Atlantic Multi-decadal oscillation & Spring/Summer U.S. Rainfall

U.S. ENSO Precipitation During Winter Pre/Post 1997-98 El Nino
11/03/2018, 2:19 pm EST
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Discussion: According to the National Center for Atmospheric Research (NCAR) the Atlantic multi-decadal oscillation (AMO) is an ocean cycle of natural variability occurring in the North Atlantic Ocean with an estimated period of 60-80 years (split evenly by both the cool and warm phase). Climate scientists generally agree that the long-term warm cycle of the AMO began following the historic 1997-98 El Nino episode and remains in-place now (Fig. 1). The most recent cool cycle of the AMO was present from approximately 1966 to 1995 (Fig. 2). Based on our understanding of the AMO period another 10 years (or so) of +AMO is anticipated.

The AMO phase contributes to annual and seasonal U.S. precipitation patterns. Of considerable interest is the relationship climate signals have to anomalous precipitation during meteorological spring (MAR/APR/MAY) and summer (JUN/JUL/AUG) across the primary U.S. crop growing areas. The warm phase of the AMO is associated with wetter than normal conditions across the Midwest U.S. (Fig. 3) while the cool phase is associated with a drier climate in this region (Fig. 4).

Of course, there are other contributing factors to the rainfall anomalies presented likely lead by the more expansive polar ice cap 20-50 years ago versus the modern-day constricted polar ice cap. During the –AMO regime there is a tendency for a mid-continent upper trough entraining above normal low level moisture from the warmer-than-normal North Atlantic rinsed out across the northeast quadrant of the U.S. During the cool cycle of the AMO the cooler ocean temperatures produce lower low-level atmospheric moisture.

+AMO is forecast to continue in 2019. Once the influence of El Nino fades mid-2019 the tendency of El Nino to produce a wetter than normal southern U.S. (also) fades and the +AMO driven wet bias across the Midwest and northeast U.S. returns to summertime probabilistic climate forecasts issued by NOAA/CPC (Fig. 5-6).

New (top) Versus Old (bottom) El Nino Precipitation OCT/NOV/DEC

Fig. 1: The long-term warm phase of the Atlantic multi-decadal oscillation.

Fig. 2: The most recent long-term cool phase of the Atlantic multi-decadal oscillation.

Fig. 3: MAR to AUG 1998-2017 U.S. rainfall anomalies.

Fig. 4: MAR to AUG 1966-1995 U.S. rainfall anomalies.

Fig. 5: NOAA/CPC precipitation probability forecast for MAY/JUN/JUL 2019.

Fig. 6: NOAA/CPC precipitation probability forecast for JUN/JUL/AUG 2019.