IMPACT OF CLOBAL CHANGE ON THE COASTAL AREAS OF THE RIO DE LA PLATA

Transcription

1 IMPACT OF CLOBAL CHANGE ON THE COASTAL AREAS OF THE RIO DE LA PLATA Vicente Barros*, Inés Camilloni* ** and Angel Menendez* *AIACC Project LA 26, University of Buenos Aires ** CIMA Introduction The Rio de la Plata (RP) has unique features; it is by far the widest river in the world with a width ranging from 50 Km in front of Buenos Aires, to 250 Km at its outlet on the Atlantic Ocean, Fig 1. Its length is only 300 Km and it has a very small descent of only 0.01m / Km. These features make the RP to respond to strong southeasterly winds over the nearby areas of the Atlantic Ocean with wave tides that increase toward its inner part. Because of their origin, the largest wave tide frequently come along with destructive winds that harm coastal facilities On the Argentine coast these effects are worsen by flooding of many sectors of the coastal areas whose altitude is only a few cm above the RP mean level. These events, known as southeasterlies, produce considerable damages. The rising development of resort areas at the RP coasts as well as the occupation of other coastal areas for precarious housing by impoverished population increases the social and economical impacts of these floods. Climate change is expected to accelerate the present sea level rise, and probably alter the discharge contribution of the RP tributaries in a yet unknown manner. In addition, it could also lead to changes in the wind field over the RP and the neighboring ocean. Because of the RP shape and its small slope, these changes will modify its mean water level as well as the heights of the wave tides caused by the southeasterlies. These aspects are being explored in the context of the AIACC LA 26 Project. Although this report synthesizes the possible RP level response to other climate forcings, the focus is on only one of them, namely, the wind field. Sensitivity of the Rio de la Plata level to climate forcing The sensitivity of the level of the RP to mean sea level to wind field and discharge of its tributaries were explored with a two dimensional (vertically-integrated) hydrodynamic model. The model domain includes an extended area of its maritime front, further than the Continental Shelf, in order to have a fetch distance long enough to build up the storm surges which penetrate through the river mouth, Fig. 1. Its grid resolution is 2.5 km. The model implementation was based on the HIDROBID software (Menéndez 1990). Its calibration followed various steps, namely, comparison with: i) water level data from the Tide Tables; ii) recorded water level data for a relatively calm wind situations; iii) recorded water level and velocity for relatively storm-free period); and iv) recorded water level for storm events. A short overview of the sensitive experiments conducted with the model is depicted in Fig. 1. According with model simulations, sea level rise propagates inside the RP almost without modification. It is only slightly reduced at Buenos Aires port and even in the more inner part of the River, at the Martin García island, is only reduced a 10 %. This is an outcome of the RP huge width, and of its almost null slope. Two different scenarios of the RP discharge are presented in Fig. 1. One of them with 75,000 m3/s corresponds to the largest discharges observed during the last century. It affects considerably the inner part of the RP, being comparable to the expected sea level rise during the next hundred years. These largest discharges last for many months and therefore are likely to be concurrent with southeasterly events enhancing their flooding effects. Besides, these largest floods have increased their frequency since the 1970-decade worsening the inner RP coast vulnerability. The 30,000 m3/s case shows the possible effect of an extraordinary increase of about 50% in the mean annual discharge. According to the model outcome, only a minor effect on the mean level could be expected at Martin Garcia, but no further downstream, as this effect would be negligible at Buenos Aires. Thus, changes in the mean discharge of the La Plata basin seem no important regarding the mean RP level. However, the largest discharge events may have a considerable influence in the inner part of the RP. Model simulations indicates that the level response to the increase of the easterly component of wind is felt with more intensity in the RP than on the ocean coast and increases towards the interior of the River. This effect results from the RP shape and it is also observed in the astronomical and wind tide surges. This sensitivity experiment was run with a stationary wind field. Since the effort of wind over surface water is nonlinear with

2 wind intensity, the result here presented must be considered as preliminary because the response to a transient wind field would be likely greater. Martín García Buenos Aires Montevideo San Clemente Mar del Plata cm m3/s m3/s Wind (FUT-PRE) Figure 1 Model domain and results from sensitivity experiments. 50 cm+ stands for a scenario of 50 cm rise in the mean sea level. Wind refers to the result of increasing the easterly component from 3 to 4 m/s. 30,000 and 75,000 m3/s stands for two tributaries contribution scenarios Trends of sea level pressure fields Surface winds are not an available standard output of the GCM scenarios. However, since this variable is strongly coupled with sea surface pressure (SLP) fields, wind fields are discussed through SLP scenarios. The first step was to evaluate the ability of the GCMs to reproduce the present features of the SLP over a region that encompasses the RP. Thus, available SRES-A2 SLP scenarios were taken from the Modelle and Daten (MOD) web-page of IPCC ( GCMs outputs were checked with the National Centers for Environmental Prediction (NCEP) and National Center for Atmospheric Research (NCEP/NCAR) reanalyzes (Kalnay et al 1996), available from the Climate Diagnostics Center web page ( Comparison with monthly and annual NCEP/NCAR fields indicates that only four models, HADCM3, CSIRO-mk2, ECHAM4, GFDL-R30, have an acceptable agreement with the observed SLP fields in southeastern South America and the adjacent ocean and were able to represent the position of the pressure systems and their annual cycle (Camilloni and Bidegain 2002). Rotated Principal Component Analysis (PCA) in t-mode was applied to mean seasonal SLP fields ( ) of both, the reanalysis and model fields, in order to find the principal models of seasonal and interannual variability. This PCA was not performed on the ECHAM4 output because its experiment started in Fig 2 shows the first two PCs of the NCEP/NCAR reanalysis. They explain about the same variance, amounting together more than 90 % total. They describe seasonal variability as the first is quite similar to winter mean SLP field with westerly circulation reaching the RP latitude and the South Atlantic high (SAH) to the north, and the second represents fairly well the summer mean SLP with an enhanced and displaced to higher latitudes SAH. The Chaco low is suggested in the first mode while is very deep in the second as in SLP winter and summer fields, respectively. Table 1 shows how their percentages of variance are seasonally distributed, confirming that

3 model 1 is more important in winter and model 2 in summer. They have a similar to summer (winter) percentage of variance in spring (autumn). The two first PCA of the three GCMs, with small differences, have 1 CPs - NCEP % 2 CPs - NCEP % ATLANTIC OCEAN ATLANTIC OCEAN Figure 2. Principal Components: model 1 (left), model 2 (right). fields similar to those of the reanalysis (not shown). Their percentage of variance are approximatelly the same as in the reanalysis as well as their seasonal variability, indicating that they represent fairly well, not only the mean annual field, but also its seasonal variability. These two first PCA carry also interannual variability as can be seen in Fig. 3 in the case of the first mode. Fig.3 shows the loading factor of the first PCA of both the NCEP/NCAR reanalysis and the GFDL fields. The negative trend is of 10 % of its initial value in the case of reanalysis and 5% in the case of the GFDL-R30 experiment. In the case of the HADCM3 and the CSIRO-mk2, the negative trend was of 5 and 15 % respectively. At the same time, the second loading factor has a positive trend, both in the reanalysis and in the MCGs (not shown). This indicates that there was a shift of the mean SLP to the south, and an increment of the summer pattern at the winter pattern expenses. In fact, the comparison of the 1960/1970 to 1990/2000 mean SLP fields indicates that there was a shift in the maximum axis pressure of 1 to 2 towards the south. This shift was captured by the GCM experiments, only forced by the increment of greenhouse gas concentrations. The displacement of the western sector of the SAH towards the south imply a considerably increment of the easterly component of wind and it could explain part of the water level increment in the RP that was 10 cm in Buenos Aires and only 6 cm in Montevideo during the last 50 years. This difference between these two trends is consistent with the model response to changes in the easterly wind component. MODEL 1 MODEL 2 NCEP HADCM3 CSIRO GFDL NCEP HADCM3 CSIRO GFDL TOTAL SUMMER AUTUMN WINTER SPRING Table 1. Total percentages of variance associated to each PC model and their respective percentages of variance for each season. Future scenarios of sea level pressure and wind fields The main SLP fields of the SRES-A2 scenarios of the four models show that the shift towards the south of the western border of the SAH continues to the Fig 4 depicts this displacement for the case of the HADCM3 experiment.

4 Figure 3. Factor loadings of model. Solid line: NNCEPP/NCAR reanalysis. Dashed line: GFDL experiments. The SLP pattern evolves according to the model experiment, but all models show an overall trend to the south of the SAH circulation of about 2 to 3 in 150 years. As displayed in Fig. 4, this trend is more pronounced to the east of the analysed region. The importance of this shift in the climate of southeastern South America can be roughly assessed considering that the present seasonal shift is about 8. One outcome of this climate change will be the increment of the easterly wind component over the RP and the consequent rise in its mean level. Since the RP is the focal area of this study, the meridional gradient of pressure over the RP area was calculated as an indication of the easterly wind component, Table 2. In the case of the HADCM3 experiment, this gradient almost doubled in 150 years. a) b) latitude -35 latitude longitude longitude Figure 4. Mean sea level pressure for the HADCM3 experiment Period Meridional SLP gradient (hpa/ 1000 Km) Table 2. Meridional gradient over the Rio de la Plata as simulated by the HADCM3 experiment

5 Social vulnerability The largest tide flood caused by a southeasterly event reached at Buenos Aires port 3.90 m over mean sea level. The number of southeasterly events that surpassed the alarm level increased during the last three decades. If in spite of that, the conservative assumption is made that in the next hundred years the largest floods are not going to be greatest than in the past century, then the combined effect of mean sea level rise, strongest mean easterly winds and eventual large discharges may lead to some flood events with water level near 5 m over present mean sea level at Buenos Aires. A large area of the Great Buenos Aires city is below the 5 m altitude cote, including an important part within the administrative boundary of the city itself. The potential vulnerable areas are not only in coastal areas of the RP, but also in the heavily populated margins of the Riachuelo-Matanzas River and along the Reconquista River, an area of many precarious settlements of an impoverished population. The total population now living in the Great Buenos Aires below the 5m cote over present mean sea level is almost 1,500,000. For the administrative units with area below 5 m over mean sea level it was developed a social vulnerability index including indicators related to the following aspects: a) demography b) living conditions of the population and c) structural production and consumption processes.. Further research To asses the vulnerability to flooding under an context of climate change, Project LA 26 will developed future scenarios of the permanent impact of the sea level rise and of transient impacts through the enhancement of tide floods provoked by the southeasterlies in a GIS base. Return periods of the floods will be calculated with high spatial resolution for present conditions and for future scenarios at different crossing times. This information, together with the social vulnerability index, will provide the basis for the development of one or more vulnerability to climate change indexes. A quantitative approach, using GCM scenarios, is being developed to assess future changes, both, in SLP and surface wind fields. Trends, like those shown in Fig. 3, appear also in the PCA of MCG results. Finally, because of the non-linear response of the water level to the surface winds, it is necessary to run long (decades) transient experiments with the hydrodynamic model to assess the response of the mean RP level to surface wind. References Camilloni, I and M. Bidegain, 2002: Climate simulation by GCM in southeastern South America. Workshop on Climate Change and the Rio de la Plata. Montevideo.Available on CD-Rom. D Onofrio, E., Fiore, M., and Romero S., 1999: Return periods of extreme winter levels estimated for some vulnerable areas of Buenos Aires. Continental Shelf Resarch, 19, Kalnay et al., 1996: The NCEP/NCAR Reanalysis 40 years -project. Bull. Amer. Soc. 77, Menéndez, A. N., 1990 : HIDROBID System for current simulations. Revista internacional de métodos numéricos para cálculo y diseño en ingeniería, Vol. 6, 1.