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The largest sea surface temperature (SST) anomalies associated with Atlantic Multidecadal Variability (AMV) occur over the Atlantic subpolar gyre, yet it is the tropical Atlantic from where the global impacts of AMV originate. Processes that communicate SST change from the subpolar Atlantic gyre to the tropical North Atlantic thus comprise a crucial mechanism of AMV. Here we use idealized model experiments to show that such communication is accomplished by an “atmospheric bridge.” Our results demonstrate an unexpected asymmetry: the atmosphere is effective in communicating cold subpolar SSTs to the north tropical Atlantic, via an immediate extratropical atmospheric circulation change that invokes slower wind‐driven evaporative cooling along the Eastern Atlantic Basin and into the tropics. Warm subpolar SST anomalies do not elicit a robust tropical Atlantic response. Our results highlight a key dynamical feature of AMV for which warm and cold phases are not opposites.

The influence of coupled model sea surface temperature (SST) climatological biases and SST projections on daily convection over the Intra‐American Seas (IAS) during the May–November rainy season are examined by clustering (k − means) daily OLR anomalies in ECHAM5 atmospheric global climate model (AGCM) experiments. The AGCM is first forced by 1980–2005 observed SSTs (GOGA), then with climatological, multi‐model mean monthly climatological SST bias from 31 CMIP5 coupled models (HIST) and projected SST changes for 2040–2059 (PROJ) and 2080–2099 (PROJ2) imposed on top of observed values. A typology of seven recurrent convection regimes is identified and consists of three dry and four wet regimes, including three regimes characterized by tropical‐midlatitude interactions between surface convection cells across the IAS and Rossby wave in the upper‐troposphere, and a regime of broad wettening typical of the ITCZ. Compared to an earlier observational study, all seven regimes are reasonably well reproduced in the HIST runs. However, the latter exhibit drier dry regimes, a less wet ITCZ‐like wet regime and a southeastward shift of convective anomalies developing across the IAS in the three other regimes, all result in a drier simulated IAS climate compared to GOGA. ECHAM5 projection runs for PROJ and PROJ2 are both characterized by the inhibition of the broad ITCZ‐like wet regime, indicating a significant trend towards more frequent dry weather. Meanwhile, convection anomalies related to tropical‐midlatitude interactions are shifted further east of the Caribbean as lead increases. These results suggest more frequent and intense extreme rainfall over the tropical Atlantic and the southeast US, while parts of the Caribbean are projected to experience drier climate. The projected drying, however, is of the same order of magnitude as results from historical SST biases, suggesting that the latter need to be considered in model projections, which might underestimate future IAS drying.

The contribution of individual aerosol species and greenhouse gases to precipitation changes during the South Asian summer monsoon is uncertain. Mechanisms driving responses to anthropogenic forcings need further characterization. We use an atmosphere‐only climate model to simulate the fast response of the summer monsoon to different anthropogenic aerosol types and to anthropogenic greenhouse gases. Without normalization, sulfate is the largest driver of precipitation change between 1850 and 2000, followed by black carbon and greenhouse gases. Normalized by radiative forcing, the most effective driver is black carbon. The precipitation and moisture budget responses to combinations of aerosol species perturbed together scale as a linear superposition of their individual responses. We use both a circulation‐based and moisture budget‐based argument to identify mechanisms of aerosol and greenhouse gas induced changes to precipitation and find that in all cases the dynamic contribution is the dominant driver to precipitation change in the monsoon region.