Passive tracers and active dynamics: A model study of hydrography and circulation in the northern North Atlantic

Abstract

[1] 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 twentieth 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.

CMIP5 Model Intercomparison of Freshwater Budget and Circulation in the North Atlantic

“…They confirm that the subpolar gyre and Atlantic meridional overturning circulation (AMOC) intensified concomitantly from the early 1970s to the mid-1990s and decreased thereafter (H at un et al 2005;Mauritzen et al 2006;Deshayes and Frankignoul 2008). These studies also indicate that recent interannual to decadal variability of subpolar gyre circulation was consonant with atmospheric fluctuations of wind and buoyancy forcing.…”

Importance of heat transport and local air‐sea heat fluxes for Barents Sea climate variability

“…[12] For a more detailed description of the model physics and the performance, we refer the reader to previous studies [Nilsen et al, 2003;Bentsen et al, 2004;Gao et al, 2004;Drange et al, 2005;Hátun et al, 2005aHátun et al, , 2005bEldevik et al, 2005;Mauritzen et al, 2006;Orre et al, 2007]. In general, the model system has demonstrated good skills in simulating the ocean circulation and thermodynamics in the Atlantic and Arctic waters.…”

The role of salinity in the decadal variability of the North Atlantic meridional overturning circulation

“…Detailed descriptions of the model, its circulation and water property characteristics have been previously published (e.g., Drange et al, 2005;Hatun et al 2005a and b;Mauritzen et al 2006;Deshayes et al 2007;DF08). We refer the interested reader to these sources, and provide a more brief description here.…”

“…The simulation has been used in a number of studies which noted favorable comparisons with hydrographic observations, in spite of diminished exchanges of dense waters between the Nordic Seas and the subpolar domain (e.g., Hátún et al 2005;Mauritzen et al 2006). In the model, the MOC is primarily driven by deep convection in the subpolar gyre (Deshayes and Frankignoul 2008, hereafter DF08).…”