Tropical climates - The Global Climate System: Patterns, Processes, and Teleconnections - page 62

N N

Figure 3.3 Models of
tropical meridional
circulation. (a) Hadley Cells
on two side of the equator;
(b) two equatorial cells
separating the two Hadley
Cells; (c) and (d) a single
equatorial cell separating the
Hadley Cells. (Based upon
various sources including
Asnani 1968 )

HADLEY
CELL

HADLEY
CELL

30 ° N

30 ° N

EQUATORIAL
CELL

EQUATOR EQUATOR

HADLEY
CELL

30 S

HADLEY
CELL

°

30 ° S

S (a) S (b)

N N

(I.T.C.Z)T 1
EQUATOR
T 2

T 1

EQUATOR

(I.T.C.Z)T 2

S (c) S (d)

Over the years various models have been proposed to explain the ITCZ.
It is commonly viewed as the ascending branch between the Hadley Cells
(Figure 3.3a ). Fletcher ( 1945 ) proposed a twin equatorial cell model (Figure
3.3b ) while Asnani ( 1968 ) suggested another model with a single equatorial cell
separating the two Hadley Cells (Figures 3.3c and 3.3d ). In the single cell, the air
flows toward the equator in the lower layers and flows away from the equator
aloft, producing upward motion on one side and downward motion on the other.
Near the equator, subsidence occurs.
There have also been numerous theories and numerical modeling studies for
the explanation of the ITCZ. Charney ( 1971 ) considered that the position of the
ITCZ depends on Ekman pumping efficiency and moisture availability.
Schneider and Lindzen ( 1977 ) discovered that zonally symmetric convective
heating was crucial in determining the location of the ITCZ. Tomas et al.( 1999 )
stated that a zonally symmetric cross-equatorial pressure gradient would result
in a convergent flow at the ITCZ latitudes.
Some modeling studies reveal that the ITCZ follows the SST maximum.
Goswami et al.( 1984 ) discovered that a steady ITCZ occurred over the SST