Abstract
Interest in the “Interdecadal Pacific Oscillation (IPO)” in the global SST has surged recently on suggestions that the Pacific may be the source of prominent interdecadal variations observed in the global-mean surface temperature possibly through the mechanism of low-frequency modulation of the interannual El Nino-Southern Oscillation (ENSO) phenomenon. IPO was defined by performing empirical orthogonal function (EOF) analysis of low-pass filtered SST. The low-pass filtering creates its unique set of mathematical problems—in particular, mode mixing—and has led to some questions, many unanswered. To understand what these EOFs are, we express them first in terms of the recently developed pairwise rotated EOFs of the unfiltered SST, which can largely separate the high and low frequency bands without resorting to filtering. As reported elsewhere, the leading rotated dynamical modes (after the global warming trend) of the unfiltered global SST are: ENSO, Pacific Decadal Oscillation (PDO), and Atlantic Multidecadal Oscillation (AMO). IPO is not among them. The leading principal component (PC) of the low-pass filtered global SST is usually defined as IPO and it is seen to comprise of ENSO, PDO and AMO in various proportions depending on the filter threshold. With decadal filtering, the contribution of the interannual ENSO is understandably negligible. The leading dynamical mode of the filtered global SST is mostly AMO, and therefore should not have been called the Interdecadal “Pacific” Oscillation. The leading dynamical mode of the filtered pan-Pacific SST is mostly PDO. This and other low-frequency variability that have the action center in the Pacific, from either the pan-Pacific or global SST, have near zero global mean.
Highlights
In the classic 1997 paper entitled “El Nino-Southern Oscillation (ENSO)-like Interdecadal Variability, 1900–1993”, Zhang, Wallace and Battisti studied in detail the leading dynamical empirical orthogonal function (EOF) of monthly sea surface temperature (SST)*, i.e. the “SST deviation”, defined as the SST field with the concurrent global mean SST subtracted from each grid point
The nature of this frequency combination is further clarified in Chen and Wallace (2016) for pan-Pacific SST* (100°E–80°W, 65°S–65°E), and by Chen et al (2017) for global SST, as “mode mixing”: In conventional EOF analysis, the principal component (PC) times series are required to be uncorrelated, and the EOFs are required to be orthogonal to each other
Interdecadal Pacific Oscillation (IPO) is often referred to as an interdecadal variability originating in the tropical Pacific that has important global influence
Summary
In the classic 1997 paper entitled “ENSO-like Interdecadal Variability, 1900–1993”, Zhang, Wallace and Battisti (hereafter ZWB) studied in detail the leading dynamical EOF of monthly SST*, i.e. the “SST deviation”, defined as the SST field with the concurrent global mean SST subtracted from each grid point. The low-frequency dynamics can be studied more systematically by first low-pass filtering the SST and performing the EOF analysis This mathematically correct approach was adopted in the usual definition of the Interdecadal Pacific Oscillation (IPO), starting from Folland et al (1999). For global SST under decadal low-pass filtering, the center of action of IPO shifts to the Atlantic, taking on the spatial shape close to that of the Atlantic Multidecadal Oscillation (AMO). This IPO is “Pacific” in name only, but this fact is often not noticed when only the PC is studied. We will discuss whether these attributes of IPO are robust
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