AS1.8

The South Atlantic Convergence Zone (SACZ), which extends from the Amazon to the southwestern South Atlantic, is one of the major precipitation systems in South America and has an important socioeconomic impact for Brazil. This study suggests the possibility of SACZ to act as a Rossby wave source using numerical simulations from a simple baroclinic model under strong El Niño basic state. Sixteen days after the perturbation, it is possible to observe wave propagation inside the subtropical latitudes of the northern hemisphere. The simulation is performed during the strong El Niño in 2015/16 austral summer, which has a intense westerly zonal flow and stationary wavenumbers 6-10 in the equatorial Atlantic region. The Rossby wave starts in the southeast Brazil, crosses the Atlantic Ocean and, embedded in the subtropical jet of the north hemisphere, extends to the subtropical latitudes over the African and Asian continents. According to the present analyses, SACZ may sometimes act as interhemispheric Rossby wave source, enabling a connection between South America and subtropical latitudes in north hemisphere over 16 days, providing there is westerly flow that allows wave propagation over the equatorial Atlantic Ocean.

How to cite: Braga, H. and Ambrizzi, T.: South Atlantic Convergence Zone as Rossby Wave Source During Strong El Niño, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2824, https://doi.org/10.5194/egusphere-egu22-2824, 2022.

Precipitation can have large dual societal impacts in regions with a dry climate. On the one hand, extreme precipitation can induce catastrophic floods, and on the other hand, replenish scarce fresh water resources. In contrast to wet extratropical regions, the atmospheric processes that lead to precipitation formation in the dry subtropics are often overlooked by the scientific community. In this study we address the role of Rossby wave breaking for annual and extreme precipitation in (semi)arid regions. To this end, we quantify the contribution of Rossby wave breaking to extreme precipitation days and annual precipitation amounts in regions with different degrees of aridity. Rossby wave breaking is represented by potential vorticity (PV) streamers and cutoffs on isentropic surfaces using ERA-Interim reanalysis data, while precipitation is used from the global precipitation measurement (GPM) integrated multi-satellite retrieval product IMERG for the period of 2001-2018. We show that the relevance of Rossby wave breaking for precipitation increases from humid to hyper arid regions. More specifically, equatorward breaking Rossby waves contribute to a large fraction of annual and extreme precipitation in regions on the pole-westward flanks of world’s most arid regions where most precipitation occurs in the cool season. In contrast, precipitation in the equator-eastward parts of these arid regions has a negative association with Rossby wave breaking, implying that the tropical forcing governs the precipitation formation which occurs in these regions predominantly in the warm season. The results suggest that breaking Rossby waves are of key importance for precipitation in (semi)arid regions that undergo a drying in a warming climate, underlining the need to better understand the response of wave breaking to global warming.

How to cite: De Vries, A. J., Armon, M., Klingmüller, K., and Portmann, R.: The role of Rossby wave breaking for annual and extreme precipitation in (semi)arid regions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8777, https://doi.org/10.5194/egusphere-egu22-8777, 2022.

The Atacama and Namib Deserts are one of the driest places in the world. They are both located west coast of their respective continents (18-28ºS), under the effects of the east margin of the subtropical anticyclones, strong subsidence and cold ocean currents. However, they also differ in terms of topography, precipitation, and humidity, being the Atacama much higher and drier than the Namib. Our understanding of how water vapor is brought to these regions and interacts with the different local circulations and topography is still limited. The objective of this study is to investigate similarities and differences of the spatio-temporal variability of water vapor between both deserts in order to assess the impact of the distinctive local factors. To this end, we use state-of-the-art satellite observations and reanalysis for a long-term perspective on total column water vapor (TCWV) as well as on the vertical distribution of humidity, temperature and on cloud structure. The analysis is aided by a one-year measurement campaign at Iquique airport (22ºS).

We found a marked seasonal cycle in the total column water vapor (TCWV) in both offshore deserts areas. While both deserts share a similar timing of the annual TCWV peak between January and March, the values of the maxima differ. The Namib surpasses the Atacama by 30%. Our analysis suggests that at least two factors contribute to the common summer maxima of the TCWV. First, warmer sea surface temperatures (SSTs) along the west coasts produce a moistening of the marine boundary layer (MBL). Second, as a consequence of the southward displacement of the subtropical anticyclones, weaker southerly winds decrease the dry advection in the MBL. The excess of humidity in the Namib is associated with a strong moisture advection feature observed in the lower part of the free-troposphere (900-750 hPa). The easterlies also transport clouds and precipitation. In the Atacama, the presence of the Andes cordillera blocks most of the potential exchange of humidity with the continent, resulting in the Pacific Ocean being the main source of moisture.

While the respective driest period presents similar TCWV amounts (~12 Kg/m2) for both deserts, it is surprising to find that it occurs later in the Atacama (spring season) than in the Namib (winter). Potential causes for this shift, such as a stronger dependence of TCWV on the SST for the Atacama, are investigated and discussed.

Furthermore, we identified a recurring atmospheric feature for the summer which exhibits a strong northerly humidity advection above the MBL. This structure is only observed in the Atacama Desert and has not been described in the literature. However, it could be a major source of humidity for the inland region in Atacama.

How to cite: Vicencio, J., Böhm, C., Löhnert, U., and Crewell, S.: Understanding atmospheric differences in the water vapor transport for the Atacama and Namib deserts, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4681, https://doi.org/10.5194/egusphere-egu22-4681, 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.

It is known that the Madden-Julian Oscillation (MJO) excites a response in the behaviour of many extratropical weather regimes at lag times of one to two weeks, acting as a key predictor in weather forecasting. Less well understood, however, is the robustness of these responses over long time scales. We begin by taking a statistical approach to assess the boreal winter response of a selection of key extratropical systems (e.g. North Atlantic Oscillation (NAO), Pacific North American (PNA) pattern) to the MJO, over two non-overlapping time periods (1974-1997 and 1997-2019). It is shown that there is significant change in both the magnitude and structure of the extratropical response signal, as a function of lag, between the two periods.

This is followed by a similar analysis applied to the 1100 year pre-industrial control run of the UKESM-1-0 coupled climate model. By breaking this period into separate 20 year segments and comparing the extratropical responses to the MJO in each segment, we show that although there is a predictable mean signal, it is overwhelmed by the internal variability in the system. Repeating this methodology with segments between 10 and 40 years in length allows us to assess sampling errors and identify the key timescales for the variability. A similar mean signal is seen with every segment length, justifying the current use of the MJO as a predictor in the extratropics, although the variability in segments of 30 and 40 years (common time periods used in many historical analyses) casts doubt on the reliability of these predictors for the future.

Recent process based analysis has shown that El Niño Southern Oscillation (ENSO) can act to modulate the Rossby wave source associated with the MJO. We investigate this using our statistical approach to assess the impact of ENSO on the MJO teleconnection patterns. In addition to this, we consider lower-frequency modes, for example Atlantic Multidecadal Variability (AMV) and the Pacific Decadal Oscillation (PDO). By compositing the extratropical response to the MJO over positive and negative phases of each of these modes, we see the individual impact of each low-frequency on the MJO teleconnections. Our work suggests updating the current MJO-extratropical predictors to include consideration of the decadal atmospheric and oceanic basic state.

How to cite: Skinner, D., Matthews, A., and Stevens, D.: Decadal variability of the extratropical response to the Madden-Julian Oscillation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1796, https://doi.org/10.5194/egusphere-egu22-1796, 2022.

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.

How to cite: Elbaum, E., Garfinkel, C. I., Adam, O., Morin, E., Rostkier-Edelstein, D., and Dayan, U.: Uncertainty in projected changes in precipitation minus evaporation: Dominant role of dynamic circulation changes and weak role for thermodynamic changes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1381, https://doi.org/10.5194/egusphere-egu22-1381, 2022.