The Atlantic Meridional Mode (AMM) is the leading mode of coupled variability in the Atlantic, and represents a distinct coupled pattern of sea surface temperature and atmospheric fields that emerges through a broad spectrum of external forcing mechanisms. Previous work has shown that the AMM may provide a dynamical explanation for the observed coherence between a number of regional atmospheric and oceanic conditions that are relevant for tropical cyclone formation.
This project aims to better understand the cause of AMM variations during boreal summer, with an emphasis on understanding the link between the AMM and tropical cyclone activity. The research will utilize designed experiments using global general circulation models, dynamical analysis of simple coupled models, analysis and interpretation of observational data, and analysis of existing global coupled model simulations of present and future climate.
Specifically, this project will address (i) What is the structure of the AMM during boreal summer? (ii) What dynamical processes control the AMM during boreal summer, and the relationship between the AMM and environmental conditions that influence hurricane activity? (iii) What external forcing mechanisms influence the AMM during boreal summer, and what is the relationship between the Atlantic Multidecadal Oscillation (AMO) and AMM, and (iv) What is the predictability of the AMM, and what processes influence that predictability?
Broader impacts of this project include encouraged integration between the disciplines of climate dynamics and tropical cyclone characteristics, training of two graduate students and one post-doctoral researcher, broader dissemination of results in national and international venues, and public availability of real-time data relating to the AMM and to tropical cyclone activity.
OVERVIEW: The Atlantic Meridional Mode (AMM) is a important source of ocean and atmosphere climate variation in the Tropical Atlantic. Previous work on AMM variability has focused on understanding its variation and mechanism during boreal spring, when it is most energetic. However, recent research has shown that AMM variations during the Atlantic hurricane season – boreal summer and fall – are associated with environmental conditions that strongly influence characteristics of seasonal tropical cyclone (hurricanes) activity. This project undertook a systematic study of the mechanisms that force and maintain the AMM during boreal summer and fall, and the mechanisms underpinning the relationships with hurricane genesis, track, and intensity. Intellectual Merit: The Intellectual Merit of this project is in its combination of observational analyses, designed model experiments, and development of theory to better understand robust features of AMM variations during boreal summer and fall, and their relationship with tropical cyclone variations. These three methods for investigating the AMM resulted in the following major findings: Observational analysis: We analyzed environmental conditions associated with boreal summer and fall AMM variations and showed that the AMM is related to environmental conditions that impact tropical cyclone variations. Furthermore, we identified that variations in environmental conditions associated with the AMM have a strong influence on tropical cyclone tracks, which could lead to societally relevant seasonal predictability of tropical cyclone tracks. We also found, while looking at AMM predictability, that mid- to high-latitude sea surface temperature (SST) anomalies emerge as precursors to AMM variations. These mid- to high-latitude SST anomalies provide skill for AMM predictions during boreal summer and fall, while ENSO tends to provide AMM predictive skill in boreal spring. Designed Global Climate Model Simulations: We directly simulated the effect of AMM-related SST variations on environmental conditions in the tropical Atlantic, and confirmed the causality from the AMM to environmental conditions that are relevant for tropical cyclone variations. We also simulated AMM variations during boreal summer using a simple representation of the tropical Atlantic ocean, which provided insight into the dynamics of AMM variations during boreal summer and fall. The same model formulation was used to investigate the potential evolution of mid- to high-latitude SST anomalies into tropical Atlantic AMM variations, and results confirmed the observed relationship between the two regions. Our project also evaluated the simulation of tropical cyclone variations by global climate models using two methods: direct estimation of cyclones in the models and the use of synthetic tracks. Development of Theory: A simple coupled model was developed to investigate the growth and propagation of AMM variations during boreal summer and fall. The simple theoretical model verified many of the observed and modeled features of AMM variability, including the importance of windspeed-induced-evaporation in generating AMM variations, the role of mid- to high-latitude SST anomalies in generating AMM variations, and the role of mid-latitude atmospheric variability in providing an external forcing for AMM variations. Broader Impacts: This project contributed to the training and development of three graduate students (one Ph.D. completed, and two Ph.D.’s in progress), one post-doctoral research associate, and one undergraduate student researcher. Additionally, findings from this analysis were disseminated through research and public lectures, through contributions to community synthesis reports (e.g. Intergovernmental Panel on Climate Change reports), and through participation in the US CLIVAR Hurricane Working Group. Results from this project also broadly inform efforts at seasonal-to-interannual prediction.
