Atlantic Multidecadal Oscillation

Atlantic Multidecadal Oscillation
Name	Atlantic Multidecadal Oscillation
Location	North Atlantic Ocean
Also known as	AMO
Type	Climate variability
Cause	Ocean dynamics
Effect	Regional temperature and precipitation patterns
First observed	Late 20th century
Related	Atlantic meridional overturning circulation

Atlantic Multidecadal Oscillation. It is a climate cycle characterized by long-term fluctuations in the sea surface temperature of the North Atlantic Ocean. These fluctuations occur over periods of several decades, typically 60 to 80 years, and influence weather patterns across the Northern Hemisphere. The phenomenon is a key mode of natural climate variability with significant implications for regional climates.

Definition and characteristics

The Atlantic Multidecadal Oscillation is defined by coherent, basin-wide changes in North Atlantic sea surface temperatures, alternating between warm and cool phases. It is typically described as a low-frequency oscillation, with an amplitude of about 0.4°C. The spatial pattern resembles a horseshoe, with the strongest temperature anomalies in the subpolar Labrador Sea and the subtropical Gulf Stream region. The index used to track it is often derived by detrending data to remove the long-term signal of global warming.

Causes and mechanisms

The primary proposed mechanism involves fluctuations in the strength of the Atlantic meridional overturning circulation, a major system of ocean currents. Variations in this circulation alter the northward transport of heat, leading to changes in sea surface temperatures. Other contributing factors may include interactions with atmospheric circulation patterns like the North Atlantic Oscillation and internal ocean dynamics such as Rossby wave propagation. The role of external forcings, such as changes in solar irradiance or volcanic eruptions, remains a topic of ongoing scientific investigation.

Observational evidence and measurement

Instrumental records from ships, buoy networks, and satellites provide evidence for the oscillation extending back to the late 19th century. Key datasets include those from the Met Office Hadley Centre and the National Oceanic and Atmospheric Administration. Paleoclimatic proxies, such as tree ring records from the United States and corals from the Caribbean Sea, suggest its existence for several centuries. The index is calculated by averaging sea surface temperatures over the North Atlantic, often using methods developed by researchers at the University of Washington.

Impacts on climate and weather

The phase of the Atlantic Multidecadal Oscillation strongly influences the frequency and intensity of Atlantic hurricane activity, with warm phases correlating to more active seasons. It affects summer precipitation patterns over North America, Europe, and the Sahel region of Africa. The oscillation has been linked to droughts in the Midwestern United States, such as those during the Dust Bowl, and can modulate the effects of El Niño–Southern Oscillation. It also impacts marine ecosystems, including fisheries in the North Sea.

Relationship to other climate phenomena

The Atlantic Multidecadal Oscillation interacts with and can modulate other major climate modes. Its warm phase can amplify the positive phase of the North Atlantic Oscillation, influencing winter weather in Europe. There is evidence of teleconnections with the Pacific Decadal Oscillation, though the mechanisms are complex. It is distinct from, but can be confused with, the forced response to anthropogenic greenhouse gas emissions. Understanding its relationship with global warming trends is a critical area for climate model projections.

History of research

The concept emerged from work in the 1990s by scientists including Michael E. Mann and Thomas Delworth, who identified multidecadal variability in Atlantic climate records. Early studies analyzed historical data from the Comprehensive Ocean-Atmosphere Data Set and outputs from the Geophysical Fluid Dynamics Laboratory model. The term "Atlantic Multidecadal Oscillation" was formally coined in a seminal 2000 paper published in the journal Science. Subsequent research has utilized advanced models from the Coupled Model Intercomparison Project to better understand its drivers and global impacts.

Category:Climate oscillations Category:Atlantic Ocean Category:Climatology