An Investigation into the Role of External Forcing and Ocean Coupling on the Relationship between the Atlantic Multidecadal Oscillation and Vertical Wind Shear in the Main Development Region

Observations show that multidecadal variability in vertical wind shear over the historical period coincides with changes in sea surface temperature (SST). It has previously been suggested that the cause of this correlation is changes in ocean circulation. Here we test the roles of internal atmospheric and oceanic circulation changes, as well as transient historical forcing (i.e. CO2 and aerosols). We analyze large ensembles using the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM). The fully coupled model consists of a 42- member ensemble (LENS-Fully Coupled), while the slab ocean model (SOM) consists of a 10-member ensemble (LENS-SOM) simulated over the historical period, from 1920-2005. We examine the role of ocean circulation by comparing the relationship between sea vertical wind shear in the Main Development Region (MDR) (80°W-20°W, 10°N-20°N) and the AMO in the LENS-Fully Coupled and LENS-SOM experiments. The role of forcing on vertical wind shear is examined by comparing the ensemble mean to the ensemble spread. We find that the model produces a correlation between the AMO and Vertical Wind Shear Variability that is comparable to observations, but the correlation appears to be mainly driven by transient historical forcing. The inclusion of ocean circulation in the Fully-Coupled ensembles does not improve the correlation values, but instead, degrades the relationship. This occurs particularly at high frequencies, which suggests that it is mainly due to ENSO.