by Judith Curry
Kevin Trenberth famously stated in the CRU emails:
The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.
For additional background, refer to this previous Climate Etc. post Where’s the missing heat?
In a new paper, Trenberth and collaborators argue that the ‘missing’ heat is sequestered in the ocean, below 700 m.
Distinctive climate signals in reanalysis of global ocean heat content
Magdalena Balmaseda, Kevin Trenberth, Erland Kallen
Abstract. The elusive nature of the post-2004 upper ocean warming has exposed uncertainties in the ocean’s role in the Earth’s energy budget and transient climate sensitivity. Here we present the time evolution of the global ocean heat content for 1958 through 2009 from a new observational-based reanalysis of the ocean. Volcanic eruptions and El Niño events are identified as sharp cooling events punctuating a long-term ocean warming trend, while heating continues during the recent upper-ocean-warming hiatus, but the heat is absorbed in the deeper ocean. In the last decade, about 30% of the warming has occurred below 700 m, contributing significantly to an acceleration of the warming trend. The warming below 700 m remains even when the Argo observing system is withdrawn although the trends are reduced. Sensitivity experiments illustrate that surface wind variability is largely responsible for the changing ocean heat vertical distribution.
Accepted for publication in Geophysical Research Letters, [link] to abstract.
The main figure under discussion is this one:
Figure 1: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.
So . . . has Trenberth found the ‘missing’ heat? To evaluate this, we need to dig into the ORAS4 ocean reanalysis. For background on reanalyses, see this previous Climate Etc. post reanalyses.org. Here is the paper describing the reanalyses.
Evaluation of the ECMWF Ocean Reanalysis ORAS4
Magdalena Balmaseda, Kristian Mogensen, Anthony Weaver
Abstract. A new operational ocean reanalysis (ORAS4) has been implemented at ECMWF. It spans the period 1958 to present. This paper describes its main components and evaluates its quality. The adequacy of ORAS4 for the initialization of seasonal forecasts is discussed, along with the robustness of some prominent climate signals.
ORAS4 has been evaluated using different metrics, including comparison with observed ocean currents, RAPID-derived transports, sea level gauges, and GRACE-derived bottom pressure. Compared to a control ocean model simulation, ORAS4 improves the fit to observations, interannual variability, and seasonal forecast skill. Some problems have been identified, such as the underestimation of meridional overturning at 26◦N, the magnitude of which is shown to be sensitive to the treatment of the coastal observations.
ORAS4 shows a clear and robust shallowing trend of the Pacific Equatorial thermocline. It also shows a clear and robust nonlinear trend in the 0-700m ocean heat content, consistent with other observational estimates. Some aspects of these climate signals are sensitive to the choice of SST product and the specification of the observation-error variances. The global sea level trend is consistent with the altimeter estimate, but the partition into volume and mass variations is more debatable, as inferred by discrepancies in the trend between ORAS4- and GRACE-derived bottom pressure.
Submitted to the Quarterly Journal of the Royal Meteorological Society. [link] to full text.
JC comments: Ocean analysis (ocean models that assimilate observations ) is used to initialize ocean models are critical for making weather forecasts at timescales beyond 10 days using coupled atmosphere/ocean models. Ocean RE-analysis uses the same version of an ocean model to assimilate cleaned up historical ocean observations into a homogeneous analysis of historical ocean states. The second paper that describes the reanalysis process. It is a complex and sophisticated undertaking.
To what extend should we have confidence in the reanalysis results? Based upon verification statistics, there is clearly some advantages to the reanalysis relative to the raw observations. However the big issue is whether we can infer reliable global trends from the reanalysis, owing to changes in the observing system (not just for the ocean, but for the surface fluxes derived from atmospheric reanalyses), and uncertainties in the overall methodology. The surprising finding is the apparent sequestration of heat in the global ocean starting circa 2000, which has been accompanied by a flattening of the trend of upper ocean temperatures since 2003. Is this real, or an artifact of the reanalysis process? We don’t know, there is a debate underway in the oceanographic and climate communities on this topic.
Lets assume for the moment that the sequestration of heat in the deep ocean since 2000 is robust. What might be the cause of this, and what is the physical mechanism? Well, the only conceivable mechanism is associated with ocean circulations, driven internally by thermohaline processes or by the wind driven circulation. The reanalyses can be used in principle to diagnose what is going here in terms of which ocean basin/region is leading this effect, seasonality, etc. So what happened circa 2000? Well, there was climate shift associated with a massive reorganization of circulation patterns in the Pacific. Is this of relevance? Who knows, no one has done the analysis on this to my knowledge.
So has Trenberth found the ‘missing’ heat? His recent paper is inconclusive on this. The important point to me is that the new ocean reanalysis products support much more comprehensive diagnostics of heat transfer and storage in the ocean, including dynamical mechanisms in the context of overall climate dynamics. Focusing on a global trend from the reanalysis data isn’t how to extract the useful content from these data, IMO.