© 2014 Macmillan Publishers Limited. All rights reserved. ARTICLES PUBLISHED ONLINE: 17 AUGUST 2014 | DOI: 10.1038/NCLIMATE2341 Seasonal aspects of the recent pause in surface warming Kevin E. Trenberth * , John T. Fasullo, Grant Branstator and Adam S. Phillips Factors involved in the recent pause in the rise of global mean temperatures are examined seasonally. For 1999 to 2012, the hiatus in surface warming is mainly evident in the central and eastern Pacific. It is manifested as strong anomalous easterly trade winds, distinctive sea-level pressure patterns, and large rainfall anomalies in the Pacific, which resemble the Pacific Decadal Oscillation (PDO). These features are accompanied by upper tropospheric teleconnection wave patterns that extend throughout the Pacific, to polar regions, and into the Atlantic. The extratropical features are particularly strong during winter. By using an idealized heating to force a comprehensive atmospheric model, the large negative anomalous latent heating associated with the observed deficit in central tropical Pacific rainfall is shown to be mainly responsible for the global quasi- stationary waves in the upper troposphere. The wave patterns in turn created persistent regional climate anomalies, increasing the odds of cold winters in Europe. Hence, tropical Pacific forcing of the atmosphere such as that associated with a negative phase of the PDO produces many of the pronounced atmospheric circulation anomalies observed globally during the hiatus. A lthough the 2000s are by far the warmest decade on record, the rate of increase of global mean temperature since 2000 has slowed, regardless of the data source 1 (see Fig. 1, and also Supplementary Fig. 1 for northern winter aspects). A linear fit to the global mean temperatures after 1970 is quite good, and the biggest outlier is actually 1998, which was affected by substantial heat coming out of the ocean in association with the 1997/1998 El Niño event 2,3 . Hence, the post-1998 perspective (Fig. 1) is contrived because it depends on the choice of the starting year. Nevertheless, it is vital to understand related interannual and decadal variability reflected in Fig. 1 and its regionality. The strongest pause in the rise in global mean surface temperatures is in the northern winter (Supplementary Fig. 1), and the main places that warming has not occurred is in much of the central and eastern Pacific Ocean 1 and over northern continents, especially Eurasia 4 . Here we explore the teleconnections that are key to understanding the global structure of the various atmospheric anomalies associated with the warming hiatus, taking into account their seasonality to better determine the atmospheric forcings and responses, and understand the northern winter changes. This also provides an important perspective on the driving forces behind the patterns, and assists in discerning consequences from causes. Several analyses of the factors involved in the apparent hiatus in the rise of global mean surface temperatures after about 2000 have been performed. Although small contributions have come from changes in the total solar irradiance as the Sun has gone into a quieter phase since about 2004 (ref. 5) and from aerosols in both the stratosphere (from small volcanic eruptions) and more regionally from pollution in the troposphere 6 , more than half of the cause of the hiatus is apparently associated with internal climate variability 1,7–9 that we explore here. Pacific decadal variability: seasonal signals The phenomenon playing the main role is the PDO (refs 1,9), alternatively known from a slightly different perspective as the Interdecadal Pacific Oscillation, although it may not be a 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1970 1980 1990 Year 2000 2010 −0.1 Global mean temperature anomalies (°C) Base period 1900−1999 Figure 1 | Global mean surface temperature from NOAA, as anomalies relative to 1900–1999 plotted with linear trends for 1970–2013 (blue) and 1998–2013 (red). quasi-linear mode of natural variability 10 . The PDO was in a negative phase before 1976, but became positive from 1976 to 1998, a period coinciding with strong increases in global mean surface temperatures 1,11 . Then it switched to a negative phase in 1999 coinciding with the pause in upward trend in global mean surface temperatures. However, since 1999, the deeper ocean below 700m has taken up more heat and there has not been a reduction in the Earth’s energy imbalance 3,8 . The PDO pattern (Fig. 2) emerges from an analysis of the departures from the global mean of sea surface temperature (SST) monthly anomalies using a core region from 20 ◦ to 70 ◦ N, 110 ◦ E to 100 ◦ W for an empirical orthogonal function analysis 1,11 . The base period is 1900–2012. The PDO/Interdecadal Pacific Oscillation has a Pacific-wide pattern in both surface and subsurface temperatures with an El Niño-like pattern throughout the tropics and strong extratropical links in both hemispheres (Fig. 2). The subsequent analysis only uses the PDO to provide markers for specifying the last two climate regimes: 1999–2012 versus 1976–1998 (ref. 1). This is more robust than using short-term linear trends 9 , although National Center for Atmospheric Research, PO Box 3000, Boulder, Colorado 80307, USA. *e-mail: trenbert@ucar.edu NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1