The role of the North Atlantic overturning and deep ocean for multi-decadal global-mean-temperature variability

Abstract

Abstract. Earth's climate exhibits internal modes of variability on various timescales. Here we investigate multi-decadal variability of the Atlantic meridional overturning circulation (AMOC), Northern Hemisphere sea-ice extent and global mean temperature (GMT) in an ensemble of CMIP5 models under control conditions. We report an inter-annual GMT variability of about ±0.1 • C originating solely from natural variability in the model ensemble. By decomposing the GMT variance into contributions of the AMOC and Northern Hemisphere sea-ice extent using a graph-theoretical statistical approach, we find the AMOC to contribute 8 % to GMT variability in the ensemble mean. Our results highlight the importance of AMOC sea-ice feedbacks that explain 5 % of the GMT variance, while the contribution solely related to the AMOC is found to be about 3 %. As a consequence of multi-decadal AMOC variability, we report substantial variations in North Atlantic deep-ocean heat content with trends of up to 0.7 × 10 22 J decade −1 that are of the order of observed changes over the last decade and consistent with the reduced GMT warming trend over this period. Although these temperature anomalies are largely densitycompensated by salinity changes, we find a robust negative correlation between the AMOC and North Atlantic deepocean density with density lagging the AMOC by 5 to 11 yr in most models. While this would in principle allow for a self-sustained oscillatory behavior of the coupled AMOCdeep-ocean system, our results are inconclusive about the role of this feedback in the model ensemble.

Changes in Global Ocean Bottom Properties and Volume Transports in CMIP5 Models under Climate Change Scenarios

“…have been found in relation to surface property changes (e.g., Jahn and Holland 2013) or intermediate depths changes (Schleussner et al 2014). We found that future changes in density in the deep oceans too are linked with a weakening of bottom and deep water volume transports.…”

Global Contributions of Incoming Radiation and Land Surface Conditions to Maximum Near‐Surface Air Temperature Variability and Trend

“…On longer time scales, atmospheric greenhouse gas concentrations (Intergovernmental Panel on Climate Change, 2013), sea surface temperatures (Hoerling et al, 2008), and climate oscillations (Chylek et al, 2016; Schleussner et al, 2014; Stolpe et al, 2017) contribute to temperature variability and trends by influencing the above‐mentioned immediate drivers. For example, expressing local near‐surface air temperature as a linear function of global mean temperature, over 90% of the local multidecadal temperature trend can be explained (Sutton et al, 2015).…”

Using Causal Effect Networks to Analyze Different Arctic Drivers of Midlatitude Winter Circulation

“…This algorithm is a modified version of the Peter Spirtes and Clark Glymour (PC) algorithm (Spirtes et al 2000), which was first applied to climate research by Ebert-Uphoff and Deng (2012) to study interactions between major climate modes. Causal discovery approaches have since been used to study atmospheric flows (Deng and Ebert-Uphoff 2014), causal relationships in the Walker cell in the tropics , the monsoonal dynamics in the Pacific-Indian Ocean (Runge et al 2015), and decadal ocean circulation in the Atlantic (Schleussner et al 2014).…”