☁️Meteorology Unit 12 – Global Circulation Patterns and Climate Zones

Key Concepts

• Earth's atmosphere circulates in response to uneven heating from the sun creating wind patterns and pressure systems
• Global wind patterns are influenced by the Coriolis effect deflecting winds to the right in the Northern Hemisphere and left in the Southern Hemisphere
• Hadley, Ferrel, and Polar cells are the major atmospheric circulation cells that transport heat and moisture around the globe
  • Hadley cells are located near the equator and are responsible for the trade winds
  • Ferrel cells are located in the mid-latitudes and are associated with the westerlies
  • Polar cells are located near the poles and are associated with the polar easterlies
• High-pressure systems are associated with descending air, clear skies, and generally fair weather while low-pressure systems are associated with ascending air, cloudy skies, and precipitation
• Ocean currents play a significant role in redistributing heat and moisture around the planet influencing regional climates (Gulf Stream, Kuroshio Current)
• Climate zones are classified based on temperature and precipitation patterns (Köppen climate classification system)
• Climate change is altering global circulation patterns leading to more extreme weather events and shifts in regional climates

Atmospheric Circulation Basics

• Atmospheric circulation is driven by the unequal heating of the Earth's surface by the sun creating temperature and pressure gradients
• Warm air rises near the equator creating low pressure while cold air sinks near the poles creating high pressure
• The rising and sinking of air creates convection cells that transport heat and moisture around the planet
• The Coriolis effect caused by the Earth's rotation deflects winds to the right in the Northern Hemisphere and left in the Southern Hemisphere
  • The Coriolis effect is strongest near the poles and weakest near the equator
• The Hadley, Ferrel, and Polar cells are the three main convection cells in the Earth's atmosphere
• The Intertropical Convergence Zone (ITCZ) is a low-pressure belt near the equator where the trade winds converge and air rises
• The subtropical high-pressure belts are located around 30° north and south of the equator where air descends creating dry, stable conditions (Sahara Desert, Australian Outback)

Major Global Wind Patterns

• The trade winds are the prevailing easterly winds that blow from the subtropical high-pressure belts towards the equator
  • The trade winds are responsible for transporting moisture from the oceans to the continents
• The westerlies are the prevailing winds that blow from west to east in the mid-latitudes between 30° and 60° north and south of the equator
  • The westerlies are associated with the Ferrel cells and are strongest in the winter hemisphere
• The polar easterlies are the prevailing winds that blow from east to west near the poles poleward of 60° north and south of the equator
  • The polar easterlies are associated with the Polar cells and are relatively weak compared to the other wind patterns
• The jet streams are narrow bands of strong winds that flow from west to east in the upper atmosphere
  • The polar jet stream is located between the Ferrel and Polar cells around 60° north and south of the equator
  • The subtropical jet stream is located between the Hadley and Ferrel cells around 30° north and south of the equator
• Monsoon winds are seasonal winds that reverse direction between summer and winter due to changes in temperature and pressure gradients over land and sea (Indian Monsoon, East Asian Monsoon)

Pressure Systems and Their Influence

• High-pressure systems are characterized by descending air, clear skies, and generally fair weather
  • Anticyclones are high-pressure systems that rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere
• Low-pressure systems are characterized by ascending air, cloudy skies, and precipitation
  • Cyclones are low-pressure systems that rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere
• The Bermuda-Azores High is a semi-permanent high-pressure system located over the Atlantic Ocean that influences weather patterns in North America and Europe
• The Aleutian Low is a semi-permanent low-pressure system located over the Aleutian Islands in the North Pacific that influences weather patterns in North America and Asia
• The Siberian High is a semi-permanent high-pressure system that develops over Siberia in the winter and influences weather patterns in Asia and Europe
• The Icelandic Low is a semi-permanent low-pressure system that develops over Iceland in the winter and influences weather patterns in Europe and the North Atlantic
• Pressure systems can create local wind patterns such as sea breezes and land breezes, as well as mountain and valley breezes

Ocean Currents and Climate

• Ocean currents are large-scale flows of water in the ocean that are driven by wind patterns, temperature and salinity gradients, and the Coriolis effect
• Surface currents are wind-driven and flow in the upper layer of the ocean while deep ocean currents are density-driven and flow in the deep ocean
• The global conveyor belt is a system of deep ocean currents that circulate water around the planet redistributing heat and nutrients
• The Gulf Stream is a warm ocean current that originates in the Gulf of Mexico and flows along the east coast of North America towards Europe
  • The Gulf Stream moderates the climate of Western Europe making it warmer than other regions at similar latitudes
• The Kuroshio Current is a warm ocean current that flows along the east coast of Japan and influences the climate of the western North Pacific
• The California Current is a cold ocean current that flows along the west coast of North America and influences the climate of the western United States
• Upwelling is a process where deep, cold, nutrient-rich water is brought to the surface by wind-driven currents (Peruvian Upwelling, Benguela Upwelling)
  • Upwelling regions are highly productive and support large populations of marine life

Climate Zone Classification

• The Köppen climate classification system is the most widely used system for classifying the world's climates based on temperature and precipitation patterns
• The five main climate groups in the Köppen system are tropical, dry, temperate, continental, and polar
  • Tropical climates are characterized by high temperatures and heavy rainfall throughout the year (Amazon Rainforest, Congo Basin)
  • Dry climates are characterized by low precipitation and high evaporation rates (Sahara Desert, Gobi Desert)
  • Temperate climates are characterized by moderate temperatures and precipitation with distinct warm and cold seasons (Eastern United States, Western Europe)
  • Continental climates are characterized by large temperature variations between summer and winter with moderate precipitation (Siberia, Canadian Prairies)
  • Polar climates are characterized by extremely cold temperatures and low precipitation throughout the year (Antarctica, Greenland)
• Each main climate group is further divided into subcategories based on seasonal variations in temperature and precipitation
• The Köppen system uses letters to denote the main climate groups and subcategories (Af = tropical rainforest, BWh = hot desert, Cfb = temperate oceanic)
• Other climate classification systems include the Thornthwaite system, which is based on potential evapotranspiration, and the Trewartha system, which is a modified version of the Köppen system

Regional Climate Characteristics

• Regional climates are influenced by a combination of factors including latitude, altitude, proximity to water bodies, ocean currents, and atmospheric circulation patterns
• Coastal regions tend to have milder temperatures and higher humidity compared to inland regions due to the moderating influence of the ocean (Mediterranean climate, Pacific Northwest)
• Mountainous regions tend to have cooler temperatures and higher precipitation compared to surrounding lowlands due to orographic lifting (Rocky Mountains, Andes Mountains)
  • The windward side of a mountain range receives more precipitation than the leeward side due to the rain shadow effect
• Desert regions are characterized by low precipitation, high temperatures, and large diurnal temperature variations (Mojave Desert, Kalahari Desert)
• Polar regions are characterized by extremely cold temperatures, low precipitation, and seasonal variations in daylight hours (Arctic tundra, Antarctic ice sheet)
• Monsoon regions are characterized by seasonal reversals in wind direction and precipitation patterns (South Asia, West Africa)
  • The summer monsoon brings heavy rainfall and humid conditions while the winter monsoon brings dry, cool conditions
• Urban areas can create their own microclimates due to the urban heat island effect, where temperatures are higher compared to surrounding rural areas (Tokyo, New York City)

Climate Change and Global Patterns

• Climate change refers to long-term changes in temperature, precipitation, and other climate variables due to natural and anthropogenic factors
• The Earth's climate has varied naturally over geological timescales due to changes in solar output, volcanic activity, and orbital variations (Milankovitch cycles)
• Anthropogenic climate change is caused by human activities that increase greenhouse gas concentrations in the atmosphere, primarily through the burning of fossil fuels and land-use changes
  • The main greenhouse gases are carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O)
• The greenhouse effect is the process by which greenhouse gases absorb and re-emit infrared radiation, warming the Earth's surface and lower atmosphere
• Global warming is the long-term trend of rising average global temperatures due to the enhanced greenhouse effect
  • The Earth's average surface temperature has increased by approximately 1.0°C since pre-industrial times
• Climate change is altering global circulation patterns, leading to changes in temperature and precipitation patterns, sea level rise, and more frequent and intense extreme weather events (heatwaves, droughts, floods, hurricanes)
• The Arctic is warming at a faster rate than the rest of the planet due to polar amplification, leading to a decrease in sea ice extent and thickness
• Climate models project continued warming and changes in precipitation patterns throughout the 21st century, with the magnitude of change depending on future greenhouse gas emissions scenarios (Representative Concentration Pathways)