Penland, C., and L. Matrosova, 2006: "Studies of El Niño and interdecadal variability in tropical sea surface temperatures using a nonnormal filter." J. Climate, 19, 5796-5815, doi: 10.1175/JCLI3951.1.
Abstract
A dynamically based filter is used to separate tropical sea surface temperatures (SSTs) into three components: the evolving El Niño signal, the global tropical trend, and the background. The components thus isolated are not independent. On the contrary, this procedure allows us to see the importance of the interdecadal signal to the predictability of El Niño.
The data filtered in this way reveal El Niño signals in the equatorial Indian Ocean and in the north tropical Atlantic Ocean that are remarkably similar. A signature of El Niño in the south tropical Atlantic leads Niño-3.4 SST anomalies by about 9 months. The time series of a global tropical trend is found to have a very smooth parabolic structure. In unfiltered data, this trend conspires with El Niño to obscure a meridional tropical Atlantic dipole, which is significant in the filtered background SST data.