chapter - Atmospheric and Oceanic Science

Low-frequency variability
14.1. Background on low frequency variability
It is well known that the Tropical Pacific is dominated by a single mode of
interannual climate variability, which reflects in the ocean-atmosphere coupling
expressed by El Niño/Southern Oscillation (ENSO) phenomenon. Interannual climate
variations in many parts of the globe are closely related to this mode.
However, the Pacific climate contains another mode (or modes) of variability similar
to the ENSO, but varying in a decadal to multi-decadal scale. Among these
modes a recurring anomalous mode of the ocean-atmosphere system in the Pacific
with dominant multi-decadal sign in the North Pacific has been known since the
end of the 1980s (Nitta and Yamada 1989; Trenberth 1990; Trenberth and Hurrel
1994; Tanimoto et al. 1993; Latif and Barnett 1994; Mantua et at. 1997; Minobe
1997, 1999; Enfield and Mestas-Nuñez 1999). This mode shows significant climate
teleconnections and it is commonly referred to as the Pacific (inter-) Decadal
Oscillation (PDO) mode (Mantua et al. 1997; Zhang et al. 1997).
The high PDO phase or the warm PDO regime (WPDO) features anomalously
cold surface waters in the western and central North Pacific and warmer than
normal surface waters in the central and eastern Tropical Pacific and along the West
coast of Americas (e.g., Zhang et al. 1997; Mantua et al. 1997; Zhang et al. 1998;
Enfield and Mestas-Nuñez 1999). The low PDO phase or the cold PDO regime
(CPDO) features nearly reversed patterns. The inter-decadal variability of the
ocean-atmosphere system in the North Pacific determines the duration of the PDO
regimes, which were cold during the 1900-1924 and 1947-1976 periods and warm
during the 1925-1946 period and from 1977 to mid-1990s (Mantua et al. 1997).
Mantua et al. (1997) suggested that the PDO constitutes the background of the
interannual ENSO variability. In agreement, several authors showed that PDO
modulates El Niño (EN) and La Niña (LN) effects in certain regions of the globe
(Gershunov and Barnett 1998; McCabe and Dettinger 1999; Gutzler et al. 2002;
Krishnan and Sugi 2003). Gershunov and Barnett (1998) found that EN- (LN-)
related dry/wet (wet/dry) conditions over the northwestern/southwestern North
America tends to be strong and more consistent during the WPDO (CPDO) regime.
Since the PDO regimes last about 20-30 years, the information on the ENSO
effects stratified accordingly to the PDO phases may have a potential use to
improve the climate forecasting. So, this section revises the ENSO-related rainfall
anomaly patterns over southern South America, but taking into account the PDO
phases. The associated anomaly patterns for the sea surface temperature (SST) and
sea level pressure (SLP) will also be obtained and discussed.
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