Seasonality of MJO Impacts on Precipitation Extremes Over the Western U.S.

November 21, 2022

A new paper entitled “Seasonality of MJO Impacts on Precipitation Extremes Over the Western U.S.” was recently published in the Journal of Hydrometeorology and authored by CW3E researcher Jiabao Wang, Mike DeFlorio (CW3E), Bin Guan (UCLA/JPL), and Chris Castellano (CW3E). As part of CW3E’s 2019-2024 Strategic Plan, CW3E seeks to improve understanding of the subseasonal-to-seasonal (S2S) predictability of extreme weather over the western U.S. and develop a comprehensive understanding of the physical mechanisms and statistical characteristics of extreme precipitation events to inform current and future resource and risk management. This study (Wang et al. 2022) discovered strong seasonality in boreal winter western U.S. extreme precipitation that is modulated by seasonality in the Madden-Julian oscillation (MJO) and its impacts on the large-scale circulation and atmospheric river (AR) activity over the North Pacific and western North America. This research was funded by the California Department of Water Resources Atmospheric River Program Phase III and the National Aeronautics and Space Administration.

This study provides observational evidence of MJO impacts on extreme precipitation intensity, frequency, and duration over the western U.S. in boreal winter. In general, the MJO impacts on extreme precipitation intensity and duration are more uncertain than its impacts on extreme precipitation frequency. A robust increase in extreme precipitation frequency relative to climatological conditions over most of the western U.S. is expected when the MJO is in the western Pacific (Phases 6-7), and opposite changes are observed when the MJO is located over the Indian Ocean and Maritime Continent. The above MJO influence, however, is characterized by strong seasonality, with an increase in extreme precipitation frequency mainly found in late autumn/early winter (October-December; OND) over California, and weaker or opposite response found in late winter (January-March; JFM) following an MJO in Phases 6-7. This seasonality largely originates from the different amplitudes and patterns of both the MJO and the North Pacific jet, which are weaker and located/retreated more northwestward in OND compared to JFM. This leads to different responses in MJO teleconnections including moisture transport and AR activity that contribute to the different changes in precipitation extremes (summarized in Fig. 1). As a result, the moisture transport in OND brings moisture from the Pacific Ocean towards the western U.S., leading to an increase in AR activity and extreme precipitation frequency over California. On the other hand, moisture transport in JFM is diverted more poleward towards Alaska, leading to a decrease in AR activity and extreme precipitation over the western U.S.

The findings in this study suggest a value for future studies to consider the seasonal dependence of the MJO-precipitation extreme relationship. The results also have implications for the source of S2S predictions, which has potential value to stakeholders including water resource managers.

Figure 1: The schematic diagram of extratropical response (precipitation extremes, 500-hPa geopotential height anomalies, anomalous moisture transport, and AR activity) to MJO phases 6-7 in (top) OND and (bottom) JFM. Results are derived from the 5-9-day lagged average after the active MJO day at day 0. The green triangle indicates the center longitude of enhanced MJO convection at day 0. The jet (mean 250-hPa zonal wind as the basic state) interval is 20 m s^-1 starting at 30 m s^-1. The relative magnitude of each component is indicated by the difference in the thickness and length, and the variables shown may not be entirely precise in location and pattern.

Wang, J., DeFlorio, M. J., Guan, B., & Castellano, C. M. (2022). Seasonality of MJO Impacts on Precipitation Extremes Over the Western U.S. Journal of Hydrometeorology (published online ahead of print 2022). https://doi.org/10.1175/JHM-D-22-0089.1