Atlantic Multidecadal Variability

Atlantic Multidecadal Variability
Giorgiogp2 · CC BY-SA 3.0 · source
Name	Atlantic Multidecadal Variability
Period	Multidecadal
Region	North Atlantic

Atlantic Multidecadal Variability is a naturally occurring pattern of sea surface temperature and associated climate fluctuations in the North Atlantic that operates on multidecadal timescales. It manifests as coherent warm and cold phases influencing atmospheric circulation, hurricane activity, and regional climate across the North Atlantic basin. Studies by observational programs, paleoclimate reconstructions, and climate modeling efforts have linked these variations to oceanic processes, external forcings, and interactions with atmospheric modes.

Definition and Characteristics

Atlantic Multidecadal Variability is characterized by multidecadal oscillations in North Atlantic sea surface temperature with spatial patterns centered on the subpolar and subtropical gyres. Key attributes include phase durations of roughly 20–40 years, basin-scale coherence, and fingerprints in sea level pressure, surface salinity, and ocean heat content. Instrumental analyses from the Hadley Centre datasets, National Oceanic and Atmospheric Administration archives, and Plymouth Marine Laboratory compilations reveal distinct positive and negative phases that align with changes in the Atlantic subpolar gyre and subtropical circulation. Characteristic indices derived by researchers at Columbia University, Princeton University, and the Woods Hole Oceanographic Institution are used to quantify amplitude and phase, often correlating with observations from the Rosenstiel School of Marine and Atmospheric Science and the Scripps Institution of Oceanography.

Observed History and Reconstruction

Instrumental records beginning in the 19th century show prominent warm phases in the 1920s–1960s and after the late 1990s, with cold phases documented in the early 20th century and mid-to-late 20th century. Paleoclimate reconstructions using proxy archives from the Greenland Ice Sheet Project, Camp Century, coral records from Bermuda, tree rings from Scotland and Iceland, and sediment cores from the Mackenzie River delta extend the history back centuries to millennia. Reconstructions by teams at Lamont–Doherty Earth Observatory, University of Copenhagen, and the Max Planck Institute for Meteorology synthesize multiproxy evidence confirming multidecadal variability during the Medieval Climate Anomaly and Little Ice Age intervals. Instrumental efforts by the UK Met Office and NOAA Paleoclimatology Program have been cross-validated against PAGES initiatives and archives curated at the British Geological Survey.

Physical Mechanisms and Drivers

Proposed mechanisms emphasize coupled ocean–atmosphere dynamics, including variations in the Atlantic Meridional Overturning Circulation, modulation by increased freshwater input from Greenland melt and Arctic rivers, and feedbacks involving surface heat fluxes and oceanic gyre adjustments. Process studies from the Geophysical Fluid Dynamics Laboratory, National Center for Atmospheric Research, and European Centre for Medium-Range Weather Forecasts attribute part of the variability to changes in wind stress associated with atmospheric modes observed around Reykjavík and the Azores High. External forcings, including volcanic eruptions recorded by the Sierra Nevada ash layers and anthropogenic sulfate aerosol emissions tracked by International Maritime Organization shipping records, have been implicated in phase shifts. Internal stochastic variability described in work at MIT and Yale University interacts with boundary conditions influenced by ice sheet dynamics at Greenland, the Labrador Sea convection variability, and teleconnections with the Mediterranean and tropical Atlantic processes documented by researchers at Universidad de las Palmas de Gran Canaria.

Climate Impacts and Teleconnections

Phases correlate with modulations of Atlantic hurricane frequency and intensity reported by the National Hurricane Center and the Cuban Meteorological Institute, links to Sahel rainfall fluctuations observed by the United Nations Environment Programme and the Food and Agriculture Organization, and impacts on European summer temperature patterns monitored by the European Environment Agency. Teleconnections extend to North American drought patterns studied at Iowa State University and heatwave occurrences assessed by the Met Office Hadley Centre. Influences on Arctic sea ice trends have been explored by the Norwegian Polar Institute and Alfred Wegener Institute, while marine ecosystem responses have been documented by the National Marine Fisheries Service and the International Council for the Exploration of the Sea.

Detection, Attribution, and Predictability

Detection employs statistical techniques such as empirical orthogonal functions applied to datasets from the Global Ocean Data Assimilation Experiment and reanalyses from the ECMWF Reanalysis and NOAA-CIRES collaborations. Attribution studies separate internal variability from external forcings using detection frameworks developed at IPCC assessment centers and methodologies from the US Global Change Research Program. Predictability assessments derive from initialized decadal prediction systems run at the Met Office Hadley Centre, Japanese Meteorological Agency, and CNRM laboratories; skillful forecasts depend on accurate representation of ocean heat uptake and the Atlantic Meridional Overturning Circulation as simulated by groups at NCAR and GFDL.

Modeling and Future Projections

Global coupled models in the Coupled Model Intercomparison Project ensembles produce a range of outcomes for future Atlantic multidecadal behavior under scenarios evaluated by the Intergovernmental Panel on Climate Change. Some simulations project continued multidecadal fluctuations modulated by greenhouse gas forcing from inventories compiled by the International Energy Agency and aerosol scenarios developed by the World Meteorological Organization. Model intercomparison studies coordinated by PCMDI and research consortia at INGV and CSIRO assess biases in Atlantic representation, with experiments indicating that shifts in the Atlantic Meridional Overturning Circulation and freshwater forcing from Greenland could alter amplitude and phase. Ongoing campaigns by the Argo program and sustained observations from the R/V Knorr and NOAA Ship Ronald H. Brown aim to reduce uncertainty in projections.

Category:Climate variability