Steady Cool Phase of the Pacific Oscillation, but will it last?
Big season in North Atlantic tropics ahead. But tropics are cooling.
Discussion: According to NOAA in February, the most recent update on Marine Heatwave NEP22A indicates strengthening and possible eastward expansion to the U.S. West Coast by autumn 2022. If so, the cool phase Pacific decadal oscillation (-PDO) would weaken or end. However, the current northeast Pacific basin SSTA is +0.43C (due to a combined warm NEP22A and cool -PDO signature) and the 30-day trend is cooler (-0.19C). Global SSTA forecasts by NCEP CFS V2, ECMWF and IMME models indicate NEP22A shifts toward but does not quite reach the North America West Coast by August. The consensus of global models indicates the -PDO regime likely weakens by Q3/2022 which implies La Nina should also end by that time. As usual, -PDO has run parallel to the Nino34 SSTA during the 2020-22 La Nina regime.
Leading seasonal tropical cyclone forecasters (Colorado State University, Climate Impact Company and Tropical storm Risk U.K.) are agreeable to another big year across the North Atlantic tropics (18-19 tropical storms, 8-9 hurricanes and 4 major hurricanes). HOWEVER, a curve ball has developed in April…cooling off of the main development region for hurricanes. Currently, the tropical North Atlantic (TNA) index is -0.33 (Fig. 3) and is cooling off steadily (-0.22 change in 30 days; Fig. 4). The basin-wide SSTA for the North Atlantic is right at normal which is markedly cooler than previous months and heading into April. The Atlantic multi-decadal oscillation is now 7th coolest in the 202-22 warm period for this time of year.
Global SSTA forecast models are generally too warm in the North Atlantic basin which biases their JUN/JUL/AUG forecasts too warm. A careful eye on global SSTA forecasts heading toward the warm season for adjustments and whether the expected warming of the North Atlantic to fuel buoyant seasonal tropical cyclone forecasts is required.
Fig. 1-2: Northeast Pacific SSTA and the PDO/Nino34 index for 2020-22.
Fig. 3-4: The tropical North Atlantic index is cooling based on current observations and the 30-day trend.