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L'Heureux M.L.,5830 University Research Court | Lee S.,Pennsylvania State University | Lyon B.,International Research Institute for Climate and Society
Nature Climate Change | Year: 2013

The Pacific Walker circulation is a large overturning cell that spans the tropical Pacific Ocean, characterized by rising motion (lower sea-level pressure) over Indonesia and sinking motion (higher sea level-pressure) over the eastern Pacific. Fluctuations in the Walker circulation reflect changes in the location and strength of tropical heating, so related circulation anomalies have global impacts. On interannual timescales, the El Niño/Southern Oscillation accounts for much of the variability in the Walker circulation, but there is considerable interest in longer-term trends and their drivers, including anthropogenic climate change. Here, we examine sea-level pressure trends in ten different data sets drawn from reanalysis, reconstructions and in situ measurements for 1900-2011. We show that periods with fewer in situ measurements result in lower signal-to-noise ratios, making assessments of sea-level pressure trends largely unsuitable before about the 1950s. Multidecadal trends evaluated since 1950 reveal statistically significant, negative values over the Indonesian region, with weaker, positive trends over the eastern Pacific. The overall trend towards a stronger, La Niña-like Walker circulation is nearly concurrent with the observed increase in global average temperatures, thereby justifying closer scrutiny of how the Pacific climate system has changed in the historical record. © 2013 Macmillan Publishers Limited. All rights reserved.

Seager R.,Lamont Doherty Earth Observatory | Naik N.,Lamont Doherty Earth Observatory | Baethgen W.,International Research Institute for Climate and Society | Robertson A.,International Research Institute for Climate and Society | And 3 more authors.
Journal of Climate | Year: 2010

Observations, atmosphere models forced by historical SSTs, and idealized simulations are used to determine the causes and mechanisms of interannual to multidecadal precipitation anomalies over southeast South America (SESA) since 1901.About 40%of SESAprecipitation variability over this period can be accounted for by global SST forcing. Both the tropical Pacific and Atlantic Oceans share the driving of SESA precipitation, with the latter contributing the most on multidecadal time scales and explaining a wetting trend from the early midcentury until the end of the last century. Cold tropical Atlantic SST anomalies are shown to drive wet conditions in SESA. The dynamics that linkSESAprecipitation to tropicalAtlantic SSTanomalies are explored. Cold tropical Atlantic SST anomalies force equatorward-flowing upper-tropospheric flow to the southeast of the tropical heating anomaly, and the vorticity advection by this flow is balanced by vortex stretching and ascent, which drives the increased precipitation. The 1930s PampasDust Bowl drought occurred, via thismechanism, in response to warm tropical Atlantic SST anomalies. The atmospheric response to cold tropical Pacific SSTs also contributed. The tropical Atlantic SST anomalies linked to SESA precipitation are the tropical components of the Atlantic multidecadal oscillation. There is little evidence that the large trends over past decades are related to anthropogenic radiative forcing, althoughmodels project that thiswill cause amodestwetting of the climate of SESA. As such, and if the Atlantic multidecadal oscillation has shifted toward a warm phase, it should not be assumed that the long-termwetting trend in SESA will continue. Any reversal to a drier climatemore typical of earlier decades would have clear consequences for regional agriculture and water resources. © 2010 American Meteorological Society.

Delsole T.,George Mason University | Delsole T.,Center for Ocean Land Atmosphere Studies | Jia L.,Center for Ocean Land Atmosphere Studies | Tippett M.K.,International Research Institute for Climate and Society | Tippett M.K.,King Abdulaziz University
Geophysical Research Letters | Year: 2013

A multivariate regression model derived from climate model simulations is shown to produce skillful predictions of unforced, annual mean sea surface temperature variations on multiyear time scales in observations and climate model simulations. Patterns that can be predicted with skill are identified explicitly and shown to arise from a combination of persistence and coupled interactions in the Pacific Ocean. Adding the regression model predictions to an estimate of the response to anthropogenic and natural forcing yields a prediction with higher skill than either alone, demonstrating the contribution of initial condition information to skill on multiyear time scales. © 2013 American Geophysical Union. All Rights Reserved.

Pomposi C.,Columbia University | Kushnir Y.,Lamont Doherty Earth Observatory | Giannini A.,International Research Institute for Climate and Society
Climate Dynamics | Year: 2015

It is well known that the Sahel region of Africa is impacted by decadal scale variability in precipitation, driven by global sea surface temperatures. This work demonstrates that the National Center for Atmospheric Research’s Community Atmosphere Model, version 4 is capable of reproducing relationships between Sahelian precipitation variability and Indian and Atlantic Ocean sea surface temperature variations on such timescales. Further analysis then constructs a moisture budget breakdown using model output and shows that the change in precipitation minus evaporation in the region is dominated by column integrated moisture convergence due to the mean flow, with the convergence of mass in the atmospheric column mainly responsible. It is concluded that the oceanic forcing of atmospheric mass convergence and divergence to a first order explains the moisture balance patterns in the region. In particular, the anomalous circulation patterns, including net moisture divergence by the mean and transient flows combined with negative moisture advection, together explain the drying of the Sahel during the second half of the twentieth century. Diagnosis of moisture budget and circulation components within the main rainbelt and along the monsoon margins show that changes to the mass convergence are related to the magnitude of precipitation that falls in the region, while the advection of dry air is associated with the maximum latitudinal extent of precipitation. © 2014, Springer-Verlag Berlin Heidelberg.

Delsole T.,George Mason University | Delsole T.,Center for Ocean Land Atmosphere Studies | Yang X.,Geophysical Fluid Dynamics Laboratory | Tippett M.K.,International Research Institute for Climate and Society | Tippett M.K.,King Abdulaziz University
Quarterly Journal of the Royal Meteorological Society | Year: 2013

This article proposes a statistical test for whether a multi-model combination with unequal weights has significantly smaller errors than a combination with equal weights. A combination with equal weights includes the case of a no-skill model, in which all weights equal zero, and the multi-model mean, in which all weights equal 1/M, where M is the number of models. The test is applied to seasonal hindcasts of 2 m temperature and precipitation generated by five state-of-the-art coupled atmosphere-ocean models. The hypothesis of equal weights could not be rejected over 75% the globe for temperature and 90% of the land for precipitation, implying that strategies for unequal weighting of forecasts may be of value only over a relatively small fraction of the globe. The fact that the test does not require pre-specifying a specific strategy for weighting forecasts suggests that it should be useful for exploring a wide range of multi-model strategies. © 2012 Royal Meteorological Society.

Delsole T.,George Mason University | Delsole T.,Center for Ocean Land Atmosphere Studies | Tippett M.K.,International Research Institute for Climate and Society | Shukla J.,George Mason University | Shukla J.,Center for Ocean Land Atmosphere Studies
Journal of Climate | Year: 2011

The problem of separating variations due to natural and anthropogenic forcing from those due to unforced internal dynamics during the twentieth century is addressed using state-of-the-art climate simulations and observations. An unforced internal component that varies on multidecadal time scales is identified by a new statistical method that maximizes integral time scale. This component, called the internal multidecadal pattern (IMP), is stochastic and hence does not contribute to trends on long time scales; however, it can contribute significantly to short-term trends. Observational estimates indicate that the trend in the spatially averaged "well observed" sea surface temperature (SST) due to the forced component has an approximately constant value of 0.1 K decade-1, while the IMP can contribute about 60.08 K decade-1 for a 30-yr trend. The warming and cooling of the IMP matches that of the Atlantic multidecadal oscillation and is of sufficient amplitude to explain the acceleration in warming during 1977-2008 as compared to 1946-77, despite the forced component increasing at the same rate during these two periods. The amplitude and time scale of the IMP are such that its contribution to the trend dominates that of the forced component on time scales shorter than 16 yr, implying that the lack of warming trend during the past 10 yr is not statistically significant. Furthermore, since the IMP varies naturally on multidecadal time scales, it is potentially predictable on decadal time scales, providing a scientific rationale for decadal predictions. While the IMP can contribute significantly to trends for periods of 30 yr or shorter, it cannot account for the 0.8°C warming that has been observed in the twentieth-century spatially averaged SST. © 2011 American Meteorological Society.

Seth A.,University of Connecticut | Rauscher S.A.,Los Alamos National Laboratory | Rojas M.,University of Chile | Giannini A.,International Research Institute for Climate and Society | Camargo S.J.,Lamont Doherty Earth Observatory
Climatic Change | Year: 2011

Twenty-first century climate model projections show an amplification of the annual cycle in tropical precipitation with increased strength in both wet and dry seasons, but uncertainty is large and few studies have examined transition seasons. Here we analyze coupled climate model projections of global land monsoons and show a redistribution of precipitation from spring to summer in northern (North America, West Africa and Southeast Asia) and southern (South America, Southern Africa) regions. The annual cycle changes are global in scale. Two mechanisms, remote (based on tropospheric stability) and local (based on low level and surface moisture), are evaluated through the annual cycle. Increases in tropospheric stability persist from winter into spring and are reinforced by a reduction in surface moisture conditions, suggesting that in spring both remote and local mechanisms act to inhibit convection. This enhanced spring convective barrier leads to reduced early season rainfall; however, once sufficient increases in moisture (by transport) are achieved, decreases in tropospheric stability result in increased precipitation during the late rainy season. Further examination of this mechanism is needed in observations and models, as the projected changes would have substantial implications for agriculture, water management, and disaster preparedness. © 2010 Springer Science+Business Media B.V.

Tippett M.K.,International Research Institute for Climate and Society | Tippett M.K.,King Abdulaziz University | Barnston A.G.,International Research Institute for Climate and Society | Li S.,International Research Institute for Climate and Society
Journal of Applied Meteorology and Climatology | Year: 2012

The performance of the International Research Institute for Climate and Society "ENSO forecast plume" during the 2002-11 period is evaluated using deterministic and probabilistic verification measures. The plume includes multiple model forecasts of the Niño-3.4 index for nine overlapping 3-month periods beginning the month following the latest observations. Skills decrease with increasing lead time and are highest for forecasts made after the northern spring predictability barrier for target seasons occurring prior to the forthcoming such barrier. Forecasts are found to verify systematically better against observations occurring earlier than the intended forecast targets, an effect that becomes greater with increasing lead time. During the study period, the mean forecasts of dynamical models appear to slightly (and statistically insignificantly) outperform those of statistical models, representing a subtle shift from earlier studies. The mean forecasts of dynamical models have overall larger anomalies but similar errors to those of statistical models. Intermodel spread is related to forecast error in an average sense with changes in forecast error due to changes in lead and verification season being properly reflected in changes in spread. The intermodel spread underestimates the forecast error variance, to a greater extent for statistical forecasts than for dynamical ones. Year-to-year changes in plume spread provide little additional information relative to climatological ones. © 2012 American Meteorological Society.

Singh A.,Indian Institute of Technology Delhi | Acharya N.,Indian Institute of Technology Delhi | Mohanty U.C.,Indian Institute of Technology Delhi | Robertson A.W.,International Research Institute for Climate and Society | Mishra G.,Utkal University
Dynamics of Atmospheres and Oceans | Year: 2012

The objective of this present study is to analyze the predictability of all India summer monsoon rainfall (AISMR) and its dependence on lead time using general circulation model (GCM) output. For the purpose, six GCMs for the hindcast run from 1982 to 2008 are used at three different initializations viz. April (lead 2), May (lead 1), and June (lead 0) for seasonal mean rainfall of June-July-August-September (JJAS). Among these models, four of them are the coupled ocean-atmosphere GCMs (CGCMs) and the remaining two are the atmospheric GCMs (AGCMs). The analysis is made on the basis of statistical measures of predictability including climatology, interannual variability, root mean square error, correlation, signal to noise ratio, potential model predictability and index of agreement. On the basis of these measures it is found that all the GCM having the minimum prediction skill is at lead 2 compare to lead 1 and lead 0. It is also noticed that higher predictability in the lead-1 forecasts is found in coupled models whereas, the predictability of atmospheric models exhibit high in lead 0. Rather than rainfall, teleconnection of rainfall with large scale features (such as sea surface temperature, zonal wind at 850. hPa) and monsoon dynamic index (Indian monsoon index (IMI)) are also examined in GCMs. The results depicted that there is not much variation in the teleconnection pattern in two leads (lead 0 and lead 1) whereas; the dynamic index being predicted closer to the observed value at lead 1 in the CGCMs. The GCMs are also examined during four typical monsoon (excess/deficit) years, among which 1983 and 1988 are excess and 1987 and 2002 are deficit. Results indicate that the coupled (atmospheric) models capture the extreme rainfall signal in lead 1 (lead 0). The probabilistic prediction skill of GCM predicted rainfall is also evaluated which supports our initial analysis and results. © 2012 Elsevier B.V.

News Article | October 23, 2015
Site: www.washingtonpost.com

UPDATE: Emissions from Indonesian fires have continued to rise since this article was published and now are estimated to be roughly equal to Japan’s annual carbon dioxide emissions. See here. See also our coverage of Indonesian President Joko Widodo’s visit to the U.S. as his country’s climate crisis rages. Experts say that along with dramatic global coral bleaching, thousands of fires across Indonesia represents the next sign of an intensifying global El Niño event. And the consequences, in this case, could affect the entire globe’s atmosphere. That’s because a large number of Indonesia’s currently raging fires are consuming ancient stores of carbon-rich peat, which is found in wetlands featuring organic layers full of dead and partially decomposed plant life. This year, the very smoky peat burning has been simply massive — the fires are estimated to have caused $ 14 billion in damage so far, and are causing hazardous air conditions in much of the area, including nearby Singapore. Millions of people have been affected, and 120,000 have sought medical treatment for respiratory illnesses, according to Weather Underground’s Jeff Masters. Indeed, the 2015 Indonesian fire season has so far featured a stunning 94,192 fires. That’s more Indonesian fires than at the same time in 2006, a banner year both for fires and also for their carbon emissions to the atmosphere. Those emissions are more than large enough to have global consequences. Indeed, according to recent calculations by Guido van der Werf, a researcher at VU University Amsterdam in the Netherlands who keeps a database that tracks the global emissions from wildfires, this year’s Indonesian fires had given off an estimated 995 million metric tons of carbon dioxide equivalent emissions as of Oct. 14. That’s just shy of a billion metric tons, or a gigaton. The number is an estimate, of course, and subject to “substantial uncertainties” — but it’s also based on a well-developed methodology for estimating wildfire emissions to the atmosphere based upon satellite images of the fires themselves and the vegetation they consume. “Fire emissions are already higher than Germany’s total CO2 emissions, and the fire season is not over yet,” says van der Werf. He provided this figure, which allows you to compare how much carbon dioxide and other emissions Indonesian fires have put into the atmosphere each year since 1997 with the annual fossil fuel emissions profiles of various countries (including Indonesia itself): As you can see, 2015 is already a very notable year — and it could get considerably worse. Van der Werf says he thinks that for total Indonesian fire emissions, 2015 may ultimately nestle somewhere between 2006 and the truly catastrophic year of 1997. During that year, according to scientists who studied its aftermath, fire emissions from Indonesia alone were “equivalent to 13–40% of the mean annual global carbon emissions from fossil fuels.” And while the emissions from wildfires in many parts of the world are at least partially offset as trees and vegetation subsequently grow back and pull carbon back out of the air again, that’s not so much the case in Indonesia. “In Indonesia, what’s burning is for the large part, peat layers that have been deposited over thousands of years. So this is really a net source of emissions just like fossil fuels are,” says van der Werf. Thus, peat emissions are, in a sense, similar to Arctic permafrost emissions — built up over vast stretches of time, the carbon contained in thawing permafrost is also a new addition to the atmosphere if emitted. What the severe Indonesian fire years of 1997, 2006, and 2015 all have in common is that they were El Niño years. El Niño is a critical factor in exacerbating Indonesian fires, because it tends to deprive the islands of needed rains and drive drought conditions. Indeed, the Indonesian fires are “one of the first severe impacts of the strong El Niño that has been developing over the last year,” according to Columbia University’s International Research Institute for Climate and Society. And that also means the trouble probably isn’t over yet. A current forecast suggests “a strong likelihood of drier-than-normal conditions over broad areas of northern South America, the Caribbean, Indonesia and the Philippines” through December. But it’s really the combination of El Niño and certain agricultural practices — characterized as “slash and burn” by NASA — that is at play here. According to Susan Minnemeyer, who is the mapping and data manager for Global Forest Watch Fires, a project of the World Resources Institute, the blazes are the result of using fire itself to clear land for agriculture, as well as the draining of peat bogs and swamps – which makes them able to light up once they are dried out. “The forests in Indonesia are generally not flammable, so these fires are virtually all caused by people, or land clearing,” says Minnemeyer. She adds that there is “little enforcement and little capacity to actually put them out once they’ve started.” The total Indonesian fire emissions, says van der Werf, will show up in atmospheric measurements of carbon dioxide this year — and may even draw attention at the climate meetings that begin next month in Paris. After all, according to the U.N.’s Intergovernmental Panel on Climate Change, as of 2011 the world only had about 1,000 more gigatons of carbon dioxide to emit to the atmosphere if we want a two-third chance of keeping warming below 2 degrees Celsius from pre-industrial levels.

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