Intertropical Convergence Zone in the South Atlantic and

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 Quick. SCAT 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 90 S - 30 S 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 90 S-30 S. It is evident that much of the SITCZ convection is confined to the domain 90 S-30 S, 350 W-200 W. 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 (150 W – 50 W, 20 S – 20 N) 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 80 S 30 S 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 30 S and reaches its southmost extension in July at 60 S. 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 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 (350 W-200 W, 90 S-30 S) and the cold tongue region (150 W-50 W, 20 S-20 N) are indicated by rectangles. 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. 5 x 10 -6 1/s with a 5 x 10 -6 1/s contour interval. 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. Figure 4. July climatological SST, SSM/I wind divergence, and wind divergence from Lindzen and Nigam [1987] model. 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 div. U 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 100 S and 300 N – 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 350 W 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 350 W to 200 W. 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.