A tangle of climate modes and the elusiveness of march precipitation variability over the southeastern US

This study investigates March precipitation and its interannual variability over Southeastern US because March precipitation ranks the second highest and bears largest variability. Moisture budget analysis demonstrates that moisture divergence, specifically the part associated with the mean flow, dominates precipitation variability. Circulation diagnoses show that precipitation is modulated by both subtropical and extratropical modes of variability. When North Atlantic Subtropical High (NASH) strengthens, it occupies the entire southeastern, leading to downward motions and moisture divergence, thus dry years. On the contrary, wet years correspond to a weaker or normal NASH and a quasi-stationary wave train originating from the North Pacific sets up an environment of large-scale ascent over the Southeast thus steers more synoptic disturbances into the region, enhancing the activities of precipitation-generating cyclones. March precipitation over the Southeast reflects a tangle of subtropical and extratropical modes of variability, making the simulation and projection of its hydroclimate a major challenge.