The recent (early 21st century) temporary, global mean surface warming slowdown (SWS) compared to surface warming predictions by climate models has stimulated much scientific (and otherwise) debate regarding its observational constraints and its cause(s). To the extent that variability and change in ocean heat uptake, storage and redistribution play an important role in deciphering the dynamics of the SWS, obtaining a quasi-complete time-evolving description of the ocean state over recent decades provides a valuable basis for understanding and quantifying contributing processes. The global multi-decadal state estimate produced by the “Estimating the Circulation and Climate of the Ocean” (ECCO) group is the result of a least-squares fit of a global general circulation model to most of the available satellite and in-situ observations. Unlike most SWS studies to date, the estimate uses high-quality satellite altimetry as an important observational constraint, especially over the period preceding the advent of the global Argo float array. Unlike other so-called ocean reanalyses that exhibit large (and at times unphysical) spreads, the estimate reproduces well the vertical structure in ocean heat content changes apparent in the major hydrographic data sets, such as the World Ocean Atlas (WOA). We attribute this to the strict dynamical consistency and property conservation afforded by the adjoint-based estimation approach, which precludes unphysical “analysis increments” incurred in filter-based data assimilation algorithms. Availability of the full state enables detailed analysis, such as regional heat budgets, vertical heat flux patterns as a function of depth, or patterns of regional thermosteric accelerations/decelerations. Thermosteric changes do not necessarily reflect local surface heat input, but may be due to ocean dynamics. The inferred net acceleration in global mean thermosteric height is only a small residual of various regional anomalies of much larger amplitudes. ECCO confirms that despite the temporary slowdown in surface warming, subsurface warming has continued unabated over the SWS period. Results indicate that quantification of the evolution of global mean quantities during the SWS likely requires accounting for many different regions and dynamical processes.
By: Patrick Heimbach, Associate Professor, UT Austin, ICES/JSG
Click for a live broadcast
Host: Yuko Okumura, UTIG