Analysis and Prediction of Large-Scale Cold-Season Atmospheric Circulation Regimes Associated with Extreme Weather over North America

Cold-season (DJF) atmospheric circulation regimes, particularly those focused in the eastern North Pacific basin, often define significant sub-seasonal variability of sensible weather for North America. Our objective is to identify these regimes, and to improve their predictions on sub-seasonal (1-4 week) time scales.

We focus on two specific regimes in the eastern North Pacific basin. The first identifies extended periods of both low- and high-variance of the 500-hPa geopotential heights. The second regime is based upon extreme area-averaged values of condensation. Each regime’s metric is computed in the eastern North Pacific region of 35 – 60 deg N, and 130 – 155 deg W.

The 500-hPa geopotential height regime is defined as a period of at least five days during which the anomaly of the seven-day running standard deviation of the seasonally de-trended time series is continuously below the 10^th percentile (low-variance regime), or above the 90^th percentile (high-variance regime).

The condensation-based regime is defined as a period of at least four days in a consecutive 10-day period during which the anomaly of the area-averaged condensation either is below the 10^th percentile (dry regime), or above the 90^th percentile (wet regime).

Each of the regime identifications is performed using the National Centers for Environmental Prediction (NCEP) reanalysis 2 data.

Principal results of our analyses of the 500-hPa height low-variance regimes include 1) anomalously large precipitable water and condensation rate and 2) anomalously strong poleward winds located eastward of anomalously low heights and westward of anomalously high heights. The high-variance regimes are characterized by anomalously strong equatorward flow that is associated with anomalously weak static stability.

The anomalously large condensation regimes are associated with a storm track of clustered explosively developing cyclones that track poleward into southeastern Alaska. The anomalously dry regimes are preferentially associated with anomalous ridging and northeasterly tropospheric flows, favorable for deep tropospheric subsidence.

Our research links these regimes with several multiscale processes that impact North American weather, including but not limited to, anomalous surface cyclone clustering as influenced by atmospheric blocking and the phases and amplitudes of the major teleconnection indices, El Niño Southern Oscillation (ENSO) and the Madden-Julian Oscillation (MJO). Additionally, we are examining the predictability of regime maintenance and change that impacts North America, with particular emphasis on Arctic air mass generation and equatorward migration, along with poleward heat and moisture transports of subtropical air masses that trigger extreme precipitation.