Ocean Currents, Temperature and Salinity

Prerequisites: Oceanography — overview · Pressure Belts, Winds and Jet Streams

The Hook: A Warm Pacific and a Weak Monsoon

The Core Concept

Temperature of ocean water

The oceans are heated by the Sun, mostly in the top layer; they heat and cool far more slowly than land.

Factors affecting surface temperature: latitude, unequal distribution of land and water (Northern Hemisphere oceans are warmer because they touch more land), prevailing winds (offshore winds cause upwelling of cold water), and ocean currents.

Vertical structure (in tropical and middle latitudes):

At high latitudes the surface water is already cold, so there is little or no thermocline.

Salinity

Salinity is the total amount of dissolved salts in sea water, expressed in parts per thousand (‰) — grams of salt per 1,000 g of water. The average for the open ocean is about 35‰; most open-ocean values lie between 33‰ and 37‰ (NCERT). Sodium chloride is the most abundant salt.

Salinity extremes (NCERT): enclosed water bodies show the highest values — Lake Van in Türkiye (about 330‰), the Dead Sea (about 238‰) and the Great Salt Lake (about 220‰). Among seas, the land-locked Red Sea reaches about 41‰; the Mediterranean is more saline than the Atlantic (high evaporation); the Baltic Sea and the Black Sea have low salinity (large river inflow, cool climate, restricted connection to the ocean).

Ocean currents

Ocean currents are large, steady flows of water in a definite direction — “rivers in the sea”.

What drives them:

The general pattern:

• Large gyres turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
• In low and middle latitudes, warm currents flow along the east coasts of continents and cold currents along the west coasts.
• In the northern high latitudes, the pattern reverses: warm currents reach the west coasts (the North Atlantic Drift keeps Norwegian ports ice-free), and cold currents reach the east coasts (the Labrador Current off Canada).

Major currents

The North Indian Ocean is special: its currents reverse with the monsoon. In summer the south-west monsoon drift flows clockwise; in winter the north-east monsoon drift flows anticlockwise. The Somali Current off the Horn of Africa reverses direction seasonally.

The Sargasso Sea lies inside the North Atlantic gyre. It is the only “sea” without a coastline, known for floating Sargassum seaweed.

Effects of ocean currents

1. Climate: warm currents warm coastal air and bring rain; cold currents cool the air, stabilise it and cause fog and desert conditions. Cold currents border the Atacama (Humboldt), Namib (Benguela), Sahara coast (Canary) and western Australian deserts (West Australian).
2. Fishing: the richest fishing grounds lie where warm and cold currents meet (Grand Banks off Newfoundland: Gulf Stream + Labrador; off Japan: Kuroshio + Oyashio) or where upwelling brings nutrient-rich cold water to the surface (off Peru).
3. Navigation and ports: warm currents keep high-latitude ports ice-free (Norway; Murmansk in Russia benefits from the end of the North Atlantic Drift); cold currents carry icebergs (Labrador) and fog, a hazard to shipping.

Upwelling and ENSO

Along west coasts in the tropics, offshore winds push surface water away and cold, nutrient-rich water wells up from below. Off Peru, upwelling supports one of the world’s largest fisheries.

graph LR
    A[Normal year: strong trade winds] --> B[Warm water piles up near Indonesia]
    A --> C[Cold upwelling off Peru]
    D[El Nino: trade winds weaken] --> E[Warm water spreads east to Peru]
    E --> F[Upwelling and fish catch decline off Peru]
    E --> G[Rising air shifts east: drought risk in Australia, Indonesia, often weaker Indian monsoon]
    H[La Nina: trade winds strengthen] --> I[Cooler eastern Pacific, often good Indian monsoon]

Thermohaline circulation

Surface currents are wind-driven. The deep ocean is moved by density — set by temperature (thermo) and salinity (haline).

• In the North Atlantic (near Greenland) and around Antarctica, sea water becomes cold and salty (salt is left behind when sea ice forms), grows dense and sinks.
• It spreads through the deep ocean, slowly rises elsewhere, and returns as surface flow — the “global conveyor belt”, which takes centuries to complete a loop.
• The Atlantic branch, the Atlantic Meridional Overturning Circulation (AMOC), carries heat north and helps keep north-western Europe mild. The IPCC’s Sixth Assessment Report (2021) assessed that the AMOC is very likely to weaken over the 21st century as melting ice and warming make North Atlantic surface water lighter, though an abrupt collapse before 2100 was judged unlikely (medium confidence).

Memory Anchors

Cold current = Coast of desert: “Canary – Sahara, Benguela – Namib, Humboldt – Atacama, West Australian – deserts of Western Australia” → “Camels Brave Hot Wastes”.

Gyre direction: “North turns like a clock” — clockwise in the North, anticlockwise in the South (the same as the Coriolis deflection: right in the North).

Fishing grounds = Meeting points: Gulf Stream + Labrador = Grand Banks; Kuroshio + Oyashio = Japan — “Where warm meets cold, fish are sold.”

Salinity extremes: “Van Dead Salt” — Lake Van > Dead Sea > Great Salt Lake.

El Niño = Niño warms Peru — the boy brings warmth (and trouble for the monsoon); La Niña cools.

For Prelims

For Mains

• Dimensions: causes of currents (winds, Coriolis, density, coastline); pattern (gyres, east–west coast contrast); effects (climate, deserts, fisheries, navigation, ports); ocean–atmosphere coupling (ENSO, IOD, Walker circulation); climate change (warming, AMOC weakening, marine heatwaves, coral bleaching).
• Keywords: gyre, upwelling, thermocline, thermohaline circulation, Walker circulation, teleconnection, ENSO, Indian Ocean Dipole, AMOC, marine heatwave.
• Value-addition: IMD’s monsoon forecasts explicitly consider ENSO and IOD conditions; the 2023 El Niño and the below-normal monsoon that year illustrate the teleconnection, while the fact that not every El Niño year is a drought year shows the moderating role of the IOD and other factors.

Common Traps

In the News

• June 2023 — NOAA declared El Niño conditions in the tropical Pacific; India’s 2023 monsoon ended below normal (IMD).

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

• Oceanography — quick revision
• Oceanography — practice questions
• Pressure Belts, Winds and Jet Streams
• Indian Climate and the Monsoon
• Climate Change