Earlier studies have shown that Atlantic multi-decadal variability (AMV) can impact climate variability globally. However the instrumental records are short compared to the timescale of AMV, and mechanisms for these impacts are unresolved. This thesis deals with impacts of AMV on the Indo-Pacific region and over the Northern Hemisphere, investigating both the persistence of multi-decadal variability in the two phenomena and possible mechanisms for interactions between them. Coupled climate models are the main tools used in this study, in the form of output from the Coupled Model Intercomparison Project 5 (CMIP5) ensembles, a freshwater hosing experiment with the Bergen Climate Model (BCM), and partially coupled ensemble simulations with the Norwegian Earth System Model (NorESM). In addition proxy records are used to assess persistence in both AMV and its relation to regional variability in the Indo-Pacific.

The results of this thesis are presented in five papers. In the first paper a new marine- based multi-proxy reconstruction for AMV is produced and presented, showing persistent multi-decadal variability 90 years further back in time than the instrumental records. The second paper evaluates multi-decadal variability in several proxy reconstructions of the Indian summer monsoon (ISM) and compares it with different AMV reconstructions, including the marine-based record from the first paper. Multi- decadal variability is found in ISM reconstructions back to the 15th century, but the relation with AMV is not clear. The AMV-ISM relation is further investigated in CMIP5 simulations in the third paper. While none of the models capture the observed significant AMV-ISM relation in the pre-industrial control simulations, one model simulates the observed correlation in the 20th century historical ensemble, indicating that the observed relation could be externally forced. In the fourth paper of this thesis it is found that changes in North Atlantic SSTs due to variations in the thermohaline circulation can impact variability in the equatorial Atlantic and change the inter-basin relation between Atlantic and Pacific Niños. The fifth paper introduces a novel approach for investigating large-scale impacts in a coupled model, by separating radiative forced and dynamically driven variability. Ensembles of partially coupled 20th century historical simulations indicate that AMV may not have been a key contributor in Northern Hemisphere surface trends on decadal timescales.

Collectively these papers indicate that the AMV is a persistent signal of the climate system, but the impacts on multi-decadal variability in the Indo-Pacific and the Northern Hemisphere may not be as strong as previous studies suggest. AMV can modulate interannual variability and inter-basin teleconnections in the tropics, but the correlation with the ISM could be due to external forcing, and the Pacific seems to make a larger contribution to decadal trends in Northern Hemisphere climate than the Atlantic.