Scientific project that investigates the extent to which the ocean heat content has increased using the EN4 dataset.
Abstract
The Earth’s energy imbalance is responsible for driving the global warming, that is increasingly being witnessedin climate systems worldwide. As most of this energy accumulates in the oceans, Ocean Heat Content (OHC) changes can be used to assess this warming. However, limited ocean observations coverage makes estimatingchanges in the OHC a significant challenge. In this report, Met Office’s EN4 analyses are evaluated for theirability to estimate trends in global OHC change, for 1950-2018. This was again used to quantify the OHCchange and its regional variations. The analyses were found to be adequate in estimating OHC, when avoidingtime periods and regions of low observational influence as they are heavily influenced by the assumptions madein dealing with regions of no observations. The sampling error was found to be most significant at depths and inthe Southern Hemisphere. From 1970-2016, the OHC increased by 24.4× 10^22 J for the 0-2000 m layer, with 74% of the total change in the upper ocean (above 700m). The ocean heat uptake experienced an acceleration around 1997, which is likely linked to a recovery in OHC from the volcanic eruption Mt Pinatubo (1991) masking the acceleration that would have otherwise been observed. In addition, it was likely linked to a reduced samplingerror from an increase in observational coverage from the Argo programme. The warming was strongest in the Southern and tropical/subtropical Atlantic Oceans, for periods of both low and high observational influence, revealing a robust footprint of global warming.
Highlight results
Time series of ocean heat content for the 0-100, 0-300, 0-700, 0-1000, 0-2000 m layers. The seasonal variations have been smoothed with a 3 year moving average.
Time series of ocean heat content for the 0-100, 0-300, 0-700 m layers. The seasonal variations havebeen smoothed with a 1 year moving average, to capture shorter fluctuations. The sea surface temperatureanomaly is represented by a red dotted line and the mean global optical depth at 550 nm is shown in greyon a scale from 0 to 1. Very strong El Niño years and strong La Niña years are shown in red and blue,respectively.
Time series of ocean heat content change for different depth layers and the mean observationalweight used in forming the analyses. The EN4 analyses uncertainty have been plotted for the 0 to 700 m plot,but is insignificantly small when propagating the errors of the global sum. Seasonal variations are smoothedwith a 3 year moving average.
Contour map of linear trend of every grid cell. The total rate is found from summingup all cells and dividing by the global area. The percentage of heating is the fraction ofcells that has a net positive change, excluding the continents.
Difference in 5 year averages at the start and end of the time period of the zonallyintegrated heat content of the world ocean by1◦latitude belts for 50 m depth layers.