Abstract
Observational analyses of running 5 year ocean heat content trends (Ht) and net downward top of atmosphere radiation (N) are significantly correlated (r ∼ 0.6) from 1960 to 1999, but a spike in Ht in the early 2000s is likely spurious since it is inconsistent with estimates of N from both satellite observations and climate model simulations. Variations in N between 1960 and 2000 were dominated by volcanic eruptions and are well simulated by the ensemble mean of coupled models from the Fifth Coupled Model Intercomparison Project (CMIP5). We find an observation-based reduction in N of − 0.31 ± 0.21 W m−2 between 1999 and 2005 that potentially contributed to the recent warming slowdown, but the relative roles of external forcing and internal variability remain unclear. While present-day anomalies of N in the CMIP5 ensemble mean and observations agree, this may be due to a cancelation of errors in outgoing longwave and absorbed solar radiation.Key PointsObserved maximum in ocean heat content trend in early 2000s is likely spuriousNet incoming radiation (N) reduced by 0.31 ± 0.21 W m−2 during the warming pausePresent-day estimates of N may contain opposing errors in radiative components
Highlights
We show that two reanalyses of heat content trends (Ht) in the early 2000s are inconsistent with satellite observations and AMIP simulations of N and are likely unreliable, helping to explain our inability to close the energy budget
We find significant correlation (r ~ 0.6, p < 0.01) between rolling 5 year mean ocean heat content trends (Ht) in two ocean analyses and net incoming radiation at the top of the atmosphere (N) from satellite observations and AMIP model simulations over the period 1960 to 1999
Ocean reanalyses of Ht in the early 2000s are inconsistent with observation- and model-based constraints on N
Summary
The recent slowdown in the rate of global surface temperature rise [Easterling and Wehner, 2009, Knight et al, 2009] has highlighted a need for a better quantification of Earth’s energy imbalance [Hansen et al, 2011; Katsman and van Oldenborgh, 2011; Balmaseda et al, 2013; Trenberth et al, 2014] and improved understanding of the flow of energy in the climate system [Trenberth, 2009; Meehl et al, 2011; England et al, 2014]. Observations of trends in ocean heat content (hereafter Ht) potentially enable the satellite observations to be anchored to obtain absolute values [Loeb et al, 2012; Allan et al, 2014], as well as providing independent estimates of variations in N. We investigate Earth’s energy imbalance over the period since 1960 in observations, atmospheric models forced by observed sea surface temperatures and radiative changes (Atmosphere Model Intercomparison Project, AMIP), and fully coupled model simulations (Coupled Model Intercomparison Project, CMIP5). Prior to 2000 variations in Ht in ocean reanalyses are in reasonable agreement with observation-based estimates of N and with CMIP5 models, as Earth’s energy imbalance was dominated by volcanic eruptions over this period. We highlight an observation-based reduction in N that likely contributed to the recent slowdown in surface warming
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