MAURITIUS METEOROLOGICAL SERVICES CLIMATE AUGUST 2018 Introduction August 2018 was dry and warm.. Synoptic weather pattern influencing the weather caused a dry air mass to prevail over the region. The moisture content in the atmosphere as measured by the relative humidity dropped to a record low of 35% at Plaisance. Occasional windy and cloudy weather was observed during the second fortnight only. ENSO and the Indian Ocean Dipole were observed to remain in neutral phase. 1. Rainfall Fig. 1: (a) Observed rainfall (b) rainfall anomaly (mm) Fig. 2: Regional rainfall distribution (based on 23 stations) August 2018 ranks as the driest in the last 45 years, i.e. since 1974 and the third driest on record, with only 34% of its normal rain recorded. The first half was extremely dry with only 1% of the monthly mean rainfall for the fortnight recorded. During the second half, weak perturbation in the trades and frontal system contributed to the rainfall and only passing showers were recorded to the East, South and over the Central Plateau. Well below normal rainfall was recorded over the whole island with deficient rainfall of up to 180 mm at specific site over the Central Plateau. To the North and West, rainfall was below the normal by 20-40 mm. Several stations, namely in the region of Magenta, Chebel, Le Bosquet and La Chaumiere recorded nil rainfall during this month.
Surface Temperature (a) Cloud free weather on 02 (b) Perturbed weather on 17 Fig 3: Weather systems during August 2018 (c) Perturbed weather on 28 2. Surface Temperature August 2018 is the warmest August on record since 1969 (based on maximum temperature recorded at Plaisance) Fig. 4: Maximum temperature trend for August from 1969-2018
August 2018 was very warm and the weather during the first half was mainly sunny during the day with a relatively dry atmosphere (Fig. 9 and Fig. 10). The monthly mean maximum temperature was above the seasonal mean by more than 1 C in most places over Mauritius (Fig. 6(a)). The maximum temperature anomaly for the month peaks at 3.3 C at Riche en Eau and most places recording in the range of 1 C to 3 C. Moreover this warmest August on record had a significant number of days where the recorded maximum temperature stayed over the monthly mean. In the region of Riche en Eau, Sauveterre, Providence and Mon Desert Mon Tresor, nearly all days during the month was warmer than usual with the maximum temperature above by more than 2 C. On 04, at several stations new records of extreme maximum temperature was recorded notably Mon Loisir Sugar Estate 28 C (previous 27.9 C), Queen Victoria 27.7 C (previous 27.5 C), Nouvelle Decouverte 27.6 C (previous 26.3 C), Vacoas 26.3 C (previous 26 C), Belle Rive 26.6 C (previous 26.3 C), Arnaud 25.8 C (previous 25.6 C), and Providence 27.3 C (previous 26.2 C). One day earlier, that is on 03, there was a new record of the extreme maximum temperature at Mon Desert Mon Tresor 29.5 C (previous 28 C) and Riche en Eau 28.5 C (previous 28 C). Few days later, on 06, Sauveterre recorded 31 C (previous 30 C) and on 10 Ferret (located few kilometres from Pamplemousses) recorded 30.5 C (previous 29.9 C). (a) For the extreme minimum temperature anomaly, at only four places new records were set up. During the first week of the month, it was observed at Gros Cailloux with a minimum of 15 C (previous 15.8 C). Other places, where record for extreme minimum temperature was set, were on the 10 at Mon Bois 10.8 C (previous 12 C), on the 12 at Mon Desert Mon Tresor 15.5 C (previous 16.9 C) and on the 18 at Mon Loisir Sugar Estate 15.6 C (previous 18.3 C). The drops in minimum temperature were due to a light wind prevailing and clear sky in the early morning. (b) Fig. 5: (a) Maximum (b) Minimum temperature distribution
Some stations had up to 29 warm days (maximum temperature anomaly (anomax) >2 C). Stations Highest Number of anomax ( C) warm days. Riche en Eau 3.3 28 Providence 3.1 26 Sauveterre 2.9 29 MonDesertMT 2.4 23 MonDesertAlma 2.3 16 N. Decouverte 2.2 15 UnionParkMSIRI 2.2 14 ML Rouillard 2.0 15 BoisCheri 2.0 14 Queen Victoria 1.9 15 Belle Mare 1.9 13 Mon Bois 1.9 13 Fuel 1.8 13 (a) Port Louis 1.5 11 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
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 mean monthly relative humidity was below normal at Plaisance for most of August and became close to the normal by the end of the month, mainly due to an influx of moist air. At Vacoas the relative humidity was observed to be below except for the few days when the island was under the influence of perturbation in the easterlies (Fig. 9). The observed relative humidity in the range of 50 to 60 % at Plaisance gives a good indication of the persistent drier air-mass which influenced the region for most of the time. Fig 9. Daily mean sunshine hours were above by 1.2 hours at Vacoas (8.0 hours) and by 1.3 hour (7.3 hours), as shown Fig 10. Most of the time, the daily sunshine hours were observed to be above the mean except for very few occasions. Cloud cover during the passage of a frontal system led to nil sunshine hours on the 17 at Plaisance. 4. Winds The wind regime for August 2018 was mainly from the eastern sector to south eastern sector. The anticyclones which transited to the south of the Mascarene Islands influenced the wind pattern when they emerged over the mid Indian Ocean and therefore most of the time, there was a ridge of high pressure which extended over our region. Also there was very few occasions when the wind was light. These were the periods when the minimum temperature anomaly plummeted, as shown in Figure 8. Fig. 11: Wind frequency at Plaisance
5. Sea Fig. 13: Waveheight distribution in the region of Blue Bay Fig. 12: Waverose based on wave rider observation in the region of Blue Bay For August 2018, the wave heights were of the order of 2 to 4 metres. 30% of the wave height recorded were on average 2 to 3 m and almost during 50% of the events, wave heights of 3m to 4m were recorded. On rare occasions, wave heights of the order of 5 meters were recorded. The state of the sea was dominated by waves mostly from the SE direction as shown in Fig 12. The synoptic pattern maintained mostly a southerly easterly windwave over the Mascarenes. There were few occasions when South South Westerly swells influenced the region. The sea surface temperature was slightly above 24.2 C at the start of the month and gradually decreased by half a degree by the end of the month (Fig 14). Between the 10 and 15, there was a gradual change in the synoptic weather systems influencing our region. This transition is well reflected in the SST by a sharp decline and then afterwards stayed within the range of 23.7 C to 24 C. 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 Fig. 14: Sea surface temperature in the region of Blue Bay