Observations and Monitoring of Atmospheric Circulation Patterns Impacting the West Coast
Four Key Modes of Atmospheric Variability over the North Pacific Ocean
Figure 1. The loading patterns of four key modes of atmospheric variability over the North Pacific Ocean (called the "NP4 modes") identified using over 70 years of 500 mb Geopotential height anomalies from NCEP Reanalysis, shown here in the positive phase. The daily interaction of these modes produce reoccurring weather patterns over the West Coast that modulate temperature, precipitation, and Santa Ana winds over California.
Figure 2. These figures describe the daily evolution of atmospheric circulation over the Northeastern Pacific Ocean and along the West Coast during WY2024, along with associated impacts. (a) shows the daily amplitude of four key modes of atmospheric variability (i.e. the "NP4 modes" shown in Figure 1). (b) shows the cumulative mode phase. Generally, when the modes are jointly positive, atmospheric circulation is favorable to AR landfalls and when the modes are jointly negative dry conditions are more likely. (c) shows observed AR landfalls at different latitudes along the West Coast, shown as the magnitude of AR IVT at the coast. (d) and (e) show the total precipitation over Northern and Southern California, stratified by weather regime, where the y-axis gives the precipitation total as a percent of the climatological average (i.e. crossing the 100% line means Northern/Southern California has exceeded the seasonal average amount of precipitation). The weather regimes are shown in Figure 3. Precipitation is from gridMET. AR landfalls are identified using the SIO-R1 catalog.
Historical Weather Regimes 1949-2017
Figure 3a. Composites of 500 mb Geopotential height anomalies (Z500) for each of sixteen weather regimes defined in Guirguis et al. (2022). The sample size is shown in the title as the percent daily occurrence during Oct-Feb over the full record. The highlighted text in the lower left corner gives the NP4 phase combination in the following order: [Baja-Pacific, Alaskan-Pacific, Canadian-Pacific, and Offshore-CA].
Weather Regimes observed during WY2024
Figure 3b. As in Figure 3a but for WY2024. The highlighted text in the lower left corner shows the number of days in the sample. The sample size is also shown in the title as the percent of days in the season so far.
Seasonal Frequency of each Weather Regime
Figure 4. This figure shows the weather regime distribution for the historical record during Oct-Feb (climatology, gray), and for WY2024 so far.