References.

1
Intertropical Convergence Zone in the South
Atlantic and the equatorial cold tongue Semyon
A. Grodsky, James A. Carton , and Alfredo
Ruiz-Barradass Department of Meteorology,
University of Maryland, College Park, MD 20742
Summary Following Leitzke et al. 2001 and
Halpern and Hung, 2001 who have examined the
dynamics of the SITCZ in the Pacific, we explore
the potential of boundary layer processes see
also Lindzen and Nigam, 1987 in producing the
observed surface divergence fields in the south
tropical Atlantic. The seasonal appearance of a
cold tongue of SST along the equator sets up
pressure gradients within the boundary layer that
induce wind convergence in summer in the band of
latitudes of the magnitude observed. Indeed,
although our record is short a statistical
analysis suggests that year-to-year changes in
the difference in SST between the cold tongue and
the SITCZ index region explains a significant
fraction of the year-to-year variability in SITCZ
rainfall. Examination the oceanic
implications of the seasonal SITCZ shows that
there is a seasonal reduction in sea surface
salinity of at most 0.3 ppt in response to
seasonal rains. The southern tropics have long
been identified as a major source of warm water
entering the Equatorial Undercurrent and crossing
into the Northern Hemisphere Metcalf and
Stalcup, 1967, and thus playing an important
role in climate. Intriguingly, several studies
beginning with Reid 1964, have proposed the
existence of a southern counterpart to the North
Equatorial Countercurrent, which would be a
consequence of strong inhomogeneity of Ekman
pumping in this region. However, despite the wind
convergence there is little rotation in the
surface wind field in the SITCZ region (because
in distinction from the ITCZ there isnt a calm
wind zone), and thus only weak Ekman pumping
induced in the surface layers of the ocean.
Introduction Recent observations from the
QuickSCAT and Tropical Rainfall Measuring Mission
satellites, as well as a longer record of Special
Sensor Microwave Imager winds are used to
investigate the existence and dynamics of a
Southern Hemisphere partner of the Intertropical
Convergence Zone (SITCZ) in the tropical Atlantic
Ocean (see also Hastenrath and Lamb 1978 ). The
SITCZ extends eastward from the coast of Brazil
in the latitude band 90S - 30S and is associated
with seasonal precipitation exceeding an average
6 cm/month during peak months over a part of the
ocean characterized by high surface salinity. It
appears in northern summer when cool equatorial
upwelling causes an anomalous northeastward
pressure gradient to develop in the planetary
boundary layer close to the equator. The result
is a zonal band of surface wind convergence,
rainfall , and associated decrease in the ocean
surface salinity of at most 0.3 ppt. Figure 1
shows that by July the ITCZ shifts northward,
while the SITCZ is visible extending eastward
from Brazil in the band of latitudes 90S-30S. It
is evident that much of the SITCZ convection is
confined to the domain 90S-30S, 350W-200W. We
will thus use this region for the purpose of
constructing SITCZ indices of rainfall, wind
divergence, etc. The monthly evolution of
precipitation, wind convergence, and SST shown in
Fig. 2 reveals close relationship in time
between the SITCZ and the equatorial cold tongue.
The seasonal appearance of rainfall in
spring and then again in summer is evident in the
time series presented in Fig. 3. The summer
precipitation appears most closely linked to the
seasonal change in SST, between the cold tongue
region (150W 50W, 20S 20N) and the SITCZ index
region shown in Fig. 1. Modeled and observed
climatological July surface wind divergence are
compared in Fig. 4. Calculations are done with
the Lindzen and Nigam 1987 model using July
SST climatology of Reynolds and Smith 1994.
The relationship between SST and wind
convergence in the SITCZ region is examined
during the 14-year period 1988-2001 in Fig.5.
For most years (10 of 14) a roughly linear
relationship agrees with model. However, during
1990, 1992-93, and 1997 wind convergence was
absent or relatively low. Interestingly, two of
these years, 1992 and 1997, are El Nino years,
suggesting the importance of extra-basin
influences. Figure 6 shows influence of the
SITCZ precipitation on the ocean. Based on data
of Dessier and Donguy 1994, we find that the
band of latitudes between 80S-30S is
characterized by up to 0.5 ppt decrease in
salinity in July relative to January. The
monthly evolution of surface rainfall (Fig. 7)
shows that the fresh anomaly first appears in
spring at 30S and reaches its southmost extension
in July at 60S. Advection clearly plays an
important role in redistributing salinity
anomalies due to the presence of 30 cm/s
westward South Equatorial Current (Fig. 6).
Figure 1. Monthly average SST (colors), winds
(vectors), and rainfall exceeding 2 mm/day (gray)
for January, 2000 and July, 2000. Wind divergence
is contoured at two levels -510-6 1/s and 510-6
1/s with dashed and solid lines, respectively.
The SITCZ index region (350W-200W, 90S-30S) and
the cold tongue region (150W-50W, 20S-20N) are
indicated by rectangles.
Figure 5. July wind divergence in the SITCZ index
region and SST difference between the cold tongue
and the SITCZ. Dashed line is a linear fit to 10
years data excluding of 1990, 1992-93, and 1997.
Dots are divU in the SITCZ index region from LN
model calculated using 20 years of SST data.
Lower panel presents time correlation of
interannual change of the SST difference and wind
divergence calculated during summer months of the
10 years specified above.
Figure 4. July climatological SST, SSM/I wind
divergence, and wind divergence from Lindzen and
Nigam 1987 model .
Figure 2. SST, wind divergence, and rainfall
(mm/day) over the tropical Atlantic during April
- September 2000. Wind divergence and convergence
are shown with solid and dashed lines,
respectively, starting from 2.5x10-6 1/s with a
5x10-6 1/s contour interval.
References. Dessier, A., and J.R. Donguy, 1994
The sea surface salinity in the tropical Atlantic
between 100S and 300N seasonal and interannual
variations (1977 1989), Deep Sea Res., 41,
81-100. Halpern, D., and C.-W. Hung., 2001
Satellite observations of the southern Pacific
intertropical convergence zone during 1993-1998,
J. Geoph. Res., accepted. Hastenrath, S., and P.
Lamb, 1978 On the dynamics and climatology of
surface flow over the equatorial oceans, Tellus,
30, 436-448. Leitzke, C.E., C. Deser, and T.H.
Vonder Haar, 2001 Evolutionary structure of the
eastern Pacific double ITCZ based on satellite
moisture profile retrievals, J. Clim., 14,
743-751. Lindzen, R.S., and S. Nigam, 1987 On
the role of sea surface temperature gradients in
forcing low-level winds and convergence in the
tropics, J. Atmos. Sci., 44, 2418-2436. Metcalf,
W.G., and M.C. Stalcup, 1967 Origin of the
Atlantic equatorial undecurrent, J. Geoph. Res.,
72, 4959-4975. Reid, J.L., 1964 Evidence of a
South Equatorial Counter Current in the Atlantic
Ocean in July 1963, Nature, 203, 182. Reynolds,
R. W., and T. M. Smith, 1994 Improved global sea
surface temperature analyses using optimum
interpolation, J. Clim., 7, 929-948.
Figure 3. SST (a) and sea level pressure (b)
differences between the cold tongue and the
SITCZ regions Surface wind divergence (c) and
rainfall (d) in the SITCZ index region.
Figure 6. July (a) and January (b) sea surface
salinity averaged between 350W and 200 W.
Vertical bars are STD within 10 latitude bands.
Observation points for July (c) and January (e).
July surface currents (d) from historical ship
drift.
Figure 7. Latitude-time diagrams of seasonal
rainfall (TRMM) and surface salinity averaged
from 350W to 200W. Note that the two data sets
are not contemporaneous.