Regional-Scale Hydroclimate Variability

Modulation of North American Winter Hydroclimate by Upstream Atmospheric Disturbances

The main objective of this NASA-funded project is to understand how transient eddies over the North Pacific controls the winter hydroclimate variability, in particular, the occurrence of hydrological extremes, over its downstream region (i.e., western North America). On interannual timescales, major modes of variability in the coastal cyclone (synoptic eddies) activity is tightly coupled to those in the characteristics of western U.S. winter precipitation (Myoung and Deng 2009). On subseasonal timescales, the Pacific storm track responds to the Madden-Julian-Oscillation (MJO) in the tropics and an amplitude-varying dipole of the storm track anomaly develops as a result of anomalous convergence of energy flux, baroclinic conversion and energy generation due to the interaction between synoptic eddies and intraseasonal flow anomalies. This storm track anomaly drives subseasonal transitions between dry and wet regimes near the west coast of North America (Deng and Jiang 2010). Development of low-frequency transients over the eastern North Pacific in association with East Asian cold surges are capable of altering the occurrence probability of atmospheric rivers (ARs) that are responsible for most extreme rainfall and flooding events in the western U.S. (Jiang and Deng 2011).       



Time-latitude plot of the composite intraseasonal anomalies of the precipitation (contour) and the composite intraseasonal SEKE (synoptic eddy kinetic energy) anomalies (color shading) averaged over 160°W-130°W. (b) is the same as (a) except for the color  shading now corresponding to the composite intraseasonal anomalies of the 2-8 day band-pass filtered OLR variance. 



(a) Composite anomalies of the daily occurrence probability of ARs along 124°W (in percentage); (b) composite anomalies of the precipitation rate along 124°W (unit: mm/day); (c) composite anomalies of the daily occurrence probability of ARs averaged over Day 0 to Day 2 (in percentage) following the occurrence of an East Asian cold surge. (d) same as (c), but for average over Day 4 to Day 6. Areas enclosed by solid contours are significant at the 90% level.