Saturday, 18 March 2017

Improved estimates of ocean heat content from 1960 to 2015

Source: Science Advances  10 Mar 2017: Vol. 3, no. 3, e1601545, DOI: 10.1126/sciadv.1601545

Authors:

Lijing Cheng1,*, Kevin E. Trenberth2, John Fasullo2, Tim Boyer3, John Abraham4 and Jiang Zhu1

1International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China.
2National Center for Atmospheric Research, Boulder, CO 80307, USA.
3National Oceanic and Atmospheric Administration/National Centers for Environmental Information, Silver Spring, MD 20910, USA.
4University of St. Thomas, St. Paul, MN 55105, USA.
*Corresponding author. Email: chenglij@mail.iap.ac.cn

Abstract

Earth’s energy imbalance (EEI) drives the ongoing global warming and can best be assessed across the historical record (that is, since 1960) from ocean heat content (OHC) changes. An accurate assessment of OHC is a challenge, mainly because of insufficient and irregular data coverage. We provide updated OHC estimates with the goal of minimizing associated sampling error. We performed a subsample test, in which subsets of data during the data-rich Argo era are colocated with locations of earlier ocean observations, to quantify this error. Our results provide a new OHC estimate with an unbiased mean sampling error and with variability on decadal and multidecadal time scales (signal) that can be reliably distinguished from sampling error (noise) with signal-to-noise ratios higher than 3. The inferred integrated EEI is greater than that reported in previous assessments and is consistent with a reconstruction of the radiative imbalance at the top of atmosphere starting in 1985. We found that changes in OHC are relatively small before about 1980; since then, OHC has increased fairly steadily and, since 1990, has increasingly involved deeper layers of the ocean. In addition, OHC changes in six major oceans are reliable on decadal time scales. All ocean basins examined have experienced significant warming since 1998, with the greatest warming in the southern oceans, the tropical/subtropical Pacific Ocean, and the tropical/subtropical Atlantic Ocean. This new look at OHC and EEI changes over time provides greater confidence than previously possible, and the data sets produced are a valuable resource for further study.


Image 1 - Fig. 4 - Global OHC change time series. (A) OHC from 0 to 700 m (blue), 700 to 2000 m (red), and 0 to 2000 m (dark gray) from 1955 to 2015 as obtained by this study, with the uncertainty of the ±2σ interval shown in shading. All time series of the new analysis are smoothed by a 12-month running mean filter, relative to the 1997–2005 base period. (B) The new estimate is compared with an independent estimate from NCEI with its SE as dashed lines. Both OHC 0 to 700 m and OHC 700 to 2000 m are shown from 1957 to 2014. The baseline of the time series from NCEI is adjusted to the values of the current analysis within 2005–2014.


Image 2 - Fig. 5 - OHC changes from 1960 to 2015 for different ocean basins. (A) For 0 to 2000 m, (B) 0 to 700 m, and (C) 700 to 2000 m. All the time series are relative to the 1997–1999 base period and smoothed by a 12-month running filter. The curves are additive, and the OHC changes in different ocean basins are shaded in different colors.


Image 3 - Fig. 6 - Estimate of the ocean energy budget. The three major volcanic eruptions are marked. The energy budgets are relative to the 1958–1962 base period. The integrated net radiative imbalance from Allan et al. (65) estimated from the TOA is included in yellow and is multiplied by 0.93 to be comparable with the ocean energy budget. The TOA radiation is adjusted to the value of OHC within 2013–2014. The dashed gray lines encompass the 95% confidence interval.

Read more at: http://advances.sciencemag.org/content/3/3/e1601545