Semyon
A. Grodsky, and James A. Carton
(MsWord document rainfall_2.doc is here).
E-mail: senya@ocean2.umd.edu
Abstract. The
winds of the tropical Atlantic are known to have strong fluctuations on
periods of a few days. These African Waves result from instabilities in
the tropical winds crossing Northwest Africa and in some cases develop
into the tropical storms and hurricanes of the western Atlantic [Yanai
and
Mukarami, 1970; Viltard et al., 1997; Diedhiou et al.,
1998].
Here we use new surface wind observations made available by the QuickSCAT
satellite and rainfall estimates from the Tropical Rainfall Measuring Mission
(TRMM) to reexamine fluctuations in the synoptic meteorology of tropical
West Africa and the Atlantic. We find the additional presence in surface
winds and rainfall of strong quasi-two week disturbances in northern spring
and summer. These disturbances appear to result from a cycle of continental
heating, convection, and cooling that gives rise to fluctuations in the
monsoon system.
1.INTRODUCTION
Intensive
study of synoptic-scale disturbances over central and West Africa and the
tropical Atlantic has revealed the existence of two types of westward propagating
wave-like fluctuations. The first are the African Waves with zonal wavelengths
of 2,500-3,000 km, periods of 3-5 days, and westward speeds of 9-10 m/s
and which are most evident in the meridional component of winds [Carlson,
1969;
Viltard
et al., 1997]. The second type has longer 6-9 day periods, correspondingly
longer 6,000 km zonal wavelengths, and higher, 11 m/s, westward speeds
[Yanai
and Mukarami, 1970]. These latter wave-like fluctuations
have their strongest expressions in zonal surface wind and cloud cover
in the latitude band 100-200N[Oubuih et al., 1999].
Recently Grodsky and Carton
[2001] have used daily data of
the year 2000 provided by the SeaWinds scatterometer aboard QuickSCAT satellite
[Spencer et al., 2000] and Tropical rainfall estimates from the
Tropical Rainfall Measuring Mission satellite microwave sensor suite [Kummerow
et al., 1998] to report on a third class of intraseasonal disturbances
in the winds of the eastern tropical Atlantic. They occur closer to the
equator, develop inside and close to the ITCZ area during April - June,
and seem do not propagate westward. The spatial extent of the wind oscillations
changes with season and is closely related to the meridional shift of the
ITCZ. Janicot
and Sultan [2001] have also observed a quasi-periodic signal of
about 15 days in the rainfall and wind fields over West Africa based on
a longer 1968-1990 data set. The
presence of the quasi-biweekly fluctuations in the reanalysis zonal winds
over the eastern tropical Atlantic and Northwest Africa have been also
noted by Viltard et al. [1997]. Moreover, the existence of bi-weekly
oscillations in the tropical wind system is not unique to Northwest Africa
as very similar oscillation was found by Krishnamurti and Bhalme
[1976]
in the Indian monsoon. These studies provide further evidence that quasi-
biweekly period is an important time scale of oscillation in the monsoon-like
system that develops due to its own inherent dynamics.
2.RESULTS
It
is evident from Figure 1 that the pattern of winds and rainfall can undergo
rather rapid changes. The 12-day period beginning May 17 illustrates a
reversal of the normal trade winds between 20N and 80N
and east of 250W. This change brings moist maritime air eastward
onto the areas surrounding the Gulf of Guinea resulting in intermittent
heavy rainfall at least a month prior to the annual rainy season. The eastward
shift of convection causes a corresponding reduction in rainfall in the
western side of the basin (May 22 panel). During the succeeding week (late
May- first days of June) the winds return to their normal configuration,
while convection shifts westward onto the ocean and towards eastern Brazil
(not shown in Figure 1). The trade winds reversal seems to be the result
of interactions between the continental hydrologic cycle, summertime heating,
and the tropical trade winds.
 |
| Figure 1. Two-day average surface winds and rainfall. Rates exceeding 0.5 mm/hr are shaded. This pattern has switched back by 30 May. |
The
sequence of wind, rainfall, and land heating cycles leading to development
of the biweekly oscillation is further illustrated by the scheme presented
in Figure 2. During normal (eastward) trade winds the convection is shifted
seaward and sunshine over land increases the land temperature that lowers
atmospheric pressure (upper panel). Decline in air pressure over land reverses
wind direction that shifts convection towards the land bringing in maritime
moisture, rainfall and cloudiness (lower panel). This decreases surface
temperature over the land and restores (increases) atmospheric pressure,
and the trade winds switch to normal direction shown in upper panel. Grodsky
and Carton [2001] have shown using the NCEP/NCAR reanalysis of Kalnay
et al. [1996] that variations of the land temperature, sea-land pressure
difference, and rainfall over the land occur in close correspondence with
wind oscillations within the ITCZ west off the African coast.
 |
| Figure 2. The scheme of trade winds direction and convective clouds during the normal easterly winds (upper panel) and trade winds reversal (bottom panel). |
Figure
3 presents the components of the air-sea surface net heat flux along with
3-day average zonal wind. The flux data are 3 - 60 day band passed and
shown during April - June when a succession of 5 bi-weekly wind oscillations
is evident. Main contribution to net heat flux change comes from variation
in SWR and LHFL, as was supposed earlier. Both components, LHFL and SWR,
vary about ±20 W/m2. Their average values are <LHFL>=78
W/m2 and <SWR>=205 W/m2. Intraseasonal variation
of the LHFL displays correlation with zonal wind speed modulus (Figure
4, upper panel). This correlation is obvious during May when the strongest
oscillation occurred. The correlation isn?t unique because of impact of
air humidity.
 |
| Figure 3. Three day mean zonal wind, U, and 3 - 60 day band passed latent heat flux, LHFL, sensible heat flux, SHFL, long wave radiation, LWR, and short wave radiation, SWR. All values are averaged over 4N - 6N, 20W - 10W. |
Short
wave radiation is expected to vary with phase opposite to that of the zonal
wind. This regularity is observed in Figure 4 (bottom panel) beginning
16-th of May. Before this date, the relationship between SWR and zonal
wind is less definite. Possible reason for that may relate to insufficient
accuracy of the cloud parameterization used by the reanalysis model.
 |
| Figure 4. (Upper panel). Latent heat flux, LHFL, (3 - 60 day band passed, bold line) and 3-day mean zonal wind modulus, |U|, (thin line). (Bottom panel). Short wave radiation, SWR, (3 - 60 day band passed, bold line) and 3 -day mean zonal wind, U, (thin line). All values are averaged over 4N - 6N, 20W - 10W. |
3. RESUME
This
paper presents observational evidence of the existence of quasi - biweekly
oscillations in the tropical wind system over the eastern Atlantic. Oscillatory
regime related to feedbacks within the hydrologic cycle is not unique to
Northwest Africa. Lau and Bua [1998], for example, present modeling
evidence of a similar phenomenon occurring in the region of East Asia/Indochina.
These studies provide further evidence of strong interactions between continental
and maritime atmospheric processes and the hydrologic cycle.
It
is much probable that zonal wind oscillations in the tropics might force
an oceanic response. The biweekly oscillations strongly modify winds in
vicinity of the ITCZ that may influence the wind-driven circulation of
the tropical ocean. Note also that during the trade winds reversal an upwelling
favorite wind westerly blows along the northern coast of the Gulf of Guinea.
All these issues are awaiting future studies.
Carlson,
T.N, Some remarks on African disturbances and their progress over the tropical
Atlantic, Mon. Wea. Rev., 97, 716-726, 1969.
Diedhiou,
A., S. Janicot, A. Viltard, and P. de Felice, Evidence of two regimes of
easterly waves over West Africa and the tropical Atlantic, Geophys.
Res. Let., 25, 2805-2808, 1998.
Janicot,
S., and B. Sultan, Intra-seasonal modulation of convectionin
the West African monsoon, Geophys.
Res. Let., 28,
523-526, 2001.
Grodsky,S.A.,
and J.A. Carton, Coupled land/atmosphere interactions in the West African
Monsoon, Geophys. Res. Let., 28, 1503-1506, 2001.
Kalnay,
E., M. Kanamitsu, R. Kistler, W. Collins, D. Deaven, L. Gandin, M. Iredell,
S. Saha, G. White, J. Woolen, Y. Zhu, M. Chelliah, W. Ebisuzaki, W. Higgins,
J. Janowiak, K.C. Mo, C. Ropelewski, J. Wang, A. Leetmaa, R. Reynolds,
R. Jenne, and D. Joseph, The NCEP/NCAR 40-year reanalysis project, Bull.
Amer. Meteorol. Soc., 77, 437-471, 1996.
Krishnamurti,
T.N., and H.N. Bhalme, Oscillations of a monsoon system. Part I. Observational
aspect, J. Atmos. Sci., 33, 1937-1954, 1976.
Kummerow,
C., W. Barnes, T. Kozu, J. Shiue, and J. Simpson, The Tropical Rainfall
Measuring Mission (TRMM) sensor package, J. Atmos. And Ocean. Tech.,
15,
809-817, 1998.
Lau,
K-M., and W. Bua, Mechanism of monsoon-Southern Oscillation coupling: insights
from GCM experiments, Climate Dynamics, 14, 759-779, 1998.
Oubuih,
J., P. de Felice, and A. Viltard, Influence of the 6-9 day wave disturbances
on temperature, vorticity and cloud cover over the tropical Atlantic during
summer 1985, Meteorol. Atmos. Phys., 69, 137-144, 1999.
Spencer,
M.W., C.L. Wu, and D.G. Long, Improved resolution backscatter measurements
with the Sea Winds pencil-beam scatterometer, IEEE Trans. Geosci. Rem.
Sens.,
38,
2642-2652, 2000.
Viltard,
A., P. de Felice, and J. Oubuin, Comparison of the African and 6-9 day
wave-like disturbance pattern over West-Africa and the tropical Atlantic
during summer 1985, Meteorol. Atmos. Phys., 62, 91-99, 1997.
Yanai,
M., and M. Murakami, Spectrum analysis of symmetric and anti-symmetric
equatorial waves, J. Meteor. Soc. Japan, 48, 331-346, 1970.