Asymmetry of the Intertropical Convergence Zone

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There are a number of explanations of the asymmetry of the Intertropical Convergence Zone (ITCZ), known by sailors as The Doldrums.

Asymmetrical distribution of continents[edit]

ITCZ is supposed to be overlapped by the geographic equator according to the symmetric solar radiation.[1] However, in reality, ITCZ is mostly perennial northern hemisphere in the eastern Pacific and Atlantic Oceans.[2][3] It is originally explained by the asymmetrical distribution of continents. However, the distribution of land and ocean is severely asymmetric in the Indian Ocean, yet the ITCZ moves forth and back between the southern and northern hemisphere.[4] Continents surround the Indian Ocean and monsoons prevail. Where the thermocline is deeper indicates that there is a weaker interaction between atmosphere and ocean. Due to the relatively small scale and deep thermocline in Indian Ocean,there will be less asymmeric effects. To the eastern Pacific and Atlantic Ocean,because of the large scale easterly wind system and the western continents boundaries,the thermocline is definitely shallower in eastern part. Thus,the asymmetry is obvious in eastern Pacific and Atlantic Ocean. There are two factors that are acknowledged by oceanographers and meteorologists: the interactions between ocean and atmosphere and the geometries of the continents.[5]

Asymmetric SST Distribution[edit]

According to observations, the Sea Surface Temperature (SST) of ITCZ in the Northern Hemisphere is higher than the same latitude in Southern Hemisphere. We can assume that the asymmetry of ITCZ is triggered by the asymmetric SST distribution, which has been verified by General Circulation Model (GCM).[6]

Wind-evaporation-SST mechanism[edit]

What's more, because the Cross Equator SST Gradient (CESG) is southwards, the cross equator northwards wind, which decelerates trade winds of north of the equator and accelerates that of south of the equator due to Coriolis force, is originated. Thus the evaporation of northern tropics is weakened, thereby cooling down northern tropicalSST slightly. Vice versa, SST of southern part to the equator is much more reduced. Therefore, SST of northern tropical is much more higher and higher than southern tropical, by which increases the CESG.As a result, this positive feedback, which is defined as Wind-Evaporation-SST (WES) will intensify this process.[7]

Explanation of Asymmetric SST Distribution[edit]

In this way, the WES keeps ITCZ northern of the equator. And the precondition of WES is the asymmetric distribution of SST, which WES also strengthens this process. According to the observation of equatorial upwelling and obvious asymmetric of ITCZ in Pacific and Atlantic, it is inferred that it’s the equatorial upwelling that prevents the ITCZ created at the equator.[8] To explain this simply, upwelling brings up cold water to the surface, which will chill the above atmosphere and make it stable because of relative high density due to low temperature. Thus, this area of equator is totally different compared to the strong vertical convection and abundant precipitation of ITCZ. As a result, the SST is latitudinal asymmetric, which is an assumption in the above article.[1]

References[edit]

  1. ^ a b C.Wang,S.-P.Xie,and J.A.Carton (2004). "Tropical Atlantic Variability: Patterns, Mechanisms,and Impacts.". Geophysical Monograph. AGU. 
  2. ^ Hastenrath, S. (1991). "Climate dynamics of the tropics.". 488 pp., Kluwer Academic, Boston, USA,. 
  3. ^ Mitchell, T. P., and J. M.Wallace. (1992). "The annual cycle in equatorial convection and sea surface temperature.". J. Clim., 5, 1140-1156,. 
  4. ^ Shang-Ping Xie . "What Keeps the ITCZ North of the Equator? An Interim Review". 
  5. ^ Philander, S. G. H.; Gu, D.; Lambert, G.; Li, T.; Halpern, D.; Lau, N.-C.; Pacanowski, R. C. (1995). "why the itcz is mostly north of the equator". Journal of Climate, vol. 9, Issue 12, pp.2958-2972. 
  6. ^ Philander, S.G.H., et al.,1996,The role of low-level stratus clouds in keeping the ITCZ mostly north of the equator. J. Clim., 9, 2958-2972.
  7. ^ Xie, S.-P., and S.G.H. Philander, (1994). "A coupled ocean-atmosphere model of relevance to the ITCZ in the eastern Pacific,". Tellus,46A, 340-350,. 
  8. ^ Pike,A.C., (1971). "the intertropical convergence zone studied with an interacting atmosphere and ocean model". Mon.Wea.Rev.,99,469-477.