Atmospheric Environment: Fundamentals of Weather, Climate, and Global Circulation

The Complexity of Meteorology and Global Systems

Meteorology: Art or Science?

The atmospheric environment is incredibly complex. Imagine taking a sphere with a diameter of 12000 km, adding continents and oceans, and enveloping it with a mixture of gases. Then, tilt it slightly, rotate it on its axis, place it around a sun 1 million km in diameter, and subject the entire system to gravitational forces. The challenge lies in understanding and predicting how the oceans, atmosphere, land, and ice will all interact. This intricate interplay makes meteorology a challenging field.

Weather versus Climate

Understanding the difference between weather and climate is fundamental:

Weather

  • Definition: What you get on a daily basis.

  • Time Period: Short-term, typically 1 to 14 days.

  • Forecasting: Includes seasonal forecasting, such as predicting if the next winter will be drier than usual.

  • Dependence: Heavily reliant on accurate initial conditions of the ocean, land, and atmosphere. This requires extensive observations and satellite data.

  • Methodology: Utilizes data assimilation techniques to integrate various data sources into models.

Climate

  • Definition: What you expect over longer periods, representing the average weather conditions.

  • Time Frame: Long-term, typically ~30-100 years.

  • Focus: Addresses shifts in seasonal patterns, such as whether summers are getting warmer than average.

  • Drivers: Influenced by a combination of factors:

    • Natural climate variability.

    • Volcanic emissions.

    • Anthropogenic CO_2 (carbon dioxide) emissions.

Pinball Analogy

To illustrate the difference, consider a pinball machine:

  • The individual path of a single ball represents weather, which is highly sensitive to initial conditions and thus hard to predict accurately over long periods.

  • The range of paths of multiple balls, released under similar overall conditions, represents climate. While the exact path of one ball is unpredictable, the general area or distribution where many balls land is predictable.

The Global Atmospheric Circulation

To understand weather and climate, we must first grasp the global atmospheric circulation.

Hypothetical Non-Rotating Earth

Imagine a perfectly spherical Earth covered by a uniform ocean and atmosphere:

  • Heat Application: If heat is applied at the equator and the poles are frozen.

  • Convection: Warm air at the equator would rise (creating low pressure), flow towards the poles, cool, sink (creating high pressure), and then flow back towards the equator along the surface.

  • Result: This would establish a single, large circulation cell in each hemisphere, with rising air at the equator and sinking air at the poles.

Earth's Rotation and the Coriolis Effect

In reality, the Earth rotates from west to east. This rotation introduces a significant force known as the Coriolis Effect:

  • Deflection: Moving objects (including air and ocean currents) are deflected to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Actual Global Circulation Cells

Due to the Coriolis effect, the single hypothetical cells break down into three distinct circulation cells in each hemisphere:

  • Hadley Cells: Extend from the Equator to approximately 30^ ext{o}N and 30^ ext{o}S. Warm air rises at the equator, moves poleward, cools, and sinks around 30^ ext{o} latitude. Surface winds within these cells are known as Trade Winds (Northeast Trades in NH, Southeast Trades in SH).

  • Ferrel Cells: Located between 30^ ext{o} and 60^ ext{o} latitude in both hemispheres. These are indirect cells driven by the Hadley and Polar cells. Surface winds are generally Westerlies.

  • Polar Cells: Extend from 60^ ext{o} latitude to the poles. Cold, dense air sinks at the poles, flows towards 60^ ext{o} latitude, where it meets warmer air from the Ferrel cell and rises. Surface winds are typically Polar Easterlies.

  • Jet Streams: Strong, fast-moving ribbons of air in the upper troposphere, including the Polar Jet Stream and Subtropical Jet Stream, which influence weather patterns.

  • Polar Vortex: A large area of low pressure and cold air surrounding the Earth's North and South Poles.

Energy and Water Balance

Net Radiation

The Earth receives short-wave radiation from the Sun and emits long-wave radiation. The Net Radiation (measured in W/m^2) represents the balance between incoming and outgoing radiation. Positive values indicate a net gain of energy, typically near the equator, while negative values indicate a net loss, typically near the poles, varying throughout the year (e.g., December).

Sensible and Latent Heat Fluxes

Energy exchange between the surface and the atmosphere also occurs through heat fluxes:

  • Sensible Heat Flux: Transfer of heat due to temperature differences (e.g., warm ground heating the air).

  • Latent Heat Flux: Transfer of heat associated with phase changes of water (e.g., evaporation cooling the surface and warming the atmosphere upon condensation).

  • Convention: Positive values for sensible and latent heat flux indicate energy moving towards the atmosphere. Positive values for change in heat storage mean energy is moving out of storage.

Global Water Cycle Components

Maps illustrating the global distribution of key water cycle components show:

  • Precipitation: Ranging from less than 10 mm to over 400 mm. (e.g., monthly averages).

  • Runoff/Water Surplus: Ranging from less than 0.01 mm to over 100 mm.

  • P-E (Precipitation minus Evaporation): Values from -200 mm (evaporation exceeds precipitation) to 200 mm (precipitation exceeds evaporation).

  • Soil Moisture: Ranging from 0.15 cm to 0.40 cm (Data based on NCEP/NCAR Reanalysis Project, 1959-1997 Climatologies, animated by the University of Oregon).

The Global Ocean Circulation

The ocean plays a crucial role in redistributing heat and influencing climate.

Factors Affecting Ocean Water Density

Ocean water density is key to its circulation. Density ( ext{Mass} / ext{Volume}) is primarily influenced by:

  • Temperature: Cold water is denser than hot water. Near the poles, less heat from the Sun leads to low temperatures, increasing water density and causing it to sink. In the tropics, more heat from the Sun leads to higher temperatures, decreasing water density and causing it to rise.

  • Salinity: Salty water is denser than fresh water. Areas with high evaporation and low rainfall tend to have higher salinity and thus denser water that sinks. Conversely, areas with low evaporation and high rainfall have lower salinity and less dense water that rises.

Thermohaline Circulation (The Great Ocean Conveyor Belt)

This global-scale circulation is driven by differences in water density (due to temperature and salinity, hence 'thermo-haline'):

  • Mechanism: Cold, saline water (formed near the poles, especially in the North Atlantic) is very dense and sinks to the deep ocean.

  • Flow: This deep water then flows throughout the world's oceans, eventually upwelling in other regions (e.g., Pacific and Indian Oceans).

  • Surface Currents: Warmer, less dense water flows along the surface, replacing the cold, sinking water, and releasing heat to the atmosphere in certain areas.

  • Importance: This circulation is a critical component of the Earth's climate system, transporting heat and nutrients globally over very long timescales.

Regional Ocean Currents: The Leeuwin Current

  • Characteristics: A very warm ocean current that flows southward along the west coast of Western Australia (WA).

  • Influence: This current significantly modulates the climate of WA, bringing warmer waters further south than would otherwise be expected at those latitudes.

  • Other Major Currents: South Equatorial Current, East Australian Current, Antarctic Circumpolar Current.

The "Ningaloo Niño" Event (March 2011)

  • Observation: During 2010-2011, exceptionally warm ocean temperatures were observed off WA.

  • Anomalies: In March 2011, Sea Surface Temperature (SST) anomalies reached +2.0 to +2.5^ ext{o}C above the 1971-2000 average in coastal WA waters.

  • Land Impact: This warmth extended to land, with Maximum (TMAX) and Minimum (TMIN) land temperatures showing anomalies of up to +4^ ext{o}C in some regions, following a drier preceding winter.

  • Naming: The strength of this localized warming event led to it being referred to as the "Ningaloo Niño event," drawing a parallel to the more widely known El Niño phenomenon.

Large-Scale Climate Drivers: El Niño Southern Oscillation (ENSO)

ENSO is one of the most important drivers of climate variability globally, particularly for Australia.

The Walker Circulation

ENSO involves changes in the Walker Circulation, an atmospheric circulation cell along the equatorial Pacific:

  • Typical Pattern: Rising air and low pressure over the western Pacific (e.g., near Darwin, Australia) with trade winds blowing towards the west, pushing warm surface water. Sinking air and high pressure occur over the eastern Pacific (e.g., near Tahiti). This leads to a warm pool in the western Pacific and a cooler, upwelling-driven eastern Pacific cold tongue.

  • El Niño Pattern: Trade winds weaken or reverse. The warm water pool shifts eastward, leading to rising air and lower pressure in the central and eastern Pacific. Sinking air and higher pressure occur in the western Pacific (e.g., over Darwin). This results in warmer sea surface temperatures in the central and eastern equatorial Pacific.

ENSO Conditions

  • Normal Conditions: Warm surface water in the western Pacific, cooler water (cold tongue) in the eastern Pacific. The thermocline (the boundary between warm surface water and colder deep water) is deeper in the west and shallower in the east.

  • El Niño Conditions: The warm pool extends across the equatorial Pacific towards South America. The thermocline deepens in the eastern Pacific, suppressing nutrient-rich upwelling.

  • La Niña Conditions: An exaggeration of normal conditions, with even colder than average SSTs in the eastern Pacific. The thermocline becomes even shallower in the east, enhancing upwelling.

Sea Surface Temperature (SST) Anomalies

Monitoring SST anomalies (differences from average temperatures) in the equatorial Pacific is crucial for identifying ENSO phases:

  • Example (Sep 2015): Showed significant positive SST anomalies (+2 to +3^ ext{o}C) in the central and eastern equatorial Pacific, characteristic of a strong El Niño event.

  • Example (Aug 2022): Showed negative SST anomalies (-1 to -2^ ext{o}C) in the central and eastern equatorial Pacific, characteristic of a La Niña event. The Bureau of Meteorology issued a "La Niña ALERT" with a 70\% chance of La Niña forming, indicating persistent cooling and atmospheric signals.

The Southern Oscillation Index (SOI)

  • Definition: The SOI is a standardized index based on the observed sea level pressure differences between Tahiti (in the central South Pacific) and Darwin (Northern Australia).

  • La Niña: Associated with lower pressure in Darwin and higher pressure in Tahiti. This results in a positive SOI (sustained values of + ext{8} or higher).

  • El Niño: Associated with higher pressure in Darwin and lower pressure in Tahiti. This results in a negative SOI (sustained values of - ext{8} or lower).

  • Historical Data: Long-term monthly SOI data (e.g., from 1880 to 2020) illustrate the episodic nature of El Niño and La Niña events.

ENSO Influence on Australia's Climate

ENSO has a profound impact on Australia's climate:

  • El Niño: Typically leads to an increased chance of below-average rainfall and warmer temperatures across much of Australia, especially the eastern side.

  • La Niña: Typically leads to an increased chance of above-average rainfall and cooler temperatures across much of Australia, particularly the north and east.

Large-Scale Climate Drivers: Indian Ocean Dipole (IOD)

The IOD is another significant climate driver, particularly for Australia, influencing rainfall and temperature patterns.

Positive Indian Ocean Dipole (IOD)

  • Temperature Anomalies: Characterized by cooler than normal SSTs in the eastern Indian Ocean (near Western Australia/Indonesia) and warmer than normal SSTs in the western Indian Ocean (near Africa).

  • Atmospheric Circulation: Reduced convection (rising air and cloud formation) over the eastern Indian Ocean.

  • Impact on Australia: A positive IOD is associated with a reduced chance of rain across central and southern Australia.

Negative Indian Ocean Dipole (IOD)

  • Temperature Anomalies: Characterized by warmer than normal SSTs in the eastern Indian Ocean and cooler than normal SSTs in the western Indian Ocean.

  • Atmospheric Circulation: Increased convection over the eastern Indian Ocean.

  • Impact on Australia: A negative IOD is associated with an increased chance of rain across central and southern Australia.

Forecasting Next Month

Climate outlooks provide probabilities for rainfall and temperature for the coming months. For example, forecasts for September 2022 (from the Bureau of Meteorology, using the ACCESS-S2 model, base period 1981-2018, run on 29/08/2022, issued 01/09/2022) show:

  • Chance of Exceeding Median Rainfall: Maps indicate regions with varying probabilities (e.g., 20-80\% range) of receiving above-median rainfall.

  • Chance of Exceeding Median Maximum Temperature: Maps show probabilities (e.g., 20-80\% range) for above-median maximum temperatures.

  • Chance of Exceeding Median Minimum Temperature: Maps show probabilities (e.g., 20-80\% range) for above-median minimum temperatures.

Wrap-up of Part I

  • Weather versus Climate: Distinct differences in timescales, predictability, and driving forces.

  • Primary Drivers: The weather and climate we experience are largely driven by Earth's rotation, the Sun's energy, and temperature gradients within the oceans and atmosphere.

  • Interconnected Systems: The land, ocean, and atmosphere are intrinsically coupled systems. Changes in one system inevitably feed back into the others, leading to complex interactions.

  • El Niño Southern Oscillation (ENSO): One of the most important drivers of climate variability in Australia, particularly influencing the East coast's rainfall and temperature.

  • Leeuwin Current: The climate of Western Australia is significantly modulated by the anomalous warm waters of the Leeuwin Current.

Next Topics

Future discussions will delve into:

  • Air pollution.

  • CO_2 emissions.

  • Climate change.

  • The greenhouse effect.