Impact Of Sudden Stratospheric Warming Research Articles

What Drives the Spread and Bias in the Surface Impact of Sudden Stratospheric Warmings in CMIP6 Models?

Abstract This study evaluates the representation of the composite-mean surface response to Sudden Stratospheric Warmings (SSWs) in 28 CMIP6 models. Most models can reproduce the magnitude of the SLP response over the Arctic, although the simulated Arctic SLP response varies from model to model. Regarding the structure of the SLP response, most models exhibit a basin-symmetric negative NAM-like response with a cyclonic Pacific SLP response, whereas the reanalysis shows a highly basin-asymmetric negative NAO-like response without a robust Pacific center. We then explore the drivers of these model biases and spread by applying a multiple linear regression. The results show that the polar-cap temperature anomalies at 100 hPa (ΔT100) modulate both the magnitude of the Arctic SLP response and the cyclonic Pacific SLP response. Apart from ΔT100, the intensity and latitudinal location of the climatological eddy-driven jet in the troposphere also affect the magnitude of the Arctic SLP response. The compensation of model biases in these two tropospheric metrics and the good model representation of ΔT100 explains the good agreement between the ensemble mean and the reanalysis on the magnitude of the Arctic SLP response, as indicated by the fact that the ensemble mean lies well within the reanalysis uncertainty range and that the reanalysis mean sits well within the model distribution. The Niño-3.4 SST anomalies and North Pacific SST dipole anomalies together with ΔT100 modulate the cyclonic Pacific SLP response. In this case, biases in both oceanic drivers work in the same direction and lead to the cyclonic Pacific SLP response in models that is not present in the reanalysis.

Impact of sudden stratospheric warming on middle atmospheric circulation in the southern hemisphere: A comparative study

A comparative study on the impact of rare Southern Hemisphere (SH) Sudden Stratospheric Warmings (SSW) on the middle atmospheric circulation is investigated using a global reanalysis dataset. Since the SH SSW generally occur around spring equinox marking the seasonal transition, so an attempt has been made to isolate the seasonal transition effect from the actual data by carrying out deseasoning to delineate the effects mainly due to the warming. The deseasoned winds are able to extract relatively weak dynamical signatures of SSW at lower altitudes in terms of prominent zonal mean easterly forcing around the peak warming day (PWD) which are not evident from the actual winds. The influence of 2002 SSW seems to reach the troposphere in terms of easterly deseasoned forcing, which is not the case in 2019. Also the deseasoned winds in the stratosphere reveals earlier occurrence of easterly forcing in the extratropical latitudes, which progresses poleward around the PWD indicating a possible tropical precursor to the SH SSW. In general, the actual upper mesospheric wind is dominated by seasonal transition but the deseasoned winds reveal easterly and southerly forcing due to the SSW. Interestingly, the horizontal flow is found to be very different or even opposite in direction at different longitudes indicating uneven longitudinal response of the atmosphere to the warming events across the globe. Overall, the present study provides a detailed and comparative overview of the middle atmospheric circulation in terms of zonal mean flow and concomitant zonal variability during two rare major and minor SSW events in the SH.

Impact of sudden stratospheric warmings on the neutral density, temperature and wind in the MLT region

In this study, the neutral density and horizontal wind observed by the four meteor radars, as well as the temperature measured by the Microwave Limb Sounder (MLS) onboard the Aura satellite are used to examine the response of neutral density, wind, and temperature in the MLT region to the stratospheric sudden warmings (SSWs) during 2005 to 2021 in the Northern Hemisphere. The four meteor radars include the Svalbard (78.3°N, 16°E) and Tromsø (69.6°N, 19.2°E) meteor radars at high latitudes and the Mohe (53.5°N, 122.3°E) and Beijing (40.3°N, 116.2°E) meteor radars at middle latitudes. The superposed epoch analysis results indicate that: 1) the neutral density over Svalbard and Tromsø at high latitude increased at the beginning of SSWs and decreased after the zonal mean stratospheric temperature reached the maximum. However, the neutral density over Mohe at midlatitudes decreased in neutral density at the beginning of SSW and increase after the zonal mean stratospheric temperature reached the maximum. 2) The zonal wind at high latitudes show a westward enhancement at the beginning of SSWs and then shows an eastward enhancement after the stratospheric temperature reaches maximum. However, the zonal wind at midlatitudes shows an opposite variation to at high latitudes, with an eastward enhancement at the onset and changing to westward enhancements after the stratospheric temperature maximum. The meridional winds at high and midlatitudes show a southward enhancement after the onset of SSW and then show a northward enhancement after the stratospheric temperature maximum. 3) In general, the temperature in the MLT region decreased throughout SSWs. However, as the latitudes decrease, the temperature cooling appears to lag a few days to the higher latitudes, and the degree of cooling will decrease relatively.

On the Different Quasi-2-Day Wave Behaviors during Sudden Stratospheric Warming Periods

The temporal variations in the sudden stratospheric warming (SSW) events in the winter stratosphere always coincide with the quasi-2-day wave (Q2DW) in the summer mesosphere, and the impact of SSW on Q2DW is interesting but still a mystery. Major SSWs occurred in both 2006 and 2009, while the Q2DW activity was quite different. The Second Modern Era-Retrospective Analysis for Research and Applications (MERRA-2) reanalysis dataset was used to comparatively analyze these two major SSW events and elucidate the reasons for the different Q2DW behaviors. We noticed that the summer easterly jet shows a large interannual variability. We conclude that the summer mesospheric Q2DWs are modulated by the winter SSW, whereas the modulation process is also affected by the interannual variability of the summer easterly flow itself. The effects of the SSW on the Q2DWs may differ from year to year due to the variability of the summer easterly flow itself, resulting in different anomalous Q2DW behavior. This conclusion may also be true for the interannual variability of other phenomena during the SSW period.

Using large ensembles to quantify the impact of sudden stratospheric warmings and their precursors on the North Atlantic Oscillation

Abstract. Sudden-stratospheric-warming (SSW) events are often followed by significant weather and climate impacts at the surface. By affecting the North Atlantic Oscillation (NAO), SSWs can lead to periods of extreme cold in parts of Europe and North America. Previous studies have used observations and free-running climate models to try to identify features of the atmosphere prior to an SSW that can determine the subsequent impact at the surface. However, the limited observational record makes it difficult to accurately quantify these relationships. Here, we instead use a large ensemble of seasonal hindcasts. We first test whether the hindcasts reproduce the observed characteristics of SSWs and their surface signature. We find that the simulations are statistically indistinguishable from the observations, in terms of the overall risk of an SSW per winter (56 %), the frequency of SSWs with negative NAO responses (65 %), the magnitude of the NAO responses, and the frequency of wavenumber-2-dominated SSWs (26 %). We also assess the relationships between prior conditions and the NAO response in the 30 d following an SSW. We find that there is little information in the precursor state to guide differences in the subsequent NAO behaviour between one SSW and another, reflecting the substantial natural variability between SSW events. The strongest relationships with the NAO response are from pre-SSW sea level pressure anomalies over the polar cap and from zonal-wind anomalies in the lower stratosphere, both exhibiting correlations of around 0.3. The pre-SSW NAO has little bearing on its post-SSW state. The strength of the pre-SSW zonal-wind anomalies at 10 hPa is also not significantly correlated with the NAO response. Finally, we find that the mean NAO response in the first 10 d following wave-2-dominated SSWs is much more strongly negative than in wave-1 cases. However, the subsequent response in days 11–30 is very similar regardless of the dominant wavenumber. In all cases, the composite mean responses are the result of very broad distributions from individual SSW events, necessitating a probabilistic analysis using large ensembles.

Possible influence of sudden stratospheric warmings on the atmospheric environment in the Beijing–Tianjin–Hebei region

Abstract. Using European Centre for Medium-Range Weather Forecasts fifth-generation (ERA5) and second Modern-Era Retrospective analysis for Research and Applications (MERRA-2) reanalysis and surface meteorological observation data, this study explores the possible impact of sudden stratospheric warming (SSW) events on air quality in the Beijing–Tianjin–Hebei (BTH) region. Major SSW events are divided into polar vortex displacement SSW and polar vortex split SSW. As the duration of split SSW events is longer and the stratospheric signal pulses propagate further downward than displacement SSWs, subseasonal variability of the atmospheric particulates in the BTH is larger during split SSWs. The air particulate concentration is light before the SSW onset due to the enhanced perturbation in the troposphere associated with strengthened planetary waves. The air particulate concentration around the SSW onset dates begins to rise due to weakening of the tropospheric disturbance as the enhanced planetary waves enter the stratosphere. In the decaying period of the SSW, the air particulate concentration decreases as the stratospheric negative northern annular mode (NAM) signal propagates downward. Specifically, in the pre-SSW period of displacement (split) SSW events, a wavenumber-1-like (wavenumber-2-like) anomaly pattern is strengthened. The East Asian winter monsoon intensifies as the East Asian trough is deepened, especially before the split SSW event onset, leading to a cleaning period. Around the SSW onset period as the tropospheric perturbation diminishes and the East Asian winter monsoon weakens, a surge of air particulate concentration is observed. After the SSW onset, due to the downward propagation of the stratospheric negative NAM signal, cold anomalies form in northeastern East Asia, especially for split SSWs, corresponding to a cleaning period in the BHT region. The local meteorological conditions during the SSWs are also discussed.

Possible impact of sudden stratospheric warming on the intraseasonal reversal of the temperature over East Asia in winter 2020/21

The intraseasonal reversal of the temperature over East Asia and the sudden stratospheric warming (SSW) are two abrupt events during the winter of 2020/21. The characteristics of these two events and their relationship are the focused issues in this paper. In winter 2020/21, there was a transition from cold to warm over East Asia. The surface air temperature (SAT) in East Asia suddenly changed from cold to warm with long duration. The cold period lasted from 21 November 2020 to 11 January 2021, while the warm period lasted from 19 January to 28 February 2021. And the transition of the cold to the warm was from 12 to 18 January 2021.There were significant differences of the atmospheric circulations between the cold and warm periods in the upper and lower troposphere. During the cold period, both of Ural Mountain High(UH) and Siberian High(SH) were stronger than normal, the Westerly Jet (WJ) was weakened and located to the area farther southerly than usual, the northerly wind anomalies and the cold advection occurred in East Asian; vice versa during the warm period. The mutations of the atmospheric circulations in the upper and lower levels were obvious during the transition period, especially for UH and SH. A strong SSW event occurred in the stratosphere at the beginning of 2021. As a result, the stratospheric polar vortex was largely weakened and deviated to the areas over Ural Mountains and nearby, the polar westerly wind turned to easterly wind and the temperature gradient reversed. The abnormal changes of the temperature and the geopotential height over the Ural Mountains area in the stratosphere were closely related to the SSW. These abnormal signals caused by the SSW gradually transmitted from stratosphere to troposphere over the Ural Mountains and the polar region, leading to the weakening and collapse of tropospheric UH and further caused the transition from cold to warm in East Asia. The SSW may be the most critical factor leading to the turning from cold to warm over East Asia during the winter of 2020/21.

How Well Do We Know the Surface Impact of Sudden Stratospheric Warmings?

AbstractSudden stratospheric warmings (SSWs) are key to understanding and predicting subseasonal Northern Hemisphere winter climate variability. Here we study the uncertainty in the surface response to SSWs in reanalysis data by constructing synthetic composites based on bootstrapping the 39 events observed during the 1958–2019 period. We find that the well‐known responses in the North Atlantic and European regions following SSWs are consistently present, but their magnitude and spatial pattern vary considerably across the synthetic composites. We further find that this uncertainty is unrelated to stratospheric polar vortex strength and is instead the result of independent tropospheric variability. Our findings provide a basis for evaluating the fidelity of the surface response to SSWs in models.

Do Sudden Stratospheric Warmings Boost Convective Activity in the Tropics?

AbstractThe impact of sudden stratospheric warmings (SSWs) on the tropical troposphere is investigated using 5000‐years scale ensemble simulations with a 60‐km global atmospheric model to detect signals from large natural variability of convection. Stratospheric temperature and static stability decrease in the tropics, and convective activity peaks simultaneously with the tropical upwelling. Tropical cyclone intensity and accumulated cyclone energy (ACE) show a weak but statistically robust enhancement of 5% and 9%, respectively, relative to the December–March climatology. However, the relative probability of the largest 10% of ACE anomalies increases by about 30%. Extratropical waves toward the tropical troposphere, which can alter convective activity, do not show a strong systematic variation during SSWs, and no significant association with convective activity is found if the sample size is large enough.

Interaction between decadal-to-multidecadal oceanic variability and sudden stratospheric warmings.

Major sudden stratospheric warmings (SSWs) are the most important phenomena of the wintertime boreal stratospheric variability. During SSWs, the polar temperature increases abruptly, and easterlies prevail in the stratosphere. Their effects extend farther from the polar stratosphere, affecting near-surface circulation. According to observations, SSWs are not equally distributed in time, with decades experiencing very few events, while others experiencing SSWs almost every winter. Some sources of this SSW multidecadal variability can be traced back to sea surface temperature changes. Here, we investigate the effects of Pacific decadal variability (PDV) and Atlantic multidecadal variability (AMV) on SSWs. We use for the first time a large ensemble of historical experiments to examine the modulation of the frequency, tropospheric precursors, and impact of SSWs by the PDV and AMV. We find a strong impact of the PDV on the occurrence of SSWs, with a higher SSW frequency for the positive phase of the PDV. This PDV influence is mediated by constructive interference of PDV anomalies with tropospheric stationary waves. The main effect of AMV is, instead, a modulation of the tropospheric response to SSWs, a finding that can be useful for predicting the tropospheric fingerprint of SSWs.

Tracking the Stratosphere‐to‐Surface Impact of Sudden Stratospheric Warmings

AbstractSudden stratospheric warming (SSW) events are extreme atmospheric regimes which can have a signature in surface weather up to 40 days after event onset in the stratosphere. SSWs can be classified as either vortex splitting or vortex displacement events, with the nature and timing of the surface impact potentially being different between the two. In this study, using ERA40/Interim reanalysis data, we develop a simple empirical downward tracking algorithm which for the first time allows us to estimate the time of surface impact for individual SSW events. We show that the surface impact following splitting events is, on average, about 1 week earlier than following displacement events, albeit with considerable variability. By compositing tropospheric responses around the identified date of surface impact, rather than around the central stratospheric onset date as common in previous studies, we can better constrain the surface signal of SSWs. We find that while the difference in North Atlantic Oscillation anomalies between split and displacement vortices is small, surface temperature anomalies over northwest Europe and northern Eurasia are significantly colder for splitting events, particularly over the UK just prior to the surface impact date. Displacement events on average are wetter over Northwest Europe around the time of surface impact, consistent with the jet stream being displaced further south in response to split events. Our downtracking algorithm can be used with any reanalyzes and gridded model data, and therefore will be a valuable tool for use with the latest climate models.

Impact of sudden stratospheric warmings on United Kingdom mortality

AbstractSudden stratospheric warmings (SSWs) during boreal winter are one of the main drivers of sub‐seasonal climate variability in the Northern Hemisphere. Although the impact of SSW events on surface climate and climate extremes has been clearly demonstrated, the impact of the resulting climate anomalies on society has not been so widely considered. In the United Kingdom (UK), SSWs are associated with cold weather, which is linked to significant increases in mortality. This study demonstrates, for the first time, that SSWs are linked to increases in mortality in the UK. A distributed lag nonlinear model and standard parameter settings from the literature is used to construct a daily time series of UK deaths attributable to cold weather between 1991 and 2018. Weekly mortality associated with SSWs is diagnosed using a superposed epoch analysis of attributed mortality for the 15 SSW events in this period. SSW associated mortality peaks between 3 and 5 weeks after SSW central date and leads to, on average, 620 additional deaths in the same period. Given that the impacts of SSWs can be skilfully predicted on sub‐seasonal timescales, this suggests that health and social care systems could derive substantial benefit from sub‐seasonal forecasts during SSWs.

The Importance of a Properly Represented Stratosphere for Northern Hemisphere Surface Variability in the Atmosphere and the Ocean

Major sudden stratospheric warmings (SSWs) are extreme events during boreal winter, which not only impact tropospheric weather up to three months but also can influence oceanic variability through wind stress and heat flux anomalies. In the North Atlantic region, SSWs have the potential to modulate deep convection in the Labrador Sea and thereby the strength of the Atlantic meridional overturning circulation. The impact of SSWs on the Northern Hemisphere surface climate is investigated in two coupled climate models: a stratosphere-resolving (high top) and a non-stratosphere-resolving (low top) model. In both configurations, a robust link between SSWs and a negative NAO is detected, which leads to shallower-than-normal North Atlantic mixed layer depth. The frequency of SSWs and the persistence of this link is better captured in the high-top model. Significant differences occur over the Pacific region, where an unrealistically persistent Aleutian low is observed in the low-top configuration. An overrepresentation of SSWs during El Niño conditions in the low-top model is the main cause for this artifact. Our results underline the importance of a proper representation of the stratosphere in a coupled climate model for a consistent surface response in both the atmosphere and the ocean, which, among others, may have implications for oceanic deep convection in the subpolar North Atlantic.

Impact of Sudden Stratospheric Warming of 2009 on the Equatorial and Low‐Latitude Ionosphere of the Indian Longitudes: A Case Study

AbstractUsing the equatorial electrojet (EEJ)‐induced surface magnetic field and total electron content (TEC) measurements, we investigated the impact of the sudden stratospheric warming (SSW) of January 2009 on the equatorial electrodynamics and low‐latitude ionosphere over the Indian longitudes. Results indicate that the intensity of EEJ and the TEC over low latitudes (extending up to 30°N) exhibit significant perturbations during and after the SSW peak. One of the interesting features is the deviation of EEJ and TEC from the normal quiet time behavior well before the onset of the SSW. This is found to coincide with the beginning of enhanced planetary wave (PW) activity over high latitudes. The substantial amplification of the semidiurnal perturbation after the SSW peak is seen to be coinciding with the onset of new and full moons. The response of TEC to SSW is found to be latitude dependent as the near‐equatorial (NE) stations show the semidiurnal perturbation only after the SSW peak. Another notable feature is the presence of reduced ionization in the night sector over the NE and low‐latitude regions, appearing as an “ionization hole,” well after the SSW peak. The investigation revealed the existence of a quasi 16 day wave in the TEC over low latitudes similar to the one present in the EEJ strength. These results have been discussed in the light of changes in the dynamical background because of enhanced PW activity during SSW, which creates favorable conditions for the amplification of lunar tides, and their subsequent interaction with the lower thermospheric tidal fields.