This study investigates the spatiotemporal variability of the Mascarene High over the southern Indian Ocean in relation to (abnormal) weather and climate in southern Africa at interseasonal, seasonal, interannual, multidecadal, and temporal event scales. There is also significant geographic variability in the development, distribution, and movement of the Mascarene High in the southern Indian Ocean over different time periods.
INTRODUCTION AND BACKGROUND
- Introduction
- Problem statement
- Research aim and specific objectives
- Research questions
- Study area description
- Justification of the study
- Definition of key terms
- Dissertation structure
The distribution, strength and movement of the Mascarene High modulates weather patterns in southern Africa throughout the year. The blocking and future evolution of the Mascarene High also motivates further investigation of the weather system.
LITERATURE REVIEW
- Introduction
- General circulation of the atmosphere
- Characteristics of subtropical anticyclones
- Regions of subtropical anticyclones
- Trade winds, midlatitude westerlies and equatorial monsoons
- Isallobaric winds
- Continental and ridging subtropical anticyclones
- Blocking anticyclones
- The Mascarene High
- Moisture flux, meridional pressure gradient and the Angola Low
- Steering of tropical revolving systems
- Cut-off lows
- The mid-tropospheric Botswana High
- Ocean-atmosphere interactions
- Subtropical anticyclones and surface ocean currents
- El Niño Southern Oscillation
- Indian Ocean Dipole
- Subtropical Indian Ocean Dipole
- Climate change and subtropical anticyclones
- Summary
It is a reflection of the Mascarene High in the upper air, as surface highs slope northwestward with altitude. The seasonal eastward shift of the Mascarene High is modulated by the Indian Ocean subtropical dipole.
DATA DESCRIPTION AND METHODOLOGY
- Introduction
- Description and Sources of Data
- Circulation variables
- Rainfall
- Air temperature
- Outgoing Long-wave Radiation
- Sea Surface Temperatures
- Southern Oscillation Index
- Research Methods
- Principal Components Analysis
- Trend Analysis
- Correlation Analysis
- Anomalies
- Composite Analysis
- Event Scale Analysis
- Blocking Index
- Hovmöller diagram
- NCEP-NCAR reanalysis
- International Research Institute Climate Data Library
- Conformal-Cubic Atmospheric Model
- Representative Concentration Pathways
- Grid Analysis and Display System Version 2.0.2.oga.2
- Summary
In the study, the wind was used to investigate the circulation over the South Indian Ocean and South Africa influenced by the Mascarene High. The zonal wind circulation is mapped to analyze the zonal wind flow in the South Indian Ocean due to the large-scale circulation of the Mascarene High. It is the flow of wind along the longitude. v over the South Indian Ocean is plotted to provide a description of the longitudinally influenced circulation of the Mascarene High.
The study uses OLR to investigate events where Mascarene High blocks circulations over southern Africa. In this study, this technique investigates MSLP variability and cycles in the Southern Indian Ocean. Variables associated with the development, distribution and movement of the Mascarene High in the South Indian Ocean are also described in detail, including their sources.
SPATIAL PATTERNS AND TEMPORAL CHARACTERISTICS OF THE
Introduction
Spatial patterns of the Mascarene High
- Mean spatial characteristics and seasonal shifts
- PCA for MSLP over the South Indian Ocean
In addition to seasonal displacements, the average monthly migration of the Mascarene High is shown in Figure 4.5. At 850 hPa, the mean location of the Mascarene High is displaced to the northwest relative to its location at the surface (Figure 4.5). While the seasonal location of the Mascarene High varies and dominance is found in the austral summer when the weather system migrates westward (Figure 4.6). Figure 4.3: Average spatial characteristics and intensity of the Mascarene High over the South Indian Ocean a) summer and b) winter.
The second seasonal PCA mode explains 23.58% of the total variance, which consists of a poleward oriented Mascarene High location (Figure 4.7). PC1 is oriented further south of the South Indian Ocean (Figure 4.8), explaining 59.25% of the total variance. PC2 explains 13.97% of the total variance and shows dominance southwest of the South Indian Ocean (Figure 4.8).
Temporal variability of the Mascarene High pressure cell
- Annual cycle
- Interannual variability and trends
The interannual variability of the Mascarene High is dominated by a seasonal cycle with an insignificant trend (Figure 4.10).
Mean circulation
Mascarene High and Angola Low interactions
Ocean-atmosphere interactions
- ENSO
- IOD
- SIOD
There is a relationship between Niño 3.4 and the surface and midlevel zonal wind over the South Indian Ocean (Figure 4.20a and b). The relationship is dominant over large parts of the southern Indian Ocean and southern Africa. The MSLP in the Mascarene High consists of an inverse relationship with the IOD variability in the northeastern part of the South Indian Ocean (Figure 4.22).
The location of the Mascarene High is also found to be influenced by the warming of the southern Indian Ocean SSTs. Positive (negative) phases of SIOD appear to correlate with high (low) values of MSLP in the southern Indian Ocean. The relationship tends to be dominant in the southern part of the southern Indian Ocean during the austral summer (Figure 4.25a). Figure 4.25: Australian a) summer and b) winter correlation of MSLP (hPa) and SSTs over the South Indian Ocean (90% significance).
Summary
MASCARENE HIGH BLOCKING AND ANOMALOUS WEATHER EVENTS OVER
Introduction
Mascarene High blocking identification
Blocking and landfalling TCs
Mascarene High blocking caused the slow west-east propagation of the cloud belt weather system over southern Africa. The strengthening and intensification of the Mascarene High is mainly influencing the changes in weather and climate patterns in the subcontinent. The study focused on the Mascarene high pressure cell over the South Indian Ocean and how it affects the weather and climate over southern Africa.
The Mascarene High center migrates north-south (28-37°S) and west-east (60-87°E) across the southern Indian Ocean. Mascarene High blocking events cause slow west-east movements of the western wave weather systems, creating anomalous weather over southern Africa (Molekwa, 2013). A clear distinction is that this study presented future projections of the Mascarene High and weather variability in southern Africa.
The study also used CCAM to illustrate future projections of the Mascarene High under a changing climate. On the major movements in IOD in the last century, the role of Mascarene high movements.
TC Leon-Eline (1-29 February 2000)
TC Jaya (26 March – 8 April 2007)
Blocking and cut-off lows
The independent spin of the shear layer detached from the westerly wave is attributed to the persistence of the Mascarene High blocking. Slow motion of the cloud band was largely induced by Mascarene High blocking (Figure and 5.23). Similarities in the simulations indicate that there is little uncertainty in the simulated structure of the Mascarene High.
CCAM projections also suggest an extension of the Mascarene High in the future in summer (Figures 6.9 and 6.10), but not an obvious intensification signal. On average, the projections seem to indicate that there will be a decrease in rainfall patterns over southern Africa with an extension and intensification of the Mascarene High. At an event scale, the study analyzed the influence of the Mascarene High blocking over southern Africa and how this contributed to abnormal weather events.
Influences of the Southern Annular Mode (SAM) and Quasi-Biennial Oscillation (QBO) still need to be further explored in understanding in relation to the Mascarene High. What role does the Southern Annual Mode (SAM) and the Quasi-Biennial Oscillation (QBO) play on the intensity and extension of the Mascarene High.
31 October-2 November 1985
27-29 September 1987
Blocking and slow moving cloud bands
TC tracks may penetrate the subcontinent as they are driven by easterlies generated from the circulation of the Mascarene High. Annual projections of the MSLP show some uncertainty, with ensemble members predicting both a strengthening and a weakening of the Mascarene high. Mascarene High expansion and intensification has important implications in weather and climate changes across southern Africa.
In the area dominated by the Mascarene High in summer, easterly winds appear to blow. The spatiotemporal variability and climatology of the Mascarene High in the southern Indian Ocean for the present climate at interseasonal, intraseasonal, and interannual time scales were also presented. It was also found that the Indian Ocean Dipole (IOD) is inversely proportional to the height of the Mascarene (Figure 4.22).
Mascarene High projections show a future expansion and intensification of the weather system for the period 2070-2099. The Mascarene High is an important weather system for weather patterns and climatology in South Africa.
31 December 1997-2 January 1998
5-7 January 1998
Summary
In this chapter, Mascarene High blocking events were investigated and the results indicate that Mascarene High blocking induces heavy rainfall conditions over southern Africa. Propagation of cut-off lows, TCs, and cloud band events appears to be heavily influenced by a slow-moving Mascarene High. During Mascarene High blocking events, rainfall is found to be anomalously high over southern Africa and in some cases supplemented by cold conditions.
Cloud bands are found to be a major rainfall bearing system over southern Africa during the austral summer, their anomalous appearance is unique and brings heavy rainfall events over southern Africa. Over southern Africa, shear lows tend to be regional rather than widespread. f) Shear lows also cause subsequent blocking of the westerly wave as the weather system consists of slow mid-tropospheric movements over southern Africa. g) Absence of Mascarene High blocking can be directly linked to normal average weather conditions over Southern Africa. In the next chapter, the study focuses on future changes in intensity and spatial characteristics of the Mascarene High using the Conformal-Cubic Atmospheric Model (CCAM) projections.
FUTURE CHANGES IN INTENSITY AND SPATIAL CHARACTERISTICS OF THE
Introduction
Present day simulations vs. observations
The height center is associated with lower wind speeds as already seen in Figure 4.12. The RCP8.5 simulations are able to capture this feature best, with smaller wind speeds around the center for high and larger magnitudes away from the center. In winter, the northward propagation of the high is also well simulated with weaker associated winds around the center and counterclockwise motion around the high (Figure 6.5).
Most of the Indian Ocean is associated with positive values, further confirming that high pressure cells are associated with fair weather conditions. The northern parts of Madagascar are associated with negative values, indicative of systems associated with summer upwelling (Figure 6.6). In winter, the positive omega values extend further north to also include the area north of Madagascar, which was associated with negative values in summer (Figure 4.13;.
Climate periods differences (2070-2099 -1985-2014)
- MSLP differences (2070-2099) – (1985-2014)
- Wind differences (2070-2099) – (1985-2014)
- Omega differences (2070-2099) – (1985-2014)
- Rainfall differences (2070-2099) – (1985-2014)
- Rainfall and MSLP differences (2070-2099) – (1985-2014)
- Omega (500 hPa) and MSLP differences (2070-2099) – (1985-2014)
- Geopotential height (500 hPa) and MSLP differences (2070-2099) – (1985-2014)
- Temperature (°C) differences (2070-2099) – (1985-2014)
The 50th and 90th percentiles show positive values north of the area dominated by the Mascarene High, further indicating the expansion of the high pressure belt. The 10th percentile is associated with negative values for RCP4.5 and RCP8.5, indicating neither expansion nor strengthening of the high mascarene value. The 90th percentile is associated with positive values on the eastern side of the domain, as well as in the southwest, for RCP4.5 and RCP8.5.
The strengthening of easterly trade winds is linked to the future projected strengthening and extension of the Mascarene High, but our results are not conclusive on this. However, RCP 8.5 projections for the austral summer seem to indicate that this relationship will migrate eastwards in the future and this may be linked to a future extension of the Mascarene High (Figure 6.19b). These future projections can be linked to changes due to intensification and extension of the Mascerene High.
Summary
DISCUSSION AND CONCLUSIONS
- Introduction
- Discussion and synthesis of key findings
- Present climate of the Mascarene High
- Interannual variability and ocean-atmosphere interactions
- Blocking and anomalous weather over southern Africa
- Mascarene High future projections
- Conclusions
- Implications and future work
These anomalous events over southern Africa are all the result of the quasi-stationary west-east motion of the Mascarene High block. It is very important to outline that Mascarene High blocking plays a major role in causing anomalous rainfall over southern Africa. Finally, there is great significance in studying the causes of Mascarene High blockage in the future.
The future findings may provide some more new knowledge about the Mascarene high-pressure cell. The study motivates to consider other climate variations besides ENSO, IOD and SIOD that influence the Mascarene High and weather variability in southern Africa. Although seasonal patterns of Mascarene High have been investigated, the causes of Mascarene High's blocking are still inconclusive and require further investigation.
