South African Weather

1. synoptic typing smears relationships between weather systems
2. it provides generalizations that exclude complex interactions
   
3. it allows simple presentation of multi-faceted concepts
4. it does not lend itself to quantitative or detailed analysis.
   

1. Coastal Low Coastally trapped low pressure circulation between the sea and escarpment; usually starts on the west coast and propagates along the SA coast to dissipate near Mozambique. Warm, often strong offshore berg winds in front of the low; cool onshore flow behind it often with fog. Usually indicates an approaching cold front. Usually occurs in winter but can cause very dry and hot days during summer or transition seasons.

2. Mid-latittude cyclone / Westerly Wave Low / Cold Front Most distinctive change in weather occurs with passage of cold front. Sharp temperature drop, rain, and wind; if it extends over the interior can cause cold snaps here. Maximum rain over S. Cape depending on trajectory of storm system. Seldom directly causes heavy rain inland of escarpment but may induce organized convection squall lines inland (later...) Primarily a winter feature but in summer weak fronts may affect coastal regions.

3. Ridging South Atlantic High South Atlantic High ridges eastward south of the country. Advection of moist air inland from Indian ocean; mesoscale orographic forcing + changing curvature of flow + weakening inland pressure gradients  => uplift and wide spread rain over eastern region. Over W. Cape - subsidence, clear air, hot, often strong south-easters, stable conditions, lenticular clouds over the peninsula. Occurs all year round.

4. Southerly Meriodional Flow Associated with the passage of westerly low and the ridging of a well developed South Atlantic HP south of South Africa. Meridional flow (South -->North) along strong pressure gradient west of cyclone. Weaker uplift sustaining light orographic rain and squalls. Strong temperature drops as cold polar air is advected onshore. Occurs all year round.

Unusual feature of cold fronts in Cape Town Crumples against mountain chains and pulses over the peninsula chain. Pulses of cold air and rain can be observed arriving in the southern suburbs, associated with a temperature fluctuation before final drop. Causes longer residency time of rain and misery than would otherwise be the case! Causes rapid incursion of (delayed) cold high latitude air masses.

5. Cut-off Low Best seen in the upper air (at 500 hPa) as a trough that deepens until it forms a closed circulation and is displaced equatorward. Associated with strong surface convergence and upward vertical motion and therefore heavy precipitation, e.g. Laingsburg floods of '81. The occurence of these peaks in the spring and autumn. The image below (of 500 hPa geopotential heights) shows a cut-off low budding off a westerly wave (associated with a passing cold front at the surface).

6. Sub-tropical Anticyclones The dominant circulation over the interior during winter. Suppresses convection and responsible for dry highveld winters. Very little happens when this system dominates, no rain, light winds (if any), very cold nights in winter (no clouds). It can occur in summer, when this happens it can produce heat waves of 4+ days.

7. Easterly Wave Low A disturbance in the tropical easterly flow driven largely by thermal heating of continent and associated with ITCZ ,warm humid easterly flow, and an easterly jet. Over southern Africa tend to be semi-stationary during summer occuring as open waves or closed lows. Indian ocean acts as moisture source. Convergence to east of trough / low produces strong uplift and good rains usually as thunderstorms. Divergence to west produces subsidence and no rain. Strongly seasonal; absent in winter and peaks between December and February. Strong controls on precipitation in the interior over summer.

8. Tropical-Temperate Troughs Wave perturbations in westerlies (2 above) become linked with disturbances in tropical flow (7 above). Extends from the SE to the NW. Links tropics to middle latitudes and forms a channels for large-scale transfer of momentum, heat and moisture. Seen as a cloud band stretching from south of the country as far north as Namibia in which is contained convection squall lines (thunderstorms). Summer phenomenon.

9. Tropical Cyclones As opposed to mid-lattitude cyclones! Closed low pressure circulations that begin in the Indian Ocean, cross or round Madagascar and follow the Mozambique current. If they reach South Africa they may affect regions of KwaZulu and Mpumalanga; occur in summer months. Cause severe flooding and damage to regions they affect. Also known as hurricanes and typhoons. Below is pictured Tropical Cyclone Cela (15 Dec 2003) having travelled down the Mozambique channel. Image courtesy of NOAA.