The Stratospheric Sudden Warming of March 2025 Abruptly Ended the NH Stratospheric Polar Vortex
Stratospheric Sudden Warmings (SSW) disrupt the climatological Northern Hemisphere (NH) wintertime westerly flow, rapidly warming the polar night region. The sensitivity of the relatively low-density stratosphere to changing weather patterns in the troposphere creates, under the right troposphere and stratosphere flow patterns, SSW events. Stratospheric mid-winter polar warmings are considered major when the warming is strong enough to reverse the stratospheric jet, creating easterlies in place of westerlies. A final SSW occurs when the created easterlies never experience enough polar night cooling to return to westerlies but remain easterlies throughout the summer season. An early final major SSW denoted the end of the NH winter of 2024-2025.
A cold stratospheric polar region characterized most of the NH winter of 2024-2025 with a minor warming in early February, followed by the major final warming in early March (Fig. 1). The stratospheric jet decreased sharply in early February in association with the temperature rise, while it reversed (became negative) during early March, the major final SSW event. A striking feature of this major SSW wave is the record wave 1 amplitude for this late in the NH winter (Fig. 3), denoting a relatively strong stratospheric vortex displaced significantly off the pole.
GEOS-FP (Goddard Earth Observing System – Forward Processing) forecasts out to 7 days captured these temperature and wind changes. The temperature forecast delayed the February warming; however, the 7-day forecasts captured the major SSW well (Fig. 4). The corresponding major final SSW wind reversal was also well captured (Fig. 5).
GEOS-S2S (GEOS – Sub-seasonal to Seasonal) forecasts suggest a large amplitude disturbance 20 days before the wind reversal on 7 March though the longitudes vary between ensembles, as seen in the 10 hPa geopotential height forecasts (Fig. 6, upper left). At 15 days the forecasts still show uncertainties about the longitude of the disturbances (Fig. 6 upper right), however the 10-day forecasts are converging about the MERRA-2 validation heights (Fig. 6 lower left), and at 5 days out the ensembles agree well with the validation (Fig. 6 lower right).
Along with warming the pole and weakening the winds, SSW events transport air between the tropics and the polar regions. As tropical air moves north the curvature of the Earth increases the alignment between the spin of the air and the Earth’s rotation. Conserving total vorticity, both planetary and local, then requires the air to acquire an anti-cyclonic rotation, to oppose the Earth’s rotation. This is seen happening several times during February-March of 2025 (Animation 1). The Ertel Potential Vorticity (EPV) fields, calculated from wind and temperature, are well conserved following the flow, allowing EPV to act like a tracer of the atmospheric motion. The February cyclonic vortex circulation (Fig. 7, left) is much stronger than at end of March, where only a few vortex remnants exist (Fig. 7, right).
Thus, the GMAO's GEOS-FP and GEOS-S2S products are capable of forecasting and characterizing the evolution of the SSW events, as well as tracking the dramatic mixing of tropical and polar air that occurs during these events.
