Latent Heat Flux in the Agulhas Current

In-situ observation, climate reanalyses, and satellite remote sensing are used to study the annual cycle of turbulent latent heat flux (LHF) in the Agulhas Current system. We assess if the datasets do represent the intense exchange of moisture that occurs above the Agulhas Current and the Retroflection region, especially the new reanalyses as the former, the National Centers for Environmental Prediction Reanalysis 2 (NCEP2) and the European Centre for Medium-Range Weather Forecast (ECMWF) reanalysis second-generation reanalysis (ERA-40) have lower sea and less distinct surface temperature (SST) in the Agulhas Current system due to their low spatial resolution thus do not adequately represent the Agulhas Current LHF. We use monthly fields of LHF, SST, surface wind speed, saturated specific humidity at the sea surface (Qss), and specific humidity at 10 m (Qa). The Climate Forecast System Reanalysis (CFSR), the European Centre for Medium-Range Weather Forecast fifth generation (ERA-5), and the Modern-Era Retrospective analysis for Research and Applications version-2 (MERRA-2) are similar to the air–sea turbulent fluxes (SEAFLUX) and do represent the signature of the Agulhas Current. ERA-Interim underestimates the LHF due to lower surface wind speeds than other datasets. The observation-based National Oceanography Center Southampton (NOCS) dataset is different from all other datasets. The highest LHF of 250 W/m2 is found in the Retroflection in winter. The lowest LHF (~100 W/m2) is off Port Elizabeth in summer. East of the Agulhas Current, Qss-Qa is the main driver of the amplitude of the annual cycle of LHF, while it is the wind speed in the Retroflection and both Qss-Qa and wind speed in between. The difference in LHF between product are due to differences in Qss-Qa wind speed and resolution of datasets

On the role of the Agulhas Current on weather and climate of South Africa

The Agulhas Current is the strongest western boundary current of the Southern Hemisphere. The aim of this thesis is to understand the impact of ocean-atmosphere interaction in the Agulhas Current on the atmosphere and to investigate its importance for Southern African rainfall. This warm Current creates a high temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere (also called the turbulent latent heat flux). The dynamics of ocean-atmosphere interaction above the Agulhas Current and its impact on the weather and climate of Southern Africa are not well known. This is due to a) climate reanalyses that do not include the Agulhas Current and b) the lack of regional capacity in ocean-atmosphere modeling. I use ocean observations, various climate reanalyses, and several satellite remote sensing data sets to find out if the new reanalyses (cited below) do represent the intense exchange of moisture that occurs above the Agulhas Current and the Agulhas Retroflection region. The largest turbulent latent heat flux is found in the Retroflection region in winter, while the lowest is off Port Elizabeth in summer. The Climate Forecast System Reanalysis (CFSR) and the ModernEra Retrospective analysis for Research and Applications version-2 (MERRA-2) do represent the turbulent latent heat flux well when compared to high-resolution satellite data. ERA-Interim Reanalysis underestimates the turbulent latent heat flux due to reduced wind speeds. The observation-based National Oceanography Centre Southampton (NOCS) is different from the satellites and the reanalysis products because its annual cycle is reversed, and NOCS underestimates the turbulent latent heat flux compared to the former products. The study of the satellite product air-sea turbulent fluxes (SEAFLUX) shows that east of the Agulhas Current, the specific humidity difference is the main driver of the annual cycle variations of turbulent latent heat flux, while the main driver in the Retroflection is the wind speed and both the specific humidity difference and the wind speed in between (around Port Elizabeth). I use high-resolution annual mean observations from satellites, atmospheric reanalysis, and the Weather Research and Forecasting (WRF) model to show that the warm core of the Agulhas Current drives a band of precipitation along the east coast of South Africa when the Current is adjacent to the coast. To do that, I conduct a classic modeling experiment with one configuration representing the sea surface temperature (SST) of the Agulhas Current relatively well. This WRF experiment reproduces the turbulent latent heat flux well. The second simulation is with SST of the Agulhas Current reduced by up to 2°C compared to the first experiment. From a diagnostic of the pressure adjustment mechanism, results show that the warm SST of the Agulhas Current enhances the formation of coastal precipitation along and above the Current. Finally, I look at the seasonality of oceanatmosphere interaction in the Agulhas Current and its impact on Southern African precipitation. In winter, the impact of the Agulhas Current is confined to the atmosphere above it and mechanisms are similar to those described for annual mean. In summer and autumn, SST differences between the two simulations where the Agulhas Current system is more than 25°C, leads to differences in geopotential height above the ocean, extending along the eastern coast and over the land area. The higher temperature of the control simulation leads to cyclonic circulation anomalies and larger moisture flux anomalies from the Agulhas Current to the continent from the south. The analysis of the high-level moisture flux indicates that the Agulhas Current influences the rainfall and humidity flux of Southern Africa. More moisture flux is then brought inland at a higher level. In the northeast of the region, there is an export of moisture anomalies from land to the ocean, and an import of moisture anomalies from above the Agulhas Current to the landmass. This is due to the wind anomalies between the two simulations. However, the overall result leads to more precipitation over the interior of the continent. The study shows that it is important to integrate the fine structure of the ocean temperature of the Agulhas Current in modeling studies and climate reanalyses. Results of this thesis have implications for the prediction of South African weather and climate, and for understanding past and present climate

The influence of the Agulhas Current on two South African extreme weather events

Includes bibliographical references.Surface station, satellite and NCEP re-analysis data are used to examine the evolution of two severe storms that occurred over the eastern coastal regions during South Africa's summer season 1998/99. The storms in November and December were both accompanied by heavy rainfall in two widely separated locations. The storm in December proved to be more severe as it resulted in flooding while tornadoes were reported in the Umtata and Hogsback regions of the Eastern Cape. Both storms appeared to result from interaction between a continental heat low, advection of warm moist air around an anticyclone in the South-west Indian Ocean and an approaching midlevel westerly trough. NCEP derived moisture flux diagrams and back trajectories of air parcels constructed from ECMWF data suggest that the Agulhas Current region was a major source of low level moisture for both storms. TRMM satellite imagery captured heavy rainfall above the high sea surface temperatures of the Agulhas Current. TRMM measurements of rainfall and latent heat in the atmosphere show that the high sea surface temperatures of the Agulhas Current modified the mesoscale environment above the current. To what extent the mesoscale environment above the Agulhas Current modified the synoptic situations over land could be answered using regional modeling and more frequent radiosonde data

Impact of the Agulhas Current on storm development

A high-resolution atmospheric model (WRF) is used to investigate the impact of the Agulhas Current on synoptic storm development. A sensitivity experiment is conducted to analyse the influence of the Agulhas Current's sea surface temperature (SST) on rain producing, synoptic scale weather features. Two model configurations: Control (CTL) and Smooth (SMTH) are analysed to understand the effect of the Agulhas Current's SST and high latent heat fluxes on storms that develop or track over the Current. The two configurations are identical except that the SMTH simulation has the SST signature of the Agulhas reduced by smoothing out the strong SST gradients associated with the Current. This results in the Agulhas Current core having SSTs reduced by roughly 1.5°C in the SMTH configuration. Consequently, lower (100 - 150 W.m¯²) latent heat fluxes are also simulated at the Current core's location in the SMTH run. Using daily South Africa Weather Service synoptic charts from 2001 - 2005, when the model output is available, two hundred (200) synoptic scale storms are found to track over the Current. Using the TRMM 3B42 3-hourly 0.25 x 0.25° precipitation rain rate product, 70 (of the 200) are found to have produced rainfall. Five model variables are used as proxies for the storm intensity of these 70 storms. Ten storms are found to show storm intensification when passing over the Current. In the CTL simulation, of these ten storms, ten show lower 850mb geopotential heights (m), nine show higher surface wind speeds (m.sˉ¹), seven show higher rain rates (mm.hrˉ¹), eight show higher Eddy Kinetic Energy (EKE) (m².sˉ²) and nine show greater upward moisture flux at the surface (g.mˉ².sˉ¹) compared to the SMTH run once each storm has propagated over the Current. Model output analysis shows sustained or dissipating storm intensity of the other 60 storms while passing over the Current. Nonetheless, these results provide a strong case for the influence of the Agulhas Current on the intensification of synoptic scale, rain producing events

The atmospheric boundary layer above the Agulhas current

This thesis describes the atmospheric boundary layer above the Agulhas Current using shipboard meteorological measurements and rawinsonde ascents. The juxtaposition of the warm Agulhas Current and cool shelf waters is shown to have far-reaching effects on the overlying atmosphere. Air-sea fluxes of momentum, sensible and latent heat and resultant boundary layer characteristics demonstrate high horizontal inhomogeneity. The results suggest that this inhomogeneity is permanent. The spatial heat flux gradient is reflected in the overlying atmosphere by a transition in stability of the boundary layer and potential cumulus formation from the cool shelf to the warm current. For airflow perpendicular to the Agulhas Current an internal boundary layer was observed to develop at the inshore sea surface temperature front. Onshore-moving air accumulated a significant quantity of moisture during its trajectory over the current. When airflow is parallel to the current an atmospheric moisture front exists along the axis of the inshore sea surface temperature front. The mean thermodynamic structure of the atmosphere was investigated. An inversion capped the boundary layer whilst a second, higher-level subsidence inversion was found which acts to limit the vertical development of cumulus clouds and therefore the redistribution of heat and moisture above the boundary layer. The results presented in this thesis are useful in two ways. The Agulhas Current has frequently been linked to South African climate. This is the first dedicated study which quantifies and characterizes the atmospheric boundary layer in this region. Secondly, maritime airmasses are dramatically modified above the Agulhas Current. The resultant large horizontal inhomogeneity, its vertical extent and permanence suggest that its inclusion is vital to any successful climate model. Atmospheric general circulation models have been criticized for not taking into account regions of strong horizontal inhomogeneity. The results of this thesis support this argument and highlight the need for similar studies. Bibliography: pages 116-123

Impact of the Agulhas Current on Southern Africa Precipitation: A Modeling Study

Postponed access: the file will be available after 2022-05-22The Agulhas Current (AC) creates a sharp temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere. We use two simulations of the Weather Research and Forecasting (WRF) Model to show the seasonal impact of the warm core of the AC on southern Africa precipitation. In one simulation the sea surface temperature (SST) of the AC is similar to satellite observations, while the second uses satellite SST observations spatially smoothed to reduce the temperature of the core of the AC by ~1.5°C. We show that decreasing the SST of the AC reduces the precipitation of the wettest seasons (austral summer and autumn) inland. Over the ocean, reducing the SST reduces precipitation, low-level wind convergence, SST, and SLP Laplacians above the AC in all seasons, consistent with the pressure adjustment mechanism. Moreover, winter precipitation above the AC may also be related to increased latent flux. In summer and autumn, the AC SST reduction is also associated with decreased precipitation farther inland (more than 1.5 mm day−1), caused by an atmospheric circulation that decreases the horizontal moisture flux from the AC to South Africa. The reduction is also associated with higher geopotential height extending from the surface east and over the AC to the midtroposphere over southeastern Africa. The westward tilted geopotential height is consistent with the linear response to shallow diabatic heating in midlatitudes. An identical mechanism occurs in spring but is weaker. Winter rainfall response is confined above the AC.publishedVersio

Cut-off low pressure systems and extreme rainfall over South Africa

Includes bibliographical references (p. 257-271).This thesis is an investigation of cut-off low pressure systems over South Africa. These weather systems have been responsible for many of the flooding disasters that have affected South Africa, particularly the coastal regions, over recent decades. The thesis has two main objectives, namely, to construct a 30-year climatology of cut-off lows over South Africa, and to further understanding of the evolution of the low-level flow that leads to these systems producing extreme quantities of rainfall

Influence of the Indian Ocean Subtropical Dipole on the Agulhas current

Includes bibliographical references.Modern studies have successfully linked Subtropical Dipole (SIOD) events to southern Africa’s austral summer precipitation patterns, however, none have investigated the SIOD’s influence on the Greater Agulhas Current System. Here, the SIOD climatology was developed using a Regional Ocean Modeling System (ROMS) configured with GFDL-CORE v.2b reanalysis winds and heat fluxes for the 1958-2007 period. This configuration allows for a relatively accurate spatial and temporal account of the Sea Surface Temperature (SST) and Sea Surface Height (SSH) variability in the Subtropical Indian Ocean (SIO). Simulation and evaluation of SIOD events was achieved through the application of the Empirical Orthogonal Function (EOF), Wavelet Analysis and Composite Map Analysis. The EOF applied to monthly SST anomalies for the months January to December during the years 1958-2007 in the SIO resulted in the SIOD phenomenon emerging as the second EOF mode and explaining 8.93 of the total variance of the SIO. Moreover, the EOF applied only to the austral summer (JFM) months emerges the SIOD as the first EOF mode and explaining 20.84 of the total variance in the SIO. ROMS model results and statistical correlation results suggest that SIOD SST variability is neither linked to the El Nino-Southern Oscillation (ENSO) nor the Tropical Indian Ocean Dipole (IOD) phenomena, notwithstanding that SIOD events have in the past, coincided with some El Nino and La Nina events. Composite map analysis results suggest no significant influence of SIOD events on anomalous Agulhas Current SST and SSH during positive and negative SIOD years. Examination of lagged statistical correlations also showed no significant relationship between the anomalous SIOD index and the satellite derived geostrophic velocity at the core of the Agulhas Current for the period 1993-2007

Impacts of ENSO on coastal South African sea surface temperatures

The impact of El Niño Southern Oscillation (ENSO) on the Southern African inland climate is well documented and provides skill in the seasonal forecast of rainfall but little is known of the impact of ENSO on the ocean surrounding South Africa. The aim of this study is to assess the impact of ENSO on sea surface temperatures around the coast of South Africa and to calculate SST trends around the coast. I start by updating the study of Rouault et al (2010) on the very topic with an additional 10 years of data and two additional newer datasets which allow sampling closer to the coast where wind-driven upwelling is more active. The new highresolution ERA 5 reanalyzed climate dataset is also used to look at the atmospheric forcing of sea surface temperatures by ENSO. As in Rouault et al. (2010), I study five similar threedegree-long coastal regions around South Africa, namely: West Coast, South Coast, Port Elizabeth/Port Alfred, Transkei, Kwazulu-Natal and a larger offshore Agulhas Current area domain. Three SST datasets are evaluated in this study: the 1 ̊x1 ̊Optimal Interpolation sea surface temperature (OI SST) used by Rouault et al (2010), the 0.25 ̊x 0.25 ̊ Optimal Interpolation SST and the 4 km x 4 km Advanced Very High-Resolution Radiometer (AVHRR) Pathfinder SST version 5.3. The 0.25 ̊x 0.25 ̊OI SST resolvesSST anomalies better in these coastal regions as compared to 1 ̊x1 ̊ OISST. The difference in results among the three products concerning trends and correlation with ENSO is a cause for concern. The 4 km x 4 km AVHRR Pathfinder allows for SST to be extracted even closer to the coast but missing values are numerous and hamper the use of this dataset for ENSO composites and trend analyses. Results show a significant positive correlation with El Niño in summer at the monthly scale, reaching a maximum correlation of 0.45 at 3 months lag. Correlation is the highest in late summer. There is a negative correlation in the Agulhas Current area, opposite to those with ENSO and West Coast. The impact of ENSO on the coast of South Africa, West Coast and South Coast is due to change in surface wind speed with weaker upwelling favorable during El Niño leading to warmer than normal coastal water SST and stronger than normal Southeasterly winds during la Niña leading to cooler than normal coastal water. The wind perturbation is part of largescale basin-wide perturbations in the tropical Atlantic, in the South Atlantic high-pressure atmospheric system and in the westerly wind pattern of the midlatitude to the south. Non-ENSO related impact can be as important as ENSO related SST perturbation and is also linked to large scale perturbations in the South Atlantic. There is no relation between the strength of ENSO and the strength of the perturbation, and some ENSO events do not lead to the expected canonical warming or cooling. The large-scale SST perturbations seem to be caused by anomalous surface turbulent flux of latent and sensible heat and abnormal wind speed and direction. This study opens the possibility of seasonal forecasting of SST in the South Benguela upwelling system because of the positive lag correlation between ENSO and SST with ENSO leading SST