Surface currents and their distribution

Ocean currents are persistent, directed movements of seawater. They do more than move water locally: by carrying heat, dissolved substances and organisms, they connect places.

Winds drive most surface currents, with friction transferring energy from moving air to the sea surface. Earth's rotation causes an apparent deflection of movement, known as the Coriolis effect. It turns moving water to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Continents redirect these currents, helping to form large circulating systems called gyres. The major subtropical gyres turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.

Energy transfers

The oceans absorb large amounts of solar energy, especially in low latitudes. As currents redistribute that energy, they reduce the temperature contrast between the tropics and the poles.

Direct heat exchange and evaporation pass heat from the ocean to the atmosphere. Water vapour then releases latent heat when it condenses. Warm currents therefore help warm and moisten the air above them. Cold currents cool the lower atmosphere and can reduce convection. A coast's climate depends on both the current and the winds that carry air towards or away from land.

Nutrient transfers

Upwelling is the rise of deeper water towards the surface. Along some coasts, winds and the Coriolis effect cause a net movement of surface water away from the shore. Deeper water rises to take its place.

As sinking organic material decomposes, it releases nutrients into deep water. Upwelling carries those nutrients into the sunlit surface layer, where phytoplankton can use them. Greater primary production supports zooplankton, fish and larger predators, helping explain the productive fishing grounds off Peru and Namibia. Surface waters also diverge near the equator, where upwelling occurs too.

Downwelling is the sinking of surface water. It carries oxygen and dissolved substances into deeper layers, but usually doesn't replenish surface nutrients as upwelling does.

Oceanic conveyor belts

Thermohaline circulation is ocean circulation driven by differences in water density associated with temperature and salinity. Cooling increases density. Increasing salinity does too. At high latitudes, sea-ice formation leaves much of the salt behind in the surrounding water, helping make that water denser.

The conveyor system redistributes heat and ventilates the deep ocean. It also transports nutrients and carbon between surface and deep reservoirs. Its deep pathways operate over centuries to millennia, rather than the much shorter timescales of many surface currents. Changes in cooling or freshwater input can alter density and deep-water formation, linking atmospheric change to ocean circulation.

The coupled Pacific system

El Niño–Southern Oscillation (ENSO) is an irregular cycle linking changes in tropical Pacific sea-surface temperatures with changes in winds and atmospheric pressure. El Niño is the warm phase; La Niña is the cool phase. The Southern Oscillation refers to the associated atmospheric pressure changes across the tropical Pacific.

In neutral conditions, easterly trade winds push surface water westwards. Warm water builds up near Indonesia and northern Australia. Here, rising moist air encourages heavy rainfall. In the eastern equatorial Pacific, including near Peru, cooler water rises to the surface. The thermocline is the layer where temperature falls rapidly with depth. It lies deeper in the western Pacific and shallower in the east.

Above this temperature contrast, the atmosphere follows the Walker circulation. Air rises over the warm western Pacific and moves eastwards at altitude. It then sinks over the cooler eastern Pacific before returning westwards near the surface.

El Niño and La Niña

During El Niño, the trade winds weaken, allowing warm water to spread eastwards. As the thermocline deepens in the eastern Pacific, upwelled water becomes warmer and supplies fewer nutrients. Rising air and rainfall shift towards the central and eastern Pacific. The reduced east–west temperature differences can weaken the trade winds further, reinforcing the change.

ENSO events usually last for many months. They recur irregularly, often at intervals of roughly two to seven years, and La Niña does not necessarily follow every El Niño. Changes in tropical heating also alter atmospheric circulation beyond the Pacific. These distant connections are called teleconnections.

Climatic, environmental and economic effects

The effects depend on the event, season and location. A rainfall tendency doesn't predict that every place in a region will experience the same conditions.

Follow the connections: changing winds alter ocean conditions. Those changes affect rainfall or nutrient supply, with consequences for ecosystems and livelihoods. Economic outcomes depend on exposure as well as the physical event. Heavy rainfall causes greater disruption where settlements, farms or transport routes occupy flood-prone land.

Detailed examples

The class chooses its own detailed examples of both El Niño and La Niña. For each example, locate and date the event, then connect the atmospheric and oceanic changes to particular places. Support the climatic, environmental and economic consequences with evidence.

Peru in 1997–1998 provides an illustrative El Niño example. Warming in the eastern Pacific reduced the usual supply of cool, nutrient-rich water and disrupted the anchovy fishery. Parts of the normally dry coast received heavy rainfall, causing flooding and erosion that damaged roads, homes and farmland. Livelihoods were affected by both marine change and exceptional rainfall during the same event.

For La Niña, an illustrative example is Queensland, Australia, in 2010–2011. Alongside other weather influences, La Niña contributed to unusually wet conditions. Flooding affected Brisbane and extensive inland areas. It inundated farmland and disrupted transport and coal production. Erosion and sediment transfer into rivers and coastal waters were among the environmental effects. Unlike Peru's El Niño impacts, these losses centred on the enhanced western Pacific rainfall pattern.

Formation and energy supply

A hurricane is a tropical cyclone in the North Atlantic or northeastern Pacific with sustained winds of at least 119 km/h. In the northwestern Pacific, equivalent storms are called typhoons; in several other ocean basins, they are called tropical cyclones.

Several conditions must come together for a hurricane to form:

• Warm ocean water, generally around 26.5°C or above, extending deep enough to sustain the supply of heat and moisture.
• Moist air and an atmosphere that allows strong upward movement.
• An initial low-pressure disturbance around which air can converge.
• Sufficient Coriolis effect to organize rotation, normally at least about 5° from the equator.
• Low vertical wind shear, so changes in wind speed or direction with height do not disrupt the developing circulation.

The ocean supplies the storm's energy. Warm water alone, though, does not guarantee formation. Over land, hurricanes weaken as their moisture supply is cut off and friction increases. Cooler water can also weaken them, as can stronger wind shear that disrupts their structure.

Distribution

Hurricanes develop over tropical oceans, but rarely directly on the equator: the Coriolis effect there is too weak to organize their rotation. Important formation regions include the tropical North Atlantic and Caribbean Sea, as well as the northeastern Pacific. Equivalent tropical cyclones occur in the western Pacific and Indian Ocean.

Activity is seasonal. It tends to peak when ocean temperatures are highest, often after the period of strongest solar heating. Tropical easterly winds commonly carry storms westwards at first. Some later curve polewards and eastwards when they encounter different steering winds. These tracks expose particular island groups and continental coastal margins, rather than all coasts equally.

Physical impacts on coastal margins

A storm surge is an abnormal rise in sea level generated by a storm, above the expected astronomical tide. Strong winds push water towards the coast. Low atmospheric pressure also raises the sea surface, but the wind-driven contribution is generally larger.

Storm surge and waves erode beaches and dunes. They also damage buildings and ports, and move sediment. Saltwater can contaminate freshwater supplies and agricultural soils. Strong winds damage vegetation and structures. Heavy rainfall produces river flooding and, on steep coastal terrain, landslides. Coastal flooding can therefore combine water arriving from the sea with runoff from inland.

Hurricane case study

The class chooses one hurricane case study. Start by establishing the storm's date, location and track. Then explain how its physical characteristics interacted with coastal conditions to affect places and people. Use evidence to distinguish wind damage from surge and rainfall impacts, rather than treating all damage as a single process.

One illustrative case is Hurricane Maria in Puerto Rico, September 2017. Maria made landfall on 20 September as a Category 4 hurricane, with sustained winds of about 250 km/h. Destructive winds damaged homes and electricity infrastructure. Surge affected exposed coastal locations, while intense rainfall caused flooding and landslides.

Damage to power, water and transport systems prolonged the consequences beyond the storm itself. Communities lost access to safe water and reliable communication; medical services were also disrupted. The prolonged electricity failure shows why wind speed alone cannot explain the impacts on people. Damaged infrastructure linked the initial physical hazard to longer-lasting hardship.

Store, sink and source

A carbon store is a reservoir that holds carbon. In the oceans, most carbon is stored as dissolved inorganic carbon. Additional carbon is held in organisms, organic matter and seabed sediments.

A carbon sink absorbs more carbon than it releases over a given period; a carbon source releases more than it absorbs. Even a large store can act as either a sink or a source. The amount of carbon held is a different idea from the direction of transfer.

Carbon dioxide moves between the atmosphere and surface seawater, with the direction of exchange depending on the difference in CO₂ availability across the air–sea boundary. Cold water can dissolve more CO₂ than warm water. As water cools and deep water forms, dissolved carbon is transferred into the ocean interior. This is the solubility pump.

During photosynthesis, phytoplankton take up dissolved CO₂. Some of the organic material they produce sinks when organisms die or release waste. Most of this material decomposes, returning carbon to the water; some reaches deeper water or sediments. This transfer is the biological pump. Deep circulation can keep carbon away from the atmosphere for centuries before the water returns to the surface.

Why the ocean's role changes

As atmospheric CO₂ rises, the ocean has a greater tendency to absorb it. The global ocean currently acts as a net sink for human-produced CO₂ and slows its accumulation in the atmosphere. Uptake isn't the same everywhere: cold waters often favour absorption, while some upwelling regions release CO₂-rich deep water to the atmosphere.

Warming reduces the solubility of CO₂ in ocean water. Stronger stratification can restrict exchange between surface and deep water, changing nutrient supply and biological uptake as well as carbon transport. Changes in winds, upwelling and deep-water formation also affect where carbon is absorbed, stored or released. These processes can weaken uptake without necessarily turning the whole ocean into a net source.

Acidification and coral reefs

Ocean acidification is a decrease in seawater pH, largely caused by the uptake of atmospheric CO₂. Seawater can become less alkaline without becoming acidic overall.

When dissolved CO₂ reacts with water, it forms carbonic acid. The acid dissociates, increasing hydrogen-ion availability. Hydrogen ions then react with carbonate ions, leaving less carbonate available to organisms that construct calcium carbonate structures. Additional CO₂ therefore changes seawater chemistry even as the ocean helps limit atmospheric CO₂ growth.

Weaker reef structures offer marine organisms less complex habitat and may support smaller fish populations. As reefs deteriorate, they can also absorb less wave energy, leaving coastal places more exposed. Fishing and tourism livelihoods can be affected too.

Acidification is distinct from coral bleaching: the loss of corals' symbiotic algae or their pigments, commonly associated with heat stress. Acidification chiefly affects carbonate chemistry and skeletal construction, whereas warming chiefly increases thermal stress. Together, they can make reef growth and recovery more difficult.