Description
Studies have shown that the Atlantic can influence the Indian summer monsoon (ISM) on multidecadal timescales. Interannual variability in the tropical Atlantic and Pacific can also be affected by the Atlantic mean state, and these regions can strongly influence ISM on interannual timescales. The objective of this project is to investigate how changes in the Atlantic mean state can modulate the inter-basin coupling between the tropical Atlantic, Pacific and Indian Oceans, and how this in turn affects ISM. The project expects to increase the understanding of how the Atlantic and Indo-Pacific regions are interacting on multidecadal timescales and what impact the Pacific has in modulating the teleconnection between low-frequency variability of the Indian summer monsoon rainfall and Atlantic SSTs. The donor partner, University of Bergen, will contribute its know-how and many years of experience in analyzing the role of changes in the ocean background state and its impacts on Europe. The main beneficiaries of the project will be professionals working in the climate change research and the society as a whole.
Summary of project results
The instrumental records show a significant positive correlation between the Atlantic multi-decadal variability (AMV) and the Indian summer monsoon (ISM) rainfall. However, most of the state of the art general circulation models do not capture the observed relationship and proxy reconstructions are inconsistent, so the persistency of this relationship is uncertain. Other studies have shown that the Atlantic mean state can affect inter-annual variability in the tropical Atlantic, such as the Atlantic Niño, and that the Atlantic Niño can influence the ISM on inter-annual timescales. This project was needed to join together expertise from Complutense University of Madrid on the Atlantic Niño and its teleconnections and from NRERSC on the Indian monsoon variability, to be able to untangle some of these teleconnections, to understand how mean state changes can modulate the tropical interbasin coupling between the tropical Atlantic and the Indian monsoon. During this project, experiments using three different atmospheric general circulation models were analysed to investigate the influence of the Atlantic Niño on the ISM and if this relation depends on the mean background state of the climate. Two of the models are used at UCM and one is used at NERSC/UiB, but the experimental setup was exactly the same for the three models. Partners analysed 4 different simulations with the three AGCMs (SPEEDY, UCLA and CAM), which all have been previously used to investigate tropical variability. In the first experiment (CLIM1) mean observed SSTs for the period 1950-1960 when the AMV is in a warm phase are prescribed globally, in the second experiment (CLLIM2) mean SSTs for the period 1975-1985 when the AMV is in an cold phase is prescribed globally. To investigate the impact of different mean states on the effect of an Atlantic Niño, two additional experiments similar to CLI1 and CLIM2, but with an Atlantic Niño pattern added in the tropical Atlantic, ATL1 and ATL2, were analyzed. The grantee Lea Svendsen presented the results of the work on different seminars, explaining how the Atlantic Niño impacts the Indian monsoon and the dependence of this connection on the background state. A paper on the result from the model analysis performed is being prepared, and will be included in a larger paper on Atlantic Niño teleconnections, led by the group at UCM. These results are also planned to be a part of the grantee’s PhD thesis.
Summary of bilateral results
The participants from UCM and from NERSC have complementary knowledge about the Atlantic Niño and the Indian Monsson, an ideal combination of experience for investigating the topic of this project. In addition the fact that we could perform the same experiments for investigating the topic of this project. In addition the fact that we could perform the same experiments with different models, made it possible to assess model dependence of our results. The main benefit of the project is the increased understanding of the tropical interbasin teleconnection, and how they depend of the background state. Both parties involved also benefited from the newly established collaboration and the exchange of knowledge. During the project the cooperation among Belén Rodríguez and Lea Svendsen was strengthened, and this cooperation will continue in the future as the planned paper is written, Further collaboration with Teresa Losada is planned, where they will perform additional experiments similar to the ones analysed during the visit, but with different configurations of the Altantic Niño. Since different periods have displayed different types of Atlantic Niños, these experiments will help understand if this is important for the large-scale tropical interactions, and how the different Atlantic Niños impact the Indian Monsson, Partners also expect further collaboration between Julián Villamayor and Lea Svendsen, as both are doing analysis of the impact of AMV and IPO on the global monsoons in CMIP5 models. While Lea Svendsen has been investigating the impact on the Indian Monsson, Julián Villamayor has been investigating the impact on the African and South American monsoon, Methods used by partners are complementary and future collaboration is expected.