Measuring the Atlantic Multidecadal Oscillation

The Atlantic Multidecadal Oscillation (AMO) refers to the variance in North Atlantic sea surface temperatures which has a 65-80 year period with an amplitude of 0.4 ⁰C. The AMO is hypothesized to have a wide influence over North Atlantic tropical cyclone activity and neighboring continental rainfall patterns. Multiple indices used to measure the AMO have been constructed to represent the AMO’s strength and longevity. In recent years, individual AMO indices have significantly diverged from each other relative to the longer period. In this study, the physical differences between the indices will be explored in detail in order to relate the recent trend in varying measurements of the AMO.

The NOAA ESRL, Trenberth & Shea, and Klotzbach & Gray methods were three indices selected for this study with particular attention paid to the individual terms which significantly influence each of the final values. Significant differences in SST datasets between the three methods will be shown as a primary source of variability in the final calculations of AMO amplitude and period. For example, differences in spatial averaging of SSTs in the Klotzbach & Gray method versus the ESRL method will be shown as a particular source of variability. In addition, the method by which SSTs were detrended between the Trenberth & Shea method and ESRL method also induced variability between the final metrics.

In addition to resolution and detrending differences, another factor that could have induced such a difference in AMO indices is the inclusion of Sea Level Pressure (SLP) to incorporate the Thermohaline Circulation (THC) in the Klotzbach & Gray method. The region the index uses is noted to have a high correlation between SLP and SST variability, although notable periods of weak correlation were more difficult to diagnose. Finally, the recent abnormality (relative to the climatological mean) of Atlantic SST spatial variability will be discussed in context of the separate AMO index calculations. In addition to the results presented here, future work will also be addressed as a consequence of the findings presented herein.