group at the University of Colorado and the National Snow and Ice Data Center in Boulder, Colorado, the center that currently maintains the Arctic sea ice age data.

Ice in motion

Every year, sea ice forms in the winter and melts in the summer. The sea ice that survives the melt season thickens with each passing year: newly formed ice grows to about 3 to 7 feet of thickness during its first year, while multi-year ice (sea ice that has survived several melt seasons) is about 10 to 13 feet thick. The older and thicker ice is more resistant to melt and less likely to get pushed around by winds or broken up by waves or storms.

The motion of sea ice is not limited to its seasonal expansion and shrinkage: Except for coastal regions where sea ice is attached to the shore, the sea ice cap is in almost constant movement. The primary driver of sea ice movement in the Arctic is wind and there are two major features in the Arctic circulation: the Beaufort Gyre, a clockwise ice circulation that makes ice spin like a wheel in the Beaufort Sea, north of Alaska, and the Transpolar Drift Stream, which transports ice from Siberia's coast toward the Fram Strait east of Greenland, where the ice exits the Arctic basin and melts in the warmer waters of the Atlantic Ocean.

"On a week-to-week basis, there are weather systems that come through, so the ice isn't moving at a constant rate: sometimes the Beaufort Gyre reverses or breaks down for a couple weeks or so, the Transpolar Drift Stream shifts in its direction ... but the overall pattern is this one," Meier said. "Then the spring melt starts and the ice shrinks back, disappearing from the peripheral seas."

The new animation shows two main bursts of thick ice loss: the first one, starting in 1989 and lasting a few years, was due to a switch in the Arctic Oscillation, an atmospheric circulation pattern, which shrunk the Beaufort Gyre and enhanced the Transpolar Drift Stream, flushing more sea ice than usual out of the Arctic. The second peak in ice loss started in the mid-2000s.

"Unlike in the 1980s, it's not so much as ice being flushed out -though that's still going on too," Meier said. "What's happening now more is that the old ice is melting within the Arctic Ocean during the summertime. One of the reasons is that the multiyear ice used to be a pretty consolidated ice pack and now we're seeing relatively smaller chunks of old ice interspersed with younger ice. These isolated floes of thicker ice are much easier to melt."

"We've lost most of the older ice: In the 1980s, multiyear ice made up 20 percent of the sea ice cover. Now it's only about 3 percent," Meier said. "The older ice was like the insurance policy of the Arctic sea ice pack: as we lose it, the likelihood for a largely ice-free summer in the Arctic increases."

West Coast record low snowpack in 2015 influenced by high temperatures

The study suggests greenhouse gases were a major contributor to the high temperatures, which doesn't bode well for the future, according to authors of a new study published in the journal Geophysical Research Letters.

In 2015, more than 80 percent of the snow measurement sites in the region -- comprised of California, Oregon, Washington, western Nevada and western Idaho -- experienced record low snowpack levels that were a result of much warmer-than-average temperatures. Most of the previous records were set in 1977, when there just wasn't enough moisture to generate snow, according to Philip Mote, director of the Oregon Climate Change Research Institute at Oregon State University and lead author on the study.

"The 2015 snowpack season was an extreme year," Mote said. "But because of the increasing influence of greenhouse gases, years like this may become commonplace over the next few decades." Impacts of the snow drought in California, Oregon and Washington led the governors of those states to order reductions in water use and saw many ski areas, particularly those in lower elevations, struggle.

California has been in a drought since 2011 and this multi-year period of low precipitation, by some measures, is the state's most severe in 500 years. In 2015, higher temperatures combined with low precipitation, leading to one of its lowest snowpack levels on record.

Oregon and Washington experienced much higher-than-average temperatures during the 2014-15 winter but were not as dry overall as California. Oregon, in fact, was 6.5 degrees (Fahrenheit) warmer than average during that period.

"The story of 2015 was really the exceptional warmth," said Dennis Lettenmaier, distinguished professor of geography at University of California Los Angeles and co-author of the study. "Historically, droughts in the West have mostly been associated with dry winters, and only secondarily with warmth. But 2015 was different. The primary driver of the record low snowpacks was the warm winter, especially in California, but in Oregon and Washington as well."

The 2015 year was an eye-opener for the scope of the snow drought:

• A total of 454 sites in the western United States (or 81 percent of the total sites) recorded record-low snowpack levels that year;
• For 111 of the sites, the April 1 value was zero for the first time ever, essentially indicating that there was no snow left;
• The overall snowpack level on April 1 in California and Oregon was 90 percent below average.

To determine the impact of greenhouse gases, the researchers used tens of thousands of citizen computers, each running a regional climate simulation in a sort of crowd-sourced supercomputer. The researchers ran one set of simulations using actual sea surface temperatures and greenhouse gas emissions from December 2014 to September 2015.

Then they ran a series of simulations with lower greenhouse gas levels corresponding to the pre-industrial era, and teased out the impacts. A third set of simulations used modern greenhouse gases but removed the unusual pattern of sea surface temperatures in 2014-15.

"The data showed that both greenhouse gases and sea surface temperature anomalies contributed strongly to the risk of snow drought in Oregon and Washington," said Mote, a professor in OSU's College of Earth, Ocean, and Atmospheric Sciences. "The contribution of sea surface temperatures was about twice that of human influence for Oregon and Washington."

Higher sea surface temperatures led to a huge patch of warm water, dubbed "The Blob," that appeared in the northern Pacific Ocean more than two years ago. Scientists aren't sure why the blob formed, though many blame a ridge of high pressure that brought sunnier weather and less mixing of surface water with colder, deeper water.

"Some recent studies suggest that a high pressure ridge that caused warmer temperatures over land also created the blob, but our results suggest that the blob itself may also have contributed to the warm winter here," Mote said.

U.S. Winter outlook predicts warmer, drier South and cooler, wetter North

"This climate outlook provides the most likely outcome for the upcoming winter season, but it also provides the public with a good reminder that winter is just up ahead and it's a good time to prepare for typical winter hazards, such as extreme cold and snowstorms," said Mike Halpert, deputy director, NOAA's Climate Prediction Center. "Regardless of the outlook, there is always some chance for extreme winter weather, so prepare now for what might come later this winter."

Other factors that often play a role in the winter weather include the Arctic Oscillation, which influences the number of arctic air masses that penetrate into the South and create nor'easters on the East Coast, and the Madden-Julian Oscillation, which can affect the number of heavy rain events in the Pacific Northwest.

The 2016 U.S. Winter Outlook (December through February):

Precipitation

• Wetter than normal conditions are most likely in the northern Rockies, around the Great Lakes, in Hawaii and in western Alaska

• Drier than normal conditions are most likely across the entire southern U.S. and southern Alaska.

Temperature

• Warmer than normal conditions are most likely across the southern U.S., extending northward through the central Rockies, in Hawaii, in western and northern Alaska and in northern New England.

• Cooler conditions are most likely across the northern tier from Montana to western Michigan.
• The rest of the country falls into the "equal chance" category, meaning that there is not a strong enough climate signal in these areas to shift the odds, so they have an equal chance for above-, near-, or below-normal temperatures and/or precipitation.

Drought

• Drought will likely persist through the winter in many regions currently experiencing drought, including much of California and the Southwest

• Drought is expected to persist and spread in the southeastern U.S. and develop in the southern Plains.

• New England will see a mixed bag, with improvement in the western parts and persistence to the east.

• Drought improvement is anticipated in northern California, the northern Rockies, the northern Plains and parts of the Ohio Valley.

This seasonal outlook does not project where and when snowstorms may hit or provide total seasonal snowfall accumulations. Snow forecasts are dependent upon the strength and track of winter storms, which are generally not predictable more than a week in advance. However, La Nina winters tend to favor above average snowfall around the Great Lakes and in the northern Rockies and below average snowfall in the mid-Atlantic.

NOAA produces seasonal outlooks to help communities prepare for what's likely to come in the next few months and minimize weather's impacts on lives and livelihoods. Empowering people with actionable forecasts and winter weather tips is key to NOAA's effort to build a Weather-Ready Nation.

A video of NOAA's 2016 winter outlook is available here: https://www.climate.gov/news-features/videos/2016-2017-us-winter-outlook

Extreme cold winters fuelled by jet stream and climate change

The research, carried out by an international team of scientists including the University of Sheffield, has found that warming in the Arctic may be intensifying the effects of the jet stream's position, which in the winter can cause extreme cold weather, such as the winter of 2014/15 which saw record snowfall levels in New York.

Scientists previously had two schools of thought. One group believe that natural variability in the jet stream's position has caused the recent severe cold winter weather seen in places such as the Eastern United States and the UK. The other camp includes scientists who are finding possible connections between the warming of the Arctic -- such as melting sea ice, warming air temperatures, and rising sea surface temperatures -- and the emerging pattern of severe cold winter weather.

Now, Professor Edward Hanna and Dr Richard Hall from the University's Department of Geography, together with Professor. James E. Overland from the US Oceanographic and Atmospheric Administration (NOAA), have brought together a diverse group of researchers from both sides of the debate.

The researchers have found that the recent pattern of cold winters is primarily caused by natural changes to the jet stream's position; however, the warming of the Arctic appears to be exerting an influence on cold spells, but the location of these can vary from year to year.

Previous studies have shown that when the jet stream is wavy there are more episodes of severe cold weather plunging south from the Arctic into the mid-latitudes, which persist for weeks at a time. But when the jet stream is flowing strongly from west to east and not very wavy, we tend to see more normal winter weather in countries within the mid-latitudes.

"We've always had years with wavy and not so wavy jet stream winds, but in the last one to two decades the warming Arctic could well have been amplifying the effects of the wavy patterns," Professor Hanna said. He added: "This may have contributed to some recent extreme cold winter spells along the eastern seaboard of the United States, in eastern Asia, and at times over the UK (e.g. 2009/10 and 2010/11).

"Improving our ability to predict how climate change is affecting the jet stream will help to improve our long-term prediction of winter weather in some of the most highly populated regions of the world.

"This would be hugely beneficial for communities, businesses, and entire economies in the northern hemisphere. The public could better prepare for severe winter weather and have access to extra crucial information that could help make live-saving and cost-saving decisions."

The study, "Nonlinear response of mid-latitude weather to the changing Arctic" is published in the journal Nature Climate Change. The research was partly sponsored by the International Arctic Science Committee (IASC) and the Climate and Cryosphere (CliC) project of the World Climate Research Programme (WCRP).

It further cements the University's position at the forefront of climate change research and gives geography students at Sheffield access to the latest innovations in environmental science.

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Ocean rogue waves: A mystery unveiled?

Numerical simulations of prototypical rogue waves in the ocean. Top left: Normal sea state. Top right: Rogue hole. Bottom left: Rogue wave. Bottom right: Rogue wave group, also known as "three sisters". Ocean nonlinearities are only required to explain why rogue holes are even more rare than positive rogue waves. Whether rogue waves appear isolated or in a group depends on the spectral width of the sea state.

In a collaborative effort, the group of Günter Steinmeyer at the Max-Born-Institut in Berlin together with colleagues from the Leibniz-University in Hannover and the Technical University in Dortmund now report a new approach to shed more light on the rogue wave mystery. To this end, they suggest a new metric for the complexity of the wave motion, namely, the so-called phase space dimension. This metric measures the effective number of waves that interfere at one given location on the ocean surface. More importantly, they also propose a way to measure the dimension, and this measurement could readily be implemented on ships, possibly providing an early warning of rogue waves.

In fact, it seems that the capability of the ocean to form rogue waves is variable. The study suggests that the ocean surface movement is fairly simply structured throughout most of the time. Even in heavy storms, mostly conditions prevail that do not enable rogue wave formation. However, the complexity of the wave patterns may suddenly increase when crossing seas are generated, resulting in rogue-wave prone situations. Using the suggested dimensional analysis, it is exactly these rogue-wave prone situations that can be detected. Nevertheless, the individual rogue wave event remains unforeseeable. Moreover the study suggests that the ocean dynamics are ruled by linear yet still very complex dynamics.

The study therefore opens a new perspective for a better understanding of ocean rogue waves. Much research went into ocean nonlinearities, but it appears that the latter play a minor role for rogue wave formation. In contrast, winds have found very little attention in the rogue wave discussion so far. As winds are ultimately the drivers behind ocean wave formation in general, it therefore seems perfectly possible to identify rogue-wave prone situations from meteorological analysis, identifying situations that may give rise to crossing seas early on. The appearance of an individual rogue wave may remain a mystery, but at least, we may soon be able to predict the "rogueness" of ocean weather hours or days in advance.

Evaluating forecasting models for predicting rainfall from tropical cyclones

Rainfall accumulation (in millimeters) for Hurricane Irene (August 26–30, 2011). The red curve with circles represents the storm’s track.

According to the National Oceanic and Atmospheric Administration (NOAA), more than 50 percent of the deaths associated with hurricanes from 1970 to 2004 were caused by fresh water flooding. And from 1981 to 2011, hurricane damage accounted for almost half -- $417.9 billion -- of the total monetary damage from all weather and climate disasters during that same time period (adjusted for inflation to 2011 dollars).

With the goal of providing basic information to help improve preparedness and mitigation efforts, new University of Iowa-led research published online in September in the Journal of Hydrology examined how accurate current forecasting systems are in predicting rainfall from North Atlantic tropical cyclones that reach land in the United States.

Comparing five state-of-the-art weather prediction models, researchers found current models can forecast both where and how much rainfall a tropical cyclone will produce up to two days in advance. However, the forecast's accuracy decreased significantly when the prediction window increased to five days. The researchers' findings were based on 15 North Atlantic hurricanes that came within at least 500 kilometers of the U.S. coastline from 2007 to 2012.

Gabriele Villarini, UI associate professor of civil and environmental engineering and corresponding author on the paper, says researchers honed in on predicting the impacts of tropical cyclones because that information is generally more useful than typical forecasts that predict how many storms are expected in a season.

"The more specific the information we can provide is, the more useful it will be. This is why we have been moving toward predicting U.S. landfalling tropical cyclone activity and the associated rainfall," he says.

Villarini, also an associate research engineer at the UI's Iowa Flood Center, says while a 48-hour lead time is a good starting point in terms of warning, he will continue to conduct more research to improve these predictions.

"By improving our understanding of the processes that drive tropical cyclones and hurricanes, we will be better positioned to improve our ability to forecast these events and their impacts with longer and longer lead times," he says.

Gabriel Vecchi, head of the climate variations and predictability group at NOAA's Geophysical Fluid Dynamics Lab and another author on the paper, says decades of weather prediction data show that forecasts have improved -- and will improve -- as scientists learn more about hurricanes.

"We can't do anything about the past," he says. "The goal of this work is to provide better information in the future."

Vecchi, who has collaborated with Villarini on several research projects, says he values the expertise in flooding and hydrology that Villarini and the Iowa Flood Center bring to their partnership.

"This is one of these examples of interdisciplinary work that has been incredibly fruitful," he says.