Northern California is currently in the midst of its first significant rainfall of the season.  The plume of moisture currently advecting on shore can be traced all the way back to a weakened typhoon in the western Pacific.  I’ve heard a lot of chatter that this is an example of an “atmospheric river“, a strong, steady plume of moisture advected poleward out of the moist tropics.  We have come to appreciate recently that atmospheric rivers are responsible for many of the significant rainfall events here in California and on the western coasts of several continents.   But, I have to ask myself when an atmospheric river is really an atmospheric river and when it isn’t.    The TPW loop shows that this moisture is not arriving from the deep tropics, but rather, from a ex-tropical system.  The original definition of an atmospheric river makes no mention of the origins of the moisture, and as such, it is perfectly general.  But, the connotation has generally been one in which the moisture originates from the TPW maximum around the equator.  The precise definition becomes important as those of us in CA focus research efforts on the formation mechanism of atmospheric rivers and try to determine how they will change under global warming scenarios.  Therefore, I would like to propose that “rivers” such as the one currently impacting CA simply be grouped in with normal warm frontal processes and we reserve the use of “atmospheric rivers” to those features extracting moisture directly from the deep tropics.