Climate models represent variability in ocean heat content on interannual-to-decadal time scales much more realistically than previously believed, a study by an Indian scientist has revealed.
Krishna Achuta Rao, previously at Livermore's Program for Climate Model Diagnosis and Intercomparison (PCMDI), and now at IIT, Delhi, used 13 numerical climate models to find the apparent discrepancies between modelled and observed variability.
He found, the variability could be explained by accounting for changes in observational coverage and instrumentation and by including the effects of volcanic eruptions.
He said the study has cast doubt on recent findings that the top 700-meters of the global ocean cooled markedly from 2003-2005. "Our analysis shows that the 2003-2005 'cooling' is largely an artefact of a systematic change in the observing system. The previous research was based on looking at the combined ocean temperature observations from several different instrument types, which collectively appear to have a cooling effect. But if you look at the observational instruments individually, there is no cooling," he said.
As part of the study, Rao and his team also looked at the impacts of changes in ocean observing systems. Researchers in Germany had recently discovered a warm bias in the older instruments.
However, now with the introduction of new instruments called Argo floats, more complete and more reliable ocean temperature measurements have become possible.
The first Argo floats were deployed in the Atlantic in 2000 and their network has rapidly ramped up to several thousand floats with near-global coverage of the world's oceans.
"This transition from a measuring system biased warm to a more realistic one appears as a cooling. Obviously, models can't account for spurious variability caused by instrument changes," said Rao.
The study appears in the June 18 online edition of the journal Proceedings of the National Academy of Sciences (PNAS).