IPRC Research on Pacific Decadal Variability and Controversies as to Its Cause
Julian P. McCreary, Jr.
International Pacific Research Center (IPRC)
Hawaii, USA
Several ongoing modeling projects at the IPRC are investigating the dynamics of Pacific decadal variability (PDV) and ENSO decadal variability (EDV).
A group of studies points toward the importance of the Pacific Subtropical Cells (STCs) in EDV. The STCs are shallow meridional overturning cells that connect subtropical subduction regions to the upwelling region in eastern, equatorial Pacific. An oceanic GCM forced by NCEP reanalysis winds suggests that: i) subtropical SST anomalies subducted into the thermocline do NOT lead to significant equatorial SST variations at decadal time scales; ii) subtropical WIND anomalies, however, do lead to significant equatorial SST changes by changing the strength of the STCs; and iii) tropical wind anomalies are also influential. An intermediate coupled model generates Pacific decadal variability (PDV) via midlatitude processes; it also alters the strength of the North Pacific STC, thereby remotely forcing ENSO decadal variability.
Other studies indicate the importance of tropical processes. A coupled GCM that generates EDV appears to involve BOTH tropical and subtropical processes: positive feedback between the ocean and atmosphere within the tropics, and time-delayed negative feedback by subtropical Rossby waves. Finally, simple and intermediate coupled models are able to generate EDV by tropical processes alone, via nonlinearities that lead to chaos.
