Four Atmospheric Circulation Regimes Over the North Pacific and Their Relationship to California Precipitation on Daily to Seasonal Timescales

May 26, 2020

CW3E scientist Kristen Guirguis, along with other researchers from CW3E (Alexander Gershunov, Michael DeFlorio, Tamara Shulgina, Luca Delle Monache, Tom Corringham, and F. Martin Ralph) and the University of Colorado at Boulder (Aneesh Subramanian), recently published an article in Geophysical Research Letters entitled “Four atmospheric circulation regimes over the North Pacific and their relationship to California precipitation on daily to seasonal timescales.” Subseasonal-to-seasonal (S2S) predictability of atmospheric rivers and precipitation is a key thematic focus area for CW3E. S2S represents timescales of prediction that are of vital importance to our stakeholders at the California Department of Water Resources. Specifically, this publication helps to further CW3E’s aim to increase understanding and improve skill in forecasting synoptic weather precursor patterns over the western United States that modulate S2S AR and precipitation occurrence and magnitude.

The purpose of this work was to quantify the importance of four atmospheric circulation regimes previously identified in an earlier study (Guirguis et al. 2018) in driving AR activity and precipitation in California. The new study highlights the importance of interactions between these four circulation regimes (called the NP4 modes). In general, when multiple modes align in the positive phase, this reinforces onshore flow patterns over California and is associated with elevated AR activity, heavy precipitation, and historical flooding. Conversely, when multiple modes are jointly negative, this favors upstream atmospheric ridging and dry conditions for California. Seasonally, the study shows how these regimes tend to favor one phase or another in a given water year, which helps to determine if a season is wet or dry. The state of the El Niño Southern Oscillation (ENSO) is shown to stack the deck towards certain phase preferences of the NP4, but within a season there is still much variability, and the probability of extreme precipitation depends on the extent to which the NP4 modes synchronize on daily timescales to reinforce or oppose each other. The study uses multivariate statistical models to quantify the role of the NP4 modes on daily coastal vapor transport and seasonal precipitation in California. The results show the importance of the NP4 modes in driving hydrologic extremes across timescales. An improved understanding of the climate-weather linkages which cause these modes to persist in a season could help to advance S2S predictability.

Figure 1 shows the NP4 atmospheric circulation regimes. Figure 2 shows an example of the reinforcing effect when four modes align in the positive phase (deep trough and enhanced onshore flow, left) or negative phase (strong offshore ridge and anomalous offshore flow, right). Figure 3 shows the effect of joint mode phasing on daily California precipitation.

Figure 1. The NP4 modes shown in the phase associated with enhanced AR activity over California. The color scale gives temporal correlation between the associated principal component time series and 500-mb geopotential height anomalies at a location.

Figure 2. Composites of 500-mb geopotential height anomalies (shaded) and wind speed (vectors) when the NP4 modes are jointly positive (left) or jointly negative (right).

Figure 3. Average daily precipitation conditional on the joint phasing of the NP4 modes. As more modes become aligned in the positive phase, onshore flow is reinforced as in Figure 2 (left).

Guirguis, K., A. Gershunov, M.J. DeFlorio, T. Shulgina, L. Delle Monache, A.C. Subramanian, T.W. Corringham, and F.M. Ralph, 2020: Four North Pacific atmospheric circulation regimes and their relationship to California precipitation on daily to seasonal timescales. Geophys. Res. Lett., 47, e2020GL087609, https://doi.org/10.1029/2020GL087609