The Mascarenes islands consist of Mauritius, Reunion and Rodrigues and are found in the tropical belt along the latitude 20 degrees South, near the Tropic of Capricorn. Being rather small islands and surrounded by vast ocean they have a common mild tropical maritime climate throughout the year. The islands have only two seasons, summer from November to April and Winter months from June to October. October and May are transition months between the two seasons.
During summer the ITCZ shift to around 05 degrees south of the equator and this usually mark the onset the cyclonic activities in the South West Indian Ocean together with increased convective activities in the region. With the sun shining vertically along the Tropic of Capricorn, the SST rise to an optimal tempetarure where low pressure system starts to develop along the ITCZ. These low pressures usually gain strength under favorable conditions and develop into tropical cyclones. Most tropical cyclones develop around 10 degrees south in latitude and moves in a general South westerly direction before curving to south easterly below 20 degrees south in latitude. On these trajectories tropical cyclones represent a potential threat to the Mascarenes islands. The cyclones may pass close or even over the islands causing cyclonic conditions where gust exceeds 120 km/h and accompanied with heavy rainfall. This also causes storm surges along the coastal regions with very rough seas. A cyclone may affect the weather for around 5 days.
In summer, the anticyclones recede further to the south of the Mascarenes islands. This cause weakening of trades winds over the islands as the pressure gradient decreases and isobars are further apart. At certain period, winds from northern Indian Ocean start to blow over the Mascarenes islands. These northerly light winds cross the warm equatorial seas and brings very hot, humid and uncomfortable weather and the islands are likely to experience heat waves during that time. The warmest months are January and February with and maximum temperature may reach up to 34 degrees Celsius. The winds usually become light and variable at times and during the hot summer days these give rise to formation sea breeze with localized thunderstorms in the afternoon especially over Mauritius and Reunion islands.
With the rise of convective activities, there are lots of formations of thunderstorms clouds over the warm southern Indian Ocean. These clouds cluster together and forms larger scale systems such as instability lines and waves. As these systems usually moves along a pressure gradient, they at times cross over the islands. They bring along moderate to heavy thundery showers and cause flooding. Most of the annual rainfalls are registered during these events.
Under light wind conditions during most of the summer months, the sea remains mostly calm in the lagoons and slight to moderate beyond the reefs. But during adverse weather conditions such as thunderstorms the sea may become locally rough. The presence of low pressure systems in the vicinity of the Mascarenes islands may also affect the sea state by propagating swells and thus causing rough seas.
As the thermal equator shift to the northern hemisphere, the anticyclones also move closer to the south of the Mascarenes islands and are much stronger during winter in the southern hemisphere. The weather in winter is mostly dominated by weekly alternating sub tropical lows and strong anticyclones that moves from west to east just below the tropic of Capricorn. The presence of the anticyclones near the Mascarenes islands causes the pressure gradient to increase and isobars get closer. These result in moderate south easterly trades wind to blows over the islands throughout the winter.
The beginning of an anticyclone is characterised by a frontal system which is associated with the sub tropical low just before the anticyclone. The frontal system usually cross through the Mascarenes islands and may even reach the islands on st Brandon near 16 degrees south in latitude. The passage of the front brings slight to moderate rain and drizzle over the islands. Just after the front there is a significant drop in temperature below the normal as cold trades associated with the anticyclone start to blow over the islands. The winter minimum temperature may reach 10 to 12 degrees Celsius. These cold trades comes from the polar regions and comprise mostly of stratified clouds. The winds also strengthen and may sometimes reach 80 km/h for strong anticyclones. As the anticyclone continue to move to the east, the wind initially from the south westerly turns to south easterly and eventually East south easterly. Wave like disturbance in the trades cross the islands from east to west bringing slight passing showers and drizzles.
As the anticyclone moves further to the east, a ridge of high pressure extends over the Mascarenes islands up to the northern part Madagascar. This usually mark the end of the anticyclone. The ridge cause sunny spells with clear sky over the islands with light wind and milder temperatures.
During the passage of an anticyclone, the sea become rough to very rough with the strengthening of the trade winds. The presence of sub tropical lows generates heavy swells and these swells are pushed to the Mascarenes islands by the anticyclones. As these swells cross the islands sea becomes very rough and wave height may reach 7 metres or more.
Global climatic processes such as ENSO, IOD and MJO affect the weather patterns in the Indian Ocean and eventually that of the Mascarenes islands.