The Influence of the AMOC Variability on the Atmosphere in CCSM3

Abstract

The influence of the Atlantic meridional overturning circulation (AMOC) variability on the atmospheric circulation is investigated in a control simulation of the NCAR Community Climate System Model, version 3 (CCSM3), where the AMOC evolves from an oscillatory regime into a red noise regime. In the latter, an AMOC intensification is followed during winter by a positive North Atlantic Oscillation (NAO). The atmospheric response is robust and controlled by AMOC-driven SST anomalies, which shift the heat release to the atmosphere northward near the Gulf Stream/North Atlantic Current. This alters the low-level atmospheric baroclinicity and shifts the maximum eddy growth northward, affecting the storm track and favoring a positive NAO. The AMOC influence is detected in the relation between seasonal upper-ocean heat content or SST anomalies and winter sea level pressure. In the oscillatory regime, no direct AMOC influence is detected in winter. However, an upper-ocean heat content anomaly resembling the AMOC footprint precedes a negative NAO. This opposite NAO polarity seems due to the southward shift of the Gulf Stream during AMOC intensification, displacing the maximum baroclinicity southward near the jet exit. As the mode has somewhat different patterns when using SST, the wintertime impact of the AMOC lacks robustness in this regime. However, none of the signals compares well with the observed influence of North Atlantic SST anomalies on the NAO because SST is dominated in CCSM3 by the meridional shifts of the Gulf Stream/ North Atlantic Current that covary with the AMOC. Hence, although there is some potential climate predictability in CCSM3, it is not realistic.

Mechanisms of Multidecadal Atlantic Meridional Overturning Circulation Variability Diagnosed in Depth versus Density Space

“…As the AMOC intensification is driven by the positive NAO in the first place, this implies a weak positive feedback between the AMOC and the NAO, which would enhance the low-frequency power of the ocean-only mode suggested by Kwon and Frankignoul (2012). In regime 1, Frankignoul et al (2013) found no direct evidence of AMOC influence on the atmosphere. However, upper ocean heat content anomalies resembling the AMOC footprint were found to precede the negative NAO, albeit lacking robustness, which hints at a weak negative feedback that should contribute to the oscillatory character of regime 1.…”

“…The basic features of the North Atlantic ocean circulation in CCSM3 have been presented in Danabasoglu (2008), Kwon and Frankignoul (2012), Tulloch and Marshall (2012), and Frankignoul et al (2013). Here we briefly discuss the mean fields in regime 1.…”

“…14b). The heat flux damping is 20-25 W m 22 8C 21 along the North Atlantic Current path in CCSM3 (Frankignoul et al 2013), so that approximately 10 yr are needed to damp temperature (or equivalently density) anomalies in a mixed layer of about 200-m depth. On the other hand, the slightly wider and weaker eastern subpolar gyre in regime 2 fails to advect the negative density anomalies all the way to the convection site, which does not favor an oscillatory AMOC behavior ( Fig.…”

“…They concluded that the red noise-like AMOC variability is primarily an ocean-only mode stochastically forced by the NAO surface fluxes, a similar conclusion to Tulloch and Marshall's (2012) for regime 1. Frankignoul et al (2013) examined the AMOC-toatmosphere feedback and found in regime 2 that weak but significant atmospheric circulation anomalies resembling a positive NAO were following the stronger AMOC. As the AMOC intensification is driven by the positive NAO in the first place, this implies a weak positive feedback between the AMOC and the NAO, which would enhance the low-frequency power of the ocean-only mode suggested by Kwon and Frankignoul (2012).…”

Changes in the seasonal cycle of the Atlantic meridional heat transport in a RCP 8.5 climate projection in MPI-ESM

“…Furthermore, a response of the North Atlantic Oscillation (NAO) to North Atlantic SSTs has been found both in observations and model studies Frankignoul, 1999, 2002;Rodwell and Folland, 2002;Frankignoul et al, 2013;Gastineau et al, 2013;Gastineau and Frankignoul, 2015). Via the Atlantic Multidecadal Oscillation (AMO), which is thought to be associated with AMOC and OHT variability (e.g.…”