A general view of boreal spring to summer interannual variability: Emergence of evolving tropical Atlantic modes

The Atlantic Meridional Mode (AMM) and Atlantic Zonal Mode (AZM) dominate the boreal spring and summer Tropical Atlantic variability (TAV), respectively, at interannual time scales, with pronounced impacts on the climate of adjacent and remote areas. Previous studies demonstrated the existence of an AMM-AZM connection via ocean wave propagation and modulated by the local wind forcing.

Here, we use a novel approach based on Extended Maximum Covariance Analysis (EMCA) to investigate the emergence of evolving spring-to-summer TAV modes in the observational record and its multidecadal modulation. Observational and reanalysis datasets reveal that the first evolving mode corresponds to a basin-wide warming with maximum anomalies over the tropical north Atlantic during boreal spring and equatorial warm conditions in summer season. In contrast, the second evolving mode displays an inter-hemispheric SST gradient during boreal spring that persists until summer months. The first and second evolving modes can be associated with a same-sign and opposite-sign relation between the AMM and AZM, respectively.

The expansion coefficients of the evolving modes are positively and negatively correlated at decadal time scales during the observational record, suggesting the emergence of diverse spatial configurations. This multidecadal modulation coincides with different global ocean background states that resemble the Atlantic Multidecadal Variability (AMV) and Pacific Decadal Variability (PDV).

To corroborate the above-mentioned observational findings, these results will be compared with those from historical and picontrol simulations from the latest state-of-the-art CMIP6 models.