Quantitative rainfall maps over the oceanic areas of the globe were derived from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) data. Analysis of satellite derived oceanic rainfall maps reveal certain distinctive characteristics of global patterns for the years 1973-74. The main ones are (1) the forking of the Intertropical Convergence Zone in the Pacific, (2) a previously unrecognized rain area in the South Atlantic, (3) the bimodal behavior of rainbelts in the Indian Ocean and (4) the large interannual variability in oceanic rainfall. These features are discussed

Rao, M. S. V.

Theon, J. S.

English

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https://ntrs.nasa.gov/search.jsp?R=19770012746 2020-03-22T11:36:02+00:00Z
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NEW FEATURES OF GLOBAL
CLIMATOLOGY REVEALED BY
SATELLITE-DERIVED OCEANIC
RAINFALL MAPS
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M. S. V. RAO
J. S. THEON
JANUARY 1977
co
GODDARD SPACE FLIGHT CENTER
GREENBELT, MARYLAND
LIST OF ILLUSTRATIONS
Figure 1.
	
	 ESMR-derived global oceanic rainfall map, illustrating the forking
of the ITCZ.
Figure 2.	 ESIAR-derived rain-rate versus latitude — Pacific r/cean
I igure 3.	 ESMR-derived rain-rate versus latitude — Indian Ocean
Figure 4.
	
	 ESMR-derived Pacific Ocean rainfall maps for January 1973 (El
Nino year) and January 1974 (non-El Nino year)
i3
1
NEW FEATURES OF GLOBAL CLIMATOLOGY REVEALED BY
SATELLPI'E-DERIVED OCEANIC RAINFALL MAPS
M. S. V. Rao
Systems & Applied Sciences Corporation, Riverdale, MD.
J. S. Theon
NASA, Goddard Space Flight Center, Greenbelt, MD.
ABSTRACT
Analysis of satellite derived oceanic rainfall maps reveal certain distinctive
characteristics of globalpatterns for the years 1973-74. The main ones are (1)
the forking of the Intertropical Convergence Zone in the Pacific, (2) a previously
unrecognized rain area in the South Atlantic, (3) the bimodal behavior of rainbelts
in the Indian Ocean and (4) the large interannual variability in oceanic rainfall.
These interesting features are discussed.
Quantitative rainfall maps over the oceanic areas of the globe were derived
from the Nimbus 5 Electrically Scanning Microwave Radiometer (ESMR) data.
The maps for the period December, 1972 through March, 1975 were published
in the form of an Atlas (Rao, Abbott and Theon, 1976).
Several interesting aspects of global climatology are disclosed from a study
of these maps. The objective of the contribution is to describe briefly four out-
standing features.
is
2(1) The Structure of the ITCZ
Although it has been known for some time that there is a dry zone near the
Ellice Islands in the Pacific flanked by wet regions to the north and south
(Seelye, 1950) the precise structure of the ITCZ in the Pacific has remained
obscure. From the typical ESMR-derived oceanic rainfall map shown in
Figure 1, it is seen that as we move eastwards along the equatorial belt in the
Pacific, the ITCZ bifurcates in the neighborhood of longitude 170°E: the upper
branch proceeds eastwards, maintaining itself slightly north of the Equator,
whereas the lower branch rims east or southeastwards and merges with the
southern Pacific convergence zone in the vicinity of longitude 100°W,
From the quantitative observations of rainfall over the oceans (derived from
ESMR), zonally averaged rainfall rates were computed separately for each of the
three major oceanic areas of the world, viz. , the Pacific, the Atlantic and the
Indian Oceans. figure 2 reproduces the plot of the zonal averages of rainfall in
the Pacific Ocean. This analysis confirms that the ITCZ splits in two during
nearly all months of the year. However, it should be pointed out that the southern
branch and its extension, the southern Pacific trough, attain their maximum
development during the austral summer period (December through February).
(2) Previously Unrecognized Rain Area in the S. Atlantic
In the Atlantic, to the southeast of S. America, there is an extensive area
of rainfall in the revion approximately between latitudes 25 0S and 500S and
3longitudes 50°W and 25MI (see rig. 1). This rainy region was not ]mown before;
it does not appear on any existing mai, of global rainfall, probably because few
ships traverse the area. This rain area revealed in ESMR-derived maps is
possibly an extension of the southern Pacific convergence zone mentioned
previously. In other words, rainfall in the region could be produced by the
same dynamical. circulation pattern as the southern region of convergence, the
flow being modified and the rain-pattern interrupted at the land protrusion
of the S. American continent and an area to the immediate west of the land.
(3) The Bimodal Behavior of Rainbolts in the Indian Ocean
Zonally averaged rainfall rates in the Indian Ocean are displayed in Figure
3. Two distinct rain maxima are evident in the tropics between latitudes 20014
and 20°S (apart from a third extra-tropical maximum obviously related to the
polar front, far to the south at about latitude 40 0S). The maximum at northern
latitudes appears to grow at the expense of the maximum immediately to the
south of the Equator as the monsoon advances, and vice versa as it retreats.
During June. to August, the amplitude of the northern maximum is 3 times that
of the other equatorial maximum, whereas during December to February, the
southern tropical maximum grows in amplitude to 3 times the northern. This
study leads to a modification of the beliefs of two schools of thought in tropical
meteorology, one that the monsoon current is a progressive advance in the
Indian Ocean from the Southern Hemisphere across the Equator to the Southeast
Asian land-mass, and the other that monsoon rainfall is due to moisture picked
4up entirely in the Northern Hemisphere, mainly in tiro Arabian Sea. It seems
necessary to postulate a circulation mechanism involving both the hemispheres,
not necessarily demanding a regular progression of the entire monsoon air-
stream from the Southern to the Northern lIemisphere, but affevting both in
a coordinated way so as to sustain the bimodal changing wave pattern out-
lined above. This question is important (considering that the monsoon affects
the lives of millions of people), and is under further investigation.
(4) Interannual Variation
The extent of variation of rainfall over the oceans from year to year has
been largely a matter of conjecture until the present time. The weekly,
monthly, seasonally, and annually averaged maps generated from ESMR data
provide new insight into this problem. A typical example is the rainfall over
the Pacific in the month of January of the years 1973 and 1974 (Fig. 4). In
January 1973, intense rainfall occurred over a wide region along the Equator
and to the south of it. This was at the time of the El Nino phenomenon (warm
ocean current due to relaxation of upwelling along the coasts of Ecuador and
Peru), with its disastrous effect on the plankton and fish in the waters of the
Pacific off the west coast of S. America. In the corresponding month of 1974
(non-E1 Nino year), we see that the region was relatively dry. The ratio of
rainfall in the Equatorial Pacific in the period December '73-February 174, to
the rainfall in the period December 1 72-February 1 73 (see Fig. 2) is 1.6.
5This is a distinctive example of rainfall anomaly. Investigations of si;m-
ilar anomalies and their correlations are very valuable in weather as well as
climatic studies.
ACKNOWLEDGMENT. We thank Mr. William V, Abbott for his assistance in
preparing the diagrams for this contribution.
REFERENCES
Rao, M. S. V., W. V. Abbott and J. S. Theon, 1976: 'Satellite-Derived Global
Oceanic Rainfall Atlas (1973-1974),' 1 NASA Special Publication 410, 1976,
p. 31.
Seelye, C. J., 1950: 'Rainfall and Its Annual Variability Over the Central and
Southwestern Pacific, " New Zealand J. Sol. Tech., B 32, pp. 11-24.
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New features of global climatology revealed by satellite-derived oceanic rainfall maps

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