Does El Niño affect MJO-AR connections over the North Pacific and associated North American precipitation?

Abstract

This study investigates how the positive phase of the El Niño-Southern Oscillation (EN) influences the Madden-Julian Oscillation (MJO) modulation of cool-season North Pacific atmospheric rivers (ARs) and associated AR-landfall driven precipitation over North America. EN changes the key drivers of MJO-AR connections by shifting MJO-driven convection east of 180° longitude in MJO phases 6-8 and extending the northern Pacific subtropical jet eastward. Under these conditions, the MJO tropicalextratropical teleconnection is triggered east of 180° in MJO phases 7-8, and a persistent cyclonic flow anomaly develops along the United States west coast. Anomalous northeastward integrated water vapor transport (IVT) within the cyclonic flow coupled with the MJO convection over the western (phase 7) and central (phase 8) Pacific increases AR frequency, shifting it to the east over regions that do not show a relationship with EN or MJO alone. Besides enhancing AR activity, EN background conditions increase the number of AR events, their lifetime, and mean intensity from MJO phases 6 through 8, as well as the number of MJO active days, AR initiations, and ARs making landfall over North America in MJO phases 8-1. The positive precipitation anomalies and increased frequency of extreme precipitation events associated with landfalling North Pacific ARs related to MJO are also shifted to the east in EN, enhancing and extending rainfall over western North America in MJO phases 6-1. Resultsprovide new insight into the drivers of AR activity and associated precipitation along the west coast of North America with implications for improving subseasonal-to-seasonal predictions.