backgroundonthe superposed epoch analysis- nature（backgroundonthe superposed数十analysis- nature）.doc

Background on the Superposed Epoch Analysis The Superposed Epoch Analysis (SEA) technique is a statistical method used to resolve significant signal to noise problems. This is especially desired in cases where the responses to particular events (in our case large radiative forcing from explosive volcanic eruptions) may be obfuscated by noise from other competing influences that operate at similar time scales (internal El Nino variation). Through simple compositing, the SEA method involves sorting data into categories dependent on a ‘key-date’ for synchronization and then comparing the means of those categories. Given sufficient data, a common underlying (causal) response to a forcing event should theoretically emerge in the average (composite) while other noise in the data should cancel. Examples of applications of the SEA method are widespread in various scientific fields of study. For example, the SEA has been employed in studies on the relationship between climate and fire histories (Swetnam and Betancourt, 1990), magnetospheric physics (Lühr et al., 1998), the lunar cycle and rainfall (Brier and Bradley, 1964), and the relationship between volcanic activity and climate variations (Mass and Portman, 1989). Compositing can also lend insight into periodic responses that, because of the relative magnitude of the signal and associated noise, are not detected by spectral analysis. The SEA method is simple and involves basic arithmetic calculations (averaging). However, the randomization procedure used to determine statistical significance, depending on the number of iterations performed, can be computationally demanding. The method requires a random number generator for the Monte Carlo randomization procedure. We used the commercially available Matlab? statistical software package to perform our SEA. As with any statistical method, care must be taken in the application and interpretation of SEA results. For instance, the SEA can be vulnerable to lever