The Response of Water Vapor Wave Activity to Warming

This study uses a comprehensive GCM to simulate the response of the wavy water vapor field to climate change, with a focus on the west coast of North America. Large poleward excursions of column water vapor are often associated with atmospheric rivers, which produce a large portion of the most extreme precipitation and wind events along the coast. Such atmospheric river events contributed to the flooding in Northern California during the winter of 2016-2017.

Here we define the wavy water vapor field by applying a transform similar to that applied by Clare S. Huang and Noboru Nakamura to the vorticity field. Specifically, a zonally symmetric basic state total column water vapor field is first derived by applying an area-conserving coordinate transformation which rearranges the water vapor field, such that it decreases monotonically towards the pole. Departures of the local water vapor field from this basic state contribute to the local water vapor wave activity depending on both the meridional extent of the parcel from its respective basic state 'equivalent' latitude, as well as the magnitude of the departure from the basic state value at the parcel's latitude. 

Probability density functions reveal an increased occurrence of extreme water vapor wave activity events in a warmer climate over much of the west coast. We analyze semi-Lagrangian water vapor budgets to diagnose the increase to diagnose this increase. These results highlight dynamical impacts of climate change on precipitation extremes over middle latitudes. Future work will focus on the robustness of these impacts between climate models.