Climate Drivers and Global Weather Pattern Shifts

Impacts of Weakening Thermohaline Circulation

If the thermohaline circulation weakens, it would very likely cause abrupt shifts in regional weather patterns and the water cycle. These could include a southward shift in the tropical rain belt, weakening of the African and Asian monsoons, strengthening of Southern Hemisphere monsoons, and drying in Europe—impacts that would greatly alter food production worldwide.

The ENSO Natural Cycle in the Pacific Ocean

The ENSO (El Niño-Southern Oscillation) is a natural cycle in Pacific Ocean temperatures, winds, and clouds. ENSO is often behind extreme events. The phase starts at the beginning of the year and lasts until autumn; it takes about four years to return to La Niña and El Niño. Trade winds usually blow from the east to the west. These winds pile up warm water in the western Pacific. In contrast, water in the east is colder. This creates the Walker circulation.

La Niña and El Niño Phases

• La Niña: Characterized by stronger trade winds, cooling the east more (the "overdrive" version). On the west side, there are higher ocean temperatures, greater evaporation, clouds, and rain. This means a higher risk for lower daytime temperatures and increased flooding.
• El Niño: Trade winds weaken or reverse, allowing warm water to drift back. The Walker circulation breaks down, further weakening the trade winds. Once this feedback starts with the warm water shifting east, the evaporation, clouds, and rain follow, shifting away from Australia. Eastern Australia experiences a drier climate.

The Indian Ocean Dipole (IOD)

The IOD is characterized by an irregular oscillation of sea surface temperatures in the eastern and western Indian Ocean.

Phases of the IOD

• IOD Natural Phase: Warm water from the western Pacific flows between the islands of Indonesia, keeping seas to Australia's northwest warm. Air rises above this area and falls over the western half of the Indian Ocean basin, blowing westerly winds along the equator.
• IOD Positive Phase: Westerly winds weaken along the equator, allowing warm water to shift towards Africa while cool water upwells in the east. The tropical Indian Ocean becomes cooler than normal in the east and warmer than normal in the west. In southeast Australia, it could mean the loss of spring rain.
• IOD Negative Phase: Westerly winds intensify along the equator, allowing warmer waters to concentrate near Australia and upwelling in the west. The tropical Indian Ocean becomes warmer than normal in the east and cooler than normal in the west, pushing more rainfall and flooding into Australia.

Southern Annular Mode (SAM)

The Southern Annular Mode (SAM) is a ring of variability that encircles the South Pole and extends as far north as New Zealand. It is characterized by a seesaw pattern in atmospheric pressure between the Antarctic region and mid-latitudes.

Ocean Heat Absorption and Greenhouse Gases

The ocean has taken up approximately 90% of the extra energy from enhanced greenhouse gas concentrations. The rate at which oceans are taking up heat has increased in recent decades. The Earth is gaining heat due to increases in greenhouse gases in the atmosphere, and most of this heat is being absorbed by the oceans.