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Mathew Barlow, Heidi Cullen, and Bradfield Lyon

Abstract

Severe drought over the past three years (1998–2001), in combination with the effects of protracted sociopolitical disruption, has led to widespread famine affecting over 60 million people in central and southwest (CSW) Asia. Here both a regional and a large-scale mode of climate variability are documented that, together, suggest a possible forcing mechanism for the drought. During the boreal cold season, an inverse relationship exists between precipitation anomalies in the eastern Indian Ocean and CSW Asia. Suppression of precipitation over CSW Asia is consistent with interaction between local synoptic storms and wave energy generated by enhanced tropical rainfall in the eastern Indian Ocean. This regional out-of-phase precipitation relationship is related to large-scale climate variability through a subset of El Niño–Southern Oscillation (ENSO) events characterized by an enhanced signal in the warm pool region of the western Pacific Ocean. Both the prolonged duration of the 1998–2001 cold phase ENSO (La Niña) event and unusually warm ocean waters in the western Pacific appear to contribute to the severity of the drought.

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Mathew Barlow, Matthew Wheeler, Bradfield Lyon, and Heidi Cullen

Abstract

Analysis of daily observations shows that wintertime (November–April) precipitation over Southwest Asia is modulated by Madden–Julian oscillation (MJO) activity in the eastern Indian Ocean, with strength comparable to the interannual variability. Daily outgoing longwave radiation (OLR) for 1979–2001 is used to provide a long and consistent, but indirect, estimate of precipitation, and daily records from 13 stations in Afghanistan reporting at least 50% of the time for 1979–85 are used to provide direct, but shorter and irregularly reported, precipitation data. In the station data, for the average of all available stations, there is a 23% increase in daily precipitation relative to the mean when the phase of the MJO is negative (suppressed tropical convection in the eastern Indian Ocean), and a corresponding decrease when the MJO is positive. The distribution of extremes is also affected such that the 10 wettest days all occur during the negative MJO phase. The longer record of OLR data indicates that the effect of the MJO is quite consistent from year to year, with the anomalies averaged over Southwest Asia more negative (indicating more rain) for the negative phase of the MJO for each of the 22 yr in the record. Additionally, in 9 of the 22 yr the average influence of the MJO is larger than the interannual variability (e.g., the relationship results in anomalously wet periods even in dry years and vice versa).

Examination of NCEP–NCAR reanalysis data shows that the MJO modifies both the local jet structure and, through changes to the thermodynamic balance, the vertical motion field over Southwest Asia, consistent with the observed modulation of the associated synoptic precipitation. A simple persistence scheme for forecasting the sign of the MJO suggests that the modulation of Southwest Asia precipitation may be predictable for 3-week periods. Finally, analysis of changes in storm evolution in Southwest Asia due to the influence of the MJO shows a large difference in strength as the storms move over Afghanistan, with apparent relevance for the flooding event of 12–13 April 2002.

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Richard Seager, Yochanan Kushnir, Martin Visbeck, Naomi Naik, Jennifer Miller, Gerd Krahmann, and Heidi Cullen

Abstract

Numerical experiments are performed to examine the causes of variability of Atlantic Ocean SST during the period covered by the National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalysis (1958–98). Three ocean models are used. Two are mixed layer models: one with a 75-m-deep mixed layer and the other with a variable depth mixed layer. For both mixed layer models the ocean heat transports are assumed to remain at their diagnosed climatological values. The third model is a full dynamical ocean general circulation model (GCM). All models are coupled to a model of the subcloud atmospheric mixed layer (AML). The AML model computes the air temperature and humidity by balancing surface fluxes, radiative cooling, entrainment at cloud base, advection and eddy heat, and moisture transports. The models are forced with NCEP–NCAR monthly mean winds from 1958 to 1998.

The ocean mixed layer models adequately reproduce the dominant pattern of Atlantic Ocean climate variability in both its spatial pattern and time dependence. This pattern is the familiar tripole of alternating zonal bands of SST anomalies stretching between the subpolar gyre and the subtropics. This SST pattern goes along with a wind pattern that corresponds to the North Atlantic Oscillation (NAO). Analysis of the results reveals that changes in wind speed create the subtropical SST anomalies while at higher latitudes changes in advection of temperature and humidity and changes in atmospheric eddy fluxes are important.

An observational analysis of the boundary layer energy balance is also performed. Anomalous atmospheric eddy heat fluxes are very closely tied to the SST anomalies. Anomalous horizontal eddy fluxes damp the SST anomalies while anomalous vertical eddy fluxes tend to cool the entire midlatitude North Atlantic during the NAO’s high-index phase with the maximum cooling exactly where the SST gradient is strengthened the most.

The SSTs simulated by the ocean mixed layer model are compared with those simulated by the dynamic ocean GCM. In the far North Atlantic Ocean anomalous ocean heat transports are equally important as surface fluxes in generating SST anomalies and they act constructively. The anomalous heat transports are associated with anomalous Ekman drifts and are consequently in phase with the changing surface fluxes. Elsewhere changes in surface fluxes dominate over changes in ocean heat transport. These results suggest that almost all of the variability of the North Atlantic SST in the last four decades can be explained as a response to changes in surface fluxes caused by changes in the atmospheric circulation. Changes in the mean atmospheric circulation force the SST while atmospheric eddy fluxes dampen the SST. Both the interannual variability and the longer timescale changes can be explained in this way. While the authors were unable to find evidence for changes in ocean heat transport systematically leading or lagging development of SST anomalies, this leaves open the problem of explaining the causes of the low-frequency variability. Possible causes are discussed with reference to the modeling results.

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Jared Rennie, Jesse E. Bell, Kenneth E. Kunkel, Stephanie Herring, Heidi Cullen, and Azar M. Abadi

Abstract

Land surface air temperature products have been essential for monitoring the evolution of the climate system. Before a temperature dataset is included in such analyses, it is important that nonclimatic influences be removed or changed so that the dataset is considered to be homogenous. These inhomogeneities include changes in station location, instrumentation, and observing practices. Many homogenized products exist on the monthly time scale, but few daily and weekly products exist. Recently, a submonthly homogenized dataset has been developed using data and software from NOAA’s National Centers for Environmental Information. Homogeneous daily data are useful for identification and attribution of extreme heat events. Projections of increasing temperatures are expected to result in corresponding increases in the frequency, duration, and intensity of such events. It is also established that heat events can have significant public health impacts, including increases in mortality and morbidity. The method to identify extreme heat events using daily homogeneous temperature data is described and used to develop a climatology of heat event onset, length, and severity. This climatology encompasses nearly 3000 extreme maximum and minimum temperature events across the United States since 1901. A sizeable number of events occurred during the Dust Bowl period of the 1930s; however, trend analysis shows an increase in heat event number and length since 1951. Overnight extreme minimum temperature events are increasing more than daytime maximum temperatures, and regional analysis shows that events are becoming much more prevalent in the western and southeastern parts of the United States.

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Ashley A. Anderson, Teresa A. Myers, Edward W. Maibach, Heidi Cullen, Jim Gandy, Joe Witte, Neil Stenhouse, and Anthony Leiserowitz

Abstract

Local television (TV) weathercasters are a potentially promising source of climate education, in that weather is the primary reason viewers watch local TV news, large segments of the public trust TV weathercasters as a source of information about global warming, and extreme weather events are increasingly common (Leiserowitz et al.; U.S. Global Change Research Program). In an online experiment conducted in two South Carolina cities (Greenville, n = 394; Columbia, n = 352) during and immediately after a summer heat wave, the effects on global warming risk perceptions were examined following exposure to a TV weathercast in which a weathercaster explained the heat wave as a local manifestation of global warming versus exposure to a 72-h forecast of extreme heat. No main effect of the global warming video on learning was found. However, a significant interaction effect was found: subjects who evaluated the TV weathercaster more positively were positively influenced by the global warming video, and viewers who evaluated the weathercaster less positively were negatively influenced by the video. This effect was strongest among politically conservative viewers. These results suggest that weathercaster-delivered climate change education can have positive, albeit nuanced, effects on TV-viewing audiences.

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David Perkins, Ed Maibach, Ned Gardiner, Joe Witte, Bud Ward, Bernadette Woods Placky, Keith Seitter, and Heidi Cullen

Abstract

As American Meteorological Society (AMS) members who study Americans’ understanding of climate change and who are engaged in programs to educate Americans about climate change, we want our AMS colleagues to realize their key role in public education. In this article we make the case that 1) AMS members are well positioned to play important leadership roles in educational outreach on climate change, 2) the public wants to learn more about climate change, 3) there is a need for more effective public engagement efforts, and 4) we have successful outreach and educational models that we can start using today.

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Xiaoquan Zhao, Edward Maibach, Jim Gandy, Joe Witte, Heidi Cullen, Barry A. Klinger, Katherine E. Rowan, James Witte, and Andrew Pyle

TV weathercasters are well positioned to educate Americans about the relationships among weather, climate, and climate change. Through a collaboration involving TV meteorologists, climatologists, and social scientists, we produced a series of educational segments to assess the impact of such an education. The educational segments were branded “Climate Matters” and aired over one year during the nightly weather segment on WLTX TV (Columbia, South Carolina). Prior to airing, we conducted a telephone survey of adult TV news viewers in the Columbia media market using random digit dialing (n = 1,068) to establish baseline measures; respondent screening was used to sample approximately equal numbers of WLTX viewers and viewers of competing stations. Approximately one year later, we resurveyed all available members of the baseline cohort (n = 502) and an independent sample of randomly selected residents (n = 910). The longitudinal data showed that—after controlling for baseline measures, demographics, and political orientation—viewers of Climate Matters were more likely to hold a range of science-based beliefs about climate change. A similar pattern of associations was observed in the independent sample. In short, Climate Matters improved the understanding of climate change among local TV viewers in a manner consistent with the educational content. The results of this field experiment suggest that when TV weathercasters educate their viewers about climate change, viewers gain a more science-based understanding of the issue.

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Bernadette Woods Placky, Edward Maibach, Joe Witte, Bud Ward, Keith Seitter, Ned Gardiner, David Herring, and Heidi Cullen

Abstract

Local TV meteorologists are optimally positioned to educate the public about the local implications of global climate change: They have high public trust as a source of climate science information, local TV is the #1 source of weather information in America, and most weathercasters have relevant scientific training and excellent communication skills. Surveys show that most TV meteorologists would like to report on climate change, but lack of time, lack of broadcast-quality graphics, and lack of access to appropriate experts are barriers that inhibit such coverage.

With funding from the National Science Foundation and philanthropic foundations, we developed Climate Matters as an educational resources program to help interested local TV meteorologists educate their viewers about the local impacts of global climate change. Currently, the program provides more than 160 participating weathercasters nationwide with weekly localized broadcast-ready graphics and script ideas, short videos, and opportunities for brief (hour-long webinars) and more intensive (day-long seminars) professional development sessions—at no cost to participating weathercasters. We aim to more than double participation in the program over the next several years.

This article will chronicle the development of Climate Matters over the past five years—beginning with a pilot test at a single news station in Columbia, South Carolina, that was shown to be effective at helping viewers better understand climate change and culminating in a comprehensive national educational resource program that is available to all interested weathercasters.

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Sjoukje Philip, Sarah F. Kew, Geert Jan van Oldenborgh, Friederike Otto, Sarah O’Keefe, Karsten Haustein, Andrew King, Abiy Zegeye, Zewdu Eshetu, Kinfe Hailemariam, Roop Singh, Eddie Jjemba, Chris Funk, and Heidi Cullen

Abstract

In northern and central Ethiopia, 2015 was a very dry year. Rainfall was only from one-half to three-quarters of the usual amount, with both the “belg” (February–May) and “kiremt” rains (June–September) affected. The timing of the rains that did fall was also erratic. Many crops failed, causing food shortages for many millions of people. The role of climate change in the probability of a drought like this is investigated, focusing on the large-scale precipitation deficit in February–September 2015 in northern and central Ethiopia. Using a gridded analysis that combines station data with satellite observations, it is estimated that the return period of this drought was more than 60 years (lower bound 95% confidence interval), with a most likely value of several hundred years. No trend is detected in the observations, but the large natural variability and short time series means large trends could go undetected in the observations. Two out of three large climate model ensembles that simulated rainfall reasonably well show no trend while the third shows an increased probability of drought. Taking the model spread into account the drought still cannot be clearly attributed to anthropogenic climate change, with the 95% confidence interval ranging from a probability decrease between preindustrial and today of a factor of 0.3 and an increase of a factor of 5 for a drought like this one or worse. A soil moisture dataset also shows a nonsignificant drying trend. According to ENSO correlations in the observations, the strong 2015 El Niño did increase the severity of the drought.

Open access
Edward Maibach, Raphael Mazzone, Robert Drost, Teresa Myers, Keith Seitter, Katharine Hayhoe, Bob Ryan, Joe Witte, Ned Gardiner, Susan Hassol, Jeffrey K. Lazo, Bernadette Placky, Sean Sublette, and Heidi Cullen

Abstract

Findings from the most recent surveys of TV weathercasters—which are methodologically superior to prior surveys in a number of important ways—suggest that weathercasters’ views of climate change may be rapidly evolving. In contrast to prior surveys, which found many weathercasters who were unconvinced of climate change, newer results show that approximately 80% of weathercasters are convinced of human-caused climate change. A majority of weathercasters now indicate that climate change has altered the weather in their media markets over the past 50 years, and many feel there have also been harmful impacts to water resources, agriculture, transportation resources, and human health. Nearly all weathercasters—89%—believe their viewers are at least slightly interested in learning about local impacts. The majority of weathercasters are interested in reporting on local impacts, including extreme precipitation and flooding, drought and water shortages, extreme heat events, air quality, and harm to local wildlife, crops and livestock, and human health; and nearly half had reported on the local impacts in at least one channel over the past 12 months. Thus, it appears that a strong majority of weathercasters are now convinced that human-caused climate change is happening, many feel they are already witnessing harmful impacts in their communities, and many are beginning to explore ways of educating their viewers about these local impacts of global climate change. We believe that the role of local climate educator will soon become a normative practice for broadcast meteorologists—adding a significant and important new role to their job descriptions.

Open access