Structure and Maintenance Mechanisms of the Mascarene High in Austral Winter

Abstract The Mascarene High (MH), is a key component of the Asian-Africa-Australia monsoon system in austral winter (JJA). Its three-dimensional structures and maintenance mechanisms are examined in this study. It is a low-level subtropical high dominating the southern Africa and South Indian Ocean, characterized by a northwestward tilt with height, which is attributed to its spatially inhomogeneous thermal structure. Large-scale subsidence characterizes the main body of the MH, with the stronger subsidence to the east than to the west. Diagnosis using the complete form of the vertical vorticity tendency equation shows that the anticyclonic structure of the MH, which can be described by the distribution of meridional wind, is maintained mainly by the vertical gradient of diabatic heating, change in static stability, and friction dissipation. In particular, a combination of sensible heating and longwave radiative cooling results in a vertical decreasing gradient of diabatic heating in the lower troposphere. It generates the stronger southerlies over the subtropical South Indian Ocean than over the southern Africa. Meanwhile, over the South Indian Ocean, the increasing static stability as a result of the downward transport of a more stable atmosphere partly offsets the effect of the vertical gradient of diabatic heating, and southerlies still prevail there. Over the southern Africa, topographic friction dissipation induces northerlies, balancing the effect of the vertical gradient of diabatic heating with a stronger magnitude, and northerlies prevail.

Download Full-text

Variability in the Mozambique Channel Trough and Impacts on Southeast African Rainfall

Journal of Climate ◽

10.1175/jcli-d-19-0267.1 ◽

2020 ◽

Vol 33 (2) ◽

pp. 749-765 ◽

Cited By ~ 3

Author(s):

Rondrotiana Barimalala ◽

Ross C. Blamey ◽

Fabien Desbiolles ◽

Chris J. C. Reason

Keyword(s):

Indian Ocean ◽

Austral Summer ◽

Western Indian Ocean ◽

Strong Relationship ◽

Ocean Basin ◽

Moist Convection ◽

South Indian Ocean ◽

Mozambique Channel ◽

Mascarene High ◽

South Indian

AbstractThe Mozambique Channel trough (MCT) is a cyclonic region prominent in austral summer in the central and southern Mozambique Channel. It first becomes evident in December with a peak in strength in February when the Mozambique Channel is warmest and the Mascarene high (MH) is located farthest southeast in the Indian Ocean basin. The strength and the timing of the mean MCT are linked to that of the cross-equatorial northeasterly monsoon in the tropical western Indian Ocean, which curves as northwesterlies toward northern Madagascar. The interannual variability in the MCT is associated with moist convection over the Mozambique Channel and is modulated by the location of the warm sea surface temperatures in the south Indian Ocean. Variability of the MCT shows a strong relationship with the equatorial westerlies north of Madagascar and the latitudinal extension of the MH. Summers with strong MCT activity are characterized by a prominent cyclonic circulation over the Mozambique Channel, extending to the midlatitudes. These are favorable for the development of tropical–extratropical cloud bands over the southwestern Indian Ocean and trigger an increase in rainfall over the ocean but a decrease over the southern African mainland. Most years with a weak MCT are associated with strong positive south Indian Ocean subtropical dipole events, during which the subcontinent tends to receive more rainfall whereas Madagascar and northern Mozambique are anomalously dry.

Download Full-text

Influence of the Subtropical High and Storm Track on Low-Cloud Fraction and Its Seasonality over the South Indian Ocean

Journal of Climate ◽

10.1175/jcli-d-17-0229.1 ◽

2018 ◽

Vol 31 (10) ◽

pp. 4017-4039 ◽

Cited By ~ 16

Author(s):

Ayumu Miyamoto ◽

Hisashi Nakamura ◽

Takafumi Miyasaka

Keyword(s):

Indian Ocean ◽

Storm Track ◽

Cloud Fraction ◽

Subtropical High ◽

The South ◽

South Indian Ocean ◽

Mascarene High ◽

South Indian ◽

Low Cloud ◽

Storm Track Activity

Abstract The south Indian Ocean is characterized by enhanced midlatitude storm-track activity around a prominent sea surface temperature (SST) front and unique seasonality of the surface subtropical Mascarene high. The present study investigates the climatological distribution of low-cloud fraction (LCF) and its seasonality by using satellite data, in order to elucidate the role of the storm-track activity and subtropical high. On the equatorward flank of the SST front, summertime LCF is locally maximized despite small estimated inversion strength (EIS) and high SST. This is attributable to locally augmented sensible heat flux (SHF) from the ocean under the enhanced storm-track activity, which gives rise to strong instantaneous wind speed while acting to relax the meridional gradient of surface air temperature. In the subtropics, summertime LCF is maximized off the west coast of Australia, while wintertime LCF is distributed more zonally across the basin unlike in other subtropical ocean basins. Although its zonally extended distribution is correspondent with that of LCF, EIS alone cannot explain the wintertime LCF enhancement, which precedes the EIS maximum under continuous lowering of SST and enhanced SHF in winter. Basinwide cold advection associated with the wintertime westward shift of the subtropical high contributes to the enhancement of SHF, especially around 15°–25°S, while seasonally enhanced storm-track activity augments SHF around 30°S. The analysis highlights the significance of large-scale controls, particularly through SHF, on the seasonality of the climatological LCF distribution over the south Indian Ocean, which reflect the seasonality of the Mascarene high and storm-track activity.

Download Full-text

Climate of Southern Africa

Oxford Research Encyclopedia of Climate Science ◽

10.1093/acrefore/9780190228620.013.513 ◽

2017 ◽

Cited By ~ 5

Author(s):

C.J.C. Reason

Keyword(s):

South Africa ◽

Tropical Cyclone ◽

Indian Ocean ◽

Southern Africa ◽

Regional Climate ◽

Western Boundary ◽

South Indian Ocean ◽

South Indian ◽

African Climate ◽

Land Mass

Southern Africa extends from the equator to about 34°S and is essentially a narrow, peninsular land mass bordered to its south, west, and east by oceans. Its termination in the mid-ocean subtropics has important consequences for regional climate, since it allows the strongest western boundary current in the world ocean (warm Agulhas Current) to be in close proximity to an intense eastern boundary upwelling current (cold Benguela Current). Unlike other western boundary currents, the Agulhas retroflects south of the land mass and flows back into the South Indian Ocean, thereby leading to a large area of anomalously warm water south of South Africa which may influence storm development over the southern part of the land mass. Two other unique regional ocean features imprint on the climate of southern Africa—the Angola-Benguela Frontal Zone (ABFZ) and the Seychelles-Chagos thermocline ridge (SCTR). The former is important for the development of Benguela Niños and flood events over southwestern Africa, while the SCTR influences Madden-Julian Oscillation and tropical cyclone activity in the western Indian Ocean. In addition to South Atlantic and South Indian Ocean influences, there are climatic implications of the neighboring Southern Ocean. Along with Benguela Niños, the southern African climate is strongly impacted by ENSO and to lesser extent by the Southern Annular Mode (SAM) and sea-surface temperature (SST) dipole events in the Indian and South Atlantic Oceans. The regional land–sea distribution leads to a highly variable climate on a range of scales that is still not well understood due to its complexity and its sensitivity to a number of different drivers. Strong and variable gradients in surface characteristics exist not only in the neighboring oceans but also in several aspects of the land mass, and these all influence the regional climate and its interactions with climate modes of variability. Much of the interior of southern Africa consists of a plateau 1 to 1.5 km high and a narrow coastal belt that is particularly mountainous in South Africa, leading to sharp topographic gradients. The topography is able to influence the track and development of many weather systems, leading to marked gradients in rainfall and vegetation across southern Africa. The presence of the large island of Madagascar, itself a region of strong topographic and rainfall gradients, has consequences for the climate of the mainland by reducing the impact of the moist trade winds on the Mozambique coast and the likelihood of tropical cyclone landfall there. It is also likely that at least some of the relativity aridity of the Limpopo region in northern South Africa/southern Zimbabwe results from the location of Madagascar in the southwestern Indian Ocean. While leading to challenges in understanding its climate variability and change, the complex geography of southern Africa offers a very useful test bed for improving the global models used in many institutions for climate prediction. Thus, research into the relative shortcomings of the models in the southern African region may lead not only to better understanding of southern African climate but also to enhanced capability to predict climate globally.

Download Full-text

SST variability in the South Indian Ocean and associated circulation and rainfall patterns over Southern Africa

Meteorology and Atmospheric Physics ◽

10.1007/bf01026637 ◽

1998 ◽

Vol 66 (3-4) ◽

pp. 243-258 ◽

Cited By ~ 9

Author(s):

C. J. C. Reason ◽

C. R. Godfred-Spenning

Keyword(s):

Indian Ocean ◽

Southern Africa ◽

The South ◽

South Indian Ocean ◽

Sst Variability ◽

South Indian ◽

Rainfall Patterns

Download Full-text

Radiative impacts of low-level clouds on the summertime subtropical high in the South Indian Ocean simulated in a coupled general circulation model

Journal of Climate ◽

10.1175/jcli-d-20-0709.1 ◽

2021 ◽

pp. 1-52

Author(s):

Ayumu Miyamoto ◽

Hisashi Nakamura ◽

Takafumi Miyasaka ◽

Yu Kosaka

Keyword(s):

Indian Ocean ◽

General Circulation ◽

Circulation Model ◽

South Indian Ocean ◽

Coupled General Circulation Model ◽

Low Level ◽

Mascarene High ◽

South Indian ◽

Radiative Impacts ◽

Low Cloud

AbstractOver the South Indian Ocean, the coupled system of the subtropical Mascarene high and lowlevel clouds exhibits marked seasonality. To investigate this seasonality, the present study assesses radiative impacts of low-level clouds on the summertime Mascarene high with a coupled general circulation model. Comparison between a fully coupled control simulation and a “no low-cloud simulation,” where the radiative effects of low-level clouds are artificially turned off, demonstrates that they act to reinforce the Mascarene high. Their impacts are so significant that the summertime Mascarene high almost disappears in the no low-cloud experiment, suggesting their essential role in the existence of the summertime Mascarene high. As the primary mechanism, lowered seasurface temperature by the cloud albedo effect suppresses deep convective precipitation, inducing a Matsuno-Gill type response that reinforces the high, as verified through an atmospheric dynamical model diagnosis. Associated reduction of high-top clouds, as well as increased low-level clouds, augments in-atmosphere radiative cooling, which further reinforces the high. The present study reveals that low-level clouds constitute a tight positive feedback system with the subtropical high via sea-surface temperature over the summertime South Indian Ocean.

Download Full-text

Circulation pattern controls of wet days and dry days in Free State, South Africa

Meteorology and Atmospheric Physics ◽

10.1007/s00703-021-00822-0 ◽

2021 ◽

Author(s):

Chibuike Chiedozie Ibebuchi

Keyword(s):

South Africa ◽

Indian Ocean ◽

Atmospheric Circulation ◽

Southern Africa ◽

Austral Summer ◽

Free State ◽

Austral Spring ◽

Study Region ◽

Southwest Indian Ocean ◽

South Indian

AbstractAtmospheric circulation is a vital process in the transport of heat, moisture, and pollutants around the globe. The variability of rainfall depends to some extent on the atmospheric circulation. This paper investigates synoptic situations in southern Africa that can be associated with wet days and dry days in Free State, South Africa, in addition to the underlying dynamics. Principal component analysis was applied to the T-mode matrix (variable is time series and observation is grid points at which the field was observed) of daily mean sea level pressure field from 1979 to 2018 in classifying the circulation patterns in southern Africa. 18 circulation types (CTs) were classified in the study region. From the linkage of the CTs to the observed rainfall data, from 11 stations in Free State, it was found that dominant austral winter and late austral autumn CTs have a higher probability of being associated with dry days in Free State. Dominant austral summer and late austral spring CTs were found to have a higher probability of being associated with wet days in Free State. Cyclonic/anti-cyclonic activity over the southwest Indian Ocean, explained to a good extent, the inter-seasonal variability of rainfall in Free State. The synoptic state associated with a stronger anti-cyclonic circulation at the western branch of the South Indian Ocean high-pressure, during austral summer, leading to enhanced low-level moisture transport by southeast winds was found to have the highest probability of being associated with above-average rainfall in most regions in Free State. On the other hand, the synoptic state associated with enhanced transport of cold dry air, by the extratropical westerlies, was found to have the highest probability of being associated with (winter) dryness in Free State.

Download Full-text