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This work is available at http://www.eos.ubc.ca/books/Practical_Meteorology/ . 11 GENERAL CIRCULATION Contents A spatial imbalance between radiative inputs and outputs exists for the earth-ocean-atmosphere Key Terms 330 system. The earth loses energy at all latitudes due A Simplified Description of the Global Circulation 330 to outgoing infrared (IR) radiation. Near the trop- Near the Surface 330 ics, more solar radiation enters than IR leaves, hence Upper-troposphere 331 there is a net input of radiative energy. Near Earth’s Vertical Circulations 332 poles, incoming solar radiation is too weak to totally Monsoonal Circulations 333 offset the IR cooling. The net result is differential Radiative Differential Heating 334 heating, creating warm equatorial air and cold po- North-South Temperature Gradient 335 lar air (Fig. 11.1a). Global Radiation Budgets 336 This imbalance drives the global-scale general Radiative Forcing by Latitude Belt 338 circulation of winds. Such a circulation is a fluid- General Circulation Heat Transport 338 dynamical analogy to Le Chatelier’s Principle of Pressure Profiles 340 chemistry. Namely, an imbalanced system reacts Non-hydrostatic Pressure Couplets 340 in a way to partially counteract the imbalance. The Hydrostatic Thermal Circulations 341 continued destabilization by radiation causes a gen- Geostrophic Wind & Geostrophic Adjustment 343 eral circulation of winds that is unceasing. Ageostrophic Winds at the Equator 343 You might first guess that buoyancy causes air Definitions 343 in the warm regions to rise, while cold air near Geostrophic Adjustment - Part 1 344 the poles sink (Fig. 11.1b). Instead, the real general Thermal Wind Effect 345 circulation has three bands of circulations in the Definition of Thickness 346 Northern Hemisphere (Fig. 11.1c), and three in the Thermal-wind Components 346 Case Study 348 Southern. In this chapter, we will identify charac- Thermal Wind & Geostrophic Adjust. - Part 2 349 teristics of the general circulation, explain why they exist, and learn how they work. Explaining the General Circulation 350 Low Latitudes 350 High Latitudes 352 B DPPM C DPPM Mid-latitudes 352 Monsoon 356 Jet Streams 357 XBSN XBSN Baroclinicity & the Polar Jet 359 Angular Momentum & Subtropical Jet 360 XBSN XBSN Types of Vorticity 362 Relative-vorticity Definition 362 Absolute-vorticity Definition 363 DPPM DPPM Potential-vorticity Definition 363 Isentropic Potential Vorticity Definition 364 D DPPM 1PMBSDFMM Horizontal Circulation 365 3PTTCZXBWFT Extratropical Ridges & Troughs 367 Barotropic Instability 367 )BEMFZDFMM Baroclinic Instability 371 XBSN XBSN Meridional Transport by Rossby Waves 374 )BEMFZDFMM Three-band Global Circulation 376 3PTTCZXBWFT A Metric for Vertical Circulation 377 1PMBSDFMM Effective Vertical Circulation 377 DPPM Earth image Ekman Spiral of Ocean Currents 378 Figure 11.1 credit: NASA Review 379 Radiative imbalances create (a) warm tropics and cold poles, in- Homework Exercises 380 ducing (b) buoyant circulations. Add Earth’s rotation, and (c) three circulation bands form in each hemisphere. 329 330 cHAPTER 11 • General CIRCULATION OPSUIQPMF / Key Terms QPMBS IJHIMBUJUVEFT Lines of constant latitude are called parallels, MBUJUVEF X TVCQPMBS and winds parallel to the parallels are identified as MP G / O zonal flows (Fig. 11.2). Lines of constant longitude M JB B E NJE J O FYUSBUSPQJDBM are called meridians, and winds parallel to the me- S P MBUJUVEFT F J E ridians are known as meridional flows. N J S F TVCUSPQJDBM Between latitudes of 30° and 60° are the mid- / N MPXMBUJUVEFT latitudes. High latitudes are 60° to 90°, and low USPQJDBMPS latitudes are 0° to 30°. Each 1° of latitude = 111 km. FRVBUPS FRVJUPSJBM Tropics, subtropics, subpolar, and polar re- [POBMGMPX MPXMBUJUVEFT gions are as shown in Fig. 11.2. Regions not in the TVCUSPQJDBM tropics are called extratropical; namely, poleward UIQBSBMMFM 8 of about 30°N and about 30°S. M 4 8 P NJE O FYUSBUSPQJDBM For example, tropical cyclones such as hur- H MBUJUVEFT J UV ricanes are in the tropics. Low-pressure centers 4 E F (lows, as indicated by L on weather maps) outside TVCQPMBS IJHIMBUJUVEFT of the tropics are called extratropical cyclones. QPMBS In many climate studies, data from the months of June, July, and August (JJA) are used to repre- 4 sent conditions in N. Hemisphere summer (and S. TPVUIQPMF Hemisphere winter). Similarly, December, January, February (DJF) data are used to represent N. Hemi- Figure 11.2 sphere winter (and S. Hemisphere summer). Global key terms. A SCIENTIFIC PERSPECTiVe • idealiza- tions of nature A simplified descripTion of The Natural atmospheric phenomena often involve the GlobAl circulation superposition of many different physical processes and scales of motion. Large-scale average conditions Consider a hypothetical rotating planet with are said to be caused by zeroth-order processes. no contrast between continents and oceans. The Dominant variations about the mean are controlled climatological average (average over 30 years; by first-order processes. Finer details are caused see the Climate chapter) winds in such a simpli- by higher-order processes. Sometimes insight is possible by stripping away fied planet would have characteristics as sketched the higher-order processes and focusing on one or in Figs. 11.3. Actual winds on any day could dif- two lower orders. Equations that describe such sim- fer from this climatological average due to transient plified physics are known as toy models. With the weather systems that perturb the average flow. Also, right simplifications, some toy models admit analyti- monthly-average conditions tend to shift toward the cal solutions. Compare this to the unsimplified phys- summer hemisphere (e.g., the circulation bands shift ics, which might be too complicated to solve analyti- northward during April through September). cally (although numerical solutions are possible). A rotating spherical Earth with no oceans and with uniform temperature is one example of a zeroth- near the surface order toy model. A first-order toy model might add Near-surface average winds are sketched in Fig. the north-south temperature variation, while neglect- 11.3a. At low latitudes are broad bands of persistent ing east-west and continent-ocean variations. With easterly winds (U ≈ –7 m s–1) called trade winds, even more sophistication, seasonal or monthly varia- named because the easterlies allowed sailing ships tions might be explained. We will take the approach to conduct transoceanic trade in the old days. in this chapter to start with zeroth-order models to These trade winds also blow toward the equa- focus on basic climate concepts, and then gradually tor from both hemispheres, and the equatorial belt add more realism. John Harte (1988) wrote a book demonstrating the of convergence is called the intertropical conver- utility of such toy models. It is “Consider a Spherical gence zone (ITCZ). On average, the air at the ITCZ Cow”, by University Science Books. 283 pages. is hot and humid, with low pressure, strong upward R. STULL • PraCTICAL METEOROLOGY 331 air motion, heavy convective (thunderstorm) pre- B /FBS4VSGBDF cipitation, and light to calm winds except in thun- 1PMBS)JHI derstorms. This equatorial trough (low-pressure ) belt) was called the doldrums by sailors whose - 4VCQPMBS-PXT - - / sailing ships were becalmed there for many days. At 30° latitude are belts of high surface pressure called subtropical highs (Fig. 11.3a). In these belts ) - 8FTUFSMJFT are hot, dry, cloud-free air descending from higher in the troposphere. Surface winds in these belts are ) ) 4VCUSPQJDBM)JHIT ) ) / also calm on average. In the old days, becalmed sail- 5SBEF8JOET ing ships would often run short of drinking water, /PSUI&BTUFSMJFT causing horses on board to die and be thrown over- board. Hence, sailors called these miserable places - EPMESVNT - *5$; - DBMN - the horse latitudes. On land, many of the world’s 4PVUI&BTUFSMJFT deserts are near these latitudes. 5SBEF8JOET In mid-latitudes are transient centers of low pres- sure (mid-latitude cyclones, L) and high pressure ) ) 4VCUSPQJDBM)JHIT ) ) 4 (anticyclones, H). Winds around lows converge 8FTUFSMJFT (come together) and circulate cyclonically — coun- ) - terclockwise in the N. Hemisphere, and clockwise in the S. Hemisphere. Winds around highs diverge - 4VCQPMBS-PXT - - 4 (spread out) and rotate anticyclonically — clock- ) wise in the N. Hemisphere, and counterclockwise in 1PMBS)JHI the S. Hemisphere. The cyclones are regions of bad weather (clouds, rain, high humidity, strong winds) and fronts. The anticyclones are regions of good weather (clear skies or fair-weather clouds, no pre- C /FBS5SPQPQBVTF cipitation, dry air, and light winds). 1PMBS-PX The high- and low-pressure centers move on av- - erage from west to east, driven by large-scale winds / from the west. Although these westerlies dominate the general circulation at mid-latitudes, the surface USPVHI winds are quite variable in time and space due to the 1PMBS+FU sum of the westerlies plus the transient circulations SJEHF around the highs and lows. )) )) 4VCUSPQJDBM+FU )) )) / Near 60° latitude are belts of low surface pres- sure called subpolar lows. Along these belts are light to calm winds, upward air motion, clouds, cool temperatures, and precipitation (as snow in winter).