Lecture 1 Part 6

Sand and Rainfall

There is an inverse relationship between the Southern Annular Mode (SAM) and rainfall in regions affected by tropical cyclones and cold fronts, such as Southwest Western Australia (WA). In regions where rainfall is linked to high-pressure systems, like WA, there is an in-phase relationship between SAM and rainfall. High-pressure systems rotate anticlockwise in the Southern Hemisphere, bringing rain from the Southern Ocean onto the WA coast.

Anticyclones and Weather

Anticyclones are associated with fair weather conditions due to descending air. Descending air suppresses vertical motion preventing cloud formation. To get cloud, vertical motion is required, which can be forced by convection.

  • Convection: Positively buoyant masses of air rise vertically, cooling as they ascend. In anticyclones, subsidence suppresses convection, preventing air parcels from rising far enough to reach the lifting condensation level (LCL). The lifting condensation level is the altitude at which air must be lifted for condensation to occur and clouds to form.

As anticyclones track from west to east, they can bring moisture onto the South Australian coast. Negative SAM in spring may increase bushfire risk in Eastern Australia due to increased westerly flow.

  • 2019 Bushfires: Westerlies prevailed through early January, leading to bushfires being driven through the landscape, such as at Prygin Prygin Beach. Negative SAM can also result in increased snow depth and persistence due to cooler air masses.

Climate Drivers

  • SAM (Southern Annular Mode): Influences the position of westerly winds and storm tracks in the Southern Hemisphere.
  • DMI (Dipole Mode Index): Measures the Indian Ocean Dipole.
  • SOI (Southern Oscillation Index): Measures the intensity of the Walker circulation and ENSO (El Niño-Southern Oscillation). It's the mean sea level pressure difference between Tahiti and Darwin, indicating the pressure gradient across the Pacific.

Blocking Anticyclones

Blocking occurs when a large anticyclone sits over New Zealand, stopping the natural progression of weather systems from west to east.

Influence on Rainfall

The climate driver with the highest correlation to monthly rainfall varies by season across the continent:

  • Summer: SOI influences rainfall over most of the continent.
  • Autumn: SAM and blocking become more important, particularly along the coast.
  • Winter: SOI is important, with some SAM influence on the West Coast. Blocking is important in the south, preventing cold fronts from moving across.
  • Spring: Blocking becomes most important. SOI continues to have a significant impact. The Indian Ocean Dipole has a smaller impact, mainly in the southeast.

Only correlations with a 95% significance level are included.

Longer-Term Climate Variability

  • ENSO: Operates on timescales of 2-6 years, switching between El Niño and La Niña phases. Recently, ENSO has shown unusual behavior, getting stuck in preferred modes (La Niña or El Niño).
  • SAM: Most dynamic on weekly time scales but influences weather over months.
  • Indian Ocean Dipole: Inter-annual, switching between negative, positive, and neutral phases.

Multi-Decadal Oscillations

  • Atlantic Multi-Decadal Oscillation: Influences weather through teleconnections to the Pacific.
  • Interdecadal Pacific Oscillation (IPO) / Pacific Decadal Oscillation (PDO): Oscillates on timescales of 40-60 years and modulates ENSO, making it stronger or weaker.

The Pacific Decadal Oscillation (PDO) is defined as the second EOF mode of global SSTs in a particular region.

Impact of PDO on ENSO

PDO modulates ENSO by influencing the intensity of La Niña and El Niño events.

  • 2010-2012: La Niña events and flooding in the Brisbane River were amplified by the PDO phase.
  • Late 1970s - 2008: The PDO phase made El Niños dominant, causing drought in Eastern Australia (the millennium drought). Brisbane's water levels dropped to 10-12%, leading to the construction of desalination plants and an interconnected water grid (at a cost of 5-6 billion dollars).
    Water purification systems were considered, including a wastewater recycling plant at Luggett Point, but faced public hesitation.

Historical Impacts of PDO Phases

  • 1924-1945 (Negative Phase): Minimal temperature change in Australia. Dry conditions in certain regions.
  • Subsequent Negative Phase: Very dry conditions, potentially lasting until 2007. This phase amplifies El Niño events.
  • More Recent Phase: Warm waters through the western Pacific, cool waters in the eastern Pacific, creating a La Niña pattern and leading to wet conditions over the Australian continent, including floods in 2010-2012.

During the PDO negative phase, the Australian continent progressively dried out, and land surface and groundwater supplies diminishing and required a phase shift to replenish.

Influence of Global Warming

Global warming is influencing teleconnections and may intensify El Niño or La Niña events:

  • Modeling studies show inconsistency on whether El Niño or La Niña will become more dominant.
  • Likely increase in frequency of El Niño and positive Indian Ocean Dipole events.
  • Potential increase in Madoki-type ENSO events, where sea surface temperature changes occur in the Equatorial Central Pacific rather than off the coast of South America.

Other Impacts of Global Warming

  • Hadley Cell Expansion: The subtropical ridge (axis of high-pressure systems) has migrated south by about 300 kilometers in the last 30 years. The Mediterranean climates may shift south, potentially disappearing as the climate system moves south.

  • Walker Circulation: Becoming weaker, and tropical cyclogenesis will move further east.

  • Thunderstorm Hotspots: Springtime thunderstorm activity may be shifting from Southeast Queensland to Northeast New South Wales.

The Hadley cell are convective circulations that ascend near the equator, transport air north and south, descend at the subtropics, and circulate back.

There are three cells:

  • Hadley cell
  • Ferrel cell
  • Polar cell

Reading Materials

  • Papers on changing El Niño and the Southern Oscillation in a warming climate.
  • Book by Andy and Nigel Still on weather and climate in Australia.
  • Paper by Risbee on remote drivers of rainfall variability in Australia.
  • Papers on the Southern Annular Mode (SAM).