Zonal extent of oceans, high‐latitude fresh water supplies and the thermohaline circulation

• Published in: Quarterly Journal of the Royal Meteorological Society
• Main Author: Cai, Wenju
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1998
• Subjects: Pacific; Schopf; North Atlantic
• Online Access: http://dx.doi.org/10.1002/qj.49712454708; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1002%2Fqj.49712454708; https://rmets.onlinelibrary.wiley.com/doi/pdf/10.1002/qj.49712454708

Abstract One of the most distinctive features of the present‐day global thermohaline circulation is the absence in the North Pacific Ocean, and the presence in the North Atlantic Ocean, of deep‐water formation. In this paper, the different zonal extents and the associated high‐latitude fresh water supplies of the Pacific and the Atlantic when under a zonally uniform fresh water flux forcing are examined, as a possible cause for the asymmetry. A series of numerical experiments using the Bryan–Cox–Semtner ocean general circulation model is performed. In order to achieve a true independence, the model configuration and the model forcing are so constructed that zonal extent is the only built‐in difference between the two oceans. Three configurations are used. They are (a) an idealized Atlantic Ocean (narrow zonal extent) alone, (b) an idealized Pacific Ocean (wider zonal extent) alone, and (c) a connected Pacific–Atlantic system. Upon spin‐up, the model ocean is forced by a zonally uniform Schopf boundary‐condition for temperature and a zonally uniform fresh water flux for salinity. Both forcings are diagnosed from a spin‐up. All diagnosed fresh water fluxes have net precipitation in polar regions and net evaporation in low‐latitude regions as observed. This feature means that the high‐latitude model Pacific is subject to a fresh water flux twice that in the high‐latitude model Atlantic. This causes the northern sinking cell in the model Pacific of the spin‐up state to collapse; a southern sinking cell settles in. The process is similar to that in many model studies examining the response of the thermohaline circulation to a high‐latitude freshening. By contrast, the northern sinking cell in the North Atlantic of the spin‐up state persists. In the connected Pacific–Atlantic system, these produce the ‘conveyor belt’ type circulation, and the asymmetry between the North Atlantic and the North Pacific. It is found that the high‐latitude fresh water supply is the crucial parameter, and that the difference in basin width ...