Drought-Alleviating Heavy Precipitation in California During Winter 2022–2023

This presentation will overview the heavy rainfall that resulted in significant drought relief and areas of flooding in California during winter 2022–2023. Especially heavy rainfall occurred between 22 December 2022 and 17 January 2023, and again between 21 February and 2 March 2023 in conjunction with record-breaking, low-elevation snowfall. The first heavy precipitation event resulted subsequent to massive high-latitude ridge building near the Dateline in mid-December 2022 that culminated in a massive Rex block. Upstream, a parade of strong and highly amplified 500-hPa troughs deepened southeastward across eastern Asia and into the WPAC. A deep low-latitude trough that formed near the Dateline on 17 December deepened southeastward and was characterized by a -6 sigma standardized height anomaly on 20 December near 20°N and 160°W. This intense low latitude trough enabled tropical moisture characterized by precipitable water values > 40+ mm to be pulled northeastward along atmospheric rivers (ARs) on its eastern side to near 40 N beginning on 20 December. The leading edge of this initial tropical moisture surge reached the West Coast on 22 December. An anomalously strong trans-NPAC 500-hPa subtropical jet stream (STJ) was established between 20–40°N in the wake of this first West Coast tropical moisture surge. This comparatively low-latitude STJ served as a conduit for successive trans-Pacific progressive trough passages and associated ARs that produced numerous heavy rain episodes in California from the latter part of December 2022 through much of January 2023.

The second heavy precipitation event occurred in conjunction with a highly amplified upper-level flow pattern that facilitated the north-south exchange of tropical and polar air masses. Repeated cold surges from eastern Asia into the WPAC facilitated multiple strong surface cyclogenesis events in the poleward exit region of the associated WPAC jet stream. These WPAC cyclogenesis events in turn facilitated strong high-latitude downstream ridge building that extended poleward to Alaska across the EPAC with subsequent downstream troughing over western North America. Strong northwesterly flow between the aforementioned EPAC ridge and the western North American trough permitted multiple baroclinic troughs and cold-core cutoff cyclones to plunge southeastward to southern California and the desert Southwest. Although these aforementioned baroclinic troughs and cold-core cutoff cyclones did not have access to ARs of tropical and subtropical origin they were able to access EPAC moisture. Robust moisture flux convergence, assisted by upslope onshore flow, sustained heavy precipitation and low-level elevation heavy snow across much of California on the forward flank of numerous individual cold-core migratory upper-level troughs and cutoff cyclones.

Four testable hypotheses for the aforementioned twin surges of drought-alleviating heavy precipitation in California are as follows: 1) Persistent lower-latitude cold surges off the coast of Asia favored enhanced baroclinicity over the western and central subtropical NPAC and supported an anomalously strong trans-NPAC STJ, 2) An anomalously strong zonally elongated, low-latitude STJ enabled tropical moisture from the southwestern NPAC to be transported ENE from its source region to the EPAC and California, (3) A broad diffluent jet-exit region flow pattern east of the Dateline favored the creation of an axis of dilatation quasi-parallel to the West Coast of North America that acted as a locus of episodic moisture flux convergence and low-level frontogenesis, and 4) repeated cyclogenesis events in the northern Gulf of Alaska associated with eastern Asia cold surges beginning in mid-February 2023 facilitated strong ridge building over Alaska and deep trough development over western North America that permitted numerous cold-core upper-level troughs to reach California in strong northwesterly flow. The first and third hypotheses are supported by an anomalously strong time-mean NPAC jet stream that featured a well-defined diffluent jet-exit region off the coast of California. The second hypothesis is supported by the presence of an anomalous tropical moisture “freeway” that extended from the southwest NPAC east-northeastward to California. The fourth hypothesis is supported by persistent cyclone-induced ridging over Alaska and anomalously strong northwesterly flow along the west coast of North America that permitted troughs of polar origin to reach southern California and northern Mexico. The relative contributions of the dynamical and thermodynamical processes mentioned in these four hypotheses to drought-alleviating heavy California rainfall during the 2022–2023 winter season will be the focus of this presentation.