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

Abstract

An OGCM hindcast is used to investigate the linkages between North Atlantic Ocean salinity and circulation changes during . The focus is on the eastern subpolar region consisting of the Irminger Sea and the eastern North Atlantic where a careful assessment shows that the simulated interannual to decadal salinity changes in the upper 1500 m reproduce well those derived from the available record of hydrographic measurements. In the model, the variability of the Atlantic meridional overturning circulation (MOC) is primarily driven by changes in deep water formation taking place in the Irminger Sea and, to a lesser extent, the Labrador Sea. Both are strongly influenced by the North Atlantic Oscillation (NAO). The modeled interannual to decadal salinity changes in the subpolar basins are mostly controlled by circulation-driven anomalies of freshwater flux convergence, although surface salinity restoring to climatology and other boundary fluxes each account for approximately 25% of the variance. The NAO plays an important role: a positive NAO phase is associated with increased precipitation, reduced northward salt transport by the wind-driven intergyre gyre, and increased southward flows of freshwater across the Greenland-Scotland ridge. Since the NAO largely controlled deep convection in the subpolar gyre, fresher waters are found near the sinking region during convective events. This markedly differs from the active influence on the MOC that salinity exerts at decadal and longer timescales in most coupled models. The intensification of the MOC that follows a positive NAO phase by about 2 years does not lead to an increase in the northward salt transport into the subpolar domain at low frequencies because it is cancelled by the concomitant intensification of the subpolar gyre which shifts the subpolar front eastward and reduces the northward salt transport by the North Atlantic Current waters. This differs again from most coupled models, where the gyre intensification precedes that of the MOC by several years.3

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

“…In NCAR and MPI-M, surface fluxes gain importance in the FW budget. The high correlation between FW content changes and FW convergence is consistent with hindcast simulations (e.g., Frankignoul et al 2009) and the preindustrial control run of another climate model (Wu and Wood 2008). Direct observations of oceanic full-depth circulation are too rare to evaluate FW convergence to the subpolar gyre and check whether this result is realistic.…”

North Atlantic Subpolar Gyre Surface Variability (1895–2009)

“…Thierry et al (2007); for the changes in eastern Atlantic, cf. Hä tun et al (2005) and Frankignoul et al (2009), mostly from model studies]. Advective signals in the upper layer are also detected mostly along the path of the NAC and its branches in the subpolar gyre from the salinity and temperature variability of 10 yr or less [Belkin et al (1998) for the Great Salinity Anomalies (GSA); Reverdin et al (1997); Holliday et al (2008)], and have also been commented on in model studies by Hä kkinen (1993,2002).…”

“…This hints that, on these decadal time series, surface changes are indicative of large advective changes associated with the ebbing and flowing of more subtropical water to the northern latitudes. Model studies for the late 1950s to the early 2002 period (Hä tun et al 2005;Frankignoul et al 2009) reinforce this dominant advective source of the variability and relate the recent change to gyre intensity and shape changes. This is also suggested in the long hydrographic time series (Holliday et al 2008).…”

Extinction of the northern oceanic deep convection in an ensemble of climate model simulations of the 20th and 21st centuries

“…In present-day climate conditions, it has been shown that the variability of deep convection in the Labrador Sea is largely driven by the North Atlantic Oscillation (NAO); through its influence on the wintertime buoyancy loss to the atmosphere (Latif et al 2006;Frankignoul et al 2009) (with positive NAO conditions promoting convective mixing). Furthermore, the transport of freshwater from the Fram Strait along the Greenland coast into the Labrador Sea is also known as a contributing factor to deep convection (Holland et al 2001;Jungclaus et al 2005;Koenigk et al 2006).…”

“…Furthermore, the transport of freshwater from the Fram Strait along the Greenland coast into the Labrador Sea is also known as a contributing factor to deep convection (Holland et al 2001;Jungclaus et al 2005;Koenigk et al 2006). In a warmer climate, however, the deep convective mixing that occurs in the Labrador Sea and the Irminger Sea is expected to decline due to a warming and a freshening of the surface (Latif et al 2006;Deshayes et al 2007; Koenigk et al 2007;Frankignoul et al 2009). …”