Play all audios:
While atmospheric rivers share a similar origin — moisture supply from the tropics — atmospheric instability of the _jet stream_ allows them to curve poleward in different ways. No two
atmospheric rivers are exactly alike. What particularly interests climate scientists, including us, is the collective behavior of atmospheric rivers. Atmospheric rivers are commonly seen in
the _extratropics_, a region between the latitudes of 30 and 50 degrees in both hemispheres that includes most of the continental U.S., southern Australia and Chile. Our study shows that
atmospheric rivers have been shifting poleward over the past four decades. In both hemispheres, activity has increased along 50 degrees north and 50 degrees south, while it has decreased
along 30 degrees north and 30 degrees south since 1979. In North America, that means more atmospheric rivers drenching British Columbia and Alaska. A GLOBAL CHAIN REACTION One main reason
for this shift is changes in sea surface temperatures in the _eastern tropical Pacific_. Since 2000, waters in _the eastern tropical Pacific have had a cooling tendency_, which affects
atmospheric circulation worldwide. This cooling, often associated with _La Nina conditions_, pushes atmospheric rivers toward the poles. The poleward movement of atmospheric rivers can be
explained as a chain of interconnected processes. During La Nina conditions, when sea surface temperatures cool in the eastern tropical Pacific, the _Walker circulation_ — giant loops of air
that affect precipitation as they rise and fall over different parts of the tropics — strengthens over the western Pacific. This stronger circulation causes the _tropical rainfall belt_ to
expand. The expanded tropical rainfall, combined with changes in atmospheric eddy patterns, results in _high-pressure anomalies_ and _wind patterns_ that steer atmospheric rivers farther
poleward.