OBSERVATATIONS AND
MECHANISMS OF
INTERANNUAL VARIABILITY
IN WEST AFRICA
SHARON NICHOLSON
FLORIDA STATE UNIVERSITY
METEOROLOGY DEPT.
MARCH 2007
OBSERVATIONS AND
MECHANISMS OF INTERANNUAL
VARIABILITY OVER WEST AFRICA
I. INTRODUCTION - THE ITCZ VS. TROPICAL RAINBELT
II. A NEW PARADIGM FOR UNDERSTANDING VARIABILITY
ATMOSPHERIC CIRCULATION
LONG-TERM VARIABILITY OF RAINFALL
CONCEPTUAL MODEL OF VARIABILITY
III. WET AND DRY MODES OVER THE SAHEL
IV. MECHANISMS OF VARIABILITY - GENERIC
INTENSIFICATION/WEAKENING OF RAINBELT - IN PROGRESS
NORTH/SOUTH DISPLACEMENT OF THE RAINBELT
V. INERTIAL INSTABILITY - LINKS TO ATLANTIC
VI. APPLICATION TO DECADAL VARIABILITY
I – CLASSIC VIEW: THE ITCZ OVER AFRICA
Seasonal Excursion of the Tropical Rainbelt (“ITCZ”)
REVISED VIEW
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The ITCZ over West Africa is a surface
zone of wind convergence.
It is NOT the main mechanism of
rainfall production over West Africa.
“TROPICAL RAIN BELT”
II – ATMOSPHERIC CIRCULATION (AUGUST)
“CENTERS OF ACTION” - JET STREAMS
AFRICAN EASTERLY JET
AEJ
TROPICAL EASTERLY JET
TEJ
II – ATMOSPHERIC CIRCULATION (AUGUST)
MEAN VERTICAL CROSS-SECTION, AUGUST ZONAL WIND
II – ATMOSPHERIC CIRCULATION (AUGUST)
VERTICAL MOTION – DECOUPLING OF ITCZ / TROPICAL RAINBELT
Area between the
AEJ and TEJ axes
NOTE: Decoupling
of ITCZ / rainbelt
ITCZ
Guinea Coast
II – ATMOSPHERIC CIRCULATION (AUGUST)
VERTICAL MOTION – 3 SOURCES OF ASCENT
II – ATMOSPHERIC CIRCULATION (AUGUST)
AEW TRACKS
II – ATMOSPHERIC CIRCULATION (AUGUST)
CONVERGENCE/DIVERGENCE AT VARIOUS LEVELS
Near surface
AEJ
TEJ
INTERANNUAL VARIABILITY OVER
WEST AFRICA
A NEW PARADIGM
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
CONCEPTUAL MODEL: VARIATIONS IN TROPICAL RAINBELT
FOUR TYPES OF YEARS: 1950, 1955, 1983, 1984
FACTORS CONTROLLING RAINBELT CHARACTERISTICS
WET AND DRY MODES - INDEPENDENT DYNAMIC MODES
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
CONCEPTUAL MODEL:
VARIATIONS IN THE TROPICAL RAINBELT
LOCATION – WHERE IT RAINS
INTENSITY – HOW MUCH IT RAINS
LATITUDINAL EXTENT – SEASON LENGTH
(“WIDTH” OF RAINBELT)
2 PRIMARY MECHANISMS OF VARIABILITY
INTENSIFICATION / WEAKENING
NORTH / SOUTH DISPLACEMENT
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
SOUTH
INTENSE
NORTH
WEAK
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
FACTORS CONTROLLING RAINBELT CHARACTERISTICS
QUESTIONS
1.
WHAT DETERMINES THE WIDTH (I.E., LATITUDINAL EXTENT) OF
THE TROPICAL RAINBELT?
SEPARATION OF AXES OF AEJ / TEJ
2.
WHAT DETERMINES THE LOCATION (I.E., LATITUDE) OF THE
TROPICAL RAINBELT OVER WEST AFRICA?
AEJ LATITUDE / LOW LEVEL WESTERLY JET
3.
WHAT DETERMINES THE INTENSITY OF THE TROPICAL
RAINBELT?
INTENSITY OF THE TEJ / VERTICAL WIND SHEAR
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
WET AND DRY MODES
Speculation: Feedback from enhanced rainfall
Strong latent heat release at TEJ level
Intensification of TEJ via coupling and latent heat
Further intensification of rainbelt intensity
WET MODE
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Strong westerlies
Strong TEJ
AEJ displaced poleward
Strong vertical shear
Strong tropical rainbelt
Strong upper/lower
troposphere coupling
Waves extend from lower to
upper troposphere
DRY MODE
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Weak Westerlies
Weak TEJ
AEJ displaced equatorward
Weak vertical shear
Weak tropical rainbelt
Weak upper/lower troposphere
coupling
Waves confined to lower
atmosphere
III – INTERANNUAL VARIABILITY OVER WEST AFRICA
MECHANISMS OF VARIABILITY
NORTH / SOUTH DISPLACEMENT OF
RAINBELT / AEJ
(THIS TALK)
INTENSIFICATION / WEAKENING
(IN PROGRESS)
IV - INERTIAL INSTABILITY – SWITCH TO WET MODE
IV - INERTIAL INSTABILITY – SWITCH TO WET MODE
PREREQUISITE 1 – STRONG SST/ SLP GRADIENTS IN WET YEARS
V - APPLICABILITY TO DECADAL VARIABILITY
LINK BETWEEN
PRESSURE GRADIENT
SPEED OF WESTERLY JET
LATITUDE OF ZERO ABSOLUTE
VORTICITY CONTOUR
IV - INERTIAL INSTABILITY – SWITCH TO WET MODE
RELATIONSHIP BETWEEN TEJ AND
WESTERLIES
18
16
14
12
10
8
6
4
2
0
-40
-35
-30
-25
Speed of TEJ
-20
-15
-10
Speed of Low Level Westerlies
HISTOGRAMS OF SPEEDS
V - APPLICABILITY TO DECADAL VARIABILITY
MEAN ANOMALIES FOR THE 1950’s & 1980’s
POSITIVE
ANOMALIES
V - APPLICABILITY TO DECADAL VARIABILITY
VERTICAL
MOTION
V - APPLICABILITY TO DECADAL VARIABILITY
CROSS-EQUATORIAL PRESSURE GRADIENT
CONCLUSIONS
MAJOR FACTORS IN INTERANNUAL VARIABILITY OVER WEST AFRICA
INTENSITY OF THE TEJ
LOCATION OF THE AEJ
LATITUDINAL SEPARATION OF THE JET AXES
THESE ARE CONSISTENT WITH THE SPATIAL PATTERNS OF VARIABILITY
THESE INFLUENCE
INTENSITY OF VERTICAL MOTION
INTENSITY OF RAINFALL IN THE RAINBELT
LATITUDINAL EXTENT/LOCATION OF RAINBELT
WAVE ACTIVITY OVER WEST AFRICA
AN IMPORTANT MECHANISM IN VARIABILITY IS INERTIAL INSTABILITY
THIS MECHANISM PROVIDES AN IMPORTANT LINK TO THE ATLANTIC
THIS MECHANISM IS ALSO APPLICABLE TO DECADAL VARIABILITY
IV - INERTIAL INSTABILITY – SWITCH TO WET MODE
PREREQUISITES 4 – DIVERGENCE EQUATORWARD OF CONTOUR,
CONVERGENCE POLEWARD
(NOTE ALSO: STRONG AEJ/TEJ CIRCULATION IN WET YEARS)