Abstract
Abstract Outputs from coupled general circulation models (CGCMs) are used in examining tropical Pacific decadal variability (TPDV) and their relationships with El Niño–Southern Oscillation (ENSO). Herein TPDV is classified as either ENSO-induced TPDV (EIT) or ENSO-like TPDV (ELT), based on their correlations with a decadal modulation index of ENSO amplitude and spatial pattern. EIT is identified by the leading EOF mode of the low-pass filtered equatorial subsurface temperature anomalies and is highly correlated with the decadal ENSO modulation index. This mode is characterized by an east–west dipole structure along the equator. ELT is usually defined by the first EOF mode of subsurface temperature, of which the spatial structure is similar to ENSO. Generally, this mode is insignificantly correlated with the decadal modulation of ENSO. EIT closely interacts with the residuals induced by ENSO asymmetries, both of which show similar spatial structures. On the other hand, ELT is controlled by slowly varying ocean adjustments analogous to a recharge oscillator of ENSO. Both types of TPDV have similar spectral peaks on a decadal-to-interdecadal time scale. Interestingly, the variances of both types of TPDV depend on the strength of connection between El Niño–La Niña residuals and EIT, such that the strong two-way feedback between them enhances EIT and reduces ELT. The strength of the two-way feedback is also related to ENSO variability. The flavors of El Niño–La Niña with respect to changes in the tropical Pacific mean state tend to be well simulated when ENSO variability is larger in CGCMs. As a result, stronger ENSO variability leads to intensified interactive feedback between ENSO residuals and enhanced EIT in CGCMs.
Highlights
Tropical Pacific decadal variability (TPDV) is an important component of global-scale climate variability (Mantua et al 1997; Zhang et al 1997), which is known to be related to the Pacific–North America pattern (Wallace and Gutzler 1981) in the Northern Hemisphere as well as the atmospheric circulation near the Southern Ocean and Antarctica (Garreaud and Battisti 1999)
We analyzed the outputs of long-term CMIP3 coupled general circulation models (CGCMs) simulations to examine the relationship between El Nino–Southern Oscillation (ENSO) and two leading modes of tropical Pacific decadal variability (TPDV)
We confirmed that the leading variability mode on interannual time scales was ENSO in all selected CGCMs
Summary
Tropical Pacific decadal variability (TPDV) is an important component of global-scale climate variability (Mantua et al 1997; Zhang et al 1997), which is known to be related to the Pacific–North America pattern (Wallace and Gutzler 1981) in the Northern Hemisphere as well as the atmospheric circulation near the Southern Ocean and Antarctica (Garreaud and Battisti 1999). The origin of the dipole-like mode of TPDV is suggested to be the statistical residual for asymmetric anomaly patterns associated with El Nino and La Nina events (Rodgers et al 2004; Cibot et al 2005; Schopf and Burgman 2006; Sun and Yu 2009) Those studies demonstrated that the residuals, owing to asymmetries between El Nino and La Nina in their spatial patterns and amplitudes, could rectify into a background condition and lead a slowly varying variability. Choi et al (2012) argued that the statistical residuals as a result of El Nino–La Nina asymmetry representing an east–west dipole structure could feed into the tropical Pacific mean state Both the residual pattern due to ENSO asymmetry and the mean state pattern correlated to the decadal modulation of ENSO show similar east– west dipole-like structures, suggesting a strong dynamic relationship between them that is worthy of further investigation.
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