Accuracy of Global Upper Ocean Heat Content Estimation Expected from Present Observational Data Sets

The simplest global mapping method and dense data coverage for the global oceans by the latest observation network ensure an estimate of global ocean heat content (OHC) within a satisfac- tory uncertainty for the last 60 years. The observational database conditionally presented a level high enough for practical use for the global OHC estimation when applying bias corrections of expendable bathythermograph, assuming that the other severe observational biases are not included in the database. Uncertainties in annual global mean temperatures averaged vertically from the surface to 1,500 m are within 0.01 K for the period from 1955 onward, when only sampling errors are taken into account. Those in annual mean global OHC of an improved objective analysis for 0−1,500 m depth is 16 ZJ on average throughout the period. Com- pared to previous studies, the new objective analysis provides a higher estimation of the global 0−1,500 m OHC trend for a lon - ger period from 1955 to 2015, which is an increase of 350 ± 57 ZJ with a 95% confidence interval. ( Citation M., T. Suzuki, and K. Sato, 2017: Accuracy of global upper ocean heat content estimation expected from present observational data sets. Profile Program (GTSPP), and the Argo Project, were used for estimating global OHC. Observations in the latter two data sets were used prior to WOD13, as they were updated later than WOD13. In addition, the duplicates in WOD13 were discarded in the following calculations. For easy handling and analyzing observed data distributed irregularly in space and time, observed anomalies relative to the climatology were arithmetically aver- aged monthly from 1945 to 2015 in 5°-longitude and 5°-latitude boxes allocated over the global oceans. A typical spatial decor- relation scale ranges from 300 to 900 km among literature (Lev-itus et al. 2000; Ishii et al. 2003; Willis et al. 2003; Ingleby and Huddleston 2007), and therefore, the 5°-box mean values are regarded as locally representative. The monthly box mean val- ues of observed profiles were computed after taking averages of data observed by each ship or buoy in each 10-day interval and in each 5°-longitude and 5°-latitude square at 28 levels from surface to 3,000 m depth; biases from localized data in space and time and from specific ships or buoys are minimized. The climatology used here is mean temperature fields of IK09 for the period from 1961 to 2005. The climatology is defined on a 1-degree grid. This minimizes spatial interpolation errors in computing 5-degree box mean values rather than the case of 5-degree climatology, particularly in areas where the horizontal temperature gradients are large. Vertically, the boxes were placed in the upper 3,000 m and the box heights, i.e., thickness, varied from 5 to 50 m in the upper