Atmospheric Circulation, Coriolis Effect, and Storms

Characteristics of the Atmosphere

• Atmospheric Pressure and Air Movement

  • Air, or wind, moves from areas of high atmospheric pressure (P) to areas of low atmospheric pressure (P).

  • Sinking air warms due to compression.

  • Rising air cools due to expansion.

  • The location of wet and dry areas on Earth's surface is determined by these pressure changes.

• Adiabatic Processes

  • An adiabatic process is one that occurs without heat exchange with the surroundings.

• Pressure Zones and Air Movement

  • Warm surface air creates low pressure zones: air rises and expands.

  • Cool upper troposphere air creates high pressure zones: air sinks and compresses.

• Troposphere Dynamics (Review)

  • A column of cool, dense air leads to:

    • High pressure at Earth's surface.

    • Sinking air.

    • Molecules close together.

  • A column of warm, less dense air leads to:

    • Low pressure at Earth's surface.

    • Rising air.

    • Molecules far apart.

  • Convection cells illustrate this: warm air expands and cools as it rises, while cool air compresses and warms as it sinks.

The Coriolis Effect

• Significance

  • Crucial for understanding global circulation patterns of the atmosphere and oceans.

  • Affects ocean and atmosphere circulation, as well as storm systems.

• Nature of the Coriolis Effect

  • It is an effect, not a force.

  • It arises from the ground moving relative to airborne or waterborne objects.

  • Frame of reference is vital for understanding it.

• Deflection Patterns

  • Deflects a moving mass (water, air) to the right in the Northern Hemisphere (NH).

  • Deflects a moving mass to the left in the Southern Hemisphere (SH).

• Mechanism: Differential Rotational Velocity of Earth

  • The Earth rotates faster at the Equator than it does at the poles because it is 'wider' (laterally) at the Equator.

  • Examples of Rotational Speeds:

    • Quito (at the Equator): $ext{1,668 km/hr}$.

    • Buffalo (further north): $ext{1,260 km/hr}$.

  • This change in velocity with latitude is the true cause of the Coriolis effect.

• Impact of Coriolis - Example (Missiles/Messages)

  • North Pole to New Orleans (NH): More significant deflection due to the larger difference in rotational velocity ($ext{0 km/hr}$ at North Pole vs. $ext{~1,400 km/hr}$ at New Orleans).

  • Galápagos (Equator) to New Orleans (NH): Less deflection because New Orleans ($ext{~1,400 km/hr}$) is traveling at a more similar speed to the Galápagos ($ext{~1,600 km/hr}$) compared to the North Pole.

  • Key Points:

    • Rotational velocity does not change linearly with latitude.

    • No Coriolis effect at the Equator.

    • Maximum Coriolis effect at the poles.

• Observed vs. Fictional Wind Patterns

  • On a non-spinning Earth, air would move directly from poles to equator and vice versa.

  • Due to the Coriolis effect, global wind patterns are deflected, creating complex circulation.

Global Atmospheric Circulation Patterns

• Global Wind Belts

  • These patterns create global wind belts that are bent to the right in the NH and to the left in the SH.

• Circulation Cells (Driven by Latitudinal Air Circulation)

  • Three atmospheric circulation cells per hemisphere:

    1. Hadley Cell:

       • Hot equatorial air rises, causing precipitation (low pressure).

       • Air forced away from the equator in the cool upper troposphere.

       • Air descends at $30^ ext{o}$ latitudes (known as Horse Latitudes), creating dry air and high pressure zones.

       • More descending air forces air away from the equator.

    2. Ferrel Cell: Located between $30^ ext{o}$ and $60^ ext{o}$ latitude.

    3. Polar Cell:

       • Cold, dry air sinks at the poles, creating high pressure (polar deserts).

       • Air warms up, rises, and drops precipitation as it moves away from the poles.

  • Characteristics of Cells:

    • Horizontal air movement within cells.

    • Vertical air movement at boundaries.

• Jet Streams (Quick Digression)

  • Fast-moving, relatively narrow, meandering air currents in the upper troposphere (approximately $10 ext{ – }16 ext{ km}$ altitude).

  • Polar jet stream is considerably stronger (up to $ext{400 km/hr}$ ) than the subtropical jet stream.

  • Can merge at different points in space and time, and experience interruptions.

• Major Wind Patterns and Belts

  • Trade Winds:

    • Air moves from subtropical highs ($30^ ext{o}$ latitude - high pressure) towards the equator (low pressure).

    • Deflected to the NE to SW in the NH and SE to NW in the SH.

    • Named for their historical role in hastening maritime trade, often linking the Age of Discovery to the Caribbean.

    • Driven by the Hadley cell.

  • Subtropical Highs / Horse Latitudes ($30^ ext{o}$ latitude):

    • Zones of sinking, dry air, resulting in light and variable winds.

    • Historically, sailors threw livestock (horses) overboard in these calm zones to conserve water.

    • High pressure zones.

    • Juncture of the Hadley and Ferrel cells.

    • Associated with tropical forests (Hadley cell rising arm), temperate forests (Ferrel cell), and desert regions ($30^ ext{o}$ latitude and poles).

  • Westerlies:

    • Found between $30^ ext{o}$ and $60^ ext{o}$ latitude.

    • Deflected from west to east.

  • Roaring 40s, Furious 50s, Screaming 60s (Southern Hemisphere):

    • Strong westerly winds found poleward of $40^ ext{o}$ S latitude.

    • Driven by the Ferrel cell.

    • The only continuous lateral current around the world because there is less landmass to obstruct flow.

    • Creates the circumpolar vortex.

    • Partially responsible for Antarctica's extreme cold.

    • Old sailor's proverb: "Below $40^ ext{o} ext{S}$ there is no law, below $50^ ext{o} ext{S}$ there is no God!"

    • Historically used for "The Clipper Route" from England to Australia and back.

• Intertropical Convergence Zone (ITCZ)

  • An area of low atmospheric pressure that forms where the NE trade winds converge with the SE trade winds.

  • Characterized by vertical air movement and little horizontal movement (known as the doldrums).

  • Also referred to as the climate equator.

  • Experiences light winds.

  • Annual Shift: The ITCZ shifts position annually, following the sun's path and the 'hottest' weather, which creates the most convective lifting and lowest pressure (rainstorms).

  • Monsoons: The ITCZ's north-south movement and the differing heat capacities of land and water are linked to seasonal wind circulation patterns, resulting in wet summers and dry winters, particularly notable in regions like India and near Darwin, Australia.

Storms

• Definition and Characteristics

  • Regional atmospheric disturbances characterized by strong winds, often accompanied by precipitation.

  • Result from ocean-atmosphere interactions.

  • Associated with air masses.

• Air Masses and Fronts

  • Air mass: A large volume of air with nearly uniform temperature, water vapor content, and density.

  • Adapts to the characteristics of the surface below it.

  • Air masses generally do not mix; one will move over or under another.

  • Front: The boundary between two air masses, often leading to turbulence, wide moderate rain bands, or narrow intense storm bands.

• Largest Storm Systems

  • Tropical cyclones (including hurricanes, typhoons, cyclones).

  • Extratropical cyclones.

  • Both are huge rotating masses of low-pressure air where winds converge and ascend.

• Coriolis Effect on Storms

  • Storms turn counterclockwise in the Northern Hemisphere.

    • Winds are pulled into the low-pressure center but deflected to the right by the Coriolis effect.

  • Storms turn clockwise in the Southern Hemisphere.

Tropical Cyclones (Hurricanes/Typhoons/Cyclones)

• Characteristics

  • Rotating masses of warm, humid air with very strong winds and torrential rain.

  • Can be up to $ext{1000 km}$ wide and $ext{15 km}$ high, with an eye approximately $ext{15 km}$ wide.

  • Wind speeds can reach up to $ext{345 km/hr}$.

• Regional Names

  • Hurricanes: North Atlantic and eastern Pacific.

  • Typhoons: Western Pacific.

  • Cyclones: Southern Hemisphere (clockwise rotation due to Coriolis).

• Ingredients and Mechanisms of Formation

  • Location: Form within one warm, humid air mass, typically between $5^ ext{o}$ and $25^ ext{o}$ latitude.

  • Water Temperature: Sea surface temperature must be at least $26^ ext{o} ext{C}$ ($79^ ext{o} ext{F}$) to provide sufficient warm, humid air.

  • Pre-existing Disturbance: A deep low-pressure system is needed to initiate the process.

  • Energy Source: Warm water provides the energy for intensification.

  • Latent Heat Release: Energy comes from latent heat stored in evaporated moist air. As warm, wet air rises, it condenses (forming rain) and releases stored latent heat into the storm cell, which draws in more air, creating a feedback loop that intensifies the storm.

  • Dissipation: The storm weakens and dissipates when its energy source is cut off (e.g., moving over land or cooler water).

• Internal Structure

  • Warm water evaporates, and moisture rises, creating thunderstorms.

  • Winds spiral upward and outward, forming a low-pressure system over the ocean surface.

  • Clouds form in the upper atmosphere as warm air condenses.

  • The entire storm system begins to spin (counterclockwise in NH, clockwise in SH).

  • Key components: eye (calm, cool, dry air descends), eyewall (most intense winds and rains), spiral rain bands.

• Hurricane Intensity: The Saffir-Simpson Scale

  • Classifies hurricanes based on maximum sustained wind speed, with associated typical storm surge and damage levels.

  • Category 1: Winds $ext{120-153 km/hr}$, Surge $ext{1.2-1.5 m}$, Minimal damage.

  • Category 2: Winds $ext{154-177 km/hr}$, Surge $ext{1.8-2.4 m}$, Moderate damage.

  • Category 3: Winds $ext{178-209 km/hr}$, Surge $ext{2.7-3.7 m}$, Extensive damage.

  • Category 4: Winds $ext{210-249 km/hr}$, Surge $ext{4.0-5.5 m}$, Extreme damage.

  • Category 5: Winds $> ext{250 km/hr}$, Surge $> ext{5.8 m}$, Catastrophic damage.

• Hurricane Tracks (Atlantic)

  • Follow typical paths, often veering left or right around landmasses like Florida.

  • Tracks are significantly influenced by atmospheric circulation patterns.

• Storm Surges

  • An abnormal rise in seawater level during a storm, distinct from normal tides.

  • Can reach over $ext{6 m}$ (or even up to $ext{10 m}$) in the largest tropical cyclones.

  • Tides can exacerbate surges, pushing them higher and further inland.

  • Examples:

    • Hurricane Katrina (New Orleans, 2005):

      • Caused $ext{~75 billion USD}$ in damage and over $ext{1600}$ lives lost.

      • Levees breached, preventing water from draining away.

      • Not the deadliest historical hurricane.

    • Hurricane Galveston (Texas, 1900):

      • Category 4 hurricane, hit a beach resort averaging $ext{1.5 m}$ above sea level.

      • Caused a $ext{6 m}$ high storm surge, heavy rainfall, and $ext{150-200 km/hr}$ winds.

      • Estimated $ext{~6000}$ deaths due to lack of early warning systems.

    • Hurricane/Superstorm Sandy (October 2012):

      • Combination of a hurricane and a Nor'easter.

      • Category 3 at peak intensity, with a peak diameter of $ext{1,850 km}$. Largest diameter Atlantic hurricane on record.

      • One of the costliest hurricanes in US history ($ext{~69 billion USD}$ damage, $233$ deaths).

      • Coincided with very high tides (full moon - "spring tide").

      • Late season storm merged with an autumn low-pressure area in the NW Atlantic, increasing its intensity.

      • Caused a $ext{4-5 m}$ storm surge in areas unaccustomed to such events.

• Recent Hurricane Seasons (Examples)

  • 2020 Hurricane Season:

    • $ext{30}$ named storms, no Category 5, $ext{7}$ storms were Category 3 or higher.

    • Ran out of names, moved to the Greek alphabet.

    • $ext{~51 billion USD}$ damage, $ext{~420}$ deaths.

  • 2005 Hurricane Season:

    • $ext{4}$ Category 5 storms, $ext{7}$ Category 3 or higher, $ext{15}$ hurricanes.

    • $ext{~172 billion USD}$ damage, $ext{~4000}$ deaths.

  • 2017 Hurricane Season:

    • $ext{2}$ Category 5 storms, $ext{6}$ Category 3 or higher, $ext{10}$ hurricanes.

    • $ext{~300 billion USD}$ damage, $ext{~3400}$ deaths.

  • 2023 Hurricane Season:

    • $ext{20}$ named storms, $ext{7}$ hurricanes, $ext{3}$ majors.

    • $ext{2}$ Category 5 storms (Otis, Lee).

    • Otis (Mexican Pacific coast) was the strongest; Lee made landfall in Nova Scotia as a post-tropical cyclone.

    • Lee and Otis were among the $ext{5}$ fastest intensifying hurricanes on record.

    • Idalia (Category 3 landfall in Florida) caused $ext{20 billion USD}$ in damage.

  • 2024 Hurricane Season (Forecast/Early Season):

    • Forecasted $ext{18}$ named storms, $ext{11}$ hurricanes, $ext{5}$ majors, $ext{2}$ Category 5.

    • Beryl: Earliest Category 5 hurricane ever (June); remnants soaked parts of Ontario/Quebec.

    • Milton (Category 3 landfall in Florida) caused $ext{50 billion USD}$ damage.

    • Helene (Category 4 landfall in Florida/Carolinas) caused $ext{80 billion USD}$ damage and $ext{220}$ deaths.

• Rapid Intensification

  • Defined as an increase of maximum sustained windspeeds of $> ext{55 km/hr}$ in $ext{24 hours}$.

  • Approximately ext{20-30%} of all tropical cyclones undergo rapid intensification.

  • Hurricane Otis is an example, causing $ext{~16 billion USD}$ in damage.

Extratropical Cyclones and Nor'easters

• Formation and Location

  • Low-pressure systems that form between two air masses.

  • Occur at mid-latitudes (approximately $30^ ext{o}$ to $60^ ext{o}$ latitude).

  • In the NH, they typically move south from the Ferrel-Polar cell convergence low-pressure zones.

  • Associated with cloudy skies, rain, thunderstorms, and blizzards.

• Characteristics

  • Often have a characteristic 'comma' shape.

  • Can be derived from hurricanes (tropical cyclones) moving north in the autumn, merging with North Atlantic frontal systems.

  • Merging can increase the size of the system, but typically weakens the core intensity.

• Nor'easters (Atlantic Canada Example)

  • Form due to the interaction of:

    • Cold arctic air from Canada (transported southward by the polar jet stream).

    • Warm water from the Gulf Stream (moving northward from the Gulf of Mexico).

    • Cold water from the Labrador Current (moving southward).

    • The difference in air and water temperatures over the Atlantic, combined with these currents, creates the conditions for Nor'easters.

  • Characterized by strong northeasterly winds.

Summary of Key Learnings

• Understanding of the three main atmospheric circulation cells (Hadley, Ferrel, Polar) and the processes occurring at their boundaries.

• Knowledge of the three major global wind belts.

• Definition of the Coriolis effect (as an effect, not a force) and its cause (differential rotational velocity of Earth).

• Impact of the Coriolis effect on oceanography and storm systems.

• Definition, location, and reason for the existence of the Intertropical Convergence Zone (ITCZ).

• Definition of a storm.

• Distinction between tropical and extratropical cyclones.

• Formation mechanisms of Nor'easters.

• Defining characteristics of a tropical cyclone/hurricane, including necessary ingredients (e.g., warm water), the role of latent heat for energy, and why hurricanes rotate.

• Familiarity with significant Atlantic hurricanes and hurricane seasons.