HR: 10:40h
AN: U32A-02    [Abstracts]
TI: Interannual to Decadal Predictability of Tropical and North Pacific Sea Surface Temperatures
AU: * Newman, M
EM: matt.newman@noaa.gov
AF: CIRES CDC and NOAA/ESRL/PSD, 325 Broadway R/PSD1, Boulder, CO 80305, United States
AB: A multivariate empirical model is used to show that predictability of the dominant patterns of tropical and North Pacific oceanic variability, El Nino-Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), is limited to a few years despite the presence of spectral peaks on decadal timescales. The model used is a linear inverse model (LIM) derived from the observed simultaneous and one-year lag correlation statistics of July-June averaged SST from the Hadley Centre Global Sea Ice and Sea Surface Temperature (HadISST) dataset for the years 1900-2002. The model accurately reproduces the power spectra of the data, including interannual and interdecadal spectral peaks that are significant relative to univariate red noise. Eigenanalysis of the linear dynamical operator yields propagating eigenmodes that correspond to these peaks but have very short decay times and thus limited predictability. Longer-term predictability does exist, however, due to two stationary eigenmodes that are more weakly damped. These eigenmodes do not strongly correspond to the canonical ENSO and PDO patterns. Instead, one is similar to the 1900-2002 trend and might represent anthropogenic effects, while the second represents multidecadal fluctuations of a pattern that potentially represents natural decadal variability. These eigenmode patterns are robust and can also be obtained by simpler EOF analysis of 11-year running mean SST data. Predictability of these two stationary eigenmodes is significantly enhanced by tropical-North Pacific coupling. Critically, neither stationary eigenmode is well captured in either the control run or 20th century simulation of any coupled GCM in the CMIP project of the IPCC Fourth Assessment Report (AR4), perhaps because in all the GCMs tropical SST decadal variability is too weak and North Pacific SSTs are too independent of the Tropics. A key implication of this analysis is that the PDO may represent not a single physical mode but rather the sum of several phenomena, each of which represents a different red noise process with its own autocorrelation timescale and spatial pattern. The sum of these red noises can give rise to apparent PDO "regime shifts" and seeming characteristics of a long memory process. Such shifts are not predictable beyond the timescale of the most rapidly decorrelating noise, less than two years, although the expected duration of regimes may be determined from the relative amplitudes of different eigenmodes.
DE: 1605 Abrupt/rapid climate change (4901, 8408)
DE: 1610 Atmosphere (0315, 0325)
DE: 1620 Climate dynamics (0429, 3309)
DE: 4522 ENSO (4922)
SC: Union [U]
MN: 2007 Joint Assembly