What Modulates the Intensity of Synoptic-Scale Variability over the Western North Pacific during Boreal Summer and Fall?

The present study investigates the factors that affect the year-to-year change in the intensity of synoptic-scale variability (SSV) over the tropical western North Pacific (TWNP) during boreal summer and fall. It is found that the intensity of the TWNP SSV in summer is associated with the equatorial central-eastern Pacific sea surface temperature (SST) anomalies that modulate the background fields through a Rossby wave response both in the source region and along the propagation path of the synoptic-scale disturbances. In fall, the intensity of the TWNP SSV is related to an SST anomaly pattern with opposite anomalies in the equatorial central Pacific and TWNP that modulates the background fields from the equatorial central Pacific to TWNP. However, the equatorial central Pacific SST anomalies alone fail to change the intensity of the TWNP SSV as the induced background field changes are limited to the equatorial central Pacific. It is shown that tropical western Pacific SST anomalies may induce notable changes in the intensity of the TWNP SSV. The relation of the TWNP SSV to the equatorial eastern Pacific SST is weak due to opposite SST anomalies in different types of years. Both seasonal mean and intraseasonal flows provide sources of barotropic energy for the change in the intensity of the TWNP synoptic-scale disturbances in summer. Seasonal mean flow has a main contribution to the barotropic energy conversion for the change in the intensity of the TWNP synoptic-scale disturbances in fall.

Automated summary

This study investigates the factors affecting the year-to-year change in the intensity of synoptic-scale variability over the tropical western North Pacific during summer and fall. The study finds that sea surface temperature anomalies in the equatorial Pacific play a key role in modulating the background fields and intensity of synoptic-scale disturbances in the region. Furthermore, the research shows that SST anomalies in the tropical western Pacific have a notable impact on the intensity of synoptic-scale variability in the region. Opposite SST anomalies in different types of years lead to a weak relationship between the TWNP SSV and equatorial eastern Pacific SST.