Wintertime Atmospheric Response to North Atlantic Ocean Circulation Variability in a Climate Model

Abstract

Maximum covariance analysis of a preindustrial control simulation of the NCAR Community Climate System Model, version 4 (CCSM4), shows that a barotropic signal in winter broadly resembling a negative phase of the North Atlantic Oscillation (NAO) follows an intensification of the Atlantic meridional overturning circulation (AMOC) by about 7 yr. The delay is due to the cyclonic propagation along the North Atlantic Current (NAC) and the subpolar gyre of a SST warming linked to a northward shift and intensification of the NAC, together with an increasing SST cooling linked to increasing southward advection of subpolar water along the western boundary and a southward shift of the Gulf Stream (GS). These changes result in a meridional SST dipole, which follows the AMOC intensification after 6 or 7 yr. The SST changes were initiated by the strengthening of the western subpolar gyre and by bottom torque at the crossover of the deep branches of the AMOC with the NAC on the western flank of the Mid-Atlantic Ridge and the GS near the Tail of the Grand Banks, respectively. The heat flux damping of the SST dipole shifts the region of maximum atmospheric transient eddy growth southward, leading to a negative NAO-like response. No significant atmospheric response is found to the Atlantic multidecadal oscillation (AMO), which is broadly realistic but shifted south and associated with a much weaker meridional SST gradient than the AMOC fingerprint. Nonetheless, the wintertime atmospheric response to the AMOC shows some similarity with the observed response to the AMO, suggesting that the ocean-atmosphere interactions are broadly realistic in CCSM4.

Sub-decadal North Atlantic Oscillation variability in observations and the Kiel Climate Model

“…In particular, the forced atmosphere model experiment supports the existence of a positive atmosphere-ocean feedback. Atmospheric sensitivity to AMOC-related dipolar SST changes in the North Atlantic also has been recently reported from another climate model (Frankignoul et al 2015). The limited observational data also support such an atmospheric sensitivity to mid-latitudinal SST anomalies (Bryden et al 2014;Czaja and Frankignoul 1999;Czaja and Frankignoul 2002).…”

Abrupt cold events in the North Atlantic Ocean in a transient Holocene simulation

“…During event 2 the changes in ocean circulation and wind increase the transport of sea ice from the Cana- dian Shelf and Baffin Bay to the area of deep convection in the NWA. Other studies revealed that a SST anomaly pattern similar to our study can force a positive NAO phase (e.g., Czaja and Frankignoul, 2002;Frankignoul and Gastineau, 2015;Gastineau and Frankignoul, 2014). This would produce a positive feedback where the positive NAO triggers such a SST anomaly pattern and vice versa, leading the cold climate state to maintain for decades.…”

Lagged influence of Atlantic and Pacific climate patterns on European extreme precipitation

“…3e,f ). These results offer potential predictability of European extreme precipitation on a seasonal scale and in general mark a significant step forward, given the large climate variability and a limited amplitude of the atmospheric response over the Atlantic-European sector 44 , 45 . Although investigation of the underlying physical mechanisms responsible for such a long memory of atmospheric anomalies is beyond the scope of this study, it is anticipated that the following two consecutive processes are in play: the persistence of the atmospheric winter ENSO signal in the stratosphere (after upward propagation of Rossby waves from the troposphere to the stratosphere) considering its slow evolution, and the atmosphere–ocean mixed layer interaction in the North Atlantic 43 , 46 .…”