Antarctic Ozone Depletion and Trends in Tropopause Rossby Wave Breaking

Trends in summer tropopause Rossby wave breaking (RWB) are examined using meteorological reanalyses and model integrations. The reanalyses for the last 30 years show large increases in RWB on the equatorward side of the tropospheric jet and weak decreases on the poleward side. Comparable changes in RWB are found in general circulation model integrations whose stratospheric ozone differs between 1960 and 2000 levels, but not in integrations that differ only in their greenhouse gas concentrations and sea-surface temperatures. These results indicate that the formation of the ozone hole has led to changes in RWB frequency during southern summer

Cut-off lows in the Southern Hemisphere and their extension to the surface

Many cut-off low (COL) climatologies have been done throughout the Southern Hemisphere. Few have focused on COL vertical depth and their link to surface cyclones that often accompany these systems. Here we extend these climatologies in order to gain an understanding of the spatial, mobility, temporal, and seasonal variability of COL extensions towards the surface. Deep COLs (dCOLs), with extension all the way to the surface, are most frequent in the autumn months, are longer lasting, are more mobile and found most frequently situated in the high latitudes. They are usually collocated with Rossby wave breaking (RWB) on multiple isentropic surfaces. These RWB events drive high potential vorticity air into the upper troposphere. The depths of these intrusions are also shown to be critical to the development of COL extensions with dCOLs associated with deeper intrusions into the mid-troposphere. Upper-level PV features are collocated with warm surface potential temperature anomalies which can play a critical role in surface cyclogenesis. The warm surface potential temperature features, when out of phase with coupled upper tropospheric processes (surface features lagging behind upper level processes), can inhibit surfaceward extension and result in shallow COL (sCOL) development. Composite analysis shows that dCOLs that drive their own surface low development result in the simultaneous amplification of troughs throughout the troposphere, with the surface cyclone developing within a day of the COL.http://link.springer.com/journal/382hj2022Geography, Geoinformatics and Meteorolog

A quasi-geostrophic analysis of summertime southern African linear-regime westerly waves

Linear-regime westerly waves that propagate across the South African domain are often linked to well-known rainfall producing systems such as tropical temperate troughs and synoptic scale tropical low-pressure systems, and ridging South Atlantic Ocean anticyclones at the surface. It is accepted that the baroclinic waves that propagate across the domain provide the lifting mechanism that causes the required vertical motion for rainfall to occur. This study shows that there exists a jet streak embedded in these waves that is located downstream of the trough axis, to the east of which vertically upward motion is expected to occur. The entrance of the jet streak passes just south of the country, as the waves propagate past the domain. The study further shows that for this class of waves, the vertical motion that causes rainfall to occur is induced by the thermally direct transverse ageostrophic circulation that is located at this jet entrance. This is instead of the conventional upper air divergence that is located at the infection point east of the trough axis. Using a method of decomposing the Q-vector into its transverse (Qn) and shear (Qs) components, the divergence felds of which are used to decompose the vertical motion into the corresponding components, i.e 휔n and 휔s, respectively; it was shown that the vertical motion over South Africa is explained more by the former than the latter. Therefore, the uplift over the country and that located at the infection point east of the trough are dynamically distinct processes. Taking the limitations of the quasi-geostrophic framework into consideration, the study concludes that during the passage of linear-regime waves vertical motion that might lead to rainfall is caused by the circulation at the jet entrance and not the divergence in the baroclinic wav

On the Ridging of the South Atlantic Anticyclone Over South Africa: The Impact of Rossby Wave Breaking and of Climate Change

Ridging South Atlantic Anticyclones contribute an important amount of precipitation over South Africa. Here, we use a global coupled climate model and the ERA5 reanalysis to separate for the first time ridging highs (RHs) based on whether they occur together with Rossby wave breaking (RWB) or not. We show that the former type of RHs are associated with more precipitation than the latter type. The mean sea level pressure anomalies caused by the two types of RHs are characterized by distinct patterns, leading to differences in the flow of moisture-laden air onto land. We additionally find that RWB mediates the effect of climate change on RHs during the twenty-first century. Consequently, RHs occurring without RWB exhibit little change, while those occurring with RWB contribute more precipitation over the southern and less precipitation over the northeastern South Africa in the future. Key Points: - Ridging South Atlantic Anticyclones are accompanied by Rossby wave breaking (RWB) aloft in 44% of the cases - Ridging highs that are accompanied by RWB lead to more precipitation over South Africa than those that are not - Ridging highs bring more precipitation over the southern and less precipitation over the northeastern part of South Africa in the futur

Integrated climatology and trends in the subtropical Hadley cell, sunshine duration and cloud cover over South Africa

This study uses two methods to diagnose the local Hadley circulation; first the zonally averaged mass stream function, and second the stream function vector method. The two methods have been applied to the ERA‐Interim reanalysis data for the period 1979–2015, to calculate both the climatology and trends of the Hadley cell. Both diagnostics advocate downwards mass flux being dominant over the subtropics, particularly over South Africa, yet the strength of Hadley is seasonal. Contrasts have been found between linear trends of the two diagnostics. Zonally symmetric diagnostics indicate strengthening of the Hadley cell, particularly in the subtropics of the Southern Hemisphere in winter and weakening in summer. The zonally asymmetric results indicate maximum strengthening of the Hadley over South Africa to be in spring and weakening in summer. Furthermore, maximum decrease in cloud cover and increase in sunshine duration over South Africa is in spring, implying more opportunities for solar energy generation.The NationalResearch Foundation (NRF) Grant No. 102215.http://wileyonlinelibrary.com/journal/joc2020-03-30hj2019Geography, Geoinformatics and Meteorolog

Climate change and maize production in the Vaal catchment of South Africa : assessment of farmers’ awareness, perceptions and adaptation strategies

In recent years, maize production in South Africa has faced challenges related to climate change which have prompted farmers to adapt their production activities. We assessed factors informing the adaptive decision-making of maize farmers in the Vaal catchment by examining linkages between farmers’ experiences, their perceptions of climate change and the adaptation strategies they use. Data were collected through semi-structured household-level interviews, key informant interviews and focus group discussions. Catchment climate data were also collected to determine key 30 yr trends (1989-2018) and the farmers’ level of awareness about these trends. Data were analysed using descriptive statistics, Mann-Kendall (MK) test, Sen’s slope test, climate anomalies and multinomial logit modelling. Results suggest that maize farmers in the catchment are aware of climate change (95%), with many of them referring to it as ‘a shift in climate’. This perceived ‘shift’ is supported by meteorological data, as the MK test confirmed a decreasing inter-annual precipitation trend (-0.149) and a decreasing trend at the onset of the maize planting season (-0.167), with temperature showing an increasing trend (0.470). These trends have inspired the adoption of a range of timing-related responses and other farming and off-farm adaptations. Modelling results revealed farmer perception, farmer typology and the nature of maize production (rainfed) as some of the variables with a deciding influence on the nature of the adaptation employed. The study confirms the importance of understanding intersections between qualitative and quantitative variables in triggering adaptive responses. Current strategies need to be expanded and supplemented to improve resilience and prevent maladaptation.The Department of Science and Technology National Research Foundation Innovation Doctoral Scholarshiphttp://www.int-res.com/journals/cr/cr-home2022-01-28hj2021Geography, Geoinformatics and Meteorolog

Integrated assessment of the influence of climate change on current and future intra-annual water availability in the Vaal River catchment

Increasing water demand due to population growth, economic expansion and the need for development puts a strain on the supply capacity of the Vaal River catchment in South Africa. Climate change presents additional challenges in the catchment which supports the country's economic hub, more than 30% of its population and over 70% of its maize production. This study evaluates the influence of climate change on current and future intra-annual water availability and demand using a multi-tiered approach where climate scenarios, hydrological modelling and socio-economic considerations were applied. Results shows exacerbated water supply challenges for the future. Temperature increases of between 0.07 and 5 °C and precipitation reductions ranging from 0.4 to 30% for Representative Concentration Pathways (RCPs) 4.5 and 8.5, respectively, are also predicted by the end of the century. The highest monthly average streamflow reductions (8–10%) are predicted for the summer months beyond 2040. Water Evaluation and Planning (WEAP) simulations project an increase in future water requirements, gaps in future water assurance and highlight limitations in existing management strategies. The study recommends a combination of adaptation plans, climatic/non-climatic stressor monitoring, wastewater-reuse, conservation, demand management and inter-basin transfers to reduce future uncertainty in monthly water sustainability.https://iwaponline.com/jwcchj2021Geography, Geoinformatics and Meteorolog

Impact of lower stratospheric ozone on seasonal prediction systems

We conducted a comparison of trends in lower stratospheric temperatures and summer zonal wind fields based on 27 years of reanalysis data and output from hindcast simulations using a coupled oceanatmospheric general circulation model (OAGCM). Lower stratospheric ozone in the OAGCM was relaxed to the observed climatology and increasing greenhouse gas concentrations were neglected. In the reanalysis, lower stratospheric ozone fields were better represented than in the OAGCM. The spring lower stratospheric/ upper tropospheric cooling in the polar cap observed in the reanalysis, which is caused by a direct ozone depletion in the past two decades and is in agreement with previous studies, did not appear in the OAGCM. The corresponding summer tropospheric response also differed between data sets. In the reanalysis, a statistically significant poleward trend of the summer jet position was found, whereas no such trend was found in the OAGCM. Furthermore, the jet position in the reanalysis exhibited larger interannual variability than that in the OAGCM. We conclude that these differences are caused by the absence of long-term lower stratospheric ozone changes in the OAGCM. Improper representation or non-inclusion of such ozone variability in a prediction model could adversely affect the accuracy of the predictability of summer rainfall forecasts over South Africa.The Water Research Commission (project number K5/1913) and the Applied Centre for Climate and Earth Systems Science.http://www.sajs.co.zaam201

The influence of the lower stratosphere on ridging Atlantic Ocean anticyclones over South Africa

The link between Rossby wave breaking and ridging Atlantic Ocean anticyclones in the South African domain is examined using NCEP–DOE AMIP-II reanalysis data. A simple composite analysis, which used the duration of ridging events as a basis of averaging, reveals that ridging anticyclones are coupled with Rossby wave breaking at levels higher than the dynamical tropopause. Lower-stratospheric PV anomalies extend to the surface, thus coupling the ridging highs with the lower stratosphere. The anomaly extending from the 70-hPa level to the surface contributes to a southward extension of the surface negative anomaly over the Namibian coast, which induces a cyclonic flow, causing the ridging anticyclone to take a bean-like shape. The surface positive anomaly induces the internal anticyclonic flow within the large-scale pressure system, causing the ridging end to break off and amalgamate with the Indian Ocean high pressure system. Lower-stratospheric Rossby wave breaking lasts for as long as the ridging process, suggesting that the former is critical to the longevity of the latter by maintaining and keeping the vertical coupling intact.This research was partially funded by the Water Research Commission of South Africa.http://www2.ametsoc.org/ams/index.cfm/publications/journals/journal-of-climate/2019-08-01am2019Geography, Geoinformatics and Meteorolog

A quasi-geostrophic diagnosis of the zonal flow associated with cut-off lows over South Africa and surrounding oceans

The zonal flow associated with cut-off lows (COLs) comprises two jet streaks of different spatial extents. The smaller scale jet streak, located north of the COLs, forms as a result of meridional divergence of vorticity advection and it is quasi-stationary, relative to the COLs. It dissipates as the COLs do the same. The larger scale jet streak gives rise to anticyclonic and equatorward Rossby wave breaking (RWB) as it propagates southeasterly to the base of the ridge, south of the COL and then northeasterly beyond that point. As the jet streak propagates it brings with it the anticyclonic barotropic shear that causes the Rossby waves to break. Its propagation is caused by zonal momentum advection by the zonal flow from jet streak entrance to its exit. As it propagates, its northwesterly/southeasterly orientation changes to one that is more zonal to become south-westerly/northeasterly at the end of the COL life cycle. This change in orientation is due to meridional advection of zonal momentum,where the meridional flow advects momentum southward (northward) at the jet streak entrance (exit). The jet streaks form a split jet structure and the winds between the streaks is decelerated by vorticity advection convergence. Because the flow and COL (and RWB) life cycle are coupled, understanding the dynamics that underlie the changes in the COL ambient flow contributes to resolving the outstanding RWB/COL causality problem.Water Research Commissionhttp://link.springer.com/journal/3822021-08-17hj2020Geography, Geoinformatics and Meteorolog