To separate the ENSO teleconnection through the stratosphere, a nobel analysis using 3-dimensional relative vorticity has been implemented to characterize the stratospheric polar vortex and explore its interaction with the Rossby wave train forced by ENSO in the troposphere. This work reconciles previous findings in the stratospheric pathway of ENSO, and reveals why split sudden stratospheric warmings (SSWs) are more frequent during La Niña while El Niño is associated with displacement-type SSWs.
In order to investigate the QBO teleconnection several sets of idealized experiments have been conducted using CanESM5.1.
The first integration was the control run (CTL), which includes 200 years after a 100-year spin-up in ocean-coupled mode with constant radiative conditions set to the year 2005. Since CanESM does not have exhibit a spontaneous QBO, the originally proposed NOTROP experiment is no longer required. Upon completion of the CTL run, initial conditions were used to branch off nudging experiments starting on September 1st, November 1st or December 1st, depending on the specific case. For the experiments, the main nudging strategies explored are:
a) Nudging protocol
i) Vertical extension: One of the project’s objectives is to investigate the distinct roles of the upper and lower cells of the QBO. This has been explored by permanently nudging either the upper or lower cell throughout the entire winter, as well as by nudging the cells for one month and then allowing the model to run freely for the subsequent months. Additionally, the vertical extent of the nudging—either extending to the model's top or up to 10 hPa (as specified in the QBOi protocol)—was also compared.
ii) Latitudinal extension: after testing different latitudinal extensions for the nudging, three different ranges were selected for comparison: “qboi-fit”, qboi, and “qboi-20”, listed in order of increasing extension.
iii) Strength of the nudging: this can be adjusted my modulating the nudging timescale – shorter timescales correspond to stronger nudging. After testing various durations ranging from 6 to 120 hours, 24-hours nudging period was selected.
iv) Since the nudging is applied in the spectral space, it was straightforward to nudge only the zonal-mean vorticity. This approach proved sufficient, as the resulting zonal-mean winds in the tropics matched the observed values. With this method, the meridional component of the wind and waves are allowed evolve freely.
b) Target state
i) Snapshots: As an initial approach, composites of EQBO and WQBO phases were created using reanalysis events. These “snapshots” were targeted throughout the entire year for each member branched off CTL in every September. However, the amplitude of these target states was insufficient to generate an extratropical signal, so the profiles were amplified (doubled). This amplification was sufficient to produce an extratropical response.
ii) Evolving anomalies: Anomalies from the snapshots were calculated and superimposed onto the model’s climatology, resulting in a tropical state that is closer to the model’s climatology and more realistic, as it incorporates some seasonality. For this case, the anomalies used to construct the EQBO phase were amplified, but the WQBO anomalies were sufficient to generate an extratropical signal.
iii) Initialization by nudging: Rather that applying permanent nudging, only the month of October was nudged to the target state, allowing the model to run freely from November onward. This approach has the advantage that of avoiding spurious effects at the boundaries of the nudging, although the QBO takes several weeks to decay.
iv) Nudging in December: To evaluate the impact of starting the nudging when the vortex is already mature, nudging was applied in December. Two amplitudes were tested: single and double.
While the first part of the project is more process-understanding oriented, the second part is devoted to the skill assessment of the added value of the stratosphere in the NAE predictability. For this aim, one set of free evolving hindcasts in addition to several sets of idealized hindcasts experiments in which the stratosphere has been nudged to different target states have been conducted with CanESM5.1 and will be also performed using the CMCC model. This includes experiments with no stratospheric variability (nudging towards the model's climatology) experiments with "perfect" QBO (nudging towards ERA5 only in the tropics), and experiments with "perfect" stratosphere conditions (nudging towards ERA5 in the entire stratosphere). These hindcasts span the period from 1980 to 2020, with each year consisting in 10 members, all initialized in November using different initial conditions. These initial conditions correspond to those used in the seasonal prediction systems, and are also applied in the hindcast nudged experiments. This yields a total of 400 simulated years per hindcast experiment.
