08/11/2022, 4:50 pm EDT

Monthly ENSO/PDO Forecast: La Nina/-PDO to continue…adds to long-term drought.

Discussion: Since the late 1990’s the long-term cool cycle of both the El Nino southern oscillation (ENSO) and Pacific decadal oscillation (PDO) evolved. There is a tendency for each climate signal to run parallel (Fig. 1). During 1998-2021 the most dominant ENSO event is La Nina and the number of El Nino episodes is about half the long-term average. Due to the La Nina/-PDO persistence, sea surface temperature anomalies (SSTA) are frequently cooler than normal off the West Coast of North America and across the East Pacific tropics. The cooler SSTA in these regions yield lower evaporation rates and Pacific storms are rendered less powerful leaving much of the U.S. in a rainfall deficit (Fig. 2). The regions observing the worst rainfall shortages during this timeframe is coastal British Columbia, the U.S. West Coast particularly northwest California, the Southwest U.S., Gulf Coast States and parts of the Great Plains. To reverse the long-term dryness observed over many parts of the U.S., a shift from the cool cycle of ENSO and PDO is required. The last long-term warm cycle of ENSO and PDO was observed in the middle 1960’s to middle 1990’s. Currently, ENSO has entered a 3rd year of La Nina. Last week, the Nino34 SSTA region cooled to -1.0C representing moderate-strength oceanic La Nina. However, the influence on climate by La Nina 2020-22 is profound. During JUN/JUL 2022 the multivariate ENSO index (MEI) was an intense La Nina-like -2.2. Since 1979, the satellite-derived MEI indicates only one other period with a stronger -MEI than JUN/JUL 2022 which was JUL-to-OCT 2011. Although the Nino34 SSTA has wobbled in-and-out of the La Nina threshold, the MEI has featured a consistent moderate to strong La Nina signature. Fig. 1: Since the late 1990’s ENSO and PDO have mostly been in negative to neutral phase. The number of El Nino events have been about half of normal. Fig. 2: U.S. rainfall anomalies for the 1998-2021 long-term La Nina/-PDO cycle. La Nina 2020-22 is entering a third year which is rare. The catalyst to La Nina continuing and strengthening is presence of a just-evolved negative phase Indian Ocean Dipole (-IOD). The -IOD pattern has caused warming of the ocean surface to the north and northwest of Australia and is increasing convection in this region. To replace rising air currents associated with convection, trade winds near the Dateline increase to up-well cool subsurface water and re-strengthen La Nina. Due to the -IOD pattern combining with La Nina, the cold ENSO should continue to strengthen peaking in intensity during November (Fig. 3) and not weakening until early next year (Fig. 4). Due to the stronger -MEI (compared to Nino34 SSTA), the La Nina climate may linger well into 2023. Fig. 3-4: Australia Bureau of Meteorology indicates peak La Nina intensity by November 2022 and the NCEP CFS V2 ENSO projection ending La Nina early next year. Only the 1973-76 and 1998-01 La Nina regimes are sufficiently lengthy to match the 2020-22 (and beyond) cold ENSO. The average of those two lengthy La Nina regimes suggests that La Nina 2020-22 lingers through quarter 1 of 2023 and is followed by neutral ENSO for the remainder of 2023 (Fig. 5). The Matching lengthy La Nina and paralleling -PDO regimes have a less confident projection primarily due to the very intense -PDO observed in July. Analogs indicate a weaker -PDO but the current observations indicate that transition could be delayed (Fig. 6). Between the Nino34 SSTA and especially the MEI expectation through the first quarter of 2023, the most likely precipitation scenario across the U.S. favors evolution of a new drought area across the Southeast U.S. (Fig. 7). Dryness extends to the Ohio Valley and West Coast drought continues most dramatically in parched northwest California. Wet weather (above normal snowfall) is confined to the Upper Midwest. In South America fierce drought in Brazil is implied by the analog while Argentina is wetter than normal (Fig. 8). Fig. 5: Lengthy La Nina episodes are used to project ENSO phase expectations for the remainder of 2022 through 2023. La Nina ends next March and is followed by neutral phase. Fig. 6: A lengthy La Nina/-PDO analog suggests a weaker -PDO ahead and neutral phase later in 2023. However, the analog weakens -PDO too quickly therefore forecast confidence is low.   Fig. 7: The precipitation pattern across the U.S. for September 2022 to April 2023 based on the La Nina analog. Fig. 8: The precipitation pattern across South America for September 2022 to April 2023 based on the La Nina/-PDO analogs.