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The Shape of Continents, Air-Sea Interaction, and the Rising Branch of the Hadley Circulation

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The Shape of Continents, Air-Sea Interaction, and the Rising Branch of the Hadley Circulation To appear in The Hadley Circulation: Past, Present and Future, H. F. Diaz and R. S. Bradley (Eds.), Kluwer Academic Publishers, Dordrecht. The Shape of Continents, Air-Sea Interaction, and the Rising Branch of the Hadley Circulation Shang-Ping Xie International Pacific Research Center and Department of Meteorology, University of Hawaii Honolulu, HI 96822, USA ([email protected]) This chapter begins with a brief history of ITCZ research, and then goes on to summarize the recent progress in understanding why the ITCZ is locked in the Northern Hemisphere in the eastern Pacific and Atlantic, and how this northward-displaced ITCZ affects the space- time structure of tropical climate variability. ___________________ 1. Introduction The rest of the chapter is organized as follows. Sections 2 and 3 give historical and observational background, The differential solar radiation in the meridional direction is respectively. Section 4 investigates ocean-atmosphere the ultimate drive for the global Hadley circulation, dictating interactions that maintain the climatic asymmetry of the that its rising branch and heavy rainfall should be located near northward-displaced ITCZ, and Section 5 considers the effect the equator. This solar forcing of the atmosphere is indirect, of land-sea distribution. Section 6 discusses the climatic however, since most of solar radiation absorption takes place consequence of the northward-displaced ITCZ. Following a at the surface of Earth. Over the tropical oceans, most of the discussion of some remaining issues in Section 7, Section 8 absorbed solar energy is used for surface evaporation and the summarizes the main results. resultant water vapor is gathered by winds to fuel deep convection that is organized into zonally oriented rain bands. 2. History of the study of tropical winds and rains The ocean’s effect on tropical convection and hence the rising branch of the Hadley circulation is obvious; tropical rain belts “It is not the work of one, nor of few, but of a multitude of are anchored on the warmest waters, with spatial patterns that Observers, to bring together the experience required to can markedly deviate from the distribution of insolation. In compose a perfect and complete History of these Winds.” particular, the rain band over the eastern Pacific and Atlantic Edmond Halley (1686) Oceans, called the intertropical convergence zone (ITCZ), is mysteriously displaced to the north of the equator in the Before the invention of steam engines, knowledge of the annual-mean climatology, a distribution inexplicable from direction, speed and steadiness of sea surface winds was of solar forcing alone1. vital importance for the navigation of sailing boats. By the This chapter reviews the progress made in the past decade in late-17th century, the traffic between Europe and the New understanding the coupled ocean-atmospheric dynamics that World had grown to such a level that Halley (1686) was able govern the rising branch of the Hadley circulation and places to compile a quite accurate map of surface-wind streamlines this progress in a historical perspective. This chapter focuses for the tropical Atlantic and Indian Oceans by gathering on the ITCZ over the eastern Pacific and Atlantic, while information from navigators. Figure 1 reproduces the Atlantic Webster (2004) in this volume discusses convection in the portion of Halley’s wind map that depicts the steady trade Indo–western Pacific sector. Wang et al. (2004a) is a global winds in the Northern and Southern Hemispheres. survey of air-sea interaction and its role in climate variability, Remarkably, the southeasterly and northeasterly trades meet including a comparative view for the three tropical oceans. north of, instead of on the equator as one might expect from equatorial symmetry. The ITCZ—the modern term for the region where the trade winds meet—is displaced to the Northern Hemisphere in the annual mean. Halley wrote about 1 The latitude of the sinking branch of the Hadley circulation the ITCZ: “it were improper to say there is any Trade Winds, is not directly determined by solar radiation either. Instead, it or yet a Variable; for it seems condemned to perpetual Calms, is determined by dynamic requirements like angular attended with terrible Thunder and Lightning, and Rains so momentum conservation and baroclinic instability (Held and Hou 1980; Lindzen and Hou 1988). 1 frequent, that our Navigators from thence call this part of the Commerce 1971), the Pacific and Atlantic ITCZ appears as on Sea the Rains”. the dark ocean background a silver belt that is north of the In the ITCZ, surface air rises and in the process, the water equator in both boreal summer and winter and one of the most vapor it carries condenses, resulting in the frequent rains and visible and striking features in such satellite images. Since thunder storms Halley noted and releasing a huge amount of 1979, outgoing longwave radiation (OLR) measurements by latent heat that drives the Hadley and global circulation of the satellite infrared sensors are often used as a proxy of troposphere. Hereafter we will use the terms ITCZ 2 , precipitating deep convection that reaches great heights. A convective zone, and precipitation band interchangeably. The paradox arises: over the eastern Pacific and Atlantic, the ITCZ resides in a zone of “perpetual calms” in Halley’s words, OLR-based estimate of rainfall is too low compared to ship and is now called the Equatorial Doldrums in textbooks. As reports, which indicate substantial precipitation accompanied will become clear in Section 4a, the collocation of the by strong surface wind convergence there (Fig. 3). It turns out Doldrums with the ITCZ is the key to the mystery of their that this underestimation by the OLR-based method in the northward displacement from the equator. eastern Pacific and Atlantic ITCZ is due to the fact that the Before the late 17th century, the vast Pacific Ocean was SST (~27oC) there is significantly lower than in the Indo- much less navigated than the Atlantic and Halley had little western Pacific warm pool (SST>28oC). As a result, information on its wind distribution other than accounts that convection in the eastern Pacific and Atlantic does not reach “there is great conformity between the winds of this Sea and as high as in the western Pacific, yielding higher OLR values those of the Atlantic”. This lack of observations forced Halley (Thompson et al. 1979; J.M. Wallace 1994, pers. comm.). to draw an “analogy between” the Pacific winds “and those of More recent satellite microwave sensors, measuring quantities the Atlantic”. Interestingly, Halley did not draw a perfect more directly related to precipitation than the infrared ones, analogy with the Atlantic winds; in his map, the Pacific trades observe similar rain rates in the eastern and in the western converge on the geographical equator, rather than on northern Pacific. latitudes as in the Atlantic. Perhaps Halley or his Figure 2 shows the annual-mean precipitation climatology contemporaries had no reason to believe that the Pacific wind based on combined infrared and microwave satellite system should depart from equatorial symmetry. By the late observations. Over the continents and the Indo-western 19th century, Köppen’s (1899) atlas shows that the similarity Pacific sector, the annual-mean precipitation distribution in between the Pacific and Atlantic is greater than Halley the tropics is more or less symmetric about the equator, thought; as in the Atlantic, the Pacific trades also converge consistent with solar radiation distribution. On the seasonal onto the Northern Hemisphere even in boreal winter. timescale, the maximum rainfall in these regions moves back A reliable precipitation climatology proves more difficult to and forth across the equator following the sun (Mitchell and obtain because of sporadic nature of rains. In Bartholomew Wallace 1992). This solar control of tropical convection and Herbertson’s (1899) map of annual rainfall, the Pacific breaks down over the eastern half of the Pacific and entire Ocean was left blank. (One can nevertheless, infer a strong Atlantic, where deep convection is confined to the ITCZ north equatorial asymmetry from the depicted rainfall on the Pacific of the equator. This climatic asymmetry persists for most of coast that is over 160 inches/year in Colombia north of the the year, even during boreal winter when the solar radiation equator but less than 10 inches/year on the Peruvian coast.) south of the equator exceeds that to the north (Fig. 4). Only By the mid-20th century, Möller’s (1951) map of annual-mean for a brief period of March and April, a double ITCZ appears rainfall is very similar to modern climatology (Fig. 2), with a rain band on each side of the equator. showing that the ITCZ rain band is clearly displaced to the Located in the region where a great amount of latent heat is Northern Hemisphere over both the eastern Pacific and released to the atmosphere, the ITCZ is sometimes called the Atlantic. thermal equator. The peculiar location of the thermal equator in the eastern Pacific and Atlantic begs answers to the 3. Observational background following questions. i) Why is the ITCZ not on the equator where the annual-mean solar radiation is the maximum? ii) The advent of satellite remote sensing opened the door for Given that annual-mean solar radiation is roughly symmetric global observations of clouds in 1960s and somewhat later, about the equator, why is the ITCZ displaced north of the for observations of precipitation. In an early climatology of equator? iii) What effect does this northward displacement of reflectivity (U.S. Air Force and U.S. Department of the thermal equator have on climate variability? Two schools of thought exist regarding the first two questions. One points 2 More precisely, our definition of ITCZ refers to those to the strong hemispheric asymmetry in the landmass and its surface convergence zones over warm oceans with SST distribution and suggests that this continental asymmetry greater than the convective threshold (26-27oC).
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