A Dynamical Framework for Identifying Preferred Regions of Exciting Synoptic Disturbances under Varying Background Flows

Abstract. Synoptic disturbances shape mid-latitude storm tracks, yet their preferred origins under different background flows remain unclear. We employ a dynamical framework based on optimal mode analysis to simulate synoptic disturbances and identify regions favorable to their initial growth. The simulated modes reproduce key synoptic characteristics, and their root-mean-square amplitude captures the spatial distribution of synoptic variability. Under Northern Hemisphere climatological flow, the preferred initial region of synoptic disturbances appears over tropical–subtropical Asia, while variability is stronger in December–February (DJF) than in June–August (JJA). In the Southern Hemisphere, preferred initial regions shift from the South Atlantic–southern Africa sector in DJF to the tropical Indian Ocean–Maritime Continent in JJA. We also examine DJF background flows associated with major climate modes. During El Niño, enhanced initial activity in the tropical eastern Pacific and reduced activity in the extratropical North Pacific correspond to enhanced variability over southern North America and reduced variability farther north, respectively. Synoptic variability dipole over the South Pacific is associated with an initial meridional dipole to its west. During La Niña, Northern Hemisphere variability broadly reverses, while enhanced initial activity south of Australia and New Zealand corresponds to enhanced extratropical South Pacific variability. Contrasting Pacific Decadal Oscillation-related initial anomalies near the Aleutians and south of Australia and New Zealand are associated with downstream changes across northern mid-latitudes and the South Pacific–Southern Ocean, respectively. This study identifies dynamically favorable regions for synoptic perturbation growth, with potential implications for initial-condition ensemble forecasts.