Wednesday, 25 January 2017
The leading mode of boreal spring precipitation variability over the tropical Pacific experienced a pronounced interdecadal change around the late-1990s.The pattern before 1998 features positive precipitation anomalies over the equatorial eastern Pacific (EP) with positive principle component years. The counterpart after 1998 exhibits a westward shift of the positive center to the equatorial central Pacific (CP). Observational evidences showthat this interdecadal change in the leading modeof precipitation variability is tightly associated with a distinctive sea surface temperature (SST)anomaly pattern.The westward shift of anomalous precipitation center after 1999 is in tandem with a similar shift of maximum warming from the EP to CP.Diagnostic analyses based on a linear equation of the mixed layer temperature exhibit that an interdecadal enhancement of zonal advection (ZA) feedback process plays a vital role in the shift of the leading mode in tropical Pacific precipitation during spring. Moreover, the variability of anomalous zonal current at the upper ocean dominates the ZA feedback change, while the mean zonal SST gradient associated with a La Niña-like pattern of mean state only accounts for a trivial proportion of the ZA feedback change.It was found that the rapid decaying of the SST anomalies over the EP and the La Niña-like mean state makes it conceivable that the shift in the leading mode of the tropical precipitation anomaly only occurs in boreal spring.The largest variance of anomalous zonal current in spring might also contribute to the unique interdecadal change of tropical spring precipitation anomaly.
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