MAURITIUS METEOROLOGICAL SERVICES

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1 MAURITIUS METEOROLOGICAL SERVICES CLIMATE JULY 2018 Introduction Winter weather was clearly felt during the month of July 2018 where about seven anitcyclones transisted to the South of the Mascarene region. The mean axis of anticyclones during the first half of the month was about 35 degrees south latitude and thereafter moved equatorward along 30 degrees south in latitude in the second half. A persistent windy and gusty weather was observed over the islands in our region with the sea state remaining rough beyond the reefs for most of the month. Neutral conditions for El Nino and the Indian ocean Dipole persisted since the end of summer Rainfall Fig. 1: (a) Observed rainfall (b) rainfall anomaly (mm) Fig. 2: Regional rainfall distribution (based on 23 stations) July 2018 was slightly wetter than normal. The rainfall was mainly due to perturbation in the easterlies and frontal systems. At the end of the second fortnight warm conditions and light winds caused localised afternoon showers to the North-East, East and over the Central Plateau. Heaviest rainfall was concentrated over the Central Plateau; for instance, a monthly rainfall of up to 278 mm was recorded in the region of Valetta. Excess rainfall was received over the extreme North, part of Central and East and over most of the South and Southwest. A very pertinent l distribution was observed to the south with neighboring localities having excess and deficient rainfall. For example, the region of St. Felix Bel Air had deficient rainfall of about 124mm and few kilometers away, in the region of Union Savanne, rainfall in excess of 117 mm was recorded. Highest 24 hours rainfall was recorded at Pradier (in the region of Bois Cherie) which amounted to 73 mm

2 Surface Temperature (a) Perturbed weather on 01 (b) Perturbed weather on 08 Fig 3: Weather systems during July 2018 (c) Frontal system on Surface Temperature July 2018 is amongst the ninth warmest July on record since 1969 (based on temperature recorded at Plaisance) Fig. 4: Maximum temperature trend for July from

3 July 2018 had several distinct temperature distributions. The weather during the first week was dominated by a rather warm air mass and caused the maximum temperature to be above the seasonal mean by more than 1 C. The station at Domiane les Pailles recorded temperature anomaly of 4.8 C while over the highgrounds at Mon Desert Alma and Providence the maximum temperature was more than 3 C. The record of the extreme maximum temperature was equaled during that period, on the 6 at Domaine Les Pailles 30.2 C. Moreover, new records of extreme maximum temperature was set up on the 5 at Mon Desert Mon Tresor 28.0 C (previous 27.0 C) and the record was equaled during the same month on the 31. After the first week, a frontal system crossed the region and caused cold trades to influence the region. The effect on the maximum temperature was that the observations were close to normal. A few days afterwards, a second frontal system approached the region and Mauritius was in its warm sector on the 16 and a change occurs overnight. The maximum temperature anomaly recorded at Arnaud showed a positive anomaly of 4.2 C on the 16 and a negative anomaly of -0.1 on the 17. After the 17 the temperature anomaly was on the cooler side with most stations recording maximum temperature anomaly of -2.0 C. The cold spell was of short duration and by the 22, the maximum temperature observed was close to the seasonal mean. The last three days of the month the temperature began to be on the warmer side with maximum anomaly peaking to more than 4 C at Providence and Sauveterre. The minimum temperature anomaly had a similar trend as the maximum temperature anomaly during the first week of the month with positive anomaly of more than 4 C at Digure, Case Noyale and Ferret on the 5. For the second and third weeks of the month, the minimum temperature anomaly was below over the whole island with stations recording below 4 C anomaly, Mon Desert Alma on the 11, Albion on the 12, Gros cailloux on the 13,20,22 and 23, Mon Desert Mon Tresor on the 15,16 and 18. Albion recorded a minimum temperature anomaly below 6 C on the 19. New records of extreme minimum temperature was set up on 10 at Bagatelle 12.6 C (previous 13.8) and on the 12 at Mon Loisir Sugar Estate 15.1 C (previous 17). (a) (a) Fig. 5: (a) Maximum (b) Minimum temperature distribution

4 Some stations had up to 21 warm days (maximum temperature anomaly (anomax) >2 C). Stations Highest anomax ( C) Number of warm days. Arnaud Medine ML Rouillard Mon Bois) MonDesertAlma MonDesertMT N. Decouverte Providence Quatre-Bornes Riche en Eau (a) (b) Sauveterre UnionParkMSIRI Fig. 6: (a) Maximum (b) Minimum temperature anomaly 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 mean monthly relative humidity was close to the mean at Vacoas for most of the July and became below by the end of the month with the influx of drier air. For Plaisance the relative humidity was observed to below except for the period 15 to 17 during which the region was under the influence of a warm sector of a frontal system. The observed relative in the range of 50 to 60 % at Plaisance gave a good indication of the persistent drier air mass originating from the anticyclones especially after the passage of the cold fronts.. Daily mean sunshine hours were slightly above by 0.1 hours at Vacoas (7.2 hours) and at Plaisance, it was slightly below by 0.3 hours (5.2 hours). Cloud cover during passage of frontal system led to nil sunshine hours for the period 7-8 and This is in concurrence to the drop in maximum temperatures shown in Fig. 7 and Fig Winds The predominantly Southerly wind regime for the month of July 2018 was influenced by the anticyclones transiting to the south of our region. The fourth anticyclone peaked to 1042hPa on 18 and during the period July 2018, the combined effect of an anticyclone to the South and a low pressure to the North of our region, caused a strong wind to blow over Mauritius. By the end of the month the effect of the anti cyclone decreased, the wind gradually turned towards South Eastward while weakening. Fig. 11: Wind frequency at Plaisance

6 Fig. 13: Waveheight distribution in the region of Blue Bay 5. Sea Fig. 12: Waverose in the region of Blue Bay The state of the sea was dominated by waves mostly from the SSW to SE direction, Fig 12. Moreover, wave with highest amplitude are found not to vary a significantly from the southern component. For July 2018, 10% of the wave height recorded by the wave rider were on average 2m to 3m and slightly more than 40% of the events were 3m to 4m, Fig 13. Moreover 4m to 5m wave height accounted to nearly 30% and about 15% wave heights which exceeded the 5 meters. The sea surface temperature was near 24.8 C at the start of the month and gradually decreased to reach 24.2 C by the end of the month Fig 14. Between 17 to 19, there was frontal system which crossed the region and is well reflected in the SST by a sharp decline.. Thereafter, there was an invasion of cold air which interacted with the ocean surface and kept the SST below 24.2 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: /32, Fax: , meteo@intnet.mu Fig. 14: Sea surface temperature in the region of Blue Bay