Abstract
Abstract. Argo deployments began in the year 2000 and by November 2007, the array reached its initial goal of 3000 floats operating worldwide. In this study, Argo temperature and salinity measurements during the period 2005 to 2010 are used to estimate Global Ocean Indicators (GOIs) such as global ocean heat content (GOHC), global ocean freshwater content (GOFC) and global steric sea level (GSSL). We developed a method based on a simple box averaging scheme using a weighted mean. Uncertainties due to data processing methods and choice of climatology are estimated. This method is easy to implement and run and can be used to set up a routine monitoring of the global ocean. Over the six year time period, trends of GOHC and GSSL are 0.54 ± 0.1 W m−2 and 0.75 ± 0.15 mm yr−1, respectively. The trend of GOFC is barely significant. Results show that there is significant interannual variability at global scale, especially for GOFC. Annual mean GOIs from the today's Argo sampling can be derived with an accuracy of ±0.11 cm for GSSL, ±0.22 × 108 J m−2 for GOHC, and ±700 km3 for GOFC. Long-term trends (15 yr) of GOIs based on the complete Argo sampling for the upper 1500 m depth can be estimated with an accuracy of ±0.04 mm yr−1 for GSSL, ±0.02 W m−2 for GOHC and ±20 km3 yr−1 for GOFC – under the assumption that no systematic errors remain in the observing system.
Highlights
During the past decade, the international Argo programme has revolutionized the distribution of ocean data within the research and operational communities
A “forecast calculation” of the uncertainties of global trend estimations is given in Table 2, assuming Global Ocean Indicators (GOIs) error bars during the year 2010 while applying Eq (A1) of the Appendix. This simple calculation suggests that long-term trends of GOIs based on the complete Argo sampling could be performed with an accuracy of about ±0.04 mm yr−1 for global steric sea level (GSSL), ±0.02 W m−2 for global ocean heat content (GOHC) and ±20 km3 yr−1 for global ocean freshwater content (GOFC) trends – under the major assumption that no systematic errors remain in the observing system
GSSL, GOHC and GOFC derived from in situ observations are a useful benchmark for ocean and climate models and an important diagnostic for changes in the Earth’s climate system (Hansen et al, 2005; Levitus et al, 2005)
Summary
The international Argo programme has revolutionized the distribution of ocean data within the research and operational communities Argo provides the capability to assess global ocean heat content (GOHC) by measuring subsurface temperature. Salinity data allow an estimate of global ocean freshwater content (GOFC). Multi-decadal trends in ocean salinity have been observed on global and regional scales (e.g. Antonov et al, 2002; Boyer et al, 2005; Delcroix et al, 2007). Results shown in von Schuckmann et al (2009) document that ocean salinity and, freshwater are changing on gyre and basin scales and GOFC is characterized by large interannual changes rather than by a significant trend during the last decade. Several GSSL variations from Argo and other in situ observations have been derived over the past couple of years (e.g. Willis et al, 2008; Cazenave et al, 2009; Leuliette and Miller, 2009; von Schuckmann et al, 2009).
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