Seasonal Climate Forecast

MAURITIUS METEOROLOGICAL SERVICES

Winter 2021 - Seasonal Outlook

 

1.0 Introduction

The Winter 2021 Seasonal Outlook gives an insight of the expected evolution of the climate during the winter months in Mauritius and Rodrigues. The content of this outlook is meant to be used as general guidelines for planning purposes by stakeholders in various socio-economic sectors. Winter in Mauritius and Rodrigues has been defined as the period from May to October.

 

2.0 Background

A seasonal climate forecast aims to estimate the likelihood of a climatic event happening in the coming months. A seasonal forecast can provide information on how likely it is that the coming season will be wetter, drier, warmer or colder than normal. A seasonal climate forecast is not an attempt to forecast the detailed day-to-day evolution of weather.

Winter weather is often characterised by persistent trade winds emanating from anticyclones transiting the South Indian Ocean from west to east. Temperatures are cooler, especially during the months of June to September. May and October are generally considered as transition months.

The behaviour of meteorological and oceanic parameters such as El Nino Southern Oscillation (ENSO) and analogue years are the key factors used to generate the most likely scenario for the winter months. Global, regional and local predictors are analysed in the process of the preparation of this outlook.

 

3.0 The Past Summer

Summer 2020-2021 started with a Moderate La Nina prevailing in the Pacific Ocean and peaked in November. Afterwards, sea surface temperature in the Central Equatorial Pacific warmed up gradually towards a weak La Nina reaching neutral by April 2021. The Indian Ocean Dipole was neutral throughout summer 2020-2021.

Onset of summer rainfall was early and started as from the third week of November 2020. However, deficient rainfall was recorded for almost all summer months except December and April. Consequently, accumulative rainfall for the summer 2020-2021 was near the long-term mean.

The mean temperature was slightly above normal for December, February and March and near average for the remaining summer months.

The first named storm of the cyclone season 2020-2021 was in the second week of November 2020. Twelve named storms were observed in the South West Indian Ocean during this summer including Joshua and Marian which entered our area of responsibility from the Australian region. Out of these 12, 7 reached or exceeded the tropical cyclone intensity, 3 intensified into Severe Tropical Storms and 2 into Moderate Tropical Storms.

 

4.0 Conclusions

Based on both linear regression models and climate forecast from various climate models and considering that ENSO-neutral conditions will prevail during Southern Hemisphere winter, near normal sea surface temperature over the Mascarenes and a neutral Indian Ocean Dipole, the most likely outcome for winter 2021 is as follows:

 

(i) Observed trends of meteorological parameters in the region indicate that the onset of winter will be around the second week of May.

(ii) Normal winter rainfall (period 1981-2010) amounts to 668 mm. During winter 2021 rainfall over Mauritius isexpected to be slightly above normal for the first half of the season and slightly below for the second half. Thus, the cumulative rainfall this winter 2021 will be near normal, that is, around 630 mm. Most of the rains will be concentrated mainly over the Central Plateau, to the East and the South.

Normal rainfall is expected over Rodrigues which is around 400 mm.

(iii) Normal winter day temperature over the high grounds is around 22 and ranging from 26 degrees Celsius along the coasts. The night temperature is around 16 degrees Celsius over the central plateau and around 19 degrees Celsius along coastal regions.

This winter 2021, the mean day maximum and night minimum temperature will be slightly above normal by around 0.5 to 1.0 degree Celsius.

However, due to intra-seasonal variation, on certain occasions when the Mascarenes will be under the influence of strong anticyclones, the Islands may experience below normal day temperatures.

On a few occasions, especially due to clear sky and calm wind condition, it is likely that the minimum temperature may drop to near 11 degrees Celsius in places in Mauritius, especially end of August and September

At Rodrigues, minimum temperature may drop to near 14 degrees Celsius over the high grounds.

Table 1 below shows the forecast monthly average maximum and minimum temperatures in degrees Celsius for the different region of Mauritius and Table 2 gives the expected maximum and minimum temperature for Rodrigues.

 

Table 1: Expected monthly average maximum and minimum temperatures over Mauritius.

	May		Jun		Jul		Aug		Sept		Oct
	Max	Min	Max	Min	Max	Min	Max	Min	Max	Min	Max	Min
WEST	29	21	28	19	27	18	27	18	28	19	29	20
NORTH	29	19	27	18	26	17	27	17	28	18	29	19
EAST	27	20	26	19	25	18	25	18	26	18	27	19
SOUTH	28	21	26	20	25	19	25	18	26	19	27	20
CENTRE	25	19	24	17	23	16	23	16	24	17	25	17

Table 2: Expected monthly average maximum and minimum temperatures over Rodrigues.

	May		Jun		Jul		Aug		Sept		Oct
	Max	Min	Max	Min	Max	Min	Max	Min	Max	Min	Max	Min
PointeCanon	28	23	27	22	26	20	26	20	26	21	27	22
Plaine Corail	29	23	27	22	26	20	27	20	27	22	28	22

 

 

 

 

 

 

 

(iv) Inference to other parameters 

a) It is likely that the trade wind, mainly from the East South East, will blow at an average speed of 25 to 35 km/h.

Occasionally, particularly during the peak winter months i.e. July and August, the passage of strong anticyclones to the south of the Indian Ocean may cause strong pressure gradient over the Mascarenes, resulting in wind gusts to reach 90 km/h in the exposed areas.

b) On some occasions, the sea is likely to become very rough with heavy swells generated by the combined effect of travelling deep lows and strong anticyclones to the south of the Mascarenes. The breaking waves are likely to reach the southern and western shores of Mauritius and Rodrigues and may at times cause temporary inundation of low-lying areas.

c) In line with the findings of IPCC Fifth Assessment Report and the Special Report on Global Warming of 1.5^oC (SR15), extreme weather events including short duration high intensity rainfall, large variation in temperatures and windy conditions are very likely to occur occasionally over Mauritius and Rodrigues.

This report will be updated upon availability of fresh information.

 

Meteorological Services

St Paul Road, Vacoas

Date: 14 May 2021