Atmospheric Rivers Life Cycles: Climatology and Interannual Variability

Atmospheric rivers (ARs) are intense poleward moisture transport with filamentary structure, which is important to the global hydrology cycle. The life cycle of AR can be represented by a series of spatiotemporally connected AR features of contiguous time steps. A tracing algorithm based on the concept of depth-first search is developed to track the AR life cycles.

Using the tracing algorithm, we study the climatological features of the winter North Pacific ARs. These features include the region of frequent origins and terminations, lifetime, intensity, traveled distance, and propagation speed. The climatological maximum AR origin frequency occurs in the east coast of Asia, and the maximum AR termination frequency locates in high latitudes over the west coast of North America. The correspondence between origin and termination indicates the northeastward propagation of AR life cycles. We found that the traveled distance of an AR increases proportionally with the AR lifetime (i.e., AR that lasts longer travels farther). The AR origin frequency over the Northwest Pacific displays strong interannual variability. The 1^st leading mode shows a northwest-southeast dipole pattern, which is significantly correlated to an anomalous cyclonic circulation and local baroclinicity over the Northwest Pacific. The 2^nd leading mode is a monopole pattern, which reveals strong ENSO modulations. The corresponding AR termination, propagation track, and regional influence from the origin modes will be discussed.