Indian Ocean - Climate

The climate in the countries of the Indian Ocean depends greatly on latitude and associated winds. The Indian Ocean reaches its northernmost point in the Arabian Sea on the shores of Pakistan and Iran just north of the Tropic of Cancer at about 250N. The ocean then stretches south past the Antarctic Circle to the frigid continent of Antarctica. There is very little land south of the 30°S parallel, in great contrast to the northern hemisphere. Therefore, there is nothing to break the impact of the westerly winds in that region and they are often referred to as the "roaring forties" and "screeching fifties". The few islands in the southern Indian Ocean, such as Heard Island, Ile Amsterdam, Iles Kerguelen. and Prince Edward Islands. have very cold and inhospitable climates, and few if any inhabitants.

The area centered about 35°S (in January) and 30°S (in July), known as the horse latitudes, is characterized by warm dry air and high pressure, resulting in mild, fair weather and light variable winds year-round. However, there is no land in these latitudes to take advantage of this pleasant weather.

The next climatic zone is the doldrum belt, or the intertropical convergence zone (ICZ), a somewhat ill-defined area of calm winds, ascendihg air, and low pressure. This zone shifts latitudinally with the sun and is most distinct from December through February or March when it is centered about 10°S in the eastern part of the ocean and between 15°S and 20°S in the west. January is often a month of heavy rain due to the convergence in the doldrums of the northeast monsoon air current and the southeast trade winds, which bring moist and unstable air. It is near the ICZ that weather disturbances grow into tropical storms which threaten Madagascar, the Mascarene Islands, and the southeast coast of Africa.

When the ICZ shifts to the north from May to September, southeast trade winds become dominant between 300S and the equator. These are steady winds at about 10 to 15 knots and they bring mild weather with partly cloudy skies. The trades bring about 75 mm of rainfall to Reunion in July and 70 mm to Rodrigues.

North of the equator, the major climatic influence is the monsoon system. The Arabian Sea and the Bay of Bengal are strongly affected by this system. The northeast monsoon originates in Asia and it is very dry from November to March while it passes over India and Pakistan and the Arabian Sea. However, it picks up moisture across the Bay of Bengal and brings substantial precipitation to northeastern Sri Lanka in November and December. The southwest monsoon, on the other hand, starts in the Indian Ocean at the equator and travels over water for great distances beginning in May or June. It brings more than 360 mm of rain to the windward slopes of Sri Lanka in June. Daytime temperatures at low elevations in the monsoon region are usually in the low 30°s while nightime temperatures range from 24°C to 27°C. The southwest monsoon usually lasts through September.

There are two principal areas where tropical cyclones originate. The first is in the Bay of 3engal where cyclones tend to start before and after the southwest monsoon as the ICZ migrates through the area. Typically, about five tropical storms (wind speeds of 34 to 63 knots) originate in the Bay of Bengal per year, one or two of which mature into cyclones (wind speeds greater than 63 knots). These storms are particularly dangerous when they touch land on the densely populated north coast of the Bay. Flooding from the torrential rains causes the most damage.

The second area which gives rise to cyclones is the central Indian Ocean east of Madagascar. These storms generally move toward the southwest and then later change course to the south and southeast. About eleven tropical storms a year form in this 3rea, with about four developing into cyclones. Mauritius, Reunion, and the east coast of Madagascar are in a high risk cyclone area while the Comoro Islands and the west coast of Madagascar are in a moderate risk area. Most of the islands of the Seychelles and Maldives are rarely threatened by the tropical storms.

The Indian Ocean Dipole mode (IOD) is the leading mode of interannual variability of sea surface temperature (SST) in the tropical Indian Ocean during the boreal summer-autumn seasons. The positive phase of the IOD (+ve IOD) is characterised by negative SST anomalies to the west of Java and Sumatra and positive SST anomalies in the tropical central-western Indian Ocean. Opposite signed SST conditions are found during the negative phase of the IOD (-ve IOD). The IOD typically develops during boreal summer, peaks in autumn, and then rapidly decays in November and December when Australian summer monsoon starts. The IOD significantly affects the climate of the Indian Ocean rim countries such as eastern Africa, India and Indonesia and remotely influences the climate of southern Australia and north eastern Asia.

Development of the IOD is often linked with the El Niño-Southern Oscillation (ENSO) in the Pacific because of variations of the Walker Circulation, which has been explored by a number of studies. In general, the +ve IOD tends to occur with El Niño, and the -ve IOD with La Niña, although exceptions exist. In the latter half of 2016 Indonesia and Australia experienced extreme wet conditions and East Africa suffered devastating drought, which have largely been attributed to the occurrence of strong negative Indian Ocean Dipole (IOD) and weak La Niña. The long-term trend since around the year 1960 in sea surface and subsurface temperatures, which is characterised by warming of the tropical Indian and western Pacific and cooling in the equatorial eastern Pacific, contributed positively to the extraordinary strength of this IOD. The strong negative IOD was a key promoter of the weak La Niña of 2016.

Extreme weather conditions saw a 2019 drought end in one of the wettest rainy seasons in four decades in parts of East Africa, with mass floods killing hundreds. The year 2019 witnessed eight cyclones, the most in a single year since 1976. The Indian Ocean Dipole which has conversely led to extreme drought in Australia. Colder-than-average water in the eastern Indian Ocean and the islands of Southeast Asia — the “Maritime Continent” — had the effect of suppressing convection, meaning there is less rising air, cloud formation, and rain than usual. This contributed to the devastating fire conditions Australia experienced in late 2019 and early 2020.

Billions of locusts swarming through East Africa are the result of extreme weather swings and could prove catastrophic for a region still reeling from drought and deadly floods. Dense clouds of the ravenous insects have spread from Ethiopia and Somalia into Kenya, in the region's worse infestation in decades. The locust invasion is the biggest in Ethiopia and Somalia in 25 years, and the biggest in Kenya in 70 years. There have been six major desert locust plagues in the 1900s, the last of which was in 1987-89. The last major upsurge was in 2003-05.