2.2
Projections of a rebound in warming out of the current hiatus

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Monday, 5 January 2015: 1:45 PM
122BC (Phoenix Convention Center - West and North Buildings)
Matthew H. England, University of New South Wales, Sydney, NSW, Australia

Despite ongoing increases in atmospheric greenhouse gases, the Earth's global average surface air temperature has remained more or less steady since 2001. Mechanisms proposed to account for this slowdown in surface warming include increased ocean heat uptake, the prolonged solar minimum and changes in atmospheric water vapour and aerosols. While cool sea surface temperature in the east Pacific has been identified as a key component of the global hiatus, it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades unprecedented in observations/reanalysis data and not captured by climate models is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming via increased subsurface ocean heat uptake. The extra uptake has come about via increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering SST there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1 0.2C, which can account for much of the hiatus in surface warming observed since 2001. Simulations using coupled climate models suggest the hiatus could persist for much of the present decade if the trade wind trends continue, however a rebound of rapid warming is expected to resume once the anomalous wind trends abate.