| 英文摘要: | Surface global warming has stalled since around 2000 despite increasing atmospheric CO2. A study finds that recent strengthening of Pacific trade winds has enhanced heat transport from the surface to ocean depths, explaining most of the slowed surface warming. Despite an increasing concentration of CO2 in the atmosphere, global-mean surface temperature has been quite steady since around the turn of this century. A variety of causes have been proposed for this global warming hiatus1, which fall into two categories. First is a reduction in the top-of-atmosphere radiative imbalance, which could be a result of solar variability, stratospheric water vapour increase, an increase of airborne particles or a reduction in methane emissions to name a few. In the other category, the extra heat absorbed by the climate system is not spent warming the surface but somehow is stored elsewhere, with the ocean a likely candidate. In this issue of Nature Climate Change, Matthew England and co-workers2 report that the strengthening of Pacific trade winds during the past two decades has facilitated heat uptake by the subsurface ocean. Trade winds — prevailing surface winds blowing westwards over the tropical ocean — drive the underlying ocean circulation. Owing to the Earth's rotation, ocean surface currents turn to the right of the wind direction in the northern hemisphere and to the left in the southern hemisphere. Thus, trade winds induce divergent surface flow along the equator, pumping up cooler subsurface water in the central and eastern Pacific. They also blow warm water to the western Pacific where it piles up, and the resulting east–west pressure gradient sustains equatorward subsurface flow, which brings warmer subtropical water to the equatorial subsurface (at a depth below ~100 m). This system varies with time. When trade winds are stronger, this oceanic overturning circulation effect intensifies, cooling the surface while warming the deeper ocean. The opposite change occurs when trade winds weaken. Moreover, active atmospheric convection shifts concurrently with the sea surface temperature and feeds back to the trade-wind changes. A natural pattern of decade-to-decade variability called the Pacific Decadal Oscillation (PDO), also known as the Interdecadal Pacific Oscillation, highlights this tropical Pacific feature. The negative phase of the PDO, characterized by cooler equatorial Pacific conditions, charges heat into the subsurface ocean, and vice versa for the positive phase. The PDO was negative from the mid-1940s to 1970s, shifting to the positive phase until the late 1990s before again trending negatively so far this century (see Fig. 1). England and colleagues investigate this recent negative trend with a global ocean model. To constrain the model PDO to follow the observations, they superpose observed trends of surface winds over the Pacific from 1992 to 2011. The result accurately reproduces the active subsurface ocean heat uptake and surface cooling in the equatorial Pacific, even if increasing greenhouse gas concentrations are taken into account. Then they utilize a global ocean–atmosphere climate model forced with CO2 emissions scenarios, again constraining the model PDO in a similar way. Through the atmosphere, the influence of surface cooling spreads out from the equatorial Pacific and cools the global surface, causing the surface warming hiatus as observed3.
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