by Judith Curry

Direct determination of changes in oceanic heat content over the last 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalize e.g., the apparent “pause” in warming. – Wunsch and Heimbach

For context, see these two previous posts at Climate Etc.:

Jim Steele has a very good blog post Deep Oceans are Cooling Amidst a Sea of Uncertainty: New Research on Ocean Heat Content that points to the following paper:

Bidecadal changes in the abyssal ocean

Carl Wunsch and Patrick Heimbach

Abstract. A dynamically consistent state estimate is used for the period 1992-2011 to describe the changes in oceanic temperatures and heat content, with an emphasis on determining the noise background in the abyssal (below 2000 m) depths. Interpretation requires close attention to the long memory of the deep ocean, and implying that meteorological forcing of decades to thousands of years ago should still be producing trend-like changes in abyssal heat content. At the present time, warming is seen in the deep western Atlantic and Southern Ocean, roughly consistent with those regions of the ocean expected to display the earliest responses to surface disturbances. Parts of the deeper ocean, below 3600 m, show cooling. Most of the variation in the abyssal Pacific Ocean is comparatively featureless, consistent with the slow, diffusive approach to a steady state expected there. In the global average, changes in heat content below 2000 m are roughly 10% of those inferred for the upper ocean over the 20 year-period. A useful global observing strategy for detecting future change has to be designed to account for the different time and spatial scales manifested in the observed changes. If the precision estimates of heat content change are independent of systematic errors, determining oceanic heat uptake values equivalent to 0.1 W/m2 is possibly attainable over bidecadal periods.

[Link] to full manuscript

Excerpts from Jim Steele’s post:

Wunsch and Heimbach (2014) humbly admit that their “results differ in detail and in numerical values from other estimates, but the determining whether any are “correct” is probably not possible with the existing data sets.”

They estimate the changing states of the ocean by synthesizing diverse data sets using models developed by the consortium for Estimating the Circulation and Climate of the Ocean, ECCO. The ECCO “state estimates” have eliminated deficiencies of previous models and they claim, “unlike most “data assimilation” products, [ECCO] satisfies the model equations without any artificial sources or sinks or forces. The state estimate is from the free running, but adjusted, model and hence satisfies all of the governing model equations, including those for basic conservation of mass, heat, momentum, vorticity, etc. up to numerical accuracy.”

Their results (Figure 18. below) suggest a flattening or slight cooling in the upper 100 meters since 2004, in agreement with the -0.04 Watts/m2 cooling reported by Lyman (2014). The consensus of previous researchers has been that temperatures in the upper 300 meters have flattened or cooled since 2003,while Wunsch and Heimbach (2014) found the upper 700 meters still warmed up to 2009.

The deep layers contain twice as much heat as the upper 100 meters, and overall exhibit a clear cooling trend for the past 2 decades. Unlike the upper layers, which are dominated by the annual cycle of heating and cooling, they argue that deep ocean trends must be viewed as part of the ocean’s long term memory which is still responding to “meteorological forcing of decades to thousands of years ago”. If Balmaseda and Trenberth’s model of deep ocean warming was correct, any increase in ocean heat content must have occurred between 700 and 2000 meters, but the mechanisms that would warm that “middle layer” remains elusive.

No climate model had predicted the dramatically rising temperatures in the deep oceans calculated by the Balmaseda/Trenberth re-analysis, and oceanographers suggest such a sharp rise is more likely an artifact of shifting measuring systems. Indeed the unusual warming correlates with the switch to the Argo observing system. Wunsch and Heimbach (2013) wrote, “clear warnings have appeared in the literature—that spurious trends and values are artifacts of changing observation systems —the reanalyses are rarely used appropriately, meaning with the recognition that they are subject to large errors.”

More specifically Wunsch and Heimbach (2014) warned, “Data assimilation schemes running over decades are usually labeled “reanalyses.”Unfortunately, these cannot be used for heat or other budgeting purposes because of their violation of the fundamental conservation laws; see Wunsch and Heimbach (2013) for discussion of this important point. The problem necessitates close examination of claimed abyssal warming accuracies of 0.01 W/m2 based on such methods (e.g., Balmaseda et al., 2013).”

See Jim Steele’s post for fascinating regional details of ocean heat storage.

JC reflections

This is a very good paper (and a very good overview by Steele), and there are a number of things I like about it, particularly that they put their findings in the appropriate context of previous findings:

These results differ in detail and in numerical values from other estimates, but the determining whether any are correct” is probably not possible with the existing data sets.

An appropriate context of uncertainty is provided:

The globally integrated heat content changes involve small differences of the much larger regional changes. As existing estimates of the anthropogenic forcing are now about 0.5W/m2, the equivalent global ocean average temperature changes over 20 years are mostly slight compared to the shorter term temporal variations from numerous physical sources. Detailed attention must be paid to what might otherwise appear to be small errors in data calibration, and space-time sampling and model biases. Direct determination of changes in oceanic heat content over the last 20 years are not in conflict with estimates of the radiative forcing, but the uncertainties remain too large to rationalize e.g., the apparent “pause” in warming. The challenge is to develop observations so that future changes can be made with accuracies and precisions consistent with the conventional rule of thumb that they should be better than 10% of the expected signal.

Also, sorting out the global ocean heat content issue requires understanding the regional changes, for which the paper provides important insights.

And finally, the paper makes the important point that long memory of the deep ocean, and implying that meteorological forcing of decades to thousands of years ago should still be producing trend-like changes in abyssal heat content. We simply shouldn’t expect the deep ocean heat content to reflect year to year or even decadal variations in external forcing.

The bottom line is this, which I repeat from the previous thread Ocean heat content uncertainties:

:All in all, I don’t see a very convincing case for deep ocean sequestration of heat.