Monsoons, Ocean Currents, and Climate Circulation

A monsoon is a seasonal reversal of the prevailing winds that brings a distinct wet season and a dry season, and ocean currents are large, continuous movements of seawater that carry heat across the globe; together with the atmospheric circulation, they shape regional climates. In brief: in the South Asian monsoon, summer heating of the land creates low pressure, so moist winds blow from the ocean to the land and bring heavy rain from June to September, while in winter the wind reverses and brings dry air. Warm currents such as the Gulf Stream carry heat toward the poles and make places like northwestern Europe milder, while cold currents cool coasts and often create deserts and fog.

These systems affect the water supply, farming and weather of billions of people. This guide explains how monsoons form, how ocean currents are driven and how they influence climate, compares warm and cold currents in a table, works through calculations, outlines the history of their study and corrects common mistakes.

How does a monsoon work?

The word monsoon comes from an Arabic word meaning “season.” Monsoons occur where the land and sea heat and cool at different rates and where the belt of tropical rain (the ITCZ) moves with the seasons. The best-known is the South Asian monsoon.

• Summer (June to September): the land in India and Central Asia heats up quickly, so air rises and creates a low pressure area. The ITCZ moves north, and humid air from the Indian Ocean is drawn in as the southwest monsoon. It rises over the land and the Western Ghats and Himalaya, cools and releases heavy rain. India receives roughly 75% of its annual rain in these months, and agriculture, especially rice, depends on it. Delays or failures of the monsoon cause droughts, and very heavy rain can cause floods.
• Winter (December to February): the land cools and develops high pressure, so dry air flows out toward the warmer ocean as the northeast monsoon, giving a dry season for most of India.

Monsoon systems also affect West Africa, Australia and northern South America, and East Asia. The strength of the monsoon varies from year to year and is influenced by El Niño, the temperature of the Indian Ocean and snow cover on the Eurasian continent.

What drives ocean currents?

Surface currents are driven mostly by the prevailing winds, with the Coriolis effect and the shapes of the continents guiding the flow. In each ocean basin, they form circular patterns called gyres, which turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Important currents include the Gulf Stream and its continuation, the North Atlantic Drift, the Kuroshio in the Pacific, the Brazil Current and the East Australian Current (all warm), and the California, Humboldt (Peru), Benguela and Canary currents (all cold). Warm currents flow along the western edges of oceans toward the poles, and cold currents flow along the eastern edges toward the equator. Where winds push surface water away from a coast, cold, nutrient-rich water rises from below in upwelling, which supports some of the richest fisheries, as off Peru and Namibia.

How do currents affect climate?

Water has a very high heat capacity. The top 3 m or so of the ocean holds as much heat as the whole atmosphere above it, so the oceans store and move enormous quantities of energy. Warm currents bring heat to higher latitudes: the North Atlantic Drift helps to keep the coasts of Norway and the British Isles ice-free and milder than other places at the same latitude, such as Labrador in Canada. Cold currents cool the air above, reduce evaporation and so reduce rainfall, which contributes to coastal deserts such as the Atacama (Humboldt Current) and the Namib (Benguela Current), and they can produce coastal fog, as in California. Currents also affect the formation of hurricanes and the routes of ships.

What is the thermohaline circulation?

Deep ocean water circulates because of differences in density, which depends on temperature (thermo) and salinity (haline). In the North Atlantic, salty, cold surface water becomes dense and sinks, and flows south at depth. It is replaced by warm surface water drawn from the south, which is part of the Atlantic Meridional Overturning Circulation (AMOC). Together with sinking near Antarctica and upwelling elsewhere, it forms a global “conveyor belt” that takes hundreds to a thousand years to complete a circuit. Scientists are studying whether melting ice and changes in salinity could weaken the AMOC, with possible effects on the climate of the North Atlantic region, and how likely such a change is remains under investigation.

Warm and cold currents compared

Worked example: rainfall, heat and flow

1. Monsoon share of rainfall: a station receives 1,200 mm per year, of which 900 mm falls between June and September. The share is 900 ÷ 1,200 = 75%, and the average in those four months is 900 ÷ 4 = 225 mm per month, compared with (1,200 − 900) ÷ 8 = 37.5 mm per month in the other eight months.
2. A weak monsoon: if the monsoon rainfall is 15% below normal, it is 900 × 0.85 = 765 mm, a shortfall of 135 mm, and crops can suffer.
3. Ocean flow: ocean currents are measured in sverdrups (1 Sv = 1,000,000 m³ per second). The Gulf Stream in the Florida Straits carries about 30 Sv, while the Amazon River, the largest river, discharges about 0.2 Sv. The Gulf Stream is therefore about 30 ÷ 0.2 = 150 times larger.
4. Temperature comparison: London (51.5° N) has a January mean of about 5 °C, while Goose Bay, Labrador (53° N), at a similar latitude, has a January mean of about −15 °C. The difference of about 20 °C is due largely to the warm Atlantic water and westerly winds, compared with the cold Labrador Current and continental air.
5. Heat capacity: the specific heat of water is 4,186 J/kg·K and that of air is about 1,005 J/kg·K, so per kilogram, water stores about 4,186 ÷ 1,005 = 4.2 times as much heat for the same temperature change. Since water is about 800 times denser than air, a given volume of water stores roughly 3,500 times as much heat as the same volume of air.
6. Conveyor belt time: suppose a deep water mass travels around a global circuit of about 20,000 km (20,000,000 m) at an average speed of 1 mm per second (0.001 m/s). The time is 20,000,000 ÷ 0.001 = 20,000,000,000 seconds. With about 31,500,000 seconds in a year, that is roughly 630 years, consistent with estimates of several hundred years to a thousand years.

How have these systems been studied?

Sailors have used monsoon winds for trade across the Indian Ocean for over two thousand years. In 1770, Benjamin Franklin charted the Gulf Stream from reports of ship captains. In the 19th century, Matthew Fontaine Maury charted winds and currents, and the Challenger expedition (1872–1876) measured the temperature and depths of the oceans. Henry Stommel in the 1940s and 1950s explained why currents are strongest on the western sides of oceans, and Wallace Broecker popularized the “conveyor belt” idea in the 1980s. Today, a network of floats (Argo), satellites and moored instruments measure temperature, salinity and currents, and models are used to forecast monsoons.

Which mistakes are common?

Mistake 1: Thinking of the monsoon as just heavy rain

A monsoon is a seasonal wind reversal. The wet season is its summer phase, and the dry season is the winter phase.

Mistake 2: Assuming that warm currents are always on the east of continents

Warm currents flow along the western edges of oceans, which are the east coasts of continents, such as the Gulf Stream, and cold ones along the eastern edges of oceans, off the west coasts of continents.

Mistake 3: Crediting the Gulf Stream alone for Europe’s mild climate

The westerly winds, which carry the oceanic heat inland, and the shape of the continents also play major roles.

Mistake 4: Confusing surface currents with the deep circulation

Surface currents are driven by winds. The deep circulation is driven by differences in density and is much slower.