Williams R.G.,University of Liverpool |
Roussenov V.,University of Liverpool |
Smith D.,Hadley Center |
Lozier M.S.,Duke University
Journal of Climate | Year: 2014
Basin-scale thermal anomalies in the North Atlantic, extending to depths of 1-2km, are more pronounced than the background warming over the last 60 years.Adynamical analysis based on reanalyses of historical data from 1965 to 2000 suggests that these thermal anomalies are formed by ocean heat convergences, augmented by the poorly known air-sea fluxes. The heat convergence is separated into contributions from the horizontal circulation and the meridional overturning circulation (MOC), the latter further separated into Ekman and MOC transport minus Ekman transport (MOC-Ekman) cells. The subtropical thermal anomalies are mainly controlled by wind-induced changes in the Ekman heat convergence, while the subpolar thermal anomalies are controlled by the MOC-Ekman heat convergence; the horizontal heat convergence is generally weaker, only becoming significant within the subpolar gyre. These thermal anomalies often have an opposing sign between the subtropical and subpolar gyres, associatedwith opposing changes in themeridional volume transport driving the Ekman and MOC-Ekman heat convergences. These changes in gyre-scale convergences in heat transport are probably induced by the winds, as they correlate with the zonal wind stress at gyre boundaries. © 2014 American Meteorological Society.
Finnigan J.,CSIRO |
Harman I.,CSIRO |
Ross A.,University of Leeds |
Belcher S.,Hadley Center
Quarterly Journal of the Royal Meteorological Society | Year: 2015
Simple first-order closure remains an attractive way of formulating equations for complex canopy flows when the aim is to find analytic or simple numerical solutions to illustrate fundamental physical processes. Nevertheless, the limitations of such closures must be understood if the resulting models are to illuminate rather than mislead. We propose five conditions that first-order closures must satisfy, then test two widely used closures against them. The first is the eddy diffusivity based on a mixing length. We discuss the origins of this approach, its use in simple canopy flows and extensions to more complex flows. We find that it satisfies most of the conditions and, because the reasons for its failures are well understood, it is a reliable methodology. The second is the velocity-squared closure that relates shear stress to the square of mean velocity. Again we discuss the origins of this closure and show that it is based on incorrect physical principles and fails to satisfy any of the five conditions in complex canopy flows; consequently its use can lead to actively misleading conclusions. © 2015 Royal Meteorological Society.
Agency: GTR | Branch: NERC | Program: | Phase: Research Grant | Award Amount: 364.61K | Year: 2011
The climate system is widely accepted as warming. Much of the extra heat provided to the climate system is estimated to have been taken up by the oceans. However, this warming of the oceans is not happening uniformly. For the North Atlantic, the most well observed basin, there has been warming in the tropics and mid latitudes, but cooling at high latitudes over the last 50 years. These changes in heat content are associated with changes in atmospheric forcing from winds and surface heat fluxes. As well as the oceans changing their temperature, there are salinity changes with a general freshening at high latitudes and increase in salinity at low latitudes, perhaps associated with a strengthening in the atmospheric water cycle. The strong gyre-scale contrast in these ocean properties suggest that the wind forcing and gyre dynamics are playing an important role. The small residual density changes over the basin are reflected in changes in the dynamical signals for overturning and sea level. The ocean overturning response to these water-mass changes appears to be surprising: based on our historical analyses, there is a slightly weakening over the subtropical gyre and slightly strengthening over the subpolar gyre during the last 50 years. These overturning changes might reflect the effect of the wind forcing, where gyre-scale property changes feedback onto changes in the overturning. The effect of the winds also directly affects sea level and the interpretation of the tide gauge record: there are large-scale correlations between the interannual variations in air pressure over the central part of the ocean basins and eastern boundary sea level. In our study, we plan to test hypotheses as to how the climate variability in the North Atlantic is controlled. Our aim is (i) to extend our analyses and assimilation of the historical data; (ii) conduct model experiments designed to reveal the effect of changing winds on the gyre contrasts in temperature and salinity, on how heat content and overturning are related, and on the relationship with sea level; and (iii) assess how tide gauge records for sea level are affected by gyre dynamics and overturning in order to interpret changes in the long historical records of sea level rise in the North Atlantic.
Mearns L.O.,U.S. National Center for Atmospheric Research |
Arritt R.,Iowa State University |
Biner S.,Ouranos |
Bukovsky M.S.,U.S. National Center for Atmospheric Research |
And 16 more authors.
Bulletin of the American Meteorological Society | Year: 2012
North American Regional Climate Change Assessment Program (NARCCAP) evaluated temperature and precipitation results from six regional climate models driven by NCEP-DOE Reanalysis II boundary conditions for 1980-2004 to investigate uncertainties in regional-scale climate projections. The program included six RCMs use boundary conditions from the NCEP-DOE Reanalysis II (R2) for a 25-yr period. It also included boundary conditions provided by four AOGCMs for 30 years of current climate (1971-2000) and 30 years of a future climate (2041-70) for the Special Report on Emissions Scenarios (SRES) A2 emissions scenario. The results are found to be within the range of those found in other multiple model comparisons, seasonal temperature is relatively well produced by most models but seasonal precipitation is less so. With regard to seasonal average temperature biases over the whole domain RSM and MM5 had the lowest total RMSEs, whereas HadRM3 had the greatest overall temperature bias.
PARIS — With only three days left, tensions here are rising as countries race to resolve outstanding differences and forge an agreement that — hopefully — will set the planet on a path to avoiding the worst consequences of climate change. The goal is an agreement that would set the world on a path to limit warming to below 2 degrees Celsius, or perhaps even 1.5 degrees Celsius, above pre-industrial levels. But at a news conference here at the Le Bourget conference center Wednesday morning, scientists pointed out a factor that could make hitting these targets quite a lot harder. As the planet warms, this frozen northern soil is going to continue to thaw — and as it thaws, it’s going to release carbon dioxide and methane into the air. A lot of it, it turns out. Potentially enough to really throw off the carbon budgets that have been calculated in order to determine the maximum emissions that we can release and still have a good chance of keeping warming to 2 C or below it. [U.S. pledges more aid to poorer countries as Paris climate talks enter delicate stage] In particular, Susan Natali of the Woods Hole Research Center explained Wednesday that with a very high level of warming, permafrost emissions this century could be quite large indeed. Natali used numbers from the 2013 report of the United Nations’ Intergovernmental Panel on Climate Change, which found that humans can only emit about 275 more gigatons, or billion tons, of carbon (about 1,000 gigatons of carbon dioxide, which has a greater molecular weight) to have a greater than 66 percent chance of limiting warming to 2 degrees C. But out of that limited budget, she said, permafrost emissions could take up some 150 of those gigatons (or about 550 gigatons of carbon dioxide). “That’s on par with current U.S. rates of emission,” Natali said, which are about 1.4 gigatons of carbon per year. “So we’re talking about another emitting region that’s currently not included in our emissions scenarios.” Fortunately, even though they’re not considered to be strong enough, the current national pledges to limit global warming appear to have taken the world off a truly high emissions path. These pledges, or “intended nationally determined contributions,” could potentially limit warming to 2.7 degrees Celsius, according to the United Nations. But in an interview, Natali and her Woods Hole colleague and fellow permafrost expert Max Holmes explained that even for lower warming scenarios like this, permafrost could emit 50 gigatons of carbon (or about 180 gigatons of carbon dioxide) in this century. This is because under lower warming scenarios, only about 30 percent, rather than about 70 percent, of surface layer permafrost is expected to thaw. Another 50 gigatons out of a 275 gigaton carbon budget — or, another 180 gigatons out of a 1,000 gigaton carbon dioxide budget — would significantly constrain how much the world could emit and still have a strong chance of keeping warming below 2 degrees Celsius. Another prominent research institute, the U.K. Met Office’s Hadley Center, also recently released an assessment of how potential permafrost emissions could complicate attempts to limit global warming, and came up with numbers that are, if anything, potentially even worse. As the center put it: The feedbacks from wetlands and permafrost regions can be combined with other known processes to determine their greenhouse gas input into the atmosphere. For a global average temperature rise of 2 °C, this reduces the cumulative emissions that can be released by human actions by around 100GtC (360 GtCO2 ) in the most pessimistic simulation. This corresponds to about 10 years of anthropogenic emissions at the current rate. These numbers can’t be directly compared with the Woods Hole numbers, however, due to the inclusion of wetlands above. And Natali and Holmes also noted that permafrost emissions don’t end at 2100 — they are expected to continue after that and even get worse. “Most of the release will happen after 2100,” said Natali. That’s a big problem because the global carbon budget is fixed, and after it is exceeded there can be zero further emissions. Because carbon dioxide lasts so long in the atmosphere, you don’t get to start with a fresh budget in the next century. So permafrost emissions beyond 2100 would also have to be taken into account, and would restrict the budget even further. Permafrost is a potential carbon bomb because over thousands of years, dead plant life has been slowly swallowed up by the soil but has not decomposed. Plants pull carbon out of the atmosphere as they grow, but release it again when they die and decompose. As permafrost warms and thaws, microbes will have more ability to break down the plant life it contains, which is what will trigger a steady stream of emissions. “It’s just like you put celery in your freezer and then you turn your freezer into a refrigerator, and it starts to rot,” says Woods Hole’s Max Holmes. Many people are confused about permafrost, and think when they first hear about it that it is going to release methane, not carbon dioxide, in gigantic explosions. Actually, that’s confusing frozen subsea methane hydrates — which may or may not be destabilized by global warming, but in any case are a separate issue — with permafrost on land. The latter will lose carbon slowly, as thaw enables microbial processes that lead to decomposition. This will release both carbon dioxide and also some methane. There won’t be any explosion, says Natali — but as the numbers above show, it could still be dramatically significant to the total global carbon picture. The news about permafrost has been building in recent years, but it is still a relatively new area of scientific inquiry and one where there is much uncertainty. Thus, even as negotiators in Paris appear to be amping up their ambition and are even talking more about trying to limit global warming to 1.5 degrees C, there may be another wild card they have to contend with. On Tuesday, Secretary of State John F. Kerry called the Paris climate meeting the “demarcation point where we begin to get the job done to save the planet.” Alas, scientists are learning that the planet itself may not cooperate. The one thing that really doesn’t make sense about the climate debate in Paris For more, you can sign up for our weekly newsletter here, and follow us on Twitter here.