The present initiative is the result of a series of workshops on climate variability and global change related issues sponsored by the Inter-American Institute for Global Change Research (IAI) and the National Science Foundation (NSF) and attended by investigators from Argentina, Brazil, the United States, and Uruguay. The two general goals of these meetings were:
(1) Define the scientific priorities on climate variability and global change issues for the scientific communities of the participating countries.
(2) Promote the communication and collaboration among individual scientists and research institutions of the Americas.
This document summarizes the scientific discussions and conclusions that ensued from the regional workshops held in the cities of Baltimore (USA), Miami (USA), Rio Grande (Brazil), and Puerto Madryn (Argentina). A final document that will integrate all the components and define future analysis and field efforts will result from the next international IAI/NSF funded workshop that will take place in Brazil in April 1997.
Additional information on the IAI-sponsored meetings can be obtained at www.meto.umd.edu/~berbery/iai_wshop.html.
The SACC effort is an international (Argentina, Brazil, Uruguay, and the U.S.) and multi-disciplinary program to study the effects of climate change in the southwestern South Atlantic. The main goal of the program is:
To understand the interactive relationship of the southwestern South Atlantic sea surface temperature (SST) and the larger scale climate behavior. The SST is the oceanic variable that most influences the atmosphere, and therefore is a sensitive indicator of climate variability.
The primary objectives of the program are to:
(1) Describe the spatial and temporal scales of SST anomalies in the southwestern South Atlantic (SWA) and understand their coupling to ocean stratification and atmospheric forcing.
(2) Advance our understanding of climate variability as a result of changes in the thermohaline circulation and water mass properties of the southwestern South Atlantic.
(3) Determine the behavior of the Brazil/Malvinas Confluence and associated climate variations in response to local and remote forcing, both oceanic and atmospheric.
(4) Determine the relationship between oceanic variability and ecosystem changes in shelf and terrestrial environments in response to regional hydrology, including rainfall over South America and river runoff.
INTRODUCTION AND SCIENTIFIC JUSTIFICATION
The subtropical to temperate end of South America represents a unique climatic zone (Fig. 1). It is bound on the north by the greatest expanse of tropical rain forest on earth, the Amazon. To the west, the atmospheric flow is strongly perturbed by the presence of the Andes, leading to desert conditions extending from Patagonia in the south up the eastern edge of the mountains. The intermediate region is characterized by wet and dry savannas and one of the world's great river drainages, the Parana-Paraguay basins. Separated from these basins by coastal mountains is the highly populated Atlantic seaboard dominated by coastal lagoons from the Patos-Mirim system in southern Brazil and Uruguay to the bay at Rio de Janeiro. The region is home to a population of nearly 200 million people, most of whom are concentrated in the great commercial/industrial centers of Rio de Janeiro, Sao Paulo, Montevideo, and Buenos Aires. Much of the remaining region is dominated by agricultural activities. During the last few decades, climatic variations have had an important economic and social impact on the region. Drought periods have produced climatic change in cattle population, drained the water supplies of Buenos Aires and Montevideo, and caused shortages of hydroelectrical power. In the 1970s, there was a significant expansion of farming in the Argentine Pampas, apparently as a response to a westward shift in precipitation for some unknown reason. While the existence of these sorts of climate fluctuations over this region have been appreciated for at least 80 years, the mechanisms behind them are unclear.
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Figure 1. South America poleward of the Amazon. Depicted are the regions, major rivers, and cities discussed in the text.
It is in Walker's (1924) work that the Southern Oscillation (SO) was first described. In his search for Indian monsoon forecasting tools, Walker discovered a significant see-sawing between atmospheric pressure in Buenos Aires, Argentina and Sydney, Australia. This first description of the atmospheric SO cycle clearly ties climate conditions in the region to events in the Pacific. The mechanisms remains one of questions. This leads to important issues regarding the role of the South Atlantic in global climate change that are the major focus of this document. Regional precipitation and ocean surface anomalies have been fairly well documented. There are significant correlations between SO and rain anomalies over the drainage basin that feeds the Rio de la Plata estuary and the Patos-Mirim complex in southern Brazil that is fed from the eastern side of the highlands (Ropelewski and Halpert, 1987; Pisciottano et al., 1994). These rainfall anomalies are also associated with large positive sea surface temperature (SST) anomalies in the southern Argentine Basin. These anomalies are set up late in El Niño years (Fig. 2). They are apparently reinforced by ENSO-correlated perturbations in SST that translate around the Antarctic Circumpolar Current (White and Peterson, 1996). The anomalies are roughly a dipole between the Argentine Basin and the Weddell Sea (Fig. 3), although there is a suggestion of an advective plume extending the anomaly into the mid-Atlantic. Again, the question returns to the mechanisms involved with the moisture convergence that feeds the rain anomalies and its relationship to SST anomalies in the proximity of the Brazil/Malvinas Confluence (Fig. 4).
First, the moisture source for the rain anomaly is in question. Most of the evaporation occurring in the Pacific is probably removed by the Andes. Therefore, it is likely that the source of variability has its origins in the westerly winds coming out of the Pacific, rather than direct moisture advection. Some regional ideas hold that there is significant water vapor advection out of the
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Figure 2. Rain anomalies over Uruguay from 1961 to 1992. Shown is the first principal component of the rainfall anomaly obtained from data collected at 10 stations in Uruguay. This component represents the in-phase oscillations across Uruguay as a function of time. El Niño years are shown by circles and years of ENSO cold phase are denoted by triangles. Note that the droughts are typically associated to the cold phase and that rain periods are linked to El Niño years.
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Figure 3. Low passed SST anomalies south of 30° from the Rennell's blended SST data set. Depicted are examples of a cold anomaly (upper panel) and a warm anomaly (lower panel) in the southwestern Atlantic. Notice the circumpolar wave pattern described by White and Peterson (1996).
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Figure 4. SST at the Brazil-Malvinas Confluence for December 1989 and April 1990 (upper panel). First principal component of precipitation over Uruguay for 1988 to 1992 (lower panel). Local SST anomalies exceed 6°C in these months relative to the total record. Note the cooling associated with the large January-February precipitation anomaly in 1990. The precipitation record and SST anomalies are coherent (>0.8) and 90° out of phase over the overlap in the SST rainfall data (Podesta et al., unpublished work).
Amazon in a nocturnal jet along the mountains. Although moisture advection occurs from both tropical Brazil and the South Atlantic (Prohaska, 1976; Wang et al., 1992), the hypothesis here is that the Atlantic dominates, with much of the enhanced precipitation coming in cold fronts associated with cyclogenesis (Kousky, 1979). The South Atlantic also contributes much of the moisture into the other rain anomaly region in South America that occurs in northeastern Brazil (Hastenrath and Heller, 1977). The underlying idea then is that SST variations in the South Atlantic are important in controlling the variations in moisture fluxes. Objectives 1 and 4 are aimed at understanding the connection between SST anomalies in the South Atlantic and climate variations over the South American continent.
The oceanic transition zone between the area of subtropical influence (Brazil Current) and the area of subpolar influence (Malvinas Current) has its atmospheric counterpart in the area of climatic transition located between the upper Rio Colorado Basin and the lower Rio Negro Basin in southern Argentina (Fig. 1). This narrow continental zone separates the area of prevailing meridional moisture flux to the north from that of dominant zonal flux to the south (Prohaska, 1976). The area of transition in the southwestern Atlantic Ocean is where a number of weather phenomena combine to influence the climate of the coastal strip, affecting life conditions of densely populated areas and productivity of the main agricultural region in South America. The southwestern Atlantic Ocean between 30°S and 45°S is one of the regions with highest cyclogenetic activity within the southern hemisphere (Taljaard, 1972; Necco, 1982; Sinclair, 1994). Studies of cyclonic vortices from satellite data (e.g., Yasunari, 1977; Carleton, 1979) confirm this result. The occurrence of cut-off lows, extended precipitation, and occasional floods along the Rio de la Plata is one of the weather phenomena in the transition zone whose causes and dynamics are not yet well understood (Prohaska, 1976). Cyclones in the region have a tendency to stall and do not have the long westerly tracks seen in other regions of the southern hemisphere (Sinclair, 1994). Under objective 1 the intention is to study these systems in relationship to the influence of the Andes and the SST anomalies produced by variations in the position of the Brazil/Malvinas Confluence.
The region is also frequently influenced by high latitude blocking anticyclones over the western South Atlantic Ocean (Trenberth and Mo, 1985). These are associated with unusual precipitation along the northern coast of Patagonia (Grandoso and Nuñez, 1955) (Fig. 5). Berbery and Nuñez (1989), in an observational and numerical study of blocks in the South Atlantic, showed that the Andes mountains may be one of the contributing factors to the maintenance of blocks in the region. However, the effect of SSTs on the relationship of these phenomena to other features, such as the larger scale circulation, has not been thoroughly investigated.
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Figure 5. (Top) Depiction of a typical cyclogenesis in the southern hemisphere and in the region of interest. Southeast winds affecting the southern part of Buenos Aires (clockwise rotation around the low pressure in top panel) usually produce floods in the coastal areas. (Bottom) Two typical features of these cyclones: (a) an increased static instability as measured by the Total-totals index; and (b) an increased amount of moisture (precipitable water) along the coast. These two figures are based in analyses from the National Centers for Environmental Prediction (NCEP) and were obtained from the Web site of the Center for Ocean-Land-Atmosphere Studies (COLA).
The circulation in large scales and low frequencies can be considered as a mean state within which cyclones and other phenomena interact (Trenberth, 1991; Berbery and Vera, 1996). Changes in the mean state on time scales associated with natural variability (e.g., El Niño-Southern Oscillation, ENSO) or CO2 induced by global warming, are expected to produce changes in the behavior of the transients. Thus, an in-depth study of these issues requires first a better description of the mean annual cycle of the circulation as the primary basic state. Model simulations with doubled CO2 have shown that the greenhouse effect could produce changes in baroclinicity and eddy forcing, resulting in more intense and shifted storm tracks (Hall et al., 1994; Labraga, 1995a,b; Lambert, 1995). More reliable predictions are expected from higher resolution GCMs, nested climate modeling, and, most of all, from coupled atmosphere-ocean GCMs (Whetton et al., 1995). Coupled models can directly investigate the influence that the Brazil-Malvinas Confluence SSTs has on blocking, cyclogenesis, and regional rainfall.
Large variations in the latitude of the frontal zone, or Confluence, separating the warm Brazil Current from the cold, subantarctic Malvinas Current have been obvious since at least the Meteor Expedition in the 1920s (Defant, 1941). The large amplitude meanders extending offshore create large SST anomalies. The location of the separation of the currents from the continental shelf edge also varies up to 930 km along the coast (Legeckis and Gordon, 1982; Olson et al., 1988) (Fig. 6). The major question is the nature of the dynamics behind these variations and how they are related to the nature of the two currents and the factors that force them.
The Brazil Current is a weak western boundary current with a transport of around 20 Sv (Signorini, 1976; Gordon and Greengrove, 1986; Campos et al., 1995; Garzoli, 1993) in its surface layers. This low transport estimate assumes that these warm waters are isolated from the North Atlantic Deep Water (NADW). If this is not the case, estimates of the transport increase. Adding in the deep water flow and the contributions of local recirculations along the Brazil coast (Tsuchiya,
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Figure 6. Time series of the separation latitudes derived from RGSTA observations. (a) Brazil Current (heavy line) and Malvinas Current (thin line) separation from the 1000 m isobath. (b) The distance between the two separation locations in time. Separations are given in degrees of latitude (from Olson et al., 1988).
1985), it is possible to get estimates as high as 70 Sv (Zemba, 1991). An important question is the contribution of the thermohaline flow to the transport of near surface water that can play a role in modulation of the SST anomalies, i.e., the linkage between objectives 1 and 2. In a recent analysis of the Semtner and Chervin (1992) model, Garraffo and Kamenkovich (1996) obtained 27 Sv for the upper layer transport as compared to a Sverdrup interior transport of 35 Sv. These results validate Stommel's (1965) hypothesis on the role of the thermohaline circulation in reducing the transport. The surface expression of this transport is perhaps the most important issue. Although the Sverdrup transports are comparable, the Brazil Current has surface currents between 0.20 and 0.60 m/s as compared to 1.9 m/s or more in the Gulf Stream (Zemba, 1991; Campos et al., 1995; Leaman et al., 1989). That is to say, variations in the thermohaline circulation can have substantial impact on Brazil Current transport and, therefore, on the SST in the Confluence region (objective 2).
There are three branches to the thermohaline circulation that balance the southward transport of NADW (Reid et al., 1977). The deepest consists of modified Antarctic Bottom Water that enters the Argentine Basin from the Georgia Basin (1.0 to 3.0 Sv; Georgi, 1981; Whitworth et al., 1991). An estimated 6.8 Sv of this water mixed with NADW and Lower Circumpolar Deep Water from above flows northward into the Brazil Basin, according to McCartney and Curry (1993). This branch has varied significantly in the past decade (Coles et al., 1996), presumably due to interannual variations in conditions within the Weddell Sea (Gordon, 1982; Martinson et al., 1982; Bersch, 1988). Above the NADW a core of Antarctic Intermediate Water (AAIW) flows northward all the way into the North American Basin at 27°N. There is a great deal of debate on the strength and even the pathways of the AAIW circulation in the subtropical South Atlantic, although tracer evidence suggests it moves anticyclonically around the subtropical gyre. Some direct current measurements in the Brazil Current, however, show northward flowing AAIW (Evans and Signorini, 1985). In terms of AAIW transport, there is disagreement with Rintoul's (1991) conclusion that it provides the major return of waters to the north and the suggestion of Gordon (1982) that warm upper waters from the Indian Ocean dominate. These warm Indian Ocean thermocline waters admixed with subtropical waters already in the South Atlantic make up the third pathway. Recent work in the southeastern Atlantic (Garzoli and Gordon, 1996; Garzoli et al., 1996) indicate that while the transport of the Benguela Current in the eastern limb of the South Atlantic gyre is approximately constant, the source waters are extremely variable. This suggests that the connection between the Indian Ocean and the subtropical Atlantic is an important source of variations. Such variations occur in part because of the role of rings in accomplishing the transfer of Indian Waters out of the Agulhas (Olson and Evans, 1986; Gordon, 1986; Luthjeharms et al., 199?). These rings may themselves play a more direct role in creating fluctuations in the Brazil/Malvinas and, therefore, will be considered again in the discussion of objective 3.
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Figure 7. Surface velocity vectors in a numerical simulation of the southwestern Atlantic. The upper panel shows the circulation resulting from a relatively robust Malvinas Current (50 Sv), while the lower panel shows southward displacement of the front that occurs when the Malvinas transport is lowered to 10 Sv (from Matano, 1993).
The Brazil Current's mean separation latitude of 35.8°S is nearly 15° north of the zero in wind stress curl. This has been explained as a consequence of the collision with the southward flowing Malvinas Current (Veronis, 1973; Matano, 1993; Agra and Nof, 1993). The basic idea is that this stronger subpolar boundary current, 70 Sv in Peterson's (1992) estimates, essentially pushes the Brazil Current northward (Fig. 7). The dynamics of the two colliding jets is important to the understanding of the temporal variability of the separation latitude.
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Figure 8. Mean SST (from 1984 to 1995) for the months of January (summer, left) and July (winter, right) over the Brazil-Malvinas Confluence. Note that in January the warm core of the Brazil Current extends to approximately 40°S, while in July the current turns back to the northeast at approximately 36°S. Similar, but opposite excursions are observed in the Malvinas Current. The SST data was recorded by the Argentine Meteorological Service (Podesta et al., unpublished work).
The separation of the Brazil and Malvinas Currents from the shelf break is extremely variable (Olson et al., 1988; Garzoli and Giullivi, 1993). This variability leads to large SST variations both on a seasonal (Fig. 8) and interannual time scale. Podesta et al. (1991) and Provost et al. (1992) found that the annual signal in SST weakens to the south. The frequency of major oscillations is tied to the relative strengths of the annual cycle in the Brazil Current forced by winds in the subtropics (Provost et al., 1992), as compared to the semi-annual forcing over the subantarctic sector (Large and van Loon, 1989). The annual signal in the Brazil Current involves southward shifts in the separation latitude and an extension of warm waters into the first meander crest offshore around the first of the year (Olson et al., 1988) that corresponds to altimetric height signals over the region (Matano et al., 1993). The Malvinas separation has a semi-annual peak (Olson et al., 1988) that is also seen in the altimeter signals (Provost and Le Traon, 1993). While these results clearly implicate wind forcing, there are several other factors that can modify these annual and semi-annual signals to produce interannual variability.
Variations in SST in the Confluence and in the boundary current extensions to the east can be tied to a number of causes. These include:
· Local or basin scale changes in wind stress curl.
· Adjustment of the Malvinas to changes in the subantarctic front.
· Changes in the Brazil Current momentum to the north.
· Local dynamics within the Confluence involving recirculation and meander dynamics.
Changes in winds over the basin to the east create Rossby modes at annual period (Forbes et al., 1993), but this input is hard to isolate from the effects of variations in the strong local wind stress curl pattern or changes in atmospheric forcing to the south that can change the flows along the boundary via Kelvin mode adjustment (Smith et al., 1994; Garzoli and Giullivi, 1993). In fact, the latter can happen in response to the changes in the transport of the Circumpolar Current in the Drake Passage without any local wind input (Whitworth, 1983; Smith et al., 1994). The Brazil Current mass transport will respond to integrated wind stress curl across the basin and, under the Stommel (1965) hypothesis, to fluctuations in the thermohaline circulation. It is assumed that the thermohaline changes will be reflected in variations in the entire central water mass structure of the subtropical gyre. These water masses have a significant input of Indian Ocean waters through direct leakage (Gordon, 1986) and Agulhas rings (Olson and Evans, 1986). Finally, there is the suggestion of Byrne et al. (1995) that the rings themselves eventually join the Brazil Current and can be a direct source of forcing.
The last point above involves the details of the Confluence itself. From a dynamical point of view, the problem of the large amplitude meander pattern that occurs as the Brazil/Malvinas systems leave the coast is difficult (see Pedlosky, 1996). Most of the analyses that can be done on the problem are inherently ill-posed since they demand information from the solution to feed back into the boundary condition on the coast (Campos and Olson, 1991; Matano and Philander, 1994; Nof and Pichevin, 1996). These analyses suggest, however, that variations in the meanders can occur with periods of seasonal scale and longer.
Variations of the offshore boundary currents, local winds, and freshwater inputs from the continent dominate the circulation over the broad southeastern South American continental shelf, which produces SST anomalies that are nearly as strong as those produced offshore. In the Brazilian sector, during the southern hemisphere summer, upwelling events associated with the poleward flow of the Brazil Current causes the intrusion of cold, nutrient-rich water that increases the stability of the water column, decreasing the potential for vertical mixing (Castro Filho et al., 1987). To the south, and off the Rio de la Plata, water properties are dominated by mixing between water masses of subantarctic and subtropical origin (Miranda and Castro Filho, 1979), fresh water discharges, and local atmospheric forcing (Bakun and Parrish, 1990). The surface layers of mid- and outer-continental shelves are occupied by the Tropical Waters, whose origin can be traced to the Brazil Current, and by Sub-Antarctic Waters originated from the Malvinas Current (Ciotti et al., 1995). The combination of Tropical Waters and South Atlantic Waters forms the Subtropical Water, which is also referred to as the South Atlantic Central Water (Sverdrup et al., 1942), that flows northward in a deeper layer. The inner shelf is occupied by Coastal Water which is a mixture of the different shelf waters and freshwater outflow which is controlled by discharges from the Rio de la Plata and the Patos Lagoon. The Rio de la Plata drainage basin is the second largest in South America with more than 75% of its discharge provided by the Parana River, the remainder derived from the Uruguay River. The maximum discharges of the Rio de la Plata occur between March (autumn) and October (spring), due to different peaks in its main contributors (Fillmann, 1990). The outflow of the Patos Lagoon is about 10 times smaller in volume than those of the Rio de la Plata (Miranda, 1973), showing its maximum outflow during winter and spring (Moller et al., 1991). Pereira (1989) and Zavialov et al. (1995) found that during the summer months there is a coastal current running northward parallel to the coastal margin, against prevailing southward wind conditions, that is related to freshwater discharges from the Rio de la Plata and the Patos Lagoon. In the southern portion of the shelf, water mass characteristics are controlled by diffusion from the Magellan Straits (Krepper, 1977), solar radiation (Rivas, 1994), and the influence of the narrow belt of cold Antarctic waters associated with the equatorward flow of the Malvinas Current (Bakun and Parrish, 1990). Zyranov and Severov (1979) used hydrographic observations to calculate the baroclinic transport over this portion of the continental shelf. They estimated an increase in the circulation during the winter months related to an intensification in the atmospheric circulation. A similar result was obtained by Forbes and Garraffo (1988) who, using a one-dimensional model, found that during the winter northward transports were higher than those corresponding to the summer. Consideration of the temperature time series used to create Fig. 9 shows that these anomalous cold water extensions to the north on the shelf occur in every ENSO +1 in the record (1981-1995). The question about coastal circulation is what portion of its large SST anomalies are driven by direct forcing with the atmosphere and river runoff and what portion by cross-shelf exchanges with the bounding Brazil and Malvinas Currents.
Human scale relevance of these variations range from direct impact of drought and floods with their associated stress on agriculture, urban water supplies, and industrial activities. The entire understanding of these links demand success in both atmospheric and the better simulations of the oceanic systems that are the focus of the current IAI project and associated planning activities. Of particular interest are changes in offshore conditions that are capable of displacing or completely dispersing fishery resources in the region (Lima et al., 1996). Human health issues are also tied to episodic harmful algal blooms along the South American platform. It is interesting that these involve both subantarctic and subtropical species being moved along the shelf and outbreaks in the estuaries. Finally, there are ties to industrial issues such as navigation, water supplies and sewer discharge, and offshore oil exploration and production that will benefit from better specification of ocean and estuarine circulation in the region.
To successfully achieve the main objectives, the Implementation Plan will optimize communications between observational and modeling efforts, both in oceanography and meteorology, and will maximize the use of existing infrastructure and ongoing scientific programs of the participating countries. In particular, the objectives will be addressed by:
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Figure 9. Sea surface temperature anomaly along the middle South American continental shelf as a function of latitude and time. The figure shows the creation of a cold anomaly proceeding from south to north late in the 1983 El Niño. The propagation of the initial cold anomaly to the north is at the speed of the barotropic Kelvin wave (from Lentini , Olson, Podesta, and Campos, personal communication).
· Developing a comprehensive database of existing atmospheric and hydrographic data. This database will serve as an observational research tool and will provide initializing and forcing functions for the development of oceanic models. In particular, a regional wind database will be generated by a high resolution atmospheric model to provide better resolution of surface winds on the South Atlantic Ocean for the regional ocean models.
· Implementing an observational program that will complement the existing database and will address the specific needs of the research program. The objective of the observational program is to measure the current system of the southwestern South Atlantic, its interaction with the large-scale gyres of the interior basin and adjacent oceans, river discharges, and shelf processes. The ecology of key planktonic species in this region will be identified and described in order to predict the impact of global climate changes on the region's ecosystems. Besides their intrinsic value as diagnostic tools, these measurements will provide the basic information for a monitoring scheme, on decadal time scales, and the development of prognostic model simulations.
· Generating a hierarchy of numerical and analytical oceanic models to investigate the South Atlantic circulation, its links to the global circulation, and its sensitivity to climate change scenarios. The numerical models, coupled and uncoupled to atmospheric models, will range from the global to the regional scales and will be carried out in a coordinated effort among several research institutions.
The connection between the South Atlantic Ocean and the global circulation will be made through the collection and analysis of atmospheric and oceanic observations and numerical simulations. Meteorological databases of particular interest to this research program are: (1) global analyses produced by major forecast centers (e.g., ECMWF, NCEP); (2) observations at station locations (e.g., precipitation, river discharge); and (3) satellite estimates of surface winds, precipitation, SST, and SSH. Since global analyses provide only a coarse description of regional quantities, a hierarchy of embedded atmospheric models of increasing resolution will be developed to provide high resolution data on the regional scale. This database will in turn be used by high resolution ocean models.
As discussed in the introduction, it is necessary to understand the wind driven and thermohaline circulation of the basin and its interaction with the adjacent oceans in order to predict the effects of climate change in the South Atlantic. Observations at the basin scale will then be directed to understand the interactions of the SWA with the basin scale circulation and with the adjacent Pacific and Indian Oceans. Measurements should, therefore, monitor the Antarctic Circumpolar Current transport at the Drake Passage to determine its effects on the Confluence dynamics. This component will be mostly built on existing monitoring programs (Fig. 10), and it will be augmented in order to fulfill our specific objectives. The interaction of the SWA region with the Indian Ocean will be studied by monitoring the incoming transient flows entering the SWA. The contribution of the Agulhas eddies to the variability of the Brazil Current transport and, the possible effect on triggering of Confluence eddies, will be investigated by measuring the number of eddies entering the SWA and estimating the heat and mass contributed through this process to the western boundary current.
The observational program will include CTD/tracer (chlorofluorocarbon, CFC) measurements. The objective of these measurements is to monitor changes in the partitioning of the air-sea fluxes as they affect the properties of the mode and intermediate water waters (SAMW/AAIW). The importance of these changes in the meridional overturning circulation on long time scales will be evaluated through model simulations and related to long-term variations in SST such as ENSO and the Atlantic Oscillation. Transient tracer observations can effectively contribute to an evaluation of the variability related to the introduction of SAMW/AAIW into the southwestern Atlantic thermocline. Such changes can have a significant impact on the vertical density structure of the circulation gyres (Coles et al., 1996). The subsurface penetration of a tracer is similar to that of a perturbation due to climatically-induced changes in surface properties at the formation regions. Transient tracers have played an important role in the evolution of our description of the circulation from one of a steady state into one of significant decadal time scale variability. They can be used to evaluate how rapidly climate anomalies propagate into the interior and, together with coupled model simulations, can help in identifying the processes behind climate variability. Specifically, tracer gradients will be used to calculate effective spreading rates and pathways, and inventories can be used to calculate rates of water mass formation, and estimate dilution factors.
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Figure 10. Area of the intensive field work proposed in the implementation plan. The rectangles indicate the approximate locations of the monitoring arrays, the triangles, the locations of the existing moorings, and the grid, the domain of the regional models. At the Confluence of the currents from Brazil and Malvinas, a strong thermohaline front is formed. Changes in the location of this front has associated dramatic changes in the SST that extend to the coastal zone and which alter the local climate. Therefore, at the regional scale the emphasis will be on measuring simultaneously the transports of the Malvinas and the Brazil Currents, the location of the Confluence front, and the interaction between river discharges and shelf processes. The area in which the intensive field work is proposed is shown in Fig. 10. The Malvinas Current transport will be obtained by collecting measurements along two transects: one at the base of the current, to get the ACCs contributions (around 50°S), and one at the latitude of maximum transport before the Confluence (around 40°S). The Brazil Current transport will be measured along three transects: at the base of the current to get the input from the South Atlantic gyre (around 20°S); at the latitude where maximum impact from interior transients is expected (25°S); and at the latitude of maximum transport before the Confluence (around 35°S). A meridional array of instruments will monitor the incoming transient fluxes. Frontal motions will be monitored through remote sensing (SST, SSH), and the possible influence of deep flows (Weatherly et al., 1993) on the Confluence variability will be investigated. The impact of these hydrographic features on plankton biodiversity, abundance, and distribution in time and space (Fig. 11) will be studied. The "key" species in the southwest Atlantic ecosystem from the perspective of water mass indicators, species with economic and societal impact, and species with implications for, and relationships to, global change phenomena, will be identified and their abundance and distribution determined. In addition, the causes and consequences of persistent intense ocean color bands in the shelf-slope region will be studied.
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Figure 11. CZCS ocean color composite image showing phytoplankton pigment concentration (and, indirectly, phytoplankton biomass) in October 1979. The image shows a high phytoplankton biomass along the shelf-slope break, a feature that was interannually persistent through much of the CZCS lifetime. The high-biomass band is also associated with unusually high water-leaving radiances, possibly caused by blooms of Emiliania huxleyi, a coccolithophorid playing a significant role in CO2 sequestering and in the production of dimethylsulfopropionate (DMSP). Carbon dioxide and dimethysulfide (the breakdown product of DMSP) are climatically important gases regionally and globally.
The oceanographic research on continental shelves of the southwestern South Atlantic will be directed to understand the mechanisms which control heat and fresh water fluxes on the shelf, and their relationship to global change. This region between the estuaries along the coast and the western boundary currents at the shelf break is the dynamical interface between the terrestrial and oceanic systems. Processes in this area affect and are affected by atmospheric dynamics through both direct and indirect couplings. Mesoscale processes on the continental shelf such as wind-driven coastal upwelling (e.g., at Cabo Frio), meander-driven upwelling (e.g., along the southern Brazil and northern Argentine shelf breaks), and density-driven currents (e.g., the cold low salinity Rio de la Plata coastal plume) transport significant quantities of heat, salt, and nutrients which are key to controlling biological and physical dynamics in the region. Changes in the winds, surface heat fluxes, and riverine discharges can substantially alter the strength of these transport mechanisms. These can modify fluxes to the atmosphere and, therefore, potentially, result in a feedback to the continental shelf dynamics. Both selected observational and numerical efforts are necessary to elucidate the details of these connections among the oceanic, atmospheric, and continental shelf dynamics and should be initiated over the next few years.
A hierarchy of numerical and analytical models will be used to investigate the circulation in the SWA, in particular, and the South Atlantic in general, and to evaluate its sensitivity to climate change scenarios. The sensitivity of the model South Atlantic circulation to various climate change scenarios in the atmosphere (uncoupled model) will be first investigated in a global model (feasible with resolution of 1 to 2 degrees). The focus will be on how the modeled South Atlantic connections to the rest of the world oceans are modified under these various scenarios. In particular, the property changes in water masses and associated impact on the thermohaline circulation (strength, pathways) will be documented. Coupled runs with an atmospheric model will be performed and the results compared to the uncoupled runs with the ultimate goal of identifying the mechanisms behind any climate change in the South Atlantic. At the basin scale the objective will be to determine the impact of local and remote forcing (winds, ACC transport, Agulhas transfer) on the heat and mass balances of the South Atlantic (resolution of 1 to 1/3 degree monthly winds), and on the Brazil-Malvinas Confluence variability (high frequency daily winds, high resolution of 1/10 degree). The model domain in these experiments will include most of the South Atlantic basin. The boundary conditions will consist of either climatology or a mixture of coupled/uncoupled global model data and observations. Links with ENSO anomalies will be investigated. The effect of Agulhas eddies on the Brazil Current will be simulated in the basin scale models by inserting a string of eddies in the western part of the domain (based on satellite and on-site measurements). The inserted eddies will then be free to move westward and interact with the western boundary current. An analytical model will be developed in parallel to specifically address that issue. A simulation base (model) leading to an understanding of the consequences of broad-scale variations in water mass structure in terms of ecosystem structure/function and species-specific issues, will be developed. Efforts will be directed to document the fluctuations in ecosystems structure and function in relation to ENSO.
Regional models will be used to investigate the mesoscale variability at the Brazil/Malvinas Confluence, coastal circulation process, and the interaction between the two systems. These models will have a horizontal resolution that will range from 3-5 km, in the coastal areas, to no more than 10 km in the open ocean. Coupled to the basin scale and global models, regional models will be used to investigate the dynamical balance of the Brazil/Malvinas Confluence and its sensitivity to changes in the mass transport of the western boundary currents, local winds, and fresh water discharges. With the eddy resolving capabilities of the present models, for certain range of transport values, it is not expected to observe stable configurations of the Confluence but a pattern that oscillates with periods related to unstable waves' growth and decay, wind variability, and non-linear processes. Such situations have been reported in the observational descriptions of the Confluence (e.g., Legeckis and Gordon, 1982; Olson et al., 1988; Garzoli and Simionato, 1990). Regional models will also be used to investigate the effect of inertial recirculation cells on the variability of the system, and to estimate the effect of Agulhas eddies on the Confluence variability. With regard to coastal circulation phenomena, the first question to be answered is: What are the mechanisms by which the coastal and deep circulation interact? Attention will be focused on the following mechanisms of cross-frontal exchanges: (a) coastal upwelling at the shelf break associated with the poleward flow of the Brazil Current; (b) eddy diffusion of Brazil and Malvinas waters across the continental shelf; and (c) penetrations of Malvinas waters onto the shelf region associated with oscillations of the Brazil/Malvinas Confluence. The coastal circulation of the SWA is not only driven by exchanges with western boundary currents but also by local wind forcing, fresh water discharges, and surface fluxes of heat and water. As part of our numerical and observational work, the role of these local forcings on the coastal dynamics will be evaluated. Regional models will also be used to diagnose the impact of ENSO effects on the SWA. It has been observed that the warm phases of an ENSO event result in higher precipitation rates and substantial alterations of wind patterns on the SWA (Ciotti et al., 1995). Higher precipitation rates increase freshwater discharges to the shelf area and influence stability conditions of the coastal waters. Changes in the wind patterns can disrupt the normal seasonal cycles of the coastal circulation. The intensity and extent of these changes are presently unknown and will be investigated as part of this program.
Instruments proposed to be used include: current meter moorings, IES, pressure gauges, CTD/CTD/tracer surveys, ADCP (moored, lowered, and hull mounted), and CTD moorings (shallow). Models to be used are GFDL, MICOM and POM.
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