Atlantic multidecadal Oscillation (AMO)

The term was introduced in classical studies by Delworth and Mann (2000) and Kerr (2000), while the AMO index was formally defined by Enfield, Mestas-Nuñez, and Trimble (2001).

The Atlantic Multidecadal Oscillation (AMO) is a low-frequency variability observed in North Atlantic sea surface temperatures (SST), characterized by alternating phases of warming and cooling that span decades. This phenomenon is important for understanding both oceanic dynamics and its regional and global climate impacts (Knudsen et al., 2011).

The AMO exhibits cycles of approximately 50 to 70 years, with warm phases lasting around 30–40 years and cold phases around 20–30 years. These patterns reflect changes in North Atlantic thermohaline circulation and meridional heat transport (Zhang et al., 2019; Dima & Lohmann, 2007).

The AMO index is generally constructed by calculating the mean North Atlantic SST (roughly from 0° to 70°N) and removing the global mean or long-term trend, isolating the multidecadal variability (Knudsen et al., 2011). There are different versions of the index depending on the period considered and the trend-removal method (Sciencedirect, 2022).

The AMO broadly influences regional climate. For example, warm phases tend to produce hotter and drier summers in Europe and North America, while cold phases are associated with cooler and wetter summers (Lin, 2022). Additionally, it affects precipitation patterns in Northeast Brazil and the African Sahel by modulating the position of the Intertropical Convergence Zone (ITCZ) (Maksic et al., 2022).

AMO variability also impacts the frequency and intensity of Atlantic hurricanes due to higher SSTs and atmospheric conditions more favorable for tropical convection (Lin, 2022). Paleoenvironmental studies suggest that multidecadal patterns similar to the AMO occurred in the North Atlantic over the past 8,000 years (Knudsen et al., 2011), demonstrating its persistence over time.

Although the AMO is widely studied, its origin is debated. Part of the observed variability may result from external forcings, such as anthropogenic aerosols or volcanic eruptions, rather than purely internal oscillations (Pasini et al., 2022).

In summary, the AMO constitutes a mode of multidecadal variability in the North Atlantic, with significant impacts on regional and global climate, including temperatures, precipitation, and hurricane activity. Despite its importance, the predictability of the phenomenon is limited due to the small number of observed cycles and the complex interaction between internal and forced variability.