Climatology and Variability of Atmospheric Rivers over the North Pacific

June 09, 2017

Bryan Mundhenk

Committee: Elizabeth Barnes (Advisor), Eric Maloney (Co-Advisor), David Randall, Jay Ham (Soil and Crop Sciences)

Abstract

Atmospheric rivers (ARs) are plumes of intense water vapor transport that dominate the flux of water vapor into and within the extratropics. Upon landfall, ARs are a major source of precipitation and often trigger weather and/or hydrologic extremes. Over time, landfalling AR activity, or a lack thereof, can influence periods of regional water abundance and/or drought. Here, an objective detection algorithm is developed to identify and characterize these features using gridded fields of anomalous vertically integrated water vapor transport. Output from this algorithm enables an investigation into the relationships between tropical variability and ARs over the North Pacific.

In the first segment of this presentation, an all-season analysis of AR incidence within the North Pacific basin is performed. The variability of AR activity due to the seasonal cycle, the El NiÃ±o-Southern Oscillation (ENSO), and the Madden-Julian oscillation (MJO) is also presented. The results highlight that ARs exist throughout the year over the North Pacific. In general, the seasonal cycle manifests itself as northward and westward displacement of AR activity during boreal summer, rather than an absolute change in the number of ARs within the domain. It is also shown that changes to the North Pacific mean-state due to the ENSO cycle and the MJO may enhance or completely offset the seasonal cycle of AR activity, but that such influences on AR frequencies vary greatly based on location.

The second segment of this presentation investigates ARs at high northern latitudes. Comparatively little is known about the dynamics supporting these ARs in contrast to their mid-latitude counterparts. ARs are found to occur near the Gulf of Alaska and the U.S. West Coast with similar frequency, but with different seasonality. Composited atmospheric conditions reveal that a broad height anomaly over the northeast Pacific is influential to AR activity near both of these regions. When a positive height anomaly exists over the northeast Pacific, AR activity is often deflected poleward toward Alaska, while the U.S. West Coast experiences a decrease in AR activity, and vice versa. This tradeoff in AR activity between these two regions applies across a range of time scales, not just with respect to individual transient waves. Both ARs and height anomalies are found to be associated with Rossby wave breaking, thereby dynamically linking AR activity with broader North Pacific dynamics.

The third segment of this presentation explores the predictability of anomalous landfalling AR activity within the subseasonal time scale. An empirical prediction scheme based solely on the initial state of the MJO and the stratospheric quasi-biennial oscillation (QBO) is constructed and evaluated. This scheme is based on the premise that the MJO modulates landfalling AR activity along the west coast of North America within the subseasonal time scale by exciting large-scale circulation anomalies over the North Pacific. The QBO is found to further modulate the MJO-AR relationship. The prediction scheme reveals skillful subseasonal Â“forecasts of opportunityÂ” when knowledge of the MJO and the QBO can be leveraged to predict periods of increased or decreased AR activity. Moreover, certain MJO and QBO phase combinations provide predictive skill competitive with, or even exceeding, a state-of-the-art numerical weather prediction model.