Contrasting the Enhanced Atmospheric River Activity over the North Pacific and Atlantic Since 1950

Atmospheric rivers (ARs) play an essential role in the global water cycle and extreme weather, as it bridges the ocean to continental precipitation via mid-latitude moisture transport. Since 1950, the AR activities in the Northern Hemisphere have shown a significant increasing trend, and the increasing rate over the Atlantic is double that over the Pacific (3.3% vs 1.6% per decade). More interestingly, the number of landfall AR events over the Atlantic has increased significantly, while the total number of AR events has not changed much. It indicates that once ARs generate, they are more likely to propagate further east and make landfall over Europe in recent decades. As a result, western Europe has suffered more extreme precipitation days in recent decades. All the increases, in terms of AR landfall frequency and extreme precipitation frequency, are much weaker in the Pacific and western North America. The greater availability of water vapor in the atmosphere according to the Clausius–Clapeyron relationship could not explain this contrast. This North Atlantic-Pacific contrast in the trends of AR activity and related rainfall extremes results mainly from the different changing patterns of the westerly jet stream over the two northern oceans. Over the North Atlantic, the westerly jet stream extends eastward and intensifies significantly, providing a stronger steering flow for ARs toward Europe. On the contrary, the intensification of the Asia-Pacific jet stream is concentrated over Asia and the western North Pacific, without the enhanced steering effect when AR approaches North America.