Removing the North Pacific halocline: Effects on global climate, ocean circulation and the carbon cycle

A well-pronounced halocline is a key feature of today's subarctic North Pacific. There is indirect paleo-evidences from the last glacial termination as well as from the early and middle Pliocene that this halocline has not always been there. To study the effects of North Pacific salinity on global climate, ocean circulation and the marine carbon cycle, we perform idealized experiments using an Earth system model of intermediate complexity (LOVECLIM). Imposing a negative freshwater flux in the northern North Pacific, the halocline vanishes and a deep Pacific meridional overturning circulation (PMOC) establishes. The associated increase of meridional heat transport in the Pacific leads to a bipolar seesaw response in temperature, with warming in the North Pacific and over North America and cooling in the Southern Ocean. As a result of the formation of North Pacific deep water (NPDW), the surface branch of the global conveyor belt circulation weakens. Transport through the Indonesian Seas decreases by 50% as the warm and saline waters of the equatorial Pacific are diverted into the North Pacific. In our idealized experiments, the enhanced global deep water formation is balanced by an increase in diapycnal mixing. As a result nutrient concentrations in the euphotic zone increase by about 25% globally, leading to a 20% increase in global export production. The effect of greater export production on atmospheric pCO ₂ is, however, compensated by the enhanced transport of dissolved inorganic carbon (DIC) to the surface. As a result, the atmospheric CO ₂ concentration increases by only 5ppmv. Our results further suggest that the absence of the subarctic halocline for instance during Heinrich event 1 and the Pliocene may have exerted a strong influence on global climate and the carbon cycle.