Therefore, the ITCZ is responsible for the wet and dry seasons in the tropics. The sun crosses the equator twice a year in March and September, and consequently makes for two wet seasons each year. In December and July, when the sun is at its greatest extent north or south of the equator makes for two dry seasons.
Further away from the equator, the two wet seasons merge into one, and the climate becomes more monsoonal, with one wet season and one dry season. Because of its location just north of the equator, Nigeria's climate is characterized by the hot and wet conditions associated with the movement of the Inter-Tropical Convergence Zone ITCZ north and south of the equator. This is easily seen in the normal monthly rainfall for two cities, Kano and Lagos, separated by miles km. When the ITCZ is to the south of the equator, the north-east winds prevail over Nigeria, producing the dry-season conditions.
When the ITCZ moves into the Northern Hemisphere, the south westerly wind prevails as far inland to bring rain fall during the wet season. For better understanding, we must know about the trade winds and air masses.
Trade Winds: Easterly winds that circle the Earth near the equator. Air Masses : A volume of air defined by its temperature and water vapour content. In tropical latitudes this air mass is hot to very hot, with high relative humidity, bringing unstable weather. It appears as a band of clouds consisting of showers, with occasional thunderstorms, that encircles the globe near the equator due to the convergence of the trade winds.
What is Tide, why it occurs and how it is important for human life? Source: www. When this occurs you have a Monsoon Trough. Due to the new movement of winds they favor the genesis of vorticity around very low pressures which in turn could form more easily tropical cyclones. Seasons are the result of this. ITCZ moves toward the hemisphere with most heat, wich are either hemisphere summers. According to me ITCZ is basically the trough where both the north eastern and south western winds converge or meet.
You must know that winds move from high pressure to low pressure, therefore when the winds meet an area of low pressure is created on the landmass which in turn brings in winds with moisture causing rainfall. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams?
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Zhang , and P. Peyrille , : Annual cycle of the West African monsoon: Regional circulations and associated water vapour transport. Toma , V. Webster , : Oscillations of the intertropical convergence zone and the genesis of easterly waves. Part I: Diagnostics and theory. Tomas , R. Webster , : The role of inertial instability in determining the location and strength of convection. Trewartha , G. University of Wisconsin Press, pp.
Tschirhart , G. Uppala , S. Waliser , D. Walker , H. Ghana Meteorological Dept. Note 5, 42 pp. Note 9. Wallace , J. Watson , D. Yan , Y.
Encyclopedia of World Climatology , J. Oliver, Ed. Yang , W. Seager , M. Cane , and B. Lyon , : The annual cycle of East African precipitation. Zhang , C. Woodworth , and G. Gu , : The seasonal cycle in the lower troposphere over West Africa from sounding observations. Zipser , E. Cecil , C. Liu , S. Nesbitt , and D. Yorty , : Where are the most intense thunderstorms on Earth? The dashed line is the Congo air boundary. Schematic illustration of the revised picture of the West African monsoon Nicholson Only every third vector in the analysis is plotted.
At the bottom of each panel is the surface elevation m. Data are from ERA-Interim. The common explanation for the progression of the rainy season over Africa is the seasonal excursion of the ITCZ. The ITCZ paradigm stems from a time when tropical rainfall was assumed to be associated mainly with localized convection. Its development was also linked to the emergence of midlatitude frontal concepts. The paradigm has numerous shortcomings, including the diversity of definitions and the large number of parameters used to identify the ITCZ.
A historical look at the concept shows that its use over Africa has long been controversial, with many eminent tropical meteorologists harshly criticizing its applicability over this continent. However, the seasonal excursion of the ITCZ remains the classical explanation for African rainy seasons, especially in the equatorial region.
This article underscores the shortcomings of the paradigm in equatorial Africa by examining various aspects of the circulation associated with the spatial and temporal patterns of rainfall during the equatorial rainy seasons. The overall conclusion is that a deeper understanding of the seasonal cycle in the equatorial regions of Africa still needs to be developed.
For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy www. This article challenges the assumption that the seasonal cycle of rainfall in equatorial Africa is controlled by the seasonal excursion of the ITCZ and calls for additional research on the seasonal cycle. The intertropical convergence zone ITCZ has long been assumed to be a major control on tropical rainfall over both oceans and land. The assumed link to the tropical rainy seasons stems back to a time when tropical rainfall was thought to be produced primarily by local thunderstorms, with the ascent in the ITCZ promoting their development when thermodynamic conditions were favorable.
This assumption of purely localized convection has long been overturned, with numerous studies showing the importance of mesoscale convective systems e. In view of this evolution of our understanding of tropical rainfall, it seems meaningful to also reexamine the ITCZ concept. Here, this question is raised specifically concerning equatorial Africa, where rainy seasons occur twice annually—in the boreal spring and autumn.
This article commences with an overview of various published definitions of the ITCZ. This is followed by a historical look at the development of the image of the ITCZ over Africa, as well as controversies surrounding it. The development of the current scenario for West Africa is then described. The availability of reanalysis datasets permits the evaluation of this scenario. Here, the issue is examined for the two equatorial rainy seasons, focusing on April and November.
The ambiguity of the ITCZ concept is made clear with a brief survey of definitions in the literature. The usages differ with respect to the variable chosen to define this zone. Holton et al. Perhaps as a result of this ambiguity in definition, the tracking of the ITCZ has variously been based on the pressure minimum, surface wind convergence Grodsky et al.
The availability of satellite photos has led to the last two parameters being frequently utilized as a matter of convenience Waliser and Gautier The use of so many different parameters has been justified by the long-held assumptions that 1 the pressure minimum and rainfall maximum are collocated with each other and with the wind convergence, 2 maximum cloudiness is roughly collocated with maximum rainfall, and 3 longwave radiation is at a minimum at that location.
Unfortunately, these assumptions, especially the first one, do not stand up to close scrutiny. Even over the oceans the zone of minimum pressure does not generally coincide with that of the wind convergence or the rainfall maximum Tomas and Webster ; Toma and Webster Over West Africa even the maximum in surface convergence is some km south of the surface wind discontinuity between easterly and westerly winds Hastenrath Notably, most definitions emphasize the convergence of the trade winds.
This may be appropriate over some oceans sectors. However, for the most part, the trade winds do not exist over the tropical landmasses.
Thus, over Africa depictions of the surface ITCZ instead represent the meeting of the northeasterlies and southwesterly monsoon flow. However, the term is all too frequently applied to the rainfall maximum. Recognizing that the surface convergence zone and the rainfall maximum are not closely coupled, many authors now avoid the use of the term ITCZ in discussing rainfall over Africa. For example, in describing the rainfall maximum, Ross and Krishnamurti prefer the term equatorial rain belt.
Zhang et al. The latter term is used in this article. Clearly, over most of the tropical landmasses, and over Africa in particular, the use of the term ITCZ should be avoided except in coastal areas where the trade winds are influential. A search of the early meteorological literature uncovers just as much confusion and ambiguity about the ITCZ and its historical origin.
Hadley, however, did not mention the convergence, but it is implicit in his model of the vertical cells. The concept had definitely come into vogue by the s and s, when meteorologists attempted to apply midlatitude frontal concepts of the Bergen school to the tropics Barry and Chorley ; Palmer When the importance of wind convergence in tropical weather was realized in the s and s Barry and Chorley , the trade wind convergence was designated the intertropical convergence zone, initially abbreviated as ITC Fletcher Fig.
Citation: Bulletin of the American Meteorological Society 99, 2; Most of the work that led to the development of the ITCZ concept was based on conditions over the Pacific. Later, Bjerknes et al. Petterssen , further promoted the global picture of an ITCZ but defined it as the meeting of the trade winds. He used it to explain the basic rainfall pattern that exists over much of the tropics: bimodal rainfall seasonality in the equatorial latitudes with the twice-equatorial transit of the ITCZ, unimodal in the outer tropics when the ITCZ reaches its extreme latitudinal positions.
Variants of their maps are repeated in nearly every climatological textbook, as well as in a multitude of articles and textbooks from a broad range of disciplines. Eventually, the picture emerged of a global zone in which the pressure minimum, cloud and rainfall maxima, and low-level wind convergence coincided.
These ideas were not without controversy. Trewartha pointed out the ambiguity of the ITCZ and the rather capricious approach to delineating it. Ramage was particularly harsh in his criticism of the ITCZ concept.
They mentioned that the formation is discontinuous in time and space and not well developed in the doldrums. In summary, the origins of the ITCZ paradigm for Africa are murky, but it clearly emerged from attempts to parallel midlatitude concepts. However, its development was haphazard.
Most of those who argued for and implemented the concept were midlatitude meteorologists. Tropical meteorologists, and especially those who actually worked in Africa, harshly criticized the ITCZ paradigm and its applications, with many suggesting that it was completely wrong. Unfortunately, the use of this paradigm has persisted.
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