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Lei Zhang and Kristopher B. Karnauskas

1. Introduction The tropical Pacific trade winds (PTWs), or the surface branch of the Walker circulation, have been strengthening since the mid-1990s ( Merrifield 2011 ; England et al. 2014 ), accompanied by a prominent cooling trend in the sea surface temperature (SST) of the eastern tropical Pacific ( Kosaka and Xie 2013 ; Trenberth et al. 2014 ). It has been suggested that both the enhanced PTWs and the eastern Pacific cooling are closely connected with the observed global warming hiatus

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Robert C. Wills, Xavier J. Levine, and Tapio Schneider

1. Introduction In climate models and observations, convective mass flux and zonally anomalous tropical overturning circulations, such as the Walker circulation, weaken robustly in response to global warming ( Vecchi et al. 2006 ; Vecchi and Soden 2007 ; Merlis and Schneider 2011 ). Associated with this weakening is a reduction of tropical precipitation contrasts ( Wills and Schneider 2016 ; Wills et al. 2016 ) and a modification of extratropical Rossby waves, which influence midlatitude

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Lorenzo M. Polvani and Katinka Bellomo

Pacific Ocean and a warm pool that straddles the Indian and Pacific Oceans. This east–west gradient in SST is responsible for the overturning Walker circulation, with air rising over the warm pool and descending over the cold tongue. The Walker circulation affects wind stress, ocean dynamics, and clouds, and it can thus have major climatic impacts. A strengthening of the Walker circulation is believed to be a key driver of the recent warming hiatus ( Kosaka and Xie 2013 ; de Boisséson et al. 2014

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Timothy M. Merlis and Tapio Schneider

1. Introduction The evolution of zonal surface temperature gradients in the tropics is of interest in the study of future and past climates (e.g., Knutson and Manabe 1995 ; Fedorov et al. 2006 ). Recently, the changes in the zonally asymmetric component of the tropical overturning circulation, the Walker circulation, have been highlighted as a robust response to warming in climate change simulations with comprehensive GCMs ( Held and Soden 2006 ; Vecchi and Soden 2007 ). Here, we examine the

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Byung-Ju Sohn, Sukyoung Lee, Eui-Seok Chung, and Hwan-Jin Song

1. Introduction The Pacific Walker circulation (PWC) is characterized by rising motion over Indonesia and sinking motion over the eastern Pacific ( Bjerknes 1969 ). This pattern is associated with high sea surface temperatures (SSTs) and active convection in the western tropical Pacific and lower SSTs and suppressed convection in the eastern Pacific. As such, on interannual time scales, the Walker circulation is closely linked to the El Niño–Southern Oscillation (ENSO). Since the 1970s, however

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Scott B. Power and Greg Kociuba

1. Introduction The Walker circulation (WC) is one of the world’s most prominent and important atmospheric systems. It extends across the entire tropical Pacific Ocean, encompassing 1) the trade winds blowing from east to west; 2) air forced to rise over the western Pacific, Southeast Asia, and northern Australia through enhanced convection; 3) winds blowing counter to the trades aloft; and 4) air descending over the eastern Pacific Ocean (see, e.g., Gill 1982 ). Changes in the WC are

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Elina Plesca, Verena Grützun, and Stefan A. Buehler

atmospheric circulation (global and regional), but global circulation models (GCMs) have yet to provide a high confidence level in simulating and projecting these circulation changes ( Shepherd 2014 ). Under these considerations, this work investigates the variation in a warmer climate of one of the large-scale overturning circulations, namely, the Pacific Walker circulation. Recent studies ( Held and Soden 2006 ; Vecchi and Soden 2007 ; Chadwick et al. 2013 ; Bony et al. 2013 ) suggest that tropical

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Joanna Slawinska, Olivier Pauluis, Andrew J. Majda, and Wojciech W. Grabowski

1. Introduction The dynamics of the tropical atmosphere is dominated by the complex interplay between convective motions and circulation on the large scales. Convection accounts for most of the vertical energy transport and usually involves horizontal scales of 100 km or less. Yet, convective activity is strongly modulated by atmospheric variability on the synoptic and planetary scales, such as the Walker and Hadley circulations, the Madden–Julian oscillation, monsoons, and equatorially trapped

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Juho Iipponen and Leo Donner

over the western Pacific warm pool and radiative cooling over the eastern Pacific lead to a zonally asymmetric atmospheric circulation. Ever since his seminal paper, this circulation has been known as the Walker cell due to its pivotal role in the Southern Oscillation phenomenon of Sir Gilbert Walker. Many studies have analyzed the response of the tropical atmosphere to spatially localized convective heat sources (e.g., Webster 1972 ; Gill 1980 ; Geisler 1981 ; Hartmann et al. 1984 ; Wu et al

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Jian Ma and Jin-Yi Yu

as the eastern Pacific (EP) type, and the other in the central Pacific, referred to as the central Pacific (CP) type ( Yu and Kao 2007 ; Kao and Yu 2009 ). The CP El Niño has occurred more frequently in recent decades ( Ashok et al. 2007 ; Kao and Yu 2009 ; Yeh et al. 2009 ; Lee and McPhaden 2010 ; Yu et al. 2012 ). The dominance of the EP or CP type of El Niño has been linked to the relative strengths of the Walker and Hadley circulations ( Yu et al. 2012 ). By analyzing the relationship

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