The rôle of thermohaline circulation in climate

Abstract

This article discusses the rôle of the THC in climate, based upon the results of several numericalexperiments which use a coupled ocean-atmosphere model developed at the Geophysical FluidDynamics Laboratory of NOAA, USA. The first part of the article explores the mechanism which isresponsible for the abrupt climate change such as the Younger Dryas event using the coupled model.In response to the freshwater discharge into high north Atlantic latitudes over a period of 500 years,the THC in the Atlantic Ocean weakens, reducing surface air temperature over the northern northAtlantic Ocean, the Scandinavian Peninsula, and the circumpolar ocean and Antarctic Continent ofthe southern hemisphere. Upon the termination of the water discharge at the 500th year, the THCbegins to intensify, regaining its original intensity in a few hundred years. In addition, the suddenonset and the termination of the discharge of freshwater induces the multidecadal fluctuation in theintensity of the THC, which generates the almost abrupt change of climate. It is noted that similarbut much weaker oscillation of the THC is also evident in the control integration of the coupledmodel without freshwater forcing. The irregular oscillation of the THC mentioned above appears tobe related to the fluctuation of the Subarctic Gyre and associated east Greenland current, yieldingthe evolution of the surface salinity anomaly which resembles that of “great salinity anomaly”. Thesecond part of this article describes the response of a coupled ocean-atmospheremodel to the doublingand quadrupling of atmospheric carbon dioxide over centuries time-scale. In one integration, the CO₂ concentration increases by 1%/year (compounded) until it reaches 4× the initial value at the140th year and remains unchanged thereafter. In another integration, the CO₂ concentration alsoincleases at the rate of 1%/year until it reaches 2× the initial value at the 70th year and remainsunchanged thereafter. One of the most notable features of the CO₂-quadrupling integration is thegradual disappearance of thermohaline circulation in most of the model oceans during the first250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishesin the north Atlantic by the 250 years of the integration and remains very weak until the 900thyear. However, it begins to restore the original intensity by the 1600th year. In the CO₂-doublingintegration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlanticduring the first 150 years, but almost recovers its original intensity by the 500th year. The weakeningof the THC moderates temporarily the greenhouse warming over the north Atlantic Ocean and itsvicinity. In both numerical experiments described above, the initial weakening of the THC resultsfrom the capping of oceanic surface by relatively fresh, low-density water, which surpresses the convectivecooling of water in the sinking region of the THC.