Modeling and observations of the Atlantic Meridional Mode during the hurricane season

In this talk we present global climate model (GCM) simulations and observational analyses that demonstrate the influence of the Atlantic Meridional Mode (AMM) on tropical environmental conditions during the Atlantic hurricane season months (August through October). We run two sets of experiments using the National Center for Atmospheric Research Community Atmospheric Model version 3 (CAM3) that is either (i) forced by AMM-related SST anomalies through the Atlantic hurricane season; or (ii) is coupled to a slab ocean model (CAM3+SOM), initialized with AMM-related SST anomalies in July, and allowed to evolve through the following November. Both sets of simulations produce realistic AMM-related atmospheric circulation anomalies, confirming the causal relationship between the AMM and environmental conditions that it generates. Two key circulation anomalies include reduced vertical wind shear and a lower atmospheric pressure during a positive AMM phase in the main development region (MDR) for tropical cyclone formation. These changes are consistent with the relatively high correlation between tropical cyclone activity and genesis region and the AMM index during the hurricane season months, as shown in Kossin and Vimont (2007).

The atmospheric circulation response to the SST anomalies is baroclinic with lower-level (upper-level) low (high) pressure anomalies centered northwest of the MDR during a positive AMM. This kind of circulation acts to reinforce the already positive SST anomalies in the eastern MDR immediately north of the inter-tropical convergence zone position. This feedback loop, which involves the combination of a northward- directed pressure gradient force and a reduced latent heat flux, allows the AMM signature to persist for several months in the CAM3+SOM simulation without additional forcing.

A secondary effort of this research is to determine the cause of AMM-related SST anomalies. Though it is known that the AMM can be forced by the El Nino – Southern Oscillation and the North Atlantic Oscillation, their influence tends to be most prevalent during boreal spring. Instead, we find that the boreal summer and fall AMM can be forced by mid-latitude Atlantic SST anomalies that originate during the previous fall and eventually propagate into the subtropics. We analyze this phenomenon using initialized ensemble integrations of the CAM3+SOM and show that the key to determining the southern extent of SST propagation may be an interaction with the low-level cloud deck off the northwest African coast. Finally, we briefly highlight areas of further research, which include investigation of AMM predictability during hurricane season and also interaction between the AMM and the Atlantic Nino mode.