No. 72 June, 2017 CLIVAR Exchanges CLIVAR Ocean and Climate: Variability, Predictability and Change is the World Climate Research Programme’s core project on the Ocean-Atmosphere System Decadal Climate Variability • Figure by Adam Phillips, picture by Christophe Cassou Volume 25, No. 1 Past Global Changes Magazine Editorial doi: 10.22498/pages.25.1.1 Yochanan Kushnir, Christophe Cassou, Scott St George The study of Decadal Climate Variability and Predictability variability and greenhouse gas forcing, have devastated regional agriculture productivity, lead to loss of life and, characterize, understand, attribute, simulate, and predict perhaps arguably, to widespread societal instability and in the(DCVP) slow, is multi-year the interdisciplinary variations of scientific climate onenterprise global and to regional scales. Particular interest in decadal climate variations and their role in the global surface climate InSyria order to violent to anticipate conflict the and impacts war. of climate change, it is change stems from the need to detect and attribute the important for society to know how the climate response uneven rise in global mean surface temperatures (GMST) to anthropogenic forcing and the climate impact of since the beginning of the industrial period. The most natural variability will mix together to affect the near- recent expression of decadal variability in GMST has been term future. The study of DCVP aims to provide science- the slowdown in warming between roughly 1998 and based information to decision makers through research, 2012. This period, often termed as the “hiatus”, triggered observations, and decadal predictions. This goal remains intensive debate in the public domain, even if global challenging despite decades of research and of extensive temperatures had exhibited long undulations before, progress in observing and modeling the climate system. including two cooling events in the late 19th to early 20th Predicting the impact of internal decadal climate variability century and in the mid 20th century and two intervals is complicated by our incomplete understanding of the of rapid warming, one from about 1910 to 1940 and the nature of the underlying phenomena, in particular their other between the early 1970s and 1998. While these physical origins and their interaction with external forcing. departures from the expected warming due to the steady Existing obstacles in DCVP research thus test our ability to increase in greenhouse gas forcing have been attributed in attribute past variations to the combined role of internal part to natural (volcanic) and anthropogenic (industrial) variability and external forcing, as well as to reliably predict aerosols, there is ample evidence that long-term internal the near-term climate on global and regional scales. interactions between climate system components – the ocean and the atmosphere, in particular – have also been Progress in DCVP research can only be made through involved. international, cross disciplinary collaborations between Decadal and longer variations in sea surface temperatures the Earth’s climate at timescales of a decade or longer, (SSTs) have a rich and non-uniform spatial pattern thisscientists. area ofBecause research of the is difficultieswholly dependent to observe on and emerging model related to variations in the distribution of precipitation connections between those who perform, collect and and associated atmospheric convection in the tropics, to analyze instrumental observations of the present, those alterations in position and strength of the storm tracks who develop and analyze proxies of past climate, and with at midlatitudes, to changes in sea-ice extent at polar scientists who develop models and perform dedicated latitudes in both hemispheres, etc. Changes in atmospheric modeling experiments. To review ongoing research on circulation thus contributes to changes in regional DCVP and propose the road to future progress on the climates worldwide and importantly over the continents, subject, the International WCRP CLIVAR Project and PAGES directly affecting humans and their environment. The held an international workshop with representatives of most prominent example of the terrestrial response to these various disciplines in November of 2015, under decadal climate variability is the long-lasting decline of the patronage of the International Centre for Theoretical rainfall in the North African Sahel in the second half of the Physics in Trieste Italy. This issue of Exchanges grew out 20th century, which included the devastating famines of from the presentations and discussions features in this the 1970s & 1980s. These decadal-scale shifts have been workshop. attributed to slow variations in North Atlantic SSTs, which have also affected Atlantic tropical cyclone activity over the The articles in this issue of Exchanges were selected and same time frames. Similarly, the multi-year pulses of North reviewed by the members of the CLIVAR Working Group American droughts (e.g., the Great Plains “dust bowl” in on DCVP and the PAGES 2k Network. These contributions the 1930s and the recent protracted dry period in the are meant to provide brief reviews that address the Southwest US), which impacted lives and livelihoods in the progress made in understanding and resolving different US and northern Mexico, have been attributed to the state key issues in DCVP. We greatly appreciate the voluntary efforts made by these authors to capture the exciting and Mediterranean region, in South European countries along rapidly growing literature on the subject in these brief theof the northern tropical rim Pacific and andin the tropical Maghreb Atlantic and the Oceans. Middle For East, the summaries and hope that they will stimulate further recurrent heat waves and prolonged dry spells since the research collaboration on the subject. mid 1960s, attributed to a combination of internal decadal 1 CLIVAR Exchanges No. 72, June 2017 Past Global Changes Magazine, Volume 25, No. 1 An overview of decadal-scale sea surface temperature variability in the observational record doi: 10.22498/pages.25.1.2 Clara Deser, Adam Phillips National Center for Atmospheric Research, Boulder, USA Introduction Due to their thermal and mechanical inertia, the oceans SST data coverage play a key role in decadal-scale climate variability (DCV) and provide a potential source of initial-value oceanOur focus temperature on SST is recordsmotivated are by measured both practical near andthe Characterizing oceanic DCV is challenging, however, surfacephysical from considerations. ships-of-opportunity, On the practical starting side, with the longestbucket duepredictability to the limited for durationlow-frequency of the climateobservational fluctuations. record samples in the 19th and early 20th centuries followed combined with the sparse and irregular data coverage. by engine-intake measurements (e.g., Woodruff et al., These constraints also hinder assessments of the robustness of the patterns and timescales of DCV, and communication between the atmosphere and the ocean, understanding of the governing mechanisms. In this brief and2008). thus On represent the physical a key side, quantity SSTs forare probingthe main DCV agent (for of a note, we provide an overview of the main phenomena discussion of the upper-ocean mixed layer heat budget, of DCV in the historical sea surface temperature (SST) c.f. Deser et al., 2010). data record, discuss proposed interpretations and causal mechanisms, and highlight outstanding research questions. Fig. 1 (left column) shows maps of SST data coverage duringbased threeon the representative International 20-year Comprehensive periods spanning Ocean theAtmosphere late 19th andData 20th Set centuries:(ICOADS) (Woodruff1870-1899, et 1920-1939, al., 2008) and 1970-1989. These maps show the percentage of months with at least one measurement in a 2°latitude by 2° longitude grid box in the 20-year period indicated. We note that the instrumental coverage falls off rapidly before 1870, and that satellites provide nearly global coverage starting in the 1980s (see Woodruff et al., 2008 and Deser et al., 2010). The discrete outlines of commercial shipping routes and their changes over time are readily Broadly speaking, the North Atlantic, western South apparent, especially in the earlier time periods (Fig. 1). density of observations, with reasonable coverage back toAtlantic, approximately and northern 1870. Indian Data Ocean coverage contain in thethe highest North Pacific is limited before about 1920, in the Tropics before changingabout 1960, spatial and in coverage the Southern of SST Ocean measurements before the advent from historicalof satellite ship-based remote sensingarchives (Fig.must 1).be takenThe intouneven account and Figure 1: Distribution of sea surface temperature observations from the International Comprehensive Ocean Atmosphere Data Set. Maps show the percentage of months with at least temporal coverage of other SST data sets is available at in any analysis of DCV. Further information on the spatio- one measurement in a 2 degree latitude by 2 degree longitude climatedataguide.ucar.edu. grid box during (a) 1870-1899, (b) 1920-1939, and (c) 1970- 1989. Timeseries (1870-2015) show the percentage of grid The main phenomena of DCV boxes that have at least one observation per month within In our view, there is no unique “best” approach to the regions outlined in Fig. 1c. (d) North Pacific (20°-70°N, 110°E-100°W), (e) North Atlantic (0°-60°N, 80°W-0°W), and (f)