Quantified Objective: Determining the change in ocean heat storage within 0.1 W m−2 per decade.
Over 90 percent of the heat from anthropogenic warming has been stored in the ocean as reported in the recent IPCC report (Rhein et al., 2013). Observation of the ocean heat storage is key to understanding the heat budget of the planet and prediction of future climate. The uptake of heat by the ocean is estimated to be 0.5-1 W m−2 (Loeb et al., 2012; Trenberth and Fasullo, 2010 Trenberth et al., 2014). Detection of its change by 10-20 percent per decade is essential.
An observing system consisting of satellite altimetry (Jason-series), spaceborne gravimetry (Gravity Recovery and Climate Experiment [GRACE] series), and in-situ network of floats (Argo) has demonstrated the capability of determining the ocean heat storage from the measurement of sea level, ocean mass, and the upper ocean heat content. It should be noted however that the Argo coverage is limited to the upper 2,000 m of the ocean with poor coverage of the tropical Asian Archipelago which might be important to the analysis of the global mean sea level variability. The ocean heat is estimated from the total steric change calculated as the altimetry sea level-GRACE determined mass component. The comparison of the space determined steric sea level to Argo estimated value for the upper 2000 m is consistent within measurement errors of both systems. This is validation of the space system over decadal scale of the overlap between altimetry and GRACE.
Recent studies (Wunsch and Heimbach, 2014; Llovel et al., 2014; Purkey and Johnson, 2010) have suggested that the deep ocean heat change over the past two decades is roughly 10 percent of that of the upper ocean. The ocean heat storage estimated from the difference between altimetry and ocean mass measurement is consistent with the in situ measurement within the observational uncertainties. The three measurements provide a somewhat redundant and self-calibrating observing system.
TABLE E.1 Final Continuity Scoring for the Quantified Objective Ocean Heat Storage
|Radar altimetry + gravity||5||1||0.68||1.0||3.4|
The 1-σ uncertainty of Argo measurement of the upper 2,000 m on decadal scale is 0.1 W m−2 (von Schuckmann and Le Traon, 2011). Assuming the calibration errors of altimetry (from the tide gauge calibration) and GRACE (from the glacial isostatic adjustment correction) have time scales much longer than a decade, the 1-σ error in the full-depth steric sea level rate, computed from the difference between altimetry and GRACE observations over a decade (in which the calibration errors cancel out), is about 0.1 mm yr−1, roughly equivalent to 0.1 W m−2.
The sea level uncertainty of 0.1 mm yr−1 is estimated from the combination (root-sum-squares) of the altimetry error of 0.07 mm yr−1 and the GRACE mass component of sea level error of 0.1 mm yr−1 (Llovel et al., 2014).
The above analysis indicates that the current observing system has a capability of determining the rate of change in ocean heat storage with a 1-σ uncertainty of 0.1 W m−2 over decadal scales. From the equation for the accuracy factor of the quality metric in Section 4.3.1, the quality (accuracy factor) of the observation of ocean heat storage change for meeting the quantified objective has a score of 0.68.
Without in situ calibration, however, the performance of the space part of the observing system of altimetry and gravity might not be stable over multi-decadal scales. There has not been sufficient observation to characterize the possible long-term stability. The analysis is thus quite liberal in the sense that the long-term stability is not accounted for as well as the conversion of the uncertainty of sea level of 0.1 mm yr−1 to 0.1 W m−2 of ocean heat storage. The latter depends on the vertical distribution of the heat, which was simply assumed to be uniform in the calculation.
PROBABILITY OF SUCCESS
Satellite altimetry missions since TOPEX (Ocean Topography Experiment)/Poseidon have had lifetimes for more than twice their designed life of 5 years; the technology as well as implementation has become mature. The probability of success is rated in its highest category with a score of 1.0 (see Section 4.4 for scoring rationale).
The GRACE mission, although first of its kind, has lasted more than twice its designed life of 5 years. The probability of success is assessed to have a score of 1.0. The score for the probability of success for estimating ocean heat storage change is also estimated to be 1.0, which is the multiplication of that of altimetry (1.0) and gravimetry (1.0).
Final continuity scoring for the quantified objective is given in Table E.1 using the benefit (B) formula from Chapter 3 of B = I × U × Q × S, where I ranges from 1 to 5, U ranges from 0 to 1.0, Q ranges from 0 to 1.0, and S ranges from 0 to 1.0 (see Section 4.4 for scoring rationale).
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Purkey, S.G., and G.C. Johnson. 2010. Warming of global abyssal and deep southern ocean waters between the 1990s and 2000s: contributions to global heat and sea level rise budgets. Journal of Climate 23:6336-6351.
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