Passive tracers and active dynamics - a model study of hydrography and circulation in the northern North Atlantic

Abstract

A long-standing problem in oceanography has been to understand the relationship between what can be measured in the ocean, such as hydrography, and what cannot, such as the strength and structure of the complete Meridional Overturning Circulation (MOC) of the world oceans, commonly considered the main oceanic long-term modifier of Earth’s climate. With the aid of a 50 year simulation from a numerical ice-ocean model we have investigated this relationship in the area of the northernmost extension of the MOC, in the Subpolar and Nordic Seas, on interannual timescales. We find that variability in the northward flux of salt and temperature in this region is controlled almost entirely by the volume flux, confirming that a knowledge of the variability of the circulation strength proper is necessary. The simulated hydrographic anomalies are within the range observed in the 20th century, thus fundamental changes to the circulation were not expected nor found. It is seen that variability in either temperature or salinity does contain some information about the variability in current strength, because hydrography and circulation generally respond to the same atmospheric forcing in the North Atlantic sector. Whether it is temperature or salinity that contains the information is related to the parameter range of the equation of state at the location in question: if density depends primarily on temperature then a salinity anomaly will tend to survive and vice versa. The oceanic response involves hydrographic changes and propagation of these, gyre strength changes, and changes in the MOC.