Tropical SSTs: Natural variations or Global warming?

First of all, how is the AMO defined? It is intrinsically quite difficult to detect any multidecadal oscillation in roughly only one century of instrumental data. Simply put, separating an oscillation from trend becomes exceedingly tricky (and increasingly dependent on statistical assumptions) as the timescale of the oscillation approaches the length of the record. Unfortunately, this situation holds for the AMO, which has been attributed periodicities anywhere from 40-100 years, the latter approaching the length of available instrumental climate observations. In some earlier studies, the AMO was defined using multivariate signal detection procedures to tease oscillatory patterns apart from long-term (potentially non-linear) trends (e.g. Mann and Park, 1994) or using climate model-based estimates of forced trends to estimate a possible residual oscillatory component (e.g. Schlesinger and Ramankutty, 1994).

In some more recent studies, however, the AMO has been defined simply as the residual low-frequency pattern after linear detrending of SST observations (e.g. Goldenberg et al, 2001). The linear detrending is intended to remove any potential forced signal, under the assumption that it is linear in time. However, if the forced signal is not linear, then this procedure can produce a false apparent ‘oscillation’ purely as an artifact of the aliasing of the non-linear secular trend (Trenberth and Shea , 2006). In fact, we have very strong indications for the 20th Century that the forcings over that period have not varied in a smooth, linear fashion.

Because of the procedural difficulties in isolating the AMO signal in the instrumental record, the estimated attributes of the signal are quite sensitive to how it is defined. The earlier studies mentioned above (i.e., Mann and Park, 1994; Schlesinger and Ramankutty, 1994) found an AMO signal with a large projection onto high-latitude North Atlantic SST variations, but little projection onto tropical North Atlantic SST. This contrasts with studies using the linear detrending procedure described above, which indicate a sizeable impact of the AMO on tropical North Atlantic SST.

In a regression analysis using instrumental observations, Emanuel and Mann (2006) find that the estimated temporal history of the anthropogenic climate change signal in tropical North Atlantic SST superficially resembles the temporal pattern often ascribed to the AMO (i.e., early 20th century and 1960s-1980s cool phases, and 1930s-1950s and recent warm phase). However, they identify this irregular warming pattern with a combination of greenhouse gas warming influences and late 20th century sulphate aerosol cooling influences (which are especially large during the late boreal summer in the tropical Atlantic). It is therefore likely that the non-linear temporal history of anthropogenic tropical Atlantic warming has masquaraded as the ‘AMO’ in some studies.

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