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The rate of global mean surface temperature increase slowed during 1998– 2012. We review oceanic changes during this global warming hiatus from different but related perspectives. In one perspective, we explore the physical mechanisms for sea surface temperature patterns and highlight the role of natural variability, particularly the Interdecadal Pacific Oscillation (IPO) and the Atlantic Multidecadal Oscillation (AMO) that both have chaotic/random phases. In the other perspective, we investigate how the hiatus relates to changes in energy fluxes at the top of the atmosphere and to the three-dimensional distribution of ocean heat content change on decadal timescales. We find that the recent surface warming hiatus is associated with a transition of the IPO from a positive to negative phase and with heat redistribution between the tropical Pacific and Indian Oceans. The AMO has shifted to a positive phase since the late 1990s, inducing a La Niña-type response over the tropical Pacific via a tropic-wide teleconnection, contributing to the global warming hiatus. INTRODUCTION The annual mean atmospheric CO2 concentration at Mauna Loa, Hawaii, first exceeded 400 ppmv (parts per million by volume) in 2015 (Figure 1a). With the rapid rise in CO2, global mean surface temperature (GMST), both in the atmosphere (GMSAT) and in the ocean (GMSST), shows a long-term warming trend (Figure 1b,c). Concerns over the rapid anthropogenic warming rallied countries to ratify the Paris Agreement in December 2015 to curb greenhouse gas emissions and keep the rise of GMST in this century well below 2°C above preindustrial levels and to pursue efforts to limit the temperature increase to 1.5°C (Figueres et al., 2017). GMST fluctuates on decadal timescales against a century-long warming trend (Easterling and Wehner, 2009). The increase in GMST slowed during 1998–2012, as noted by the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5): “the rate of warming over the past 15 years (1998–2012; 0.05°C [−0.05°C to +0.15°C] per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12°C [0.08–0.14°C] per decade)” (Hartmann et al., 2013). This global warming hiatus FIGURE 1. (a) Observed annual-mean CO2 concentration at Mauna Loa. (b) Annual global mean surface air temperature (GMSAT) anomalies. (c) Annual global mean sea surface temperature (GMSST) anomalies. (d) Annual global mean 0–2,000 m ocean heat content (OHC) anomalies relative to the 1960–2015 average. Gray shading highlights the hiatus period. GMSAT is based on Hadley Centre Climate Research Unit Temperature (HadCRUT) version 4.6.0.0 (Morice et al., 2012), NOAA Merged Land Ocean Global Surface Temperature Analysis (NOAAGlobalTemp; T.M. Smith et al., 2008), and Goddard Institute for Space Studies Temperature (GISTEMP; Hansen et al., 2010). GMSST is based on Hadley Centre Sea Ice and Sea Surface Temperature (HadISST; Rayner et al., 2003), Centennial In Situ Observation-Based Estimates of the Variability of SST and Marine Meteorological Variables versions 1 (COBE-SST: Ishii et al., 2005) and 2 (COBE-SST2; Hirahara et al., 2014), and Extended Reconstructed Sea Surface Temperature versions 3b (ERSST.v3b; T.M. Smith et al., 2008), 4 (ERSST.v4; Huang et al., 2015; Liu et al., 2015), and 5 (ERSST.v5; Huang et al., 2017). OHC is based on EN4 data with Levitus et al. (2009) (EN4. l09) and Gouretski and Reseghetti (2010) (EN4.g10) corrections and Institute of Atmospheric Physics Chinese Academy of Sciences (IAP) data (Cheng et al., 2017). SPECIAL ISSUE ON OCEAN WARMING

An Ocean View of the Global Surface Warming Hiat