Susan Lozier at Duke University, US, and Richard Williams at Liverpool University, UK, and co-workers analysed temperature data from the National Oceanic Data Center World Ocean Database from 1950 to 2000. They interpreted the data by using circulation model experiments driven by realistic surface air-sea fluxes and winds over this period. The model predicts how winds, evaporation, rainfall and heat exchange with the atmosphere affect the North Atlantic Ocean's heat content over time.

The experiments showed that water in the subpolar North Atlantic Ocean (north of 45° latitude) became cooler as it exchanged heat with the air above it. In contrast, the subtropical and tropical ocean (south of 45° latitude) warmed up.

"The experiments revealed that much of the heat changes over the North Atlantic Ocean were associated with a wind-induced redistribution of heat together with a background input of heat in the tropics," Williams told environmentalresearchweb. This pattern can be explained by the large-scale change in the wind pattern – as measured by the North Atlantic Oscillation (NAO) index defined by differences in sea-level pressures. This atmospheric variability changes from year-to-year and over decades.

"Heat changes in the ocean show a much more complicated response than expected over the North Atlantic," explained Williams. "The variability in ocean heat content appears to be associated with changes in atmospheric wind forcing, which varies on a decadal timescale. At the moment, it is unclear as to which parts of this warming signal induced by the wind forcing link back to anthropogenic forcing and which parts reflect natural changes in the climate."

The researchers went on to say that they do not want their work to be used as part of a referendum as to whether greenhouse warming is happening. "Anthropogenic warming is almost certainly occurring given the wide range of global signals, including rise in surface and atmospheric temperatures, rise in sea levels, and decline in summer sea ice in the Arctic," stressed Williams. However, these "background" trends could also be being masked by changes that occur over decades on regional scales, he added.

"In our study, we have shown that regional signals in the North Atlantic have a more complex pattern than initially expected, where decadal variability might be masking any background warming trend," he said.

The team will now extend the ocean data analysis and model investigations to further examine interior ocean temperature changes.

The work was published in Sciencexpress.