Atlantic multidecadal oscillation
The Atlantic Multidecadal Oscillation (AMO) is a notable pattern of climate variability that occurs on a timescale of several decades, primarily affecting sea surface temperatures in the North Atlantic Ocean. The AMO typically cycles between warm and cool phases approximately every 50 to 70 years, with significant implications for climate patterns. The temperature differences during these phases, while relatively small at about 0.6°C, can lead to considerable exchanges of energy between the ocean and atmosphere, influencing weather and climate events globally.
Research indicates that the AMO is largely driven by thermohaline circulation, which is responsible for moving warm water into the North Atlantic. Historical patterns show that the AMO was in a warm phase from 1926 to 1963, transitioned to cool phases, and has been warm again since 1995. This oscillation has been linked to variations in drought conditions across North America and the frequency of hurricanes; for instance, warm phases tend to increase the likelihood of droughts and hurricanes.
Understanding the AMO is critical for deciphering natural climate variability and its potential impacts on agriculture, water resources, and public health. Moreover, changes in the AMO could also influence global temperature trends, with implications for climate forecasting as researchers continue to explore the mechanisms connecting North Atlantic sea surface temperatures to broader climate phenomena.
Atlantic multidecadal oscillation
Definition
The (AMO) is a global-scale mode of multidecadal climate variability (that is, it is an example of a cyclical or semicyclical pattern of climate change that repeats on a timescale on the order of several decades). The AMO is based on sea surface temperatures in the North Atlantic Ocean between the equator and 70° north latitude. Generally, the AMO is computed as a detrended ten-year running mean of these sea surface temperatures and represents variability across the entire North Atlantic basin.
The AMO exhibits a long-term, quasi-cyclic variation at timescales of fifty to seventy years. Modeling studies reveal that multidecadal variability in the North Atlantic Ocean is dominated by this single mode of sea surface temperature variability. The range in AMO values between warm and cold extremes is only about 0.6° Celsius; however, because the North Atlantic Ocean is so large, even small differences in sea surface temperatures represent extremely large exchanges in energy between the ocean and atmosphere.
The predominant hypothesis is that the AMO is primarily driven by the thermohaline circulation. This hypothesis is supported by both instrumental and climate-model studies. The is the large-scale ocean circulation that moves water among all of the world’s oceans and is driven by density differences in ocean water caused by heat and freshwater fluxes. When the thermohaline is fast, warm water is moved from tropical areas into the North Atlantic Ocean and the AMO enters a warm phase. When the thermohaline is slow, warm water is not readily moved into the North Atlantic Ocean and the AMO enters a cool phase. During the twentieth century, the AMO was in a warm phase from 1926 through 1963, and it was in cool phases from 1905 through 1925 and 1964 through 1994. In 1995, it entered another warm phase.
Research has suggested that the North Atlantic Ocean may provide information that explains significant amounts of multidecadal climate variability. For example, when the AMO is in a warm phase, the likelihood of drought in North America increases, and when the AMO is in a cool phase, the likelihood of drought in North America decreases. Analysis of major US droughts during the last century indicates that North Atlantic Ocean surface temperatures were warm during the 1930s and 1950s droughts, as well as during the dry period that began in the late 1990s. In contrast, both the early (1905-1920) and late (1965-1995) twentieth century pluvials in the western United States were associated with cool North Atlantic Ocean surface temperatures. The AMO also has been linked to the occurrence of hurricanes in the North Atlantic Ocean. During warm phases of the AMO, hurricanes are more frequent, whereas when the AMO is in a cool phase, hurricane frequency decreases.
Significance for Climate Change
Because the AMO is an important mode of multidecadal climate variability, there are a number of important implications of the AMO for the study of climate variability and change. Understanding what portion of climate variability is due to multidecadal variability, such as that driven by the AMO, allows the discrimination of changes in climate from natural variability. Knowledge of multidecadal climate variability, such as that indicated by the AMO, also has implications for defining and potentially estimating risks in agriculture, water resources, public health, and nature. Variability of the AMO also has an effect on global temperature, and the beginning of a new warm phase of the AMO in 1995 may have contributed to the strong warming of global temperatures in the years immediately following.
Some scientists suggest that if the AMO shifts into a cool phase, the cooling of North Atlantic Ocean surface temperatures may reduce the amount of global warming and may result in a leveling of global temperatures for about a decade. In addition, research has indicated that North Atlantic Ocean surface temperatures may have predictability on the order of a decade or longer, which has important implications for climate forecasting.
The actual physical mechanisms that explain the associations between the North Atlantic Ocean and global climate are still unknown, but several possible mechanisms have been recognized. North Atlantic Ocean surface temperatures may affect Northern Hemisphere atmospheric circulation, such that the frequency of zonal versus meridional atmospheric flow is modulated. Decadal-to-multidecadal variability of North Atlantic Ocean surface temperatures may be aliasing for low-frequency or lagged variations of the tropical oceans. North Atlantic Ocean surface temperatures may be influencing the location and strength of subtropical high pressures. Finally, the North Atlantic Ocean may be modulating the strength and variability of tropical Pacific Ocean surface temperatures.
Bibliography
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Knight, Jeff R., Chris K. Folland, and Adam A. Scaife. “Climate Impacts of the Atlantic Multidecadal Oscillation.” Geophysical Research Letters 33, no. 17 (September, 2006).
Lin, Jialin and Taotao Qian. "The Atlantic Multi-Decadal Oscillation." Atmosphere-Ocean, vol. 60, no. 3-4, 13 July 2022, pp. 307-337, doi.org/10.1080/07055900.2022.2086847. Accessed 20 Dec. 2024.
McCabe, G. J., M. A. Palecki, and J. L. Betancourt. “Pacific and Atlantic Ocean Influences on Multidecadal Drought Frequency in the United States.” Proceedings of the National Academy of Sciences 101 (2004): 4136-4141.
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