Is The Atlantic Multi-decadal Oscillation Changing?

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Highlight: Is the warm cycle of the past 20 years reversing?

Summary: In November 2018 the Atlantic multi-decadal oscillation (AMO) was -0.121 which is slightly negative but rivaling other infrequent months of cool AMO within what has otherwise been a 2-decade long warm period (Fig. 1). The length of the warm (or cool) cycle of the AMO has a tendency to be 2 or 3 decades. The previous cool cycle occurred in the middle 1960’s to middle 1990’s (Fig. 2). Based on historical patterns a flip from warm to cool phase of the AMO in coming years is certainly possible.

Fig. 1: The 20-year monthly plot of AMO index identifying the warm cycle although recently signs of a possible cycle change are emerging.

Fig. 2: The previous long-term cool cycle of the AMO occurred 1965-1996.

Fig. 3-4: The 1998-2017 +AMO/1965-1995 –AMO cycle influence on U.S. precipitation anomalies.

Discussion: The warm cycle of the AMO has a tendency to enhance subtropical ridging in the North Atlantic basin. A stronger than normal “Bermuda High” is common when +AMO is present. Further upstream (over the U.S.) there is a tendency for an upper trough. During the past 20 years precipitation has been above normal across the northeast quadrant of the U.S. driven largely by +AMO (Fig. 3). Conversely, when the –AMO cycle is present, low pressure has a tendency to be dominant over the oceans while upstream the Central U.S. lay beneath an upper ridge promoting dry climate (Fig. 4).

Formal forecasts of an AMO cycle change are not available. Climate scientists have made note of the tendency for a cool pool of ocean water to emerge south of Greenland in recent years while the remainder of the basin stays warmer than normal. Explanations of the cool pool are generally centered on fresh water from the Greenland ice sheet melting. This phenomenon was first recognized in 2013 and 2014 (Fig. 5) and except for 2017 has been a dominant feature in that region. The cool pool is slowing the North Atlantic oceanic thermohaline belt. The cooler AMO regime occurring now may be a sign that the cool pool south of Greenland is a catalyst to causing a basin-wide (cooler) change (Fig. 6).

Fig. 5: The May 2014 North Atlantic SSTA analysis identifying a cool pool south of Greenland caused by spring ice melt.

 

Fig. 6: The November 2018 North Atlantic basin SSTA pattern indicates areas of broad cooling while along the Gulf Stream very warm conditions continue.

If the cool cycle returns, based on history as witnessed from 1965-1995 the following North America climate patterns could return.

  • Increased risk of drought in the growing areas of the Great Plains.
  • Decreased risk of drought in the Southern U.S.
  • Increased risk of excessive rainfall in the Southeast U.S.
  • More El Nino events.
  • Less number of North Atlantic hurricanes.
  • *The tendency for warmer than normal oceans GLOBALLY and constricted polar ice cap due to climate change caused by increased CO2 levels likely renders the 1965-1995 historic (analog) references weaker.

Conclusion: The monthly AMO index cooled significantly during autumn 2018. Is this cooling a sign of a long-term change in the warm AMO cycle present since the mid-to-late 1990’s? There are no forecast models to answer this question (operational models somewhat blindly forecast anomalous warmth most of the time). However, scientists have identified (and predicted) that if a cool pool of water emerging south of Greenland due to fresh water melt from an expansive snowcap were to continue the warm oceanic conveyor belt may slow and cause basin-wide cooling. This scenario could be unfolding. A cooler North Atlantic would cause a climate change across the U.S. and Europe favoring drier annual conditions in the Great Plains and central/northern Europe and diminished North Atlantic hurricanes.