Sunday, July 21, 2013

A Looming Climate Shift: Will Ocean Heat Come Back to Haunt Us?

Average sea surface temperature trends from the climate model simulations for a) 'hiatus' decades, i.e. decades with no warming of global mean surface temperatures, and b) 'accelerated' decades, i.e. decades with greater-than-average rises in global surface temperatures. The subtropical ocean gyres (green ellipses) are key players in the downward transport of heat. The stippling indicates areas where this trend is statistically significant. From Meehl (2013).
Despite a large increase in heat being absorbed by the Earth's climate system (oceans, land & ice), the first decade of the 21st century saw a slowdown in the rate of global surface warming (surface air temperatures).

A climate model-based study, Meehl (2011), predicted that this was largely due to anomalous heat removed from the surface ocean and instead transported down into the deep ocean. This anomalous deep ocean warming was later confirmed by observations.

This deep ocean warming in the model occurred during negative phases of the Interdecadal Pacific Oscillation (IPO), an index of the mean state of the north and south Pacific Ocean, and was most likely in response to intensification of the wind-driven ocean circulation.

Meehl (2013) is an update to their previous work, and the authors show that accelerated warming decades are associated with the positive phase of the IPO. This is a result of a weaker wind-driven ocean circulation, when a large decrease in heat transported to the deep ocean allows the surface ocean to warm quickly, and this in turn raises global surface temperatures.

This modeling work, combined with current understanding of the wind-driven ocean circulation, implies that global surface temperatures will rise quickly when the IPO switches from the current negative phase to a positive phase.

A Looming Climate Shift: Will Ocean Heat Come Back to Haunt Us?

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