11: General Circulation
- Page ID
- 9602
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)A spatial imbalance between radiative inputs and outputs exists for the earth-ocean-atmosphere system. The earth loses energy at all latitudes due to outgoing infrared (IR) radiation. Near the tropics, more solar radiation enters than IR leaves, hence there is a net input of radiative energy. Near Earth’s poles, incoming solar radiation is too weak to totally offset the IR cooling, allowing a net loss of energy. The result is differential heating, creating warm equatorial air and cold polar air (Fig. 11.1a).
This imbalance drives the global-scale general circulation of winds. Such a circulation is a fluiddynamical analogy to Le Chatelier’s Principle of chemistry. Namely, an imbalanced system reacts in a way to partially counteract the imbalance. The continued destabilization by radiation causes a general circulation of winds that is unceasing.
Because buoyancy causes warmer air to rise and colder air to sink, you might guess that equator-topole overturning would exist (Fig. 11.1b). Instead, the real general circulation has three bands of circulations in the Northern Hemisphere (Fig. 11.1c), and three in the Southern. In this chapter, we will identify characteristics of the general circulation, explain why they exist, and learn how they work.
- 11.1: Key Terms
- This page introduces essential geographic and meteorological concepts, defining parallels (latitude lines) and meridians (longitude lines). It distinguishes between zonal and meridional wind flows and categorizes latitude zones: low, mid, and high. The division of tropical and extratropical regions is highlighted, along with the occurrence of tropical and extratropical cyclones.
- 11.2: A Simplified Description of the Global Circulation
- This page explains Earth's atmospheric circulation features, focusing on surface influences like trade winds and cyclones, as well as upper tropospheric westerlies and Hadley cells crucial for heat and precipitation distribution. It highlights the role of solar declination in seasonal wind variations and introduces mid-latitude Ferrel cells and polar cells, noting their weakening in summer.
- 11.3: Radiative Differential Heating
- This page explores the Earth's atmospheric circulation influenced by differential heating due to solar radiation, resulting in warmer equatorial and colder polar regions. It covers temperature gradients relating to latitude, solar insolation, and net radiative flux, highlighting the balance between incoming and outgoing radiation.
- 11.4: Pressure Profiles
- This page examines residuals in scientific measurement, highlighting how unknown factors are estimated through differences from known values while noting that measurement errors can mask signals. It also details global circulation and concepts like non-hydrostatic pressure couplets, which affect vertical motion, and hydrostatic thermal circulations driven by temperature differences.
- 11.5: Geostrophic Wind and Geostrophic Adjustment
- This page explains wind patterns at the equator and mid-latitudes, highlighting the role of Coriolis force in pressure circulation. At the equator, winds quickly equalize pressure differences, while mid-latitudes exhibit more persistent patterns due to Coriolis effects.
- 11.6: Thermal Wind Effect
- This page explores the thermal wind effect, explaining how temperature gradients influence geostrophic winds and their vertical variations. It covers the thermal wind equations and thickness between isobaric surfaces, illustrating their significance in meteorology and weather map interpretation. The relationship between thermal wind components and geostrophic winds is detailed, with examples of calculations and case studies involving cold and warm fronts.
- 11.10: Horizontal Circulation
- This page covers horizontal circulation and vorticity in fluid dynamics. It explains calculating circulation around a closed shape using tangential wind velocity and distance increments. It highlights key relationships, particularly how circulation relates to wind shear and vorticity. Two important theorems, Kelvin's and Bjerknes, are discussed in the context of different atmospheric conditions.
- 11.11: Extratropical Ridges and Troughs (Rossby Waves)
- This page covers key atmospheric dynamics, focusing on the jet stream and Rossby waves generated by temperature gradients between the equator and poles. It explains related concepts such as vorticity, barotropic and baroclinic instabilities, and static stability. The interactions of atmospheric variables are illustrated through equations and examples.
- 11.12: Three-band Global Circulation
- This page explains the structure of thermally-driven planetary circulation divided into three latitude bands: the Hadley cell in low latitudes, Rossby waves in mid-latitudes, and a weak vertical cell in high latitudes. These circulation types work in unison to transport heat and momentum, ensuring the Earth's spin rate remains almost stable. It also introduces metrics for vertical circulation and discusses the influence of ocean currents and the Ekman spiral on global heat redistribution.
- 11.13: Ekman Spiral of Ocean Currents
- This page explains how friction between the atmosphere and ocean generates surface currents influenced by the Coriolis force, resulting in the Ekman spiral. It describes the 45° turn of surface currents relative to wind direction in the Northern Hemisphere and discusses deeper currents at greater angles.
- 11.14: Review
- This page explores how solar radiation and infrared output interact with global circulation patterns, particularly focusing on the warming of the equator and the formation of thunderstorms at the ITCZ. It details the generation of the Hadley cell, the impact of Coriolis forces on winds, and the roles of mid-latitude and polar jets.
- 11.15: Homework Exercises
- This page provides a comprehensive collection of exercises and discussions related to atmospheric and climatological concepts, emphasizing meteorological dynamics and circulation. Topics include analyzing satellite imagery, calculating vorticity, exploring atmospheric layers, and understanding global circulation cells like the Hadley and Ferrel cells. The exercises involve practical applications of mathematical principles, wind behavior, jet streams, and specific phenomena such as monsoons.
- 11.7: Explaining the General Circulation
- This page explores the Earth's atmospheric circulation, covering the impact of differential heating on wind patterns in various latitudes. It explains the formation of precipitation bands and high-pressure zones, the influence of the Coriolis force, and the dynamics of polar and mid-latitude systems. The polar front and jet stream dynamics, including Rossby waves, are highlighted for their role in weather variability.
- 11.8: Jet Streams
- This page explores the dynamics of polar and subtropical jet streams, highlighting their winter characteristics and influences such as temperature gradients and Earth's rotation. The polar jet is more variable than the steady subtropical jet. It discusses the limitations of simple models in predicting wind speeds and locations, emphasizing the role of angular momentum and real atmospheric forces like turbulence in contrast to theoretical estimates.
- 11.9: Types of Vorticity
- This page covers the concepts of vorticity, including relative, absolute, and potential vorticity, along with their calculations and applications in atmospheric phenomena such as jet streams and hurricanes. It also discusses shear curvature and isentropic potential vorticity (IPV), highlighting its importance in atmospheric dynamics and stability under adiabatic conditions.