>CORRECTION: The AMO illustrations and its descriptions have been corrected to reflect the discussion in The Atlantic Multidecadal Oscillation – Correcting My Mistake.
A QUICK OVERVIEW OF THE AMO
The Atlantic Multidecadal Oscillation (AMO) is an expression of the natural quasi-periodic cycle in North Atlantic SST anomalies. The AMO is created by detrending the North Atlantic SST anomalies; that is, the monthly values of the linear trend line are subtracted from the monthly SST anomaly data. It is considered to have major impacts on Northern Hemisphere temperatures and precipitation. Its effect on the frequency and magnitudes of hurricanes is hotly debated.
A SIGNIFICANT RECENT DROP
I haven’t taken a look at the AMO for more than six months. In that time, there has been a significant decline in the AMO. Refer to Figure 1, which illustrates AMO data from the NOAA Earth Science Research Laboratory and AMO data created by detrending ERSST.v3b SST anomaly data for the North Atlantic. The magnitude of the recent drop is not unusual. Similar drops have also happened in the recent past. But the AMO is worth keeping an eye on.
The smoothed (12-month filter) long-term data, Figure 2, shows how insignificant the recent drop is in relationship to the overall variation of the AMO. Assuming that the AMO reached its cycle peak in 2004, the AMO has a long way to go before slowly declining to its minimum in ~25 years.
COMPARISON OF THE AMO TO NINO3.4 SST ANOMALIES
Figure 3 is a comparative graph of scaled NINO3.4 SST anomalies and the AMO. The first thing that stands out is the response of the AMO to the major El Nino events of 1986/87/88 and 1997/98. The time lag in the AMO varies from approximately 6 to 12 months. Do we consider the recent drop a delayed reaction to the 2007/08 La Nina event? Also note that the AMO does not respond similarly to the 1982/83 and 1991/92 El Nino events. This is due to the dominant effects of the El Chichon and Mount Pinatubo eruptions. Last quick observation: after the 2003/03 El Nino, the AMO appears to have been forced higher by something other than El Nino events (the AO possibly?), but as noted earlier, the AMO has dropped quickly for more than six months. Will it continue?
COMPARISON OF THE AMO TO THE EAST INDIAN-WEST PACIFIC SST ANOMALIES
The AMO and the SST anomalies for the East Indian-West Pacific SST anomalies are compared in Figure 4. The two datasets react similarly to ENSO events with step changes. When viewing that graph, keep in mind that the North Atlantic SST anomalies have been detrended in the AMO. So we’ll compare apples to apples next.
A comparative graph of North Atlantic SST anomalies and SST anomalies of the East Indian-West Pacific SST, Figure 5, better illustrates the similarities, especially the step changes in the datasets following the 1986/87/88 and 1997/98 El Nino events.
The processes that cause the step changes in the East Indian and West Pacific SST anomalies are discussed in:
The NOAA Earth System Research Laboratory (ESRL) Atlantic Multidecadal Oscillation (AMO) webpage is here:
The unsmoothed AMO data is here:
ERSST.v3b SST data is available through the KNMI Climate Explorer website: