1 MAURITIUS METEOROLOGICAL SERVICES CLIMATE NOVEMBER 2018 Introduction Climatologically speaking, November is a relatively dry month for Mauritius with a long term monthly mean rainfall of 78 mm. However, November 2018 was a very wet month and the mean monthly rainfall recorded islandwise was 194 mm which is 249 % of the normal. ENSO neutral condition persisted and IOD was neutral in the Indian Ocean. The Madden Julian Oscillation (MJO) was active over the equatorial Indian Ocean during the first week. It enhanced the formation of two tropical systems, Alcide and Bouchra which were named on 06 and 10 respectively. Alcide peaked to intense tropical cyclone stage and passed very close to the island of Agalega. The second storm, Bouchra evolved far over the eastern part of the cyclone basin and peaking to severe tropical storm. Thereafter, it weakened and the remnants of the system drifted in the easterlies and caused widespread rainfall over Mauritius. 1. Rainfall Fig. 2: Regional rainfall distribution (based on 23 stations) Fig. 1: (a) Observed rainfall (b) rainfall anomaly (mm) November 2018 turned out to be the second wettest November in the last 45 years. All regions received well above normal rainfall with some stations, over the Central Plateau, recording excessive rainfall of upto 270 mm. The rainfall was associated with several weather systems notably perturbation in the trades, cold fronts, instability zone and local convection (sea breeze). Further, clouds associated with remnants of Ex- Bouchra caused widespread rain over the island which were locally heavy and warranted a heavy rain warning on 27. The highest rainfall of mm was recorded at Valetta and the highest intensity occurred on 27 between 1600 to 1700 whereby 45.6 mm was recorded.
2 Surface Temperature (a) Cloud free weather on 01 (b) Sea Breeze effect on 06 (c) Perturbed weather on 15 (d) Remnants of Ex-Bouchra on 27 Fig 3: Weather systems during November Surface Temperature November 2018 is the sixth warmest November on record since 1969 (based on maximum temperature recorded at Plaisance) Fig. 4: Maximum temperature trend for November from (Plaisance).
3 November 2018 was very warm. The weather during the first few days was mainly sunny with a relatively dry atmosphere (Fig. 9 and Fig. 10). The monthly mean maximum temperature was close to above the seasonal mean over Mauritius (Fig. 6(a)). Moreover, during few days, northerly to north-north-westerly wind prevailed, thus causing higher temperature anomalies to the south-eastern and eastern part of the island. The monthly mean maximum temperature anomaly peaks at 2.6 C at Providence and Belle Mare. Many places recorded above 1 C in the mean while some places mainly to the east, north and central plateau had above 2 C on average. Moreover, some stations especially over the central plateau recorded daily maximum temperature 3 to 4 C above their normal for more than 10 days. For example, Providence recorded temperatures above 3 C for 14 days. On average, the western and part of the northern plain had a mean temperature anomaly less than 1 C. When the island experienced light wind conditions, convective clouds developments occurred in the in the western sector as well as part of the central plateau. (a) One station had a new record of extreme maximum temperature for the month of November notably at Union Park (MSIRI) with 30 C (previous 29.9 C) on the 16. Two other stations had a temperature equivalent to the previous record namely Bagatelle Dam 29.7 C and Mon Desert Mon Tresor 32 C. During the first week and by the end of the month, the drops in minimum temperature occurred due to a light wind prevailing and clear sky in the early morning. On the 06, a new record in minimum temperature was observed at Bagatelle Dam with a value of 17.4 C (previous 18.0 C). The other place, where record for extreme minimum temperature was set, was on the 27 at Mon Loisir Sugar Estate 19.8 C (previous 20.6 C). (b) Fig. 5: (a) Maximum (b) Minimum temperature distribution
4 Some stations had up to 24 warm days (maximum temperature anomaly (anomax) >2 C). Stations Highest Number of warm days. anomax ( C) Belle Mare Mon Desert MT ML Rouillard Providence Riche en Eau Queen Victoria Bois Cheri Union Park MSIRI N. Decouverte Mon Bois Mon Desert Alma Arnaud (a) Albion Grand Bassin Fig. 6: (a) Maximum (b) Minimum temperature (b) Fig. 7: Daily maximum temperature anomalies at Vacoas and Plaisance Fig. 8: Daily minimum temperature anomalies at Vacoas and Plaisance
5 3. Sunshine and Humidity 4. Winds Fig. 9: Daily Relative Humidity: Vacoas (blue) and Plaisance (red) Fig. 10: Daily sunshine hours: Vacoas (blue) and Plaisance (red) The relative humidity for November fluctuated considerably throughout the month and it reflected the variation in the weather systems that influenced the local weather. Three peaks for maximum relative humidity was observed on 6, 15 and 27 (Fig 9). These were mainly due to clouds associated with different weather system influencing the island. Overall, most of the time the relative humidity was close to above the normal. Among the few occasions when it was below, the least value was 70 % for Plaisance. Daily mean sunshine hours were above by more than 3 hours at Plaisance on four occasions as shown Fig 10. Generally, the daily sunshine hours showed considerable variations. Cloud cover during the passage of perturbed systems led to nearly nil sunshine hours on the 15 and 27 at Plaisance and Vacoas. 4. Winds Mostly easterly to east north easterly wind prevailed during the month. These could be accounted for by the persistence of anticyclones and their associated ridge of high pressure to the south of the Mascarene. There was very few occasions between the transition of the anticyclones when the wind was light. Fig. 11: Wind frequency at Plaisance
6 FORECAST FOR DECEMBER - JANUARY FEBRUARY (DJF) The period DJF represents the core summer season. Numerical models are expecting the warm anomaly over the equatorial Pacific Ocean that has been observed in October and November to strengthen further into a weak to moderate El-Nino event which is likely to peak in February In the Indian Ocean, the positive IOD will continue to transit towards neutral phase whereas SIOD will remain in a weak positive phase weakening gradually by end of forecast period. The Mascarene region will lie in a warm pool of SST for the period NDJ (Fig12). Positive SST anomalies in the South West Indian Ocean and around Diego Garcia are expected to persist. As such, the air mass within the Mascarene area especially between the eastern Malagasy coast and Mauritius is likely to remain moist and warm. Forecasts for Mauritius Fig. 12: SST anomaly in the south Indian Rainfall amount is expected to be slightly above normal for the period DJF (Fig. 12(a)). December rainfall will be slightly normal for the month (~200 mm). During January and February rainfall is also expected to be slightly above normal (~300mm and 375 mm respectively). Day time maximum temperature is likely to be above normal at most places (Fig. 12 (b)). The right of publication in print, electronic and any other form is reserved by the Meteorological Services. Any part of this document may be reproduced provided complete source is acknowledged or after authorization from: Director, Meteorological Services, St Paul; Road Vacoas, Tel: /32, Fax: , Fig. 12: SST anomaly in the south Indian