Downstream development during ridging South Atlantic Ocean anticyclones

This work is part of a broader project that aims is to understand the underlying dynamical processes of the important synoptic systems in the South Atlantic Ocean/South Africa basin.AVAILABILITY OF DATA AND MATERIALS : This study has been supported by the Water Research Foundation (Grant number: C2020/2021-00653) of South Africa and it is part of a larger project that aims to understand the underlying dynamical processes of the important synoptic systems in the South Atlantic Ocean/South Africa basin. All the data used in the study was obtained are the Fifth Generation European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) can be obtained from. https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5Ridging South Atlantic high pressure systems (ridging highs) are often accompanied by cut-off low (COL) pressure systems aloft, but may also occur without them, in which case a linear baroclinic wave would be observed propagating across the South African domain in the upper troposphere. Using 41 years of ERA-5 reanalysis data, this study documents differences between the characteristics of the prevailing dynamical processes and associated local eddy kinetic energy generation, its downstream transfer and dissipation during these two scenarios. The study shows that when COLs are present then baroclinic conversion is strong and it is confined east of the Greenwich Meridian, whereas it is located downstream of South Africa and it is much weaker, when ridging occurs without COLs. The differences in strength and locations of the baroclinic conversion are associated with the differing jet streak configurations between the two scenarios; which lead to Rossby wave breaking and the absence thereof when there are COLs and when ridging occurs without COLs, respectively. The presence of breaking during COLs leads to trans-ridge downstream development that facilitates energy transfer from the midlatitudes into the South African domain. When there are no COLs present, the trans-trough downstream development is stronger than it is across the upstream ridge. Barotropic conversion from eddy kinetic energy to mean kinetic energy occurs in the South African domain during COLs, but occurs much further downstream when there are no COLs during ridging highs. The difference in the characteristics identified in this study can be traced back to the differences in the potential vorticity anomaly structures, which are largely due to whether the waves break or do not during the evolution of ridging events in the South African domain.The Water Research Commission. Open access funding provided by University of Pretoria.http://link.springer.com/journal/382am2024Geography, Geoinformatics and MeteorologySDG-13:Climate actio

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

DATA AVAILABILITY : All the data used in the study was obtained are the Fifth Generation European Centre for Medium-Range Weather Forecasts reanalysis (ERA5) can be obtained from https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era5.Please read abstract in the article.The Water Research Foundation of South Africa.http://link.springer.com/journal/382hj2024Geography, Geoinformatics and MeteorologySDG-13:Climate actio

The greater Agulhas Current system: an integrated study of its mesoscale variability

For the purpose of developing an operational oceanography system for the greater Agulhas Current regime, a high resolution Hybrid Coordinate Ocean Model (HYCOM) has been set in a nested configuration. The intense and complex current regime poses a challenge in modelling. However, access to satellite and in-situ data with strong and persistent signals of the dynamics and mesoscale variability ensure that adequate model validation is feasible. The study concludes that HYCOM reproduces the general larger scale circulation of the greater Agulhas Current reasonably accurately in addition to the regionally specific characteristics and mesoscale variability. Furthermore, strong anticyclonic eddies occurring in the Mozambique Channel at a frequency of 5–6 per year, are found to drift southward and merge with the northern Agulhas Current. Evidence of these eddies can also be tracked further southwestwards into the southern Agulhas Current and sometimes all the way towards the Agulhas retroflection region. Operational forecasting of the greater Agulhas Current, and in particular the retroflection, must therefore adequately account for the presence and influence of the Mozambique Channel eddies, in order to forecast their evolution on time scales from days to week

Building a tropical-extratropical cloud band metbot

An automated cloud band identification procedure is developed that captures the meteorology of such events over southern Africa. This “metbot” is built upon a connected component labelling method that enables blob detection in various atmospheric fields. Outgoing longwave radiation is used to flag candidate cloud band days by thresholding the data and requiring detected blobs to have sufficient latitudinal extent and exhibit positive tilt. The Laplacian operator is used on gridded reanalysis variables to highlight other features of meteorological interest. The ability of this methodology to capture the significant meteorology and rainfall of these synoptic systems is tested in a case study. Usefulness of the metbot in understanding event to event similarities of meteorological features is demonstrated, highlighting features previous studies have noted as key ingredients to cloud band development in the region. Moreover, this allows the presentation of a composite cloud band life cycle for southern Africa events. The potential of metbot to study multiscale interactions is discussed, emphasising its key strength: the ability to retain details of extreme and infrequent events. It automatically builds a database that is ideal for research questions focused on the influence of intraseasonal to interannual variability processes on synoptic events. Application of the method to convergence zone studies and atmospheric river descriptions is suggested. In conclusion, a relation-building metbot can retain details that are often lost with object-based methods but are crucial in case studies. Capturing and summarising these details may be necessary to develop deeper process-level understanding of multiscale interactions

A climatology of drylines in the interior of subtropical Southern Africa

DATA AVAILABILITY STATEMENT : The ERA5 data (Hersbach. et al. 2018) used were downloaded from the Copernicus Climate Change Service (C3S) Climate Data Store. Satellite imagery was downloaded from EUMETSAT Data Store. Station data in this study are available on request from the South AfricanWeather Service (SAWS) (http://www.weathersa. co.za). Other surface observation data were downloaded from online (www.ogimet.com), as were monthly ERSSTv5 data for the Ni ˜no-3.4 region (Huang et al. 2017) (https://climexp. knmi.nl/).A climatology of synoptic drylines on the subtropical southern African interior plateau (SAP) is developed using ERA5 reanalysis specific humidity and surface temperature gradients and an objective detection algorithm. Drylines are found to occur regularly during spring and summer (September–March), and almost daily during December of that period, but rarely in winter. A westward shift in peak dryline frequency takes place through the summer. Drylines peak first over the eastern parts of the SAP during November with a mean of 10 drylines and then over the central (mean of 12) and western SAP (mean of 20) in December. During midsummer, drylines over the eastern SAP are negatively correlated with drylines in the west. Between 1980 and 2020, a significant correlation exists between ENSO and dryline days over the eastern (r = 0.44; p value = 0.004) and central (r = 0.41; p value = 0.008) SAP with fewer drylines (up to 10) occurring during years with increased surface moisture and more drylines (up to 45) occurring during years with decreased surface moisture. Drylines forming over the eastern parts of the SAP were more likely to move westward than drylines over the central and western parts. Onset times across the SAP show that drylines have a tendency to form during either the late morning to early afternoon (1100 and 1400 LST) or during the early evening hours (1700 and 2000 LST), suggesting that the surface heat trough (Kalahari heat low) and westward moisture transport mechanisms, such as the Limpopo low-level jet and ridging highs, are responsible for the formation of most drylines across the SAP.South African National Research Foundation (NRF) ACSyS Programme and the FLAIR programme, a partnership between the African Academy of Sciences and the Royal Society funded by the U.K. Government’s Global Challenges Research Fund.https://journals.ametsoc.org/view/journals/clim/clim-overview.xmlam2023Geography, Geoinformatics and MeteorologySDG-13:Climate actio

Downstream development during South African cut-off low pressure systems

Using 39 years of ECMWF renalysis data, an established energetics framework and simple composite analysis this study has shown that South African cut-off low (COL) pressure systems are preceded by downstream development of a baroclinic wave. Downstream development is characterised by the evolution of two energy centres, one located upstream in the midlatitudes and another downstream in the subtropics. The upstream eddy kinetic energy, which is associated with a midlatitude jet streak, develops and reaches its maximum before the formation of the closed COL cyclonic circulation. The downstream eddy kinetic energy centre maximises at the point where the closed circulation forms. The upstream eddy kinetic energy centre grows from baroclinic conversion from eddy available potential energy to eddy kinetic energy, whilst the downstream centre grows by receiving energy by means of ageostrophic geopotential fluxes that transport eddy kinetic energy in a north-eastward direction from the upstream centre. These ageostrophic geopotential fluxes are induced, increased in magnitude and directed by processes associated with Rossby wave breaking (RWB) on the midlatitude dynamical tropopause and so the downstream energy transfer connects South African COLs to midlatitude processes. The study has further shown that the baroclinic kinetic energy configuration previously associated with wet seasons over South Africa is consistent with times when COLs forms over the country.The Water Research Commission, South Africahttps://www.elsevier.com/locate/atmosreshj2022Geography, Geoinformatics and Meteorolog

Inter‐annual variability of the along‐shore Lagrangian transport success in the southern Benguela Current upwelling system

A 3-km resolution regional ocean model is used to investigate the role of wind-driven coastal circulation and mesoscale variability on the inter-annual variability of transport success in the southern Benguela between Cape Point (34°S) and St Helena Bay (32°S) from 1992 to 2011. Lagrangian particles are released within the top 100 m of the water column along an across-shore transect off Cape Point. Transport success is given by the ratio of the number of particles that reach St Helena Bay over the total number of particles released. The analysis of transport success anomalies and their relationship with the local circulation and wind forcing reveal that there is no single driver of the inter-annual variability. The transport success variability of particles released on the shelf (depths 500 m), across-shore transports induced by mesoscale eddies are the main contributors to transport success variability. Rare passage of Agulhas rings near the shelf edge can induce strong offshore advection of particles into the open ocean. In contrast, shelf-edge cyclonic eddies favour the onshore transport of particles originating from the outer shelf-edge and thus contribute to increase transport success. Plain Language Summary This study investigates the inter-annual variation of Lagrangian transport in the southern Benguela Current upwelling system using a high-resolution regional ocean model and particle tracking experiments. Transport of fish eggs and larvae by upper ocean currents is crucial for the marine ecosystem in this highly productive region since the spawning and nursery areas used by anchovies are separated by large distances ( ∼400 km). The alongshore connectivity between the Cape Peninsula and St Helena Bay from 1992 to 2011 is analysed and linked to the regional ocean circulation and wind-forcing on an inter-annual time scale. We find that transport success is influenced by several drivers including the Benguela Jet, Ekman transport, the coastal inner-shelf poleward counter-current, and occasional interactions with eddies such as Agulhas rings and shelf-edge cyclonic eddies. These findings provide a valuable information for the future studies on the role of the physical drivers that impact transport of larvae and eggs in the southern Benguela, underlining that no single driver can account solely for extreme positive or negative events in transport success

Two types of ridging South Atlantic Ocean anticyclones over South Africa and the associated dynamical processes

Using 41 years of ERA5 reanalysis, two types of ridging South Atlantic Ocean high pressure systems were identified in the South African domain. Type-N events have a zonal structure and the ridging component breaks off from the parent South Atlantic Ocean anticyclone, after extending across the South African mainland. Type-S events extend south of the mainland and then break off. The Type-N (Type-S) ridging component is weaker (stronger) leaving behind a stronger (weaker) South Atlantic Ocean high. The two types of ridging events are associated with different configurations of Rossby wave packets that propagate across the South Atlantic Ocean. Surface and upper tropospheric anomalies associated with Type-S wave packets are stronger than those associated with Type-N events and the vertical coupling of the anomalies is much stronger during Type-S events. Type-N events are associated with a double jet streak structure, with the downstream jet contributing to upward motion over the landmass by means of its thermally direct circulation at its jet entrance. The upstream jet during Type-N events induces downward motion over the southern half of South Africa as it propagates eastward. The Type-S upstream jet streak, which only appears during winter, has limited zonal extent and does not induce downward motion over the country. Type-N ridging is associated with stronger ageostrophic moisture fluxes along the southern coast leading to higher moisture content and precipitation along the south eastern and eastern coasts of South Africa.The Water Research Commission, South Africahttp://www.elsevier.com/locate/atmoshj2021Geography, Geoinformatics and Meteorolog