Cold air outbreaks (CAOs) enormously influence agriculture, environment, industry, and other socio-economic activities in East Asia. This study objectively identifies and categorizes CAOs in East Asia by employing 3-dimensional CAO detection algorithm and k-means clustering method. The analysis identifies a total of 106 CAOs which are classified into three distinct types based on core cold anomaly positions: South, Mixed, and North types. The South type, with the fewest occurrences and shortest lifetime, exhibits a weak warm lobe over Europe and northern Asian coast, with the coldest anomalies over southern China. The Mixed type, most frequent and longest-lived, accompanies the most extensive and coldest anomalies across Eurasia, uniformly distributed over East Asia. The North type shows the coldest anomalies in Northeast Asia with weaker effects in southern East Asia. The weakening trend in CAO intensity observed in East Asia is primarily due to long-term changes in the Mixed and North types. Temperature anomaly patterns and circulation variations are diverse, with the South type associated with a pre-existing strong stratospheric polar vortex state, and the Mixed and North types with a pre-existing week vortex state. Under those stratospheric backgrounds, even similar wave activities can result in dissimilar upper-tropospheric circulation anomalies over East Asia, leading to different extents and magnitudes of anomalous coldness. This study provides valuable insights into the diversity of East Asian CAOs, essential for forecasting and managing associated risks in East Asia.

A document by Tiffany A Shaw. Click on the document to view its contents.

Roughly one-third of sudden stratospheric warming (SSW) events lack a strong canonical surface impact, and this can lead to a forecast bust if a strong impact had been predicted. Hence, it is important to predict before the SSW onset if an event will propagate downward. The predictability of the downward impact of SSWs is considered in 7 subseasonal-to-seasonal forecast models for 16 major SSWs between 1998 and 2022, a larger sample size than considered by previous works. The models successfully predict which SSWs have a stronger downward impact to 100hPa, however they struggle to predict which have a stronger tropospheric impact. The downward impact is stronger if the deceleration of the 10hPa winds is better predicted. Downward impact is stronger for split and for absorbing SSWs, and is better predicted in high-top models. In contrast, there is little relationship between SSWs with above-average predictability and the subsequent downward impact.

Stratospheric Sudden Warmings (SSWs) predominantly occur in the Northern Hemisphere with only 1 major event recorded in the Southern Hemisphere in the satellite era. Investigating factors that contribute to this asymmetry can help to reveal the cause of SSWs and lead to improved forecasts. Here we use climate model simulations to investigate the impact of boundary conditions (topography and ocean circulation) on the asymmetry. Flattening topography eliminates Northern Hemisphere SSWs, while removing the ocean meridional overturning circulation reduces their frequency by half. The SSW response to boundary conditions is controlled by decrease in hemispheric asymmetry of eddy heat flux. The reduction is driven by a decrease in amplitude of both eddy meridional wind and eddy temperature, as well as an increase in the difference between their phases. The results suggest boundary conditions play an important role in shaping SSWs, especially topographic forcing, but that the boundary condition interactions are nonlinear.

Heavy precipitation events (HPEs) can lead to deadly and costly natural disasters and are critical to the hydrological budget in regions where rainfall variability is high and water resources depend on individual storms. Thus, reliable projections of such events in the future are needed. To provide high-resolution projections under the RCP8.5 scenario for HPEs at the end of the 21st century and to understand the changes in sub-hourly to daily rainfall patterns, weather research and forecasting (WRF) model simulations of 41 historic HPEs in the eastern Mediterranean are compared with “pseudo global warming” simulations of the same events. This paper presents the changes in rainfall patterns in future storms, decomposed into storms’ mean conditional rain rate, duration, and area. A major decrease in rainfall accumulation (-30% averaged across events) is found throughout future HPEs. This decrease results from a substantial reduction of the rain area of storms (-40%) and occurs despite an increase in the mean conditional rain intensity (+15%). The duration of the HPEs decreases (-9%) in future simulations. Regionally maximal 10-min rain rates increase (+22%), whereas over most of the region, long-duration rain rates decrease. The consistency of results across events, driven by varying synoptic conditions, suggests that these changes have low sensitivity to the specific large-scale flow during the events. Future HPEs in the eastern Mediterranean will therefore likely be drier and more spatiotemporally concentrated, with substantial implications on hydrological outcomes of storms.

Projections of extreme precipitation based on modern climate models suffer from large uncertainties. Specifically, unresolved physics and natural variability limit the ability of climate models to provide actionable information on impacts and risks at the regional, watershed and city scales relevant for practical applications. Here we show that the interaction of precipitating systems with local features can constrain the statistical description of extreme precipitation. These observational constraints can be used to project local extremes of low yearly exceedance probability (e.g., 100-year events) using synoptic-scale information from climate models, which is generally represented more accurately than the local-scales, and without requiring climate models to explicitly resolve extremes. The novel approach offers a path for improving the predictability of local statistics of extremes in a changing climate, independent of pending improvements in climate models at regional and local scales.

End of century projections from Coupled Model Intercomparison Project (CMIP) models show a decrease in precipitation over subtropical oceans that often extends into surrounding land areas, but with substantial intermodel spread. Changes in precipitation are controlled by both thermodynamical and dynamical processes, though the importance of these processes for regional scales and for intermodel spread is not well understood. The contribution of dynamic and thermodynamic processes to the model spread in regional precipitation minus evaporation (P-E) is computed for 48 CMIP models. The intermodel spread is dominated essentially everywhere by the change of the dynamic term, including in most regions where thermodynamic changes dominate the multi-model mean response. The dominant role of dynamic changes is insensitive to zonal averaging which removes any influence of stationary wave changes, and is also evident in subtropical oceanic regions. Relatedly, intermodel spread in P-E is generally unrelated to climate sensitivity.

An intermediate complexity moist General Circulation Model is used to investigate the sensitivity of the Quasi-Biennial Oscillation (QBO) to resolution, diffusion, tropical tropospheric waves, and parameterized gravity waves. Finer horizontal resolution is shown to lead to a shorter period, while finer vertical resolution is shown to lead to a slower period and to an accelerated amplitude in the lowermost stratosphere. More scale-selective diffusion leads to a faster and stronger QBO, while enhancing the sources of tropospheric stationary wave activity leads to a weaker QBO. In terms of parameterized gravity waves, broadening the spectral width of the source function leads to a longer period and a stronger amplitude although the amplitude effect saturates when the half-width exceeds $\sim25$m/s. A stronger gravity wave source stress leads to a faster and stronger QBO, and a higher gravity wave launch level leads to a stronger QBO. All of these sensitivities are shown to result from their impact on the resultant wave-driven momentum torque in the tropical stratosphere. Atmospheric models have struggled to accurately represent the QBO, particularly at moderate resolutions ideal for long climate integrations. In particular, capturing the amplitude and penetration of QBO anomalies into the lower stratosphere (which has been shown to be critical for the tropospheric impacts) has proven a challenge. The results provide a recipe to generate and/or improve the simulation of the QBO in an atmospheric model.

The predictability of stratospheric sudden warming (SSW) events are considered in 10 subseasonal to seasonal (S2S) forecast models for 10 SSWs over the period 1999-2009. The 10 SSWs are divided into those with above-average predictability (in one case exceeding 20 days), below-average predictability, and average predictability. The four factors that most succinctly distinguish the composite with above average predictability are an active Madden-Julian Oscillation with enhanced convection in the West Pacific, the Quasi-Biennial Oscillation phase with easterlies in the lower stratosphere, a strong SSW, and a strong pulse of wave activity in the week before the event. Other factors, such as El Nino, stratospheric preconditioning, and the morphology (split vs. displacement) are comparatively less important.

Sudden stratospheric warmings (SSWs) are impressive fluid dynamical events in which large and rapid temperature increases in the winter polar stratosphere (~0–50km) are associated with a complete reversal of the climatological wintertime westerly winds. SSWs are fundamentally caused by the breaking of planetary-scale waves that propagate upwards from the troposphere. During an SSW, the polar vortex breaks down, accompanied by rapid warming of the polar air column. This rapid warming and descent of the polar air column affects tropospheric weather, shifting jet streams, storm tracks, and the Northern Annular Mode (NAM), including increased frequency of cold air outbreaks over North America and Eurasia. SSWs affect the whole atmosphere above the stratosphere producing widespread effects on atmospheric chemistry, temperatures, winds, neutral (non-ionized) particle and electron densities, and electric fields. These effects span the surface to the thermosphere and across both hemispheres. Given their crucial role in the whole atmosphere, SSWs are also seen as a key process to analyze in climate change studies and subseasonal to seasonal predictions. This work reviews the current knowledge on the most important aspects related to SSWs from the historical background to involved dynamical processes, modelling, chemistry and impact on other atmospheric layers.

A sudden stratospheric warming (SSW) happened in September 2019 in the Southern Hemisphere (SH) with winds at 10hPa, 60°S reaching their minimum value on September 18. The evolution, favorable conditions, and predictability for this SSW event are explored. The favorable conditions include easterly equatorial quasi biennial oscillation (QBO) winds at 10hPa, solar minimum, positive Indian Ocean Dipole (IOD) sea surface temperatures (SST), warm SST anomalies in the central Pacific, and a blocking high near the Antarctic Peninsula. The predictive limit to this SSW is around 18 days in some S2S models, and more than 50% of the ensemble members forecast the zonal wind deceleration in reforecasts initialized around 29 August. A vortex slowdown in evident in some initializations from around 22 August, while initializations later than 29 August capture the wave-like pattern in the troposphere. The ensemble spread in the magnitude of the vortex deceleration during the SSW is mainly explained by the ensemble spread in the magnitude of upward propagation of waves, with an underestimated tropospheric wave amplitude leading to a too-weak weakening of the vortex. The September 2019 SH SSW did not show a near-instantaneous downward impact on the tropospheric southern annular mode (SAM) in late September and early October 2019. The Australian drought and hot weather in September possibly associated with the positive IOD might have been exacerbated by the negative SAM in October and later months due to the weak stratospheric polar vortex. However, models tend to forecast a near-instantaneous tropospheric response to the SSW.

An intermediate complexity moist General Circulation Model is used to investigate the factor(s) controlling the magnitude of the surface impact from Southern Hemisphere springtime ozone depletion. In contrast to previous idealized studies, a model with full radiation is used, which allows focus on the full range of feedbacks between incoming ultraviolet radiation and temperature variations. In addition, the model can be run with a varied representation of the surface, from a zonally uniform aquaplanet to a highly realistic configuration. The model captures the positive Southern Annular Mode response to ozone depletion evident in observations and comprehensive models in December through February. It is shown that while synoptic waves dominate the long-term poleward jet shift, the initial response includes changes in planetary waves which simultaneously moderate the polar cap cooling (i.e., a negative feedback), but also constitute nearly half of the initial momentum flux response that shifts the jet polewards. Enhanced ultraviolet absorption at the surface due to the ozone hole drives an additional negative feedback on the poleward jet shift. The net effect is that stationary waves and surface radiative effects weaken the circulation response to ozone depletion, and also delay the response until summer rather than spring when ozone depletion peaks.