Roles of MJO and Tropical–Extratropical Interactions in Subseasonal Conditions Related to Landfalling Atmospheric Rivers
Journal Article
Overview
abstract
Abstract; This study investigates the role of the Madden–Julian oscillation (MJO) and other tropical–extratropical interactions in generating atmospheric rivers (ARs) using a linear inverse model (LIM) framework. We examine subseasonal conditions that preferentially lead to landfalling ARs over Alaska, the Pacific Northwest, and California during boreal winters. We identify LIM dynamical modes that strongly project onto tropical sea surface temperature, such as El Niño–Southern Oscillation (ENSO); modes coupled with tropical heating, such as the MJO; and modes that are weakly coupled between the tropics and extratropics. The composite analysis of prolonged AR active conditions (14-day window) reveals that they are driven primarily by weakly coupled modes, with small contributions from tropically driven modes. The only significant signal from tropical heating modes is the subtropical vapor transport associated with MJO phases 6–7 for Alaska ARs. We also examine the role of the MJO in the optimal growth of initial conditions into the AR patterns. For all regions, the evolving optimals show nearly stationary phase patterns but propagating wave activity that shifts the location of maximum amplitude in time. The contributions from three mode groups are consistent across the regions, with the MJO partially contributing to the linear predictable growth, while weakly coupled modes remain the main drivers. The MJO and weakly coupled modes constructively interfere in subtropical vapor transport and destructively interfere in tropical convective activity. These findings underscore the importance of accurately resolving weakly coupled tropical–extratropical interactions to improve subseasonal AR predictions.; ; Significance Statement; Although the Madden–Julian oscillation (MJO) is often considered important for predicting atmospheric rivers on subseasonal time scales (10–30 days), our findings show that its contribution is relatively minor. Instead, other tropical–extratropical interactions that are only weakly connected to tropical heating play a larger role in driving atmospheric river activity. These weakly coupled processes tend to decay faster than MJO-related signals but are crucial for understanding and forecasting atmospheric river variability along the West Coast.;
