MAURITIUS METEOROLOGICAL SERVICES CLIMATE SEPTEMBER 2018 Introduction Synoptic weather pattern over the region portrayed September as a transition month. The month started with wintry characteristics. By mid September, in the equatorial region, under the influence of MJO in the Indian Ocean, a tropical low evolving in the vicinity of Diego Garcia deepened into a tropical depression nearly reaching naming stage during the third week. Over Mauritius, the first fortnight was wet under the influence of clouds in the easterlies. In the second half, large scale dry conditions favored long hours of sunshine and low humidity. In addition, large scale climate indices such as ENSO and IOD remained neutral. However, a warm anomaly had started to develop in the equatorial Pacific Ocean which indicated an El-Nino event developing. The South Indian ocean Dipole (SIOD in subtropical regions) was positive with a persistent tongue of warm pool south of the Mascarene Islands. 1. Rainfall Fig. 1: (a) Observed rainfall (b) rainfall anomaly (mm) Fig. 2: Regional rainfall distribution (based on 23 stations) September 2018 recorded slightly below normal rainfall, amounting to 87 mm over the island. The first half was the wetter than the second half with 87 % of the total rainfall for the month. This rainfall was mainly due to frequent influences of easterly waves and instability lines. The remaining 13 % was recorded during the second half. Deficient rainfall of up to 90 mm was observed locally over the East and the South. In addition, the central part of the island recorded rainfall associated with sea breeze by the end of the month. Moderate rainfall was also recorded to the west on the last day of the month, namely Riviere Noire 39 mm, Beau Songes 34 mm Baie du Cap 46 mm and Tamarin 45 mm within 2 hours associated with local cloud developments. This explains the above normal rainfall observed locally to the west.
Surface Temperature (a) Perturbed weather on 03 (b) Perturbed weather on 07 Fig 3: Weather systems during September 2018 (c) Cloud free weather on 19 2. Surface Temperature September 2018 is the second warmest September on record since 1969 (based on maximum temperature recorded at Plaisance) Fig. 4: Maximum temperature trend for September from 1969-2018
September 2018 temperatures was mainly close to normal in the beginning of the month and was even below normal on few occasions due to cloudiness especially on the 03 when an easterly wave crossed the island. Even though East South-easterly trades persisted over the region (Fig 11), the second fortnight was mainly dry (Fig 9) leading to long hours of sunshine (Fig 10) over the island. These conditions contributed towards a warm atmosphere (Fig 7). Mean maximum temperature was above by 1.5 0 C over the windward slopes, the extreme north and west of the island (Fig 6(a)). Highest maximum temperature anomaly was recorded at Providence with 5.2 0 C which coincided with a new record of extreme maximum temperature of 27.6 0 C observed on 17. At the remaining stations, the maximum temperature anomaly range between 2.8 to 4.5 0 C, and these were significant. Record of extreme maximum temperature was also observed at the following stations: Queen Victoria recorded 28.4 0 C on 25 (previous 27.9 0 C) and at Mon Desert Mon Tresor 30.0 0 C on 22 (previous 28.5 0 C). Beau Songes recorded 29.7 0 C on 17 and at Domaine Les Pailles 30.7 0 C on 24 (previous 29.4 0 C and 30.5 0 C respectively). The highest number of warm days was observed at Riche-en-Eau and most of the stations over the island recorded over 10 warm days. (a) The mean minimum temperature (Fig 6(b), showed mainly normal to above normal anomaly except for very few regions on the high grounds and the western coast. A cold anomaly was observed in the region of Gros Cailloux and 8 cold days were noted at the station due to radiative cooling and land breezes. The lowest minimum observed was 14.2 0 C at Sans Souci on the first day of the month. (b) Fig. 5: (a) Maximum (b) Minimum temperature distribution
Some stations had up to 20 warm days (maximum temperature anomaly (anomax) >2 C). Stations Highest anomax ( C) Number of warm days. Riche En Eau 3.8 20 Providence 5.2 19 Union Park MSIRI 4.1 16 ML Rouillard 3.8 15 Queen Victoria 3.9 15 Case Noyale 4.5 15 Nouvelle Decouverte 3.9 15 Quatre Bornes 4.3 15 MDesert MTresor 4.4 13 Albion 2.8 12 (a) (b) Beau Songes 3.9 12 M Desert Alma 3.2 12 Fig. 6: (a) Maximum and (b) Minimum temperature anomaly Fig. 7: Daily maximum temperature anomalies at Vacoas and Plaisance Fig. 8: Daily minimum temperature anomalies at Vacoas and Plaisance
3. Sunshine and Humidity 4. Winds Fig. 9: Daily Relative Humidity: Vacoas (blue) and Plaisance (red) The mean monthly relative humidity was above normal at Vacoas and Plaisance at the beginning of the month and these were associated with moisture laden easterlies, especially on the 03. Afterwards, the relative humidity followed a variable trend reducing to normal and below normal at times. It reached its lowest value of 63 % at Plaisance on the 10 under the influence of drier trades. By end of September, it went back to normal. Fig. 10: Daily sunshine hours: Vacoas (blue) and Plaisance (red) During the first week of September, on two occasions the sunshine hours were close to zero due to clouds cover on the 03. On the 07, perturbations in the easterlies crossed the island. The second week witnessed variable cloud cover over Mauritius and during the second fortnight, sunshine was mainly above the normal. Form the 17 to 19, it was 3.2 hours above normal. 4. Winds The wind regime was mainly from the eastern to south eastern sectors. The anticyclones which transited to the south of the Mascarene Islands maintained the trade winds flow over our region. However, in between the passage of these anticyclones, there were few occasions when the wind was light. Fig. 11: Wind frequency at Plaisance
5. Sea Fig. 13: Wave height distribution in the region of Blue Bay Fig. 12: Waverose based on wave rider observation in the region of Blue Bay For the month of September, 60% of the wave height recorded were on average 2m to 3m and almost 30% of the events were 3m to 4m. On rare occasions, wave heights of the order of 4 to 5 meters were recorded. The state of the sea was dominated by waves mostly from the SE direction as shown in Fig 12, and this was favoured by the prevailing synoptic weather pattern. There were very few occasions when South South Westerly swells influenced the region. Rather normal sea surface temperatures were observed in the vicinity of Mauritius but warm waters prevailed to the south of the Mascarene. A marked warm pool with anomalies of over 1.5 C is quite discernible to the south of Madagascar. Fig. 14: September sea surface temperature anomaly in the Mascarene region.
FORECAST FOR OCTOBER-NOVEMBER-DECEMBER (OND) The period OND gives an insight of the beginning of the summer season. Most climate models are predicting a weak El-Nino to settle down towards the end of December. The Indian Ocean Dipole is expected to remain neutral during the forecast period. Fig. 15: OND anomaly over the south Indian Ocean (a) Pressure and (b) sea surface temperature Pressure anomaly chart (Fig.15 (a)) shows a mark positive anomaly north of the equator extending till latitude 10 degrees South while over the Mascarene region normal mean sea level pressure is expected. Positive SST anomalies in the South West Indian Ocean and around Diego Garcia are expected to prevail. As a result, the air mass within the Mascarene area, especially between the eastern Malagasy coast and Mauritius is likely to remain warm and moist. Rainfall amount is expected to be normal for the period OND (Fig. 16(a)). October rainfall will be slightly below the normal for the month (~60 mm). Normal rainfall is expected in November (~80 mm). During December rainfall is expected to be slightly above normal (~200mm). Day time temperature is likely to be above normal over most of the island. Warmer than normal conditions are expected to be particularly felt over the Central Plateau and to the East. (Fig. 16(b)). Fig. 16: OND forecast over Mauritius (a) Rainfall (b) Temperature 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: 6861031/32, Fax: 6861033, email: meteo@intnet.mu