Chapter 11 Copyright © 2011, 2015 by Roland Stull. Meteorology for Scientists and Engineers, 3rd Ed. Global CirCulation Contents Solar radiation absorbed in the trop- ics exceeds infrared loss, causing heat Nomenclature 330 11 to accumulate. The opposite is true at A Simplified Description of the Global Circulation 330 the poles, where there is net radiative cooling. Such Near-surface Conditions 330 radiative differential heating between poles and Upper-tropospheric Conditions 331 equator creates an imbalance in the atmosphere/ Vertical Circulations 332 ocean system (Fig. 11.1a). Monsoonal Circulations 333 Le Chatelier’s Principle states that an imbal- Differential Heating 334 anced system reacts in a way to partially undo the Meridional Temperature Gradient 335 imbalance. For the atmosphere of Fig. 11.1a, warm Radiative Forcings 336 tropical air rises due to buoyancy and flows toward Radiative Forcing by Latitude Belt 338 the poles, and the cold polar air sinks and flows to- Heat Transport by the Global Circulation 338 ward the equator (Fig. 11.1b). Pressure Profiles 340 Because the radiative forcings are unremitting, Non-hydrostatic Pressure Couplets 340 a ceaseless movement of wind and ocean currents Hydrostatic Thermal Circulations 341 results in what we call the general circulation or Geostrophic Wind & Geostrophic Adjustment 343 global circulation. The circulation cannot instant- Ageostrophic Winds at the Equator 343 ly undo the continued destabilization, resulting in Definitions 343 tropics that remain slightly warmer than the poles. Geostrophic Adjustment - Part 1 344 But the real general circulation on Earth does not Thermal Wind Relationship 345 look like Fig. 11.1b. Instead, there are three bands of Thickness 345 circulations in the Northern Hemisphere (Fig. 11.1c), Thermal Wind 346 Case Study 348 and three in the Southern. In this chapter, we will Thermal Wind & Geostrophic Adjust. - Part 2 349 identify characteristics of the general circulation, ex- plain why they exist, and learn how they work. Explaining the Global Circulation 350 Low Latitudes 350 High Latitudes 352 Mid-latitudes 352 B DPME C DPME Monsoon 356 Jet Streams 357 Baroclinicity & the Polar Jet 359 IPU IPU Angular Momentum & Subtropical Jet 360 IPU IPU Vorticity 362 Horizontal Circulation 365 Mid-latitude Troughs And Ridges 367 DPME DPME Barotropic Instability & Rossby Waves 367 Baroclinic Instability & Rossby Waves 371 D DPME Meridional Transport by Rossby Waves 374 1PMBSDFMM Three-band General Circulation 376 3PTTCZXBWFT A Measure of Vertical Circulation 377 )BEMFZDFMM Effective Vertical Circulation 377 IPU IPU Ekman Spiral In The Ocean 378 )BEMFZDFMM Summary 379 3PTTCZXBWFT Exercises 380 1PMBSDFMM DPME “Meteorology for Scientists and Engineers, 3rd Edi- tion” by Roland Stull is licensed under a Creative Figure 11.1 Commons Attribution-NonCommercial-ShareAlike Earth/atmosphere system, showing that (a) differential heating 4.0 International License. To view a copy of the license, visit http://creativecommons.org/licenses/by-nc-sa/4.0/ . This work is by radiation causes (b) atmospheric circulations. Add Earth’s available at http://www.eos.ubc.ca/books/Practical_Meteorology/ . rotation, and (c) 3 circulation bands form in each hemisphere. 329 330 CHAPTER 11 GLOBAL CIRCULATION /OPSUIQPMF nomenClature QPMBS IJHIMBUJUVEFT X TVCQPMBS MBUJUVEF Latitude lines are parallels, and east-west winds MP G / O are called zonal flow (Fig. 11.2). Each 1° of latitude B M J B E J O FYUSBUSPQJDBM NJEMBUJUVEFT S = 111 km. Longitude lines are meridians, and P F J E north-south winds are called meridional flow. N J / S F TVCUSPQJDBM Mid-latitudes are the regions between about N MPXMBUJUVEFT 30° and 60° latitude. High latitudes are 60° to 90°, FRVBUPSJBM and low latitudes are 0° to 30°. FRVBUPS PSUSPQJDBM The subtropical zone is at roughly 30° latitude, [POBMGMPX MPXMBUJUVEFT and the subpolar zone is at 60° latitude, both of TVCUSPQJDBM which partially overlap mid-latitudes. Tropics span UIQBSBMMFM 4 8 the equator, and polar regions are near the Earth’s M 8 P O FYUSBUSPQJDBM NJEMBUJUVEFT poles. Extratropical refers to everything outside of H J UV the tropics: poleward from roughly 30°N and 30°S. E 4 F For example, extratropical cyclones are low- TVCQPMBS IJHIMBUJUVEFT pressure centers — typically called lows and labeled QPMBS with L — that are found in mid- or high-latitudes. Tropical cyclones include hurricanes and ty- 4TPVUIQPMF phoons, and other strong lows in tropical regions. Figure 11.2 In many climate studies, data from the months Global nomenclature. of June, July, and August (JJA) are used to repre- sent conditions in N. Hemisphere summer (and S. Hemisphere winter). Similarly, December, January, February (DJF) data are used to represent N. Hemi- sphere winter (and S. Hemisphere summer). a simplified desCription of the ON DOING sCIENCe • toy models Global CirCulation Some problems in meteorology are so complex, and involve so many interacting variables and pro- Consider a hypothetical rotating planet with cesses, that they are intimidating if not impossible to solve. However, we can sometimes gain insight into no contrast between continents and oceans. The fundamental aspects of the problem by using ideal- climatological average (average over 30 years; ized, simplified physics. Such an approximation is see the Climate chapter) winds in such a simpli- sometimes called a “toy model”. fied planet would have characteristics as sketched A rotating spherical Earth with no oceans is one in Figs. 11.3. Actual winds on any day could dif- example of a toy model. The meridional variation of fer from this climatological average due to transient temperature given later in this chapter is another ex- weather systems that perturb the average flow. Also, ample. These models are designed to capture only monthly-average conditions tend to shift toward the the dominant processes. They are toy models, not summer hemisphere (e.g., the circulation bands shift complete models. northward during April through September). Toy models are used extensively to study climate change. For example, the greenhouse effect can be ex- amined using toy models in the Climate chapter. Toy near-surface Conditions models capture only the dominant effects, and ne- Near-surface average winds are sketched in Fig. glect the subtleties. You should never use toy models 11.3a. At low latitudes are broad bands of persis- to infer the details of a process, particularly in situa- tent easterly winds (U ≈ –7 m/s) called trade winds, tions where two large but opposite processes nearly named because the easterlies allowed sailing ships cancel each other. For other examples of toy models applied to the to conduct transoceanic trade in the old days. environment, see John Harte’s 1988 book “Consider a These trade winds also blow toward the equa- Spherical Cow”, University Science Books. 283 pp. tor from both hemispheres, and the equatorial belt of convergence is called the intertropical conver- R. STULL • METEOROLOGy FOR SCIENTISTS AND ENGINEERS 331 gence zone (ITCZ). On average, the air at the ITCZ B /FBS4VSGBDF is hot and humid, with low pressure, strong upward 1PMBS)JHI air motion, heavy convective (thunderstorm) pre- ) cipitation, and light to calm winds except in thun- - 4VCQPMBS-PXT - - / derstorms. This equatorial trough (low-pressure belt) was called the doldrums by sailors whose sailing ships were becalmed there for many days. ) - 8FTUFSMJFT At 30° latitude are belts of high surface pressure called subtropical highs (Fig. 11.3a). In these belts ) ) 4VCUSPQJDBM)JHIT ) ) / are hot, dry, cloud-free air descending from higher 5SBEF8JOET in the troposphere. Surface winds in these belts are /PSUI&BTUFSMJFT also calm on average. In the old days, becalmed sail- ing ships would often run short of drinking water, - EPMESVNT - *5$; - DBMN - causing horses on board to die and be thrown over- 4PVUI&BTUFSMJFT board. Hence, sailors called these miserable places 5SBEF8JOET the horse latitudes. On land, many of the world’s 4VCUSPQJDBM)JHIT deserts are near these latitudes. ) ) ) ) 4 In mid-latitudes are transient centers of low pres- 8FTUFSMJFT sure (mid-latitude cyclones, L) and high pressure ) - (anticyclones, H). Winds around lows converge - 4VCQPMBS-PXT - - 4 (come together) and circulate cyclonically — coun- ) terclockwise in the N. Hemisphere, and clockwise in the S. Hemisphere. Winds around highs diverge 1PMBS)JHI (spread out) and rotate anticyclonically — clock- wise in the N. Hemisphere, and counterclockwise in the S. Hemisphere. The cyclones are regions of bad weather (clouds, rain, high humidity, strong winds) C /FBS5SPQPQBVTF and fronts. The anticyclones are regions of good 1PMBS-PX weather (clear skies or fair-weather clouds, no pre- - cipitation, dry air, and light winds). / The high- and low-pressure centers move on av- erage from west to east, driven by large-scale winds USPVHI from the west. Although these westerlies dominate 1PMBS+FU SJEHF the general circulation at mid-latitudes, the surface winds are quite variable in time and space due to the )) )) 4VCUSPQJDBM+FU )) )) / sum of the westerlies plus the transient circulations around the highs and lows. 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). Near each pole is a climatological region of high pressure called a polar high. In these regions )) )) 4VCUSPQJDBM+FU )) )) 4 SJEHF are often clear skies, cold dry descending air, light 1PMBS+FU winds, and little snowfall. Between each polar high USPVHI (at 90°) and the subpolar low (at 60°) is a belt of weak easterly winds, called the polar easterlies. 4 - upper-tropospheric Conditions 1PMBS-PX The stratosphere is strongly statically stable, and acts like a lid to the troposphere. Thus, vertical cir- culations associated with our weather are mostly trapped within the troposphere. These vertical Figure 11.3 circulations couple the average near-surface winds Simplified global circulation in the troposphere: (a) near the surface, and (b) near the tropopause.