Ocean heat content as a fundamental climate indicator

dr. Lijing Cheng

International Center for Climate and Environment Sciences, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China,

"Ocean heat content as a fundamental climate indicator"

Abstract

Earth’s energy imbalance (EEI) drives the ongoing global warming and can best be assessed across the historical record (i.e. since 1960) from ocean heat content (OHC) changes. Because the ocean takes up over 90% of the energy imbalance associated with increasing heat-trapping gases in the atmosphere, so OHC change is a fundamental climate indicator. Assessment of OHC change relies on in situ ocean observations. However, an accurate assessment of OHC is a challenge, because of the instrumental bias in historical ocean observations (i.e. XBT), and also insufficient and irregular data coverage (i.e. this induces sampling error). In this talk, I will review our recent progresses in estimating OHC (or EEI) via observations, and provide updated OHC estimates with the goal of minimizing associated sampling error and instrumental bias.
The inferred integrated OHC and EEI is greater than that reported in previous assessments (i.e. IPCC-AR5), 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 timescales. 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. Some examination of ENSO-related ocean energy change/redistribution will also be discussed in this talk.
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.