Forecasting extreme stratospheric polar vortex events

A tropical stratopause precursor for sudden stratospheric warmings

Dramatic meteorological phenomena in the winter polar stratosphere known as Sudden Stratospheric Warming (SSW) events are well recognized for their impacts felt across the whole atmosphere. Apart from the influence of tropospheric forcing and stratospheric control, many studies have addressed the possible role of external factors on the occurrence of SSW events. Here, with the help of reanalysis datasets, we present a hitherto unexplored connection between the tropical upper stratosphere and the polar vortex. We identify enhanced planetary wave driving around the tropical stratopause and poleward progression of the zero-wind line as early indicators for the occurrence of SSW events. We demonstrate that the poleward progression of the zero wind line results in efficient focusing of planetary waves into the polar vortex which culminates in its disruption. Statistically, nearly 70% of the SSW events that took place so far have been preceded by enhanced tropical stratopause wave driving which points towards identifying this as a potential precursor for the occurrence of SSW events. After the year 2000, significantly a greater number of SSW events have been found to be preceded by enhanced tropical stratopause wave driving.

Polar Vortex Multi-Day Intensity Prediction Relying on New Deep Learning Model: A Combined Convolution Neural Network with Long Short-Term Memory Based on Gaussian Smoothing Method

The variation of polar vortex intensity is a significant factor affecting the atmospheric conditions and weather in the Northern Hemisphere (NH) and even the world. However, previous studies on the prediction of polar vortex intensity are insufficient. This paper establishes a deep learning (DL) model for multi-day and long-time intensity prediction of the polar vortex. Focusing on the winter period with the strongest polar vortex intensity, geopotential height (GPH) data of NCEP from 1948 to 2020 at 50 hPa are used to construct the dataset of polar vortex anomaly distribution images and polar vortex intensity time series. Then, we propose a new convolution neural network with long short-term memory based on Gaussian smoothing (GSCNN-LSTM) model which can not only accurately predict the variation characteristics of polar vortex intensity from day to day, but also can produce a skillful forecast for lead times of up to 20 days. Moreover, the innovative GSCNN-LSTM model has better stability and skillful correlation prediction than the traditional and some advanced spatiotemporal sequence prediction models. The accuracy of the model suggests important implications that DL methods have good applicability in forecasting the nonlinear system and vortex spatial-temporal characteristics variation in the atmosphere.

Planetary Wave Spectrum in the Stratosphere–Mesosphere during Sudden Stratospheric Warming 2018

The planetary wave activity in the stratosphere–mesosphere during the Arctic major Sudden Stratospheric Warming (SSW) in February 2018 is discussed on the basis of microwave radiometer (MWR) measurements of carbon monoxide (CO) above Kharkiv, Ukraine (50.0° N, 36.3° E) and the Aura Microwave Limb Sounder (MLS) measurements of CO, temperature and geopotential heights. From the MLS data, eastward and westward migrations of wave 1/wave 2 spectral components were differentiated, to which less attention was paid in previous studies. Abrupt changes in zonal wave spectra occurred with the zonal wind reversal near 10 February 2018. Eastward wave 1 and wave 2 were observed before the SSW onset and disappeared during the SSW event, when westward wave 1 became dominant. Wavelet power spectra of mesospheric CO variations showed statistically significant periods of 20–30 days using both MWR and MLS data. Although westward wave 1 in the mesosphere dominated with the onset of the SSW 2018, it developed independently of stratospheric dynamics. Since the propagation of upward planetary waves was limited in the easterly zonal flow in the stratosphere during SSW, forced planetary waves in the mid-latitude mesosphere may exist due to the instability of the zonal flow.