Large-scale mixing in the atmosphere redistributes moisture as organised bands or filaments. In some tropical and subtropical monsoon regions, a substantial part of rainfall happens under moisture and cloud bands commonly referred to as convergence zones. Recent regional studies have shown that such large-scale filaments or bands of moisture and rainfall form along or in the neighborhood of mixing features known as attracting ``Lagrangian Coherent Structure'' (LCSs) - material skeletons associated with strong attraction of air parcels. However, there are still no global climatologies to support more general conclusions about the spatiotemporal relationships between mixing and precipitation and the impact of large-scale mixing on monsoons. In this study, we investigate how mixing features determine the subseasonal and seasonal rainfall variability in tropical and subtropical regions around the globe. We characterise mixing by computing the Finite-time Lyapunov Exponent (FTLE), a measure of Lagrangian deformation among neighbouring parcels, on ERA5 reanalysis data between 1980 and 2009. Attracting LCSs are identified as ridges of the FLTE. We also employ diagnostic Eulerian variables such as mean sea level pressure and mass meridional streamfunction to associate mixing with general circulation features. On the seasonal scale, we show that the strength of mixing and the frequency of LCSs modulates rainfall under the African, American and Asian convergence zones and the ITCZ. On the subseasonal scale, we focus on the influence of the Madden-Julian oscillation and the North Atlantic oscillation on the mixing regime of the Atlantic and East Pacific; we show how these oscillations control horizontal mixing as to suppress or enhance precipitation variability over the American monsoons. This first long-term global climatology of mixing and LCSs quantifies the often overlooked role of Lagrangian kinematics on the hydrological cycle and provides a powerful process-based diagnostic to investigate mechanisms of rainfall variability that does not require region-specific considerations.

How to cite: Martins Palma Perez, G., Vidale, P. L., Dacre, H., and Garcia-Franco, J.: A global climatology of the kinematical skeletons organising subtropical and tropical convergence zones, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12085, https://doi.org/10.5194/egusphere-egu22-12085, 2022.

How to cite: Rudeva, I., Holgate, C., Pepler, A., McKay, R., and Hope, P.: Extreme subtropical precipitation in Australia: reasons for decline  , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10816, https://doi.org/10.5194/egusphere-egu22-10816, 2022.

Atmospheric rivers (ARs) are generally considered to be transient and concurrent with an extratropical cyclone (Ralph et al. 2018). However, this is not necessarily the case for the ARs in the East Asian summer monsoon (EASM). Despite several climatological surveys on the EASM ARs in recent years (e.g., Park et al. 2021a), through what processes they develop is still unclear because of the complex interplay between monsoonal and extratropical circulations in the region (Horinouchi 2014; Park et al. 2021b).

In this talk, we demonstrate that the EASM ARs have different “flavors” in terms of moisture transport characteristics. By quantifying the relative contribution of high- and low-frequency components of the integrated water vapor transport anomaly (IVTA) for each AR, it is found that both components are important in East Asia summer, in contrast to the ARs in the U.S. west coast where the high-frequency component is predominant.

To investigate the synoptic condition governing the high- and low-frequency IVTA, the EASM ARs are classified into the three categories—1) transient, 2) quasi-stationary and 3) intermediate ARs—depending on the fractional contribution of high-frequency IVTA to total IVTA. While the transient ARs are driven by an extratropical cyclone in an analogy of classical ARs, the quasi-stationary ARs are associated with an anomalously enhanced monsoon flow. The intermediate ARs, which are a majority of summertime ARs in East Asia, show the confounding features of the two types. We suggest that the concept of “transient” and “quasi-stationary” AR flavors offer an important foundation in understanding the EASM ARs with a variety of underlying dynamics. Further implications and possible future works will be also discussed.

References:

Horinouchi, T., 2014: Influence of upper tropospheric disturbances on the synoptic variability of precipitation and moisture transport over summertime East Asia and the northwestern Pacific. J. Meteor. Soc. Japan, 92, 519–541, https://doi.org/10.2151/jmsj.2014-602.

Park, C., S.-W. Son, and H. Kim, 2021a: Distinct features of atmospheric rivers in the early versus late EASM and their impacts on monsoon rainfall. J. Geophys. Res. Atmos., 126, e2020JD033537, https://doi.org/10.1029/2020JD033537.

Park, C., S.-W. Son, and J.-H. Kim, 2021b: Role of baroclinic trough in triggering vertical motion during summertime heavy rainfall events in Korea. J. Atmos. Sci., 78, 1687–1702, https://doi.org/10.1175/JAS-D-20-0216.1.

Ralph, F. M., M. D. Dettinger, M. M. Cairns, T. J. Galarneau, and J. Eylander, 2018: Defining “atmospheric river”: How the glossary of meteorology helped resolve a debate. Bull. Amer. Meteor. Soc., 99, 837–839. https://doi.org/10.1175/BAMS-D-17-0157.1.

How to cite: Park, C. and Son, S.-W.: Transient versus quasi-stationary flavors of atmospheric rivers during East Asian summer monsoon, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-242, https://doi.org/10.5194/egusphere-egu22-242, 2022.

In addition to various factors over the tropics, the interannual variability of northwest Indian summer monsoon (NWISM) rainfall is also regulated by extratropical signals. We defined a subtropical westerly jet index (SWJI) based on the meridional position and intensity of 200-hPa zonal wind within [25-55°N, 40-90°E]. It is found that SWJI exhibits a significant positive correlation with summer rainfall over the NWISM region during 1951-2015. During positive (negative) SWJI years, an upper-level anticyclonic (cyclonic) anomaly over Central Asia along with positive (negative) rainfall anomaly and low-level easterly (westerly) anomalies were observed over the NWISM region. The upper-level anticyclonic (cyclonic) anomaly was accompanied by the descending (ascending) motion and warm (cold) tropospheric temperature anomalies. The anticyclonic (cyclonic) anomaly increased (decreased) the land-ocean thermal contrast by warm (cold) air advection and modified the local meridional circulation. Interannual variability of rainfall over the NWISM region is associated with the meridional position and intensity of the jet that manifest in both upper- and low-level circulation anomalies. Further analysis showed that the interannual variability of SWJI is correlated with Arctic Oscillation (AO). During the positive phase of AO, an upper-level anticyclonic anomaly appeared over Central Asia and favored convection over the NWISM region.

How to cite: Sadaf, N., Lin, Y., and Dong, W.: Relationship between the Central Asian Subtropical Westerly and Northwest Indian Summer Monsoon rainfall, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1630, https://doi.org/10.5194/egusphere-egu22-1630, 2022.

Seasonal prediction is generally skillful over the subtropical landmasses of the Southern Hemisphere during summer seasons with strong ENSO (El Niño Southern Oscillation) forcing. Skill is substantially reduced, however, during summer seasons that are ENSO neutral. Over southern Africa, forecast skill is also comparatively less for the spring and autumn seasons, and only marginally exists for winter. This seasonal cycle in predictive skill and the strong dependence of skill on ENSO forcing raise questions about the limits of predictability in the Southern Hemisphere subtropics. Here we explore these potential limits using Atmospheric Model Intercomparison Project (AMIP) simulations. These simulations are part of the larger Coupled Model Intercomparison Project Phase Six (CMIP6), and are constructed using global atmospheric models forced at their lower boundaries with historical sea-surface temperature and sea-ice reconstructions. Radiative forcing is in the form of historical greenhouse gas and ozone concentrations, as well as aerosol emissions, for the period 1979-2014 (the same historical forcings are used in ScenarioMIP of CMIP6). AMIP simulations may be regarded as providing an upper boundary of seasonal predictive skill, at least to the extent that atmospheric inter-annual variability is a response to inter-annual variations in lower-boundary and radiative forcing. AMIP simulations are initialized only once however, and don’t make use of updated initial conditions as in the case of operational seasonal forecasts. Also, although the lower boundary forcing in AMIP simulations may be regarded as ‘perfect’, important coupled processes that influence inter-annual variability may not be represented. Our focus here is on analyzing the skill of AMIP simulations in representing inter-annual atmospheric variability over the subtropical landmasses of the Southern Hemisphere, focusing on rainfall and low-level circulation. NOAA-CIRES-DOE reanalysis v3 and Climatic Research Unit (CRU) data are used for verification. The first stage of analysis consist of constructing a multi-model ensemble of AMIP simulations, with each model contributing a single ensemble member. Such an ensemble isolates to some extent the predictability that may be derived purely from boundary forcing. In the second stage of the analysis, we evaluate skill for those AMIP models for which initial-condition based ensembles have been derived, thereby incorporating the effects of model internal-variability on predictive skill. The resulting evaluations of skill confirm the results from operational seasonal forecasting, namely that a pronounced seasonal cycle in predictive skill exists over the Southern Hemisphere continents in the subtropics, with peak skill in summer in association with ENSO forcing. However, in spring and autumn and particularly in winter, circulation patterns of lower predictability originating from the Southern Ocean impact on atmospheric variability over the subtropical landmasses. Since these circulation patterns seem to be relatively unconstrained by lower boundary and atmospheric radiative forcing, it implies that predictability in the subtropics may be constrained in winter and the transition seasons by the relatively less predictable higher-latitude circulation regimes of the Southern Hemisphere.

How to cite: Engelbrecht, F. and Ndarana, T.: Predictability of inter-annual variability in the Southern Hemisphere subtropics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12010, https://doi.org/10.5194/egusphere-egu22-12010, 2022.

The Azores High is a subtropical high-pressure ridge in the North Atlantic surrounded by anticyclonic winds that steer rain-bearing weather systems. The size and intensity of the Azores High modulate the oceanic moisture transport to Europe thereby affecting hydroclimate across western Europe, especially during wintertime. While changes in the North Atlantic storm track have been linked to the variability of the North Atlantic Oscillation (NAO), we focus on North Atlantic variability with a subtropical perspective by focusing on the Azores High independently of the Icelandic Low. The subtropical perspective provides a direct understanding of regional climate variability in the western Mediterranean and reveals dramatic changes to North Atlantic climate throughout the past century and can provide insight into the impact of future warming on the dynamics of the Azores High and associated hydroclimate. Here we show that winters with an extremely large Azores High are significantly more common in the industrial era (since 1850 CE) than in preindustrial times, resulting in anomalously dry conditions across the western Mediterranean, including the Iberian Peninsula. Climate model simulations of the past millennium indicate that the industrial-era expansion of the Azores High is unprecedented throughout the last millennium (since 850 CE), consistent with proxy evidence from Portugal. Azores High expansion emerges after the end of the Little Ice Age and strengthens into the 20^th century consistent with anthropogenically-driven warming.

How to cite: Ummenhofer, C., Cresswell-Clay, N., Thatcher, D., Wanamaker, A., Denniston, R., Asmerom, Y., and Polyak, V.: Unprecedented Expansion of the Azores High due to Anthropogenic Climate Change, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3876, https://doi.org/10.5194/egusphere-egu22-3876, 2022.

How did climatic and environmental variability and stress affect past societies in an area of increasing relevance for contemporary planning and policy concerns? The Eastern Mediterranean (EM) and the Nile river basin (Nile) bear a long history of human social dynamics, making it a suitable area for exploring potential interactions between climate variability, extreme events, environmental changes and society over a variety of time scales. The areas contain abundant natural and human-historical archives that preserve information on the climate conditions and impacts on humans and ecosystems covering the past centuries to millennia. So far, the links between climate and societies are examined mainly from the proxy records or the derived paleoclimatic reconstruction perspectives, without addressing the detail of the processes and underlying dynamics that offer the regional climate model simulations. In order to improve our understanding of past climate in the EM and Nile at the regional scale, we developed a spatially high resolved fully-forced paleoclimate version of the COSMO-CLM running over the past 2500 years. All forcings used for the driving ESM, namely volcanic (stratospheric aerosol optical depth), orbital (eccentricity, obliquity, precession), solar (irradiance), land-use and greenhouse-gas changes are implemented to COSMO 5.0-clm16 (see Hartmann et al. for more details). As a starting point for exploring the relationship between climate and society over the last 2500 years, we compared the mean climate conditions (2m temperature and precipitation) of two periods that are 2400 years apart, namely BCE 400-362 and 1980-2018 CE. Overall, the results show that summer temperatures differ by up to 3 degrees between the two periods. In particular, over the tropics, the temperature differences are largest. Precipitation changes vary within the study area and the climate regimes covered. We will further analyze the dynamics and climate variability of the area over the two periods to explore more details of regional and local climate change.

How to cite: Zhang, M., Hartmann, E., Xoplaki, E., Wagner, S., and Adakudlu, M.: The climate of the Eastern Mediterranean and the Nile river basin 2500 years before present: a fully forced paleo regional climate simulation with COSMO-CLM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12333, https://doi.org/10.5194/egusphere-egu22-12333, 2022.