Transcript Document

Intertropical Convergence Zone in the South Atlantic and the equatorial cold tongue
Semyon A. Grodsky, James A. Carton , and Alfredo Ruiz-Barradass
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 80S30S 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).
Department of Meteorology, University of Maryland, College Park, MD 20742
T itle:
jan_july_port.eps
Creator:
Adobe Il lus trator(R) 9.0
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture wi ll pri nt to a
Pos tScri pt printer, but not to
other types of printers.
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 -5*10-6 1/s
and 5*10-6 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.
T itle:
s st_di v_pr_month.eps
Creator:
gxeps $Revis ion: 0.68 $
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture wi ll pri nt to a
Pos tScri pt printer, but not to
other types of printers.
T itle:
s st_di v_model_july.eps
Creator:
gxeps $Revis ion: 0.68 $
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture will pri nt to a
Pos tScript printer, but not to
other types of printers.
T itle:
div_ss t.eps
Creator:
MAT LAB, T he Mathworks , Inc .
Preview:
T his EPS pic ture was not s aved
with a preview inc luded in i t.
Comment:
T his EPS pic ture wi ll print to a
Pos tScri pt printer, but not to
other types of printers.
T itle:
c orr_dt_div_int.eps
Creator:
gxeps $Revis ion: 0.68 $
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded in i t.
Comment:
T his EPS pic ture wi ll print to a
Pos tScri pt printer, but not to
other types of printers.
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.
T itle:
/data/pac ific2/s enya/quic ks cat/pr_ss t_dp_ti me.eps
Creator:
gxeps $Revis ion: 0.68 $
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture wi ll pri nt to a
Pos tScri pt printer, but not to
other types of printers.
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 isn’t a calm
wind zone), and thus only weak Ekman pumping induced
in the surface layers of the ocean.
T itle:
s al_area_l at.eps
Creator:
MAT LAB, T he Mathworks , Inc .
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture wi ll pri nt to a
Pos tScri pt printer, but not to
other types of printers.
Figure 4. July climatological
SST, SSM/I wind divergence,
and wind divergence from
Lindzen and Nigam [1987]
model .
T itle:
/data/pac ific2/s enya/trimm/pr_s al_portrai t.eps
Creator:
gxeps $Revis ion: 0.68 $
Preview:
T his EPS pic ture was not s aved
with a preview i nc luded i n i t.
Comment:
T his EPS pic ture wi ll pri nt to a
Pos tScri pt printer, but not to
other types of printers.
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.
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 19931998, 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.
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.
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.