The Mascarene High and India’s Monsoon Explained

The Mascarene High is a semi-permanent area of high atmospheric pressure over the southern Indian Ocean, centred near Mauritius and the Mascarene Islands, and its strength helps set the pressure gradient and the winds that cross the equator to become India’s southwest monsoon. When this high is strong in the months before the rains, the south-easterly winds on its northern flank tend to be stronger, and a stronger flow towards India generally supports a more vigorous monsoon.

Most Indians never hear about this system, because it sits thousands of kilometres from the subcontinent in the southern hemisphere. Yet it is one of the invisible engines of the monsoon. Last updated: 2 November 2026. This explainer covers what the Mascarene High is, how it works, why its pressure matters for India, how it links to other monsoon features, and what scientists and forecasters do with it.

Key Takeaways

• The Mascarene High is a large, slow-moving zone of high surface pressure in the southern Indian Ocean, near the Mascarene Islands of Mauritius, Reunion and Rodrigues.
• It is strongest during the southern winter, exactly when India’s southwest monsoon is under way, so its pressure is in phase with the monsoon season.
• The pressure difference between the Mascarene High and the low-pressure heat zone over northwest India and Pakistan helps drive the cross-equatorial winds that carry moisture to India.
• A stronger Mascarene High tends to go with a stronger low-level flow and a more active monsoon, though this is a tendency and not a rule.
• The Mascarene High works with the Somali Jet, the monsoon trough and the Tibetan High, so no single feature controls the rains on its own.
• Monitoring pressure and winds over the southern Indian Ocean gives forecasters early clues about monsoon onset, strength and the Arabian Sea branch.

What is the Mascarene High?

The Mascarene High is a semi-permanent subtropical anticyclone, which means a large and persistent dome of descending air and high surface pressure, located in the southern Indian Ocean between about the latitudes of Madagascar and the open ocean to the east. It is one of the great subtropical highs that sit over each ocean basin, alongside the Azores High in the Atlantic and the Pacific highs.

The system is named after the Mascarene Islands, the volcanic archipelago of Mauritius, Reunion and Rodrigues that lies under or near its centre. It does not stay fixed. The high shifts a little north and south with the seasons, strengthens and weakens over weeks, and occasionally breaks into separate cells as cold air pushes up from the south. Meteorologists often measure its strength by averaging sea-level pressure over a box in the southern Indian Ocean and tracking how that value changes.

In any subtropical high, air sinks from high in the atmosphere, warms as it descends and spreads outward near the surface. That sinking air suppresses cloud formation, which is why the region under the Mascarene High is relatively dry and sunny, with the famous trade winds flowing steadily around its edges.

How does the Mascarene High form and move through the year?

The Mascarene High forms as part of the global Hadley circulation, in which warm air rises near the equator, flows towards the poles at height, and sinks in the subtropics at around 20 to 35 degrees latitude. Where that sinking air meets the ocean surface, pressure builds and a high-pressure cell takes shape, and over the southern Indian Ocean this cell is the Mascarene High.

Its yearly cycle is tied to the sun. In the southern summer, from December to February, the high weakens and moves a little further south, and the warm ocean to its north supports tropical cyclones in the southwest Indian Ocean. In the southern winter, from June to August, the surrounding land and ocean cool, the high strengthens and shifts slightly north, and its pressure reaches a seasonal peak.

That peak matters for India. At the very time when the Indian landmass is hottest and its surface pressure is lowest, the southern Indian Ocean is cool and its pressure is highest. This opposition sets up a large north-south pressure difference across the equator, which is the basic driver of the monsoon wind reversal.

How does the Mascarene High help create India’s southwest monsoon?

The Mascarene High helps create India’s southwest monsoon by pushing air towards the low-pressure zone over the Indian subcontinent, which sets up the south-easterly trade winds that cross the equator and become south-westerlies. Air moves from high pressure to low pressure, and the gap between the Mascarene High and the heat low over northwest India is one of the largest on the planet.

The steps run roughly as follows.

• Land heats up: By May, northwest India, Pakistan and the Thar Desert region bake, and surface pressure there falls.
• Southern ocean stays cool and high: The Mascarene High strengthens as the southern hemisphere enters winter.
• Trades flow north: South-easterly trade winds blow around the northern edge of the Mascarene High towards the equator.
• Crossing the equator: North of the equator the Coriolis effect, which bends moving air to the right in the northern hemisphere, deflects these winds into south-westerlies.
• Moisture pickup: Crossing the warm Arabian Sea, the winds absorb large amounts of water vapour.
• Rain over land: The moist air meets the Western Ghats, the monsoon trough and the heated land, rises and produces the monsoon rains.

Because the pressure gradient drives the strength of the flow, changes in the Mascarene High’s intensity or position affect how much moisture the monsoon winds can carry. This is why researchers include southern Indian Ocean pressure in studies of the monsoon, even though the rain itself falls far to the north.

Why does a stronger Mascarene High tend to mean a stronger monsoon?

A stronger Mascarene High tends to mean a stronger monsoon because higher pressure in the south steepens the cross-equatorial pressure gradient, which tends to strengthen the low-level winds blowing towards India. A stronger flow brings more moisture and more energy to the monsoon trough and the west coast.

Studies have found that the intensity of the Mascarene High in the months leading up to the monsoon, and during early summer, is statistically linked with how the rains perform over parts of India. A well-developed high in late spring is often read as a sign of a vigorous start to the season. However, the relationship is not simple, and scientists treat it as a tendency that works alongside other influences.

Several caveats apply.

• The link varies from decade to decade and between regions of India.
• Other drivers, such as the El Nino Southern Oscillation and the Indian Ocean Dipole, can strengthen or cancel the Mascarene signal.
• The position of the high, and not only its central pressure, changes how the winds approach the equator.
• Pressure patterns in the Australian region, where a separate high influences flow near Indonesia, also contribute to the cross-equatorial flow.

How is the Mascarene High connected to the Somali Jet?

The Mascarene High is connected to the Somali Jet because the jet is essentially the narrow, fast stretch of the same cross-equatorial flow that begins around the Mascarene High’s northern flank. After the south-easterly trades blow north, they bend westward along Madagascar and then funnel along the east African coast.

Near the Horn of Africa, the East African highlands, particularly the Ethiopian Highlands and the Kenyan mountains, steer and squeeze the flow into a strong low-level jet. This is the Somali Jet, sometimes called the Findlater Jet after the British pilot and meteorologist who documented it. It races across the Arabian Sea, reaching the Indian west coast as the monsoon westerlies.

The jet also whips up strong winds over the western Arabian Sea, driving coastal upwelling off Somalia and Oman that cools the sea surface and brings nutrient-rich water upwards. In this chain of cause and effect, the Mascarene High is a source region, the Somali Jet is the express lane, and the Arabian Sea branch of the monsoon is the delivery.

How does the Mascarene High compare with the Tibetan High and other monsoon features?

The Mascarene High is a low-level, southern hemisphere surface feature, while the Tibetan High is an upper-level anticyclone sitting over the Tibetan Plateau, and the two work together as parts of a single monsoon system. One feeds air in near the surface, and the other steers and vents it high in the atmosphere.

Together these pieces form a loop. Air rises over India and the Bay of Bengal, flows outward at height beneath the Tibetan High and the tropical easterly jet, sinks over the southern Indian Ocean in the Mascarene High, and returns at the surface as the trades. The Mascarene High is the sinking, returning limb of that overturning circulation.

What happens to the Mascarene High during El Nino, La Nina and the Indian Ocean Dipole?

During El Nino, La Nina and Indian Ocean Dipole events, the Mascarene High tends to shift in position or strength, and these shifts can reinforce or offset the climate signals acting on the monsoon. The details differ between events, so scientists describe the patterns as tendencies and not as fixed outcomes.

Warm and cool phases in the equatorial Pacific rearrange atmospheric pressure across the tropics through the Walker circulation. Pressure anomalies in the southern Indian Ocean can accompany these shifts, and in some years a displaced or weakened Mascarene High has gone along with a weakened cross-equatorial flow. The Indian Ocean Dipole, which is the east-west temperature difference across the tropical Indian Ocean, also modifies winds near the equator and can strengthen or weaken the monsoon flow independently of the Pacific.

This is one reason monsoon forecasting is hard. In a year with a developing El Nino, a favourable Mascarene High or a positive Indian Ocean Dipole can partly rescue the rains, while an unfavourable combination can deepen a deficit. Forecasters therefore examine a basket of indicators and not any single one.

Is climate change altering the Mascarene High?

Climate change may be altering the Mascarene High, with some studies suggesting a shift or strengthening of subtropical highs as the tropics expand and the oceans warm, but the picture is uncertain and still under research. Southern Indian Ocean pressure records are sparse compared with those over land, which makes trends difficult to pin down.

The southern Indian Ocean has been warming, and this affects the temperature gradient between ocean and land that drives the monsoon. Some modelling work points to changes in the subtropical highs of the southern hemisphere, including a southward drift of the circulation belts, while other work stresses large natural variability. For India, the practical concern is whether changes in the pressure gradient and the cross-equatorial flow will alter the timing, strength or reliability of the monsoon, an area where models still disagree in their details.

What can be said with confidence is that scientists are watching the region closely, because shifts in the Mascarene High could ripple through the monsoon, the Somali Jet and the Arabian Sea upwelling that supports fisheries.

How do forecasters use the Mascarene High in monsoon prediction?

Forecasters use the Mascarene High in monsoon prediction by tracking southern Indian Ocean pressure and the strength of the cross-equatorial winds as early indicators of onset, strength and the behaviour of the Arabian Sea branch. The India Meteorological Department (IMD) and research institutes such as the Indian Institute of Tropical Meteorology in Pune consider such large-scale features alongside ocean temperatures and satellite data.

In practice, a build-up of the Mascarene High and a strengthening of the cross-equatorial flow in May are signals watched before onset over Kerala. Weather models, including those run under the Monsoon Mission, simulate the full chain from southern Indian Ocean pressure to monsoon rain. Seasonal outlooks also draw on statistical links between earlier pressure patterns and the later rainfall.

The Mascarene High is therefore a piece of a larger puzzle. It gives context about the monsoon’s fuel supply but cannot predict daily rain, which depends on features such as the monsoon trough, low-pressure systems from the Bay of Bengal and the Madden-Julian Oscillation.

Bottom Line

The Mascarene High is the quiet partner of India’s monsoon, a dome of high pressure far south of the equator that helps drive the winds feeding the rains. Its strength, position and links to the Somali Jet and the Tibetan High make it a key part of how scientists understand the monsoon, even though it is only one of many ingredients.

Frequently Asked Questions

What is the Mascarene High in simple terms?

The Mascarene High is a large, persistent area of high pressure over the southern Indian Ocean, near Mauritius and Reunion. Air sinks within it, giving dry, sunny weather and steady trade winds. Its winds flow north and help form India’s southwest monsoon.

Why is it called the Mascarene High?

It is called the Mascarene High because it sits over the Mascarene Islands, which include Mauritius, Reunion and Rodrigues. Those islands are named after the Portuguese navigator Pedro Mascarenhas. Meteorologists use the name for the subtropical high in that part of the Indian Ocean.

How does the Mascarene High affect India’s monsoon?

The Mascarene High affects India’s monsoon by creating high pressure in the south, which, compared with the low pressure over northwest India, drives winds across the equator. Those winds turn south-westerly, pick up moisture over the Arabian Sea and bring rain. A stronger high tends to support a stronger flow.

Does a strong Mascarene High guarantee a good monsoon?

A strong Mascarene High does not guarantee a good monsoon, because it is only one influence among many. El Nino, the Indian Ocean Dipole, the Madden-Julian Oscillation and local conditions all matter. Scientists describe the Mascarene link as a tendency that varies between years and regions.

What is the difference between the Mascarene High and the Tibetan High?

The Mascarene High is a surface-level high-pressure system in the southern Indian Ocean, while the Tibetan High is an upper-atmosphere anticyclone over the Tibetan Plateau. The first supplies the low-level winds that cross the equator, and the second shapes the upper-level outflow and the tropical easterly jet. Both are parts of the monsoon circulation.

When is the Mascarene High strongest?

The Mascarene High is strongest during the southern hemisphere winter, roughly June to August, which overlaps with India’s summer monsoon. In the southern summer it weakens and shifts south. This seasonal rhythm keeps the pressure gradient across the equator strongest when India needs it most.

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