7.10 LOW-LEVEL CIRCULATION PATTERNS AND IN THE ARGENTINE . ASSOCIATION WITH SOYBEAN YIELD.

M. Laura Bettolli* , Olga C. Penalba and Walter M. Vargas#

Departamento de Ciencias de la Atmósfera y los Océanos, FCEN, UBA, , # Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET)

1. INTRODUCTION 2. METHODS AND DATA

The study of atmospheric circulation In order to perform this study anomalies of patterns and the analysis of their frequency, daily averages of 1000 hPa geopotential heights distribution and temporal variability result a for the period 1979-2001 in a 2.5° x 2.5° - useful tool for diagnostics. Revisions of longitude grid were used. The series of daily the analysis methodologies and applications to maps was obtained from the NCEP/NCAR synoptic-climatology can be found in Barry and reanalysis II. The domain chosen extends from Perry (1973), Yarnal (1993) and more recently in 60° S to 15°S and 90°W to 30°W and it was Yarnal et al. (2001). For southern South defined to study circulation patterns that affect America, Compagnucci et al. (1998) discussed Southern (Figure 1). It includes different methodologies for classification of 475 grid points. The period of months chosen synoptic situations and Compagnucci and Salles was from October to May of the next year: 243 (1997) characterised the main synoptic patterns days x 22 years = 5346 maps. These months by means of principal component analysis (PCA) were selected since major in of observed daily sea-level pressure for the central-eastern Argentina occur in this period period 1972-1983. Solman and Menéndez (Prohaska 1976) and due to the importance of (2003) classified winter daily fields of 500 hPa within the study area (Figure 1). geopotential heights for the period 1966-1999 by Principal component analysis (Green 1978; means of K-means clustering technique and Jolliffe 1986) was carried out on daily maps to studied their relationship with precipitation and determine the main synoptic types and their temperature in Argentina. Bischoff and Vargas frequency. The PCA results using the T-mode (2003) studied the 500 and 1000 hPa weather approach and the correlation matrix showed that type circulations and their relationship with some the first six principal components (PC) explained extreme climatic conditions using reanalyses more than 80 per cent of the total variance. developed by the ECWMF for the period 1980- Unrotated solutions were used in this work since 1988. This kind of synthesis are very important Varimax orthogonal rotated solutions (Richman to develop objective forecast methods and 1986) did not redistribute satisfactorily the meteorological-climatic diagnostics to interact variance explained by the PC. The direct and with others systems like agriculture or hydrology. inverse modes of the first six PC scores patterns Subsequently, in with an important determined 12 different spatial patterns for world agricultural production it is necessary to geopotencial fields (Figure 2). A 0.7 factor study the climate events and related loading value was used as a threshold to retain meteorological conditions that have a strong the daily maps that are represented fairly well by impact on the economy. Argentine Pampas these 12 synoptic patterns and the series of (central-eastern Argentina) is one of these seasonal frequencies (total frequency from regions being the third-largest producer of October to May) were calculated for each soybean crop in the world (Podesta et al. 1999). pattern. Therefore, this work aims to identify In order to link low-level circulations characteristics of the atmospheric circulation at patterns to regional rainfall in the Pampas, series low levels over southern South America in order of monthly rainfall for the period 1979-2001 were to relate these types to precipitation amounts used. The dataset was provided by the National and soybean yields in the Pampas region. Meteorological Service of Argentina. Five stations were used in the analysis since series with more than 10% missing data were eliminated. An “out of range” consistency checks on the series were performed and they were

------* Corresponding author address: M Laura Bettolli, Univ. de Buenos Aires, Departamento de Ciencias de la Atmósfera y los Océanos, Buenos Aires, Argentina, (5411) 45763364 e-mail: [email protected]

consisted from a statistical analysis. However, the five stations used are located in the soybean’s production core region and in the Table 1. Characteristics of the direct (D) and centre of the Argentine Pampas (Figure 1). The inverse (I) modes of the first six PC scores areal average of ‘seasonal precipitation’ (total patterns. SP: South Pacific; SA: South Atlantic precipitation from October to May) in the Pampas region was calculated from the monthly PC Description dataset. Pattern Trough over southern part of the PC1 D and SP Ocean-Deep SA High Post-frontal entering via -20 PC1 I the SW of generating cold air advection Intensified zonal flow in mid- and high -30 PC2 D – Trough over Patagonia and Pampas center of Argentina. Region -40 Two systems with NNW-SSE direction: High over SP Ocean to Patagonia and PC2 I Patagonia higher latitudes, Low over North of the -50 country to SA Ocean. High-pressure system centered at the PC3 D eastern part of . -60 Low-pressure system centered at -90 -80 -70 -60 -50 -40 -30 PC3 I Figure 1. The study area. Black cruces indicate southeast of the continent location of rainfall stations. High-pressure system over Argentina. PC4 D SA and SP Highs limited to lower latitudes. This work focuses on yield (estimated as Intensified SA and SP Highs, relative the ratio of total production to area harvested) as PC4 I low over Argentina, westerly flow an indicator of a crop’s vulnerability to climate restricted south of 50°S variability. The 58 series of yield used in this analysis were obtained from Argentina’s Low-pressure system centered at Secretaría de Agricultura, Ganadería, Pesca y PC5 D south of Patagonia affecting the entire Alimentación de la Nación (SAGPyA 2000) and country. they correspond to provincial departments High-pressure system entering via the North of Patagonia, intensified located in the Pampas region. The series of PC5 I annual frequency of provincial departments westerlies south of 47°S over the SP where the soybean yield was classified as a Ocean minimum or maximum were used in the study. Low-pressure system over center and north of Argentina extending to SA PC6 D Ocean. Ridge entering south of the 3. RESULTS continent. High-pressure system centered at PC6 I 3.1 Weather Patterns , trough over Patagonia

The PC spatial patterns and their associated 1000 hPa geopotential fields are presented in Figure 2 and their characteristics are summarised in Table 1. Geopotential fields are shown instead of anomalies in order to make better synoptic interpretations. The 12 patterns obtained (direct and inverse modes of the first six PC scores patterns) fit very well with the main synoptic types recognized by forecasters.

PC Direct Mode Inverse Mode 02/26/2001 04/16/1984 -20

-30 PC 1 Var: 24.9% -40

-50

-60

05/01/1984 12/17/1984 -20

-30 PC 2 Var: 21.1% -40

-50

-60

10/21/1996 03/16/2000 -20

-30 PC 3 Var: 11.4 % -40

-50

-60 10/31/1980 10/27/1986 -20

-30 PC 4 Var: 9.9 % -40

-50

-60 10/12/1991 05/23/1999 -20 PC 5 Var: 9.0% -30

-40

-50

-60

01/15/1989 01/08/1981 -20

-30 PC 6 Var: 4.3% -40

-50

-60 -90 -80 -70 -60 -50 -40 -30-90 -80 -70 -60 -50 -40 -30-90 -80 -70 -60 -50 -40 -30

Figure 2. Spatial patterns of PC scores for the first six PCs (left column) and associated geopotential fields of 1000 hPa for the direct (central column) and inverse (right column) mode. Dashed line: negative values. Var: Percentage of explained variance.

decrease with time. Meanwhile those associated Even though the analyzed period in this with the inverse modes of PC4 and PC5 as well work is relatively short to infer temporal as frequencies associated with PC3’s direct variabilities, the temporal series of seasonal mode show a weak progressive increase with frequencies of the 12 synoptic patterns were time (Figure 3). These results indicate a weak studied in order to find main characteristics. The positive ‘trend’ in the seasonal frequency of N- seasonal frequencies of fields associated with NE flow (warm and humid) situations over the PC1’s inverse mode present a progressive central Argentina and a negative one in the

seasonal frequencies of situations with cold air of situations with favorable conditions for advection over this area. The most important generating rain (warm and moist advection) and interannual variability of seasonal frequencies the gradually increase in annual precipitation in was found for the synoptic situations with an recent decades over this area (Hoffmann et al. intense/weak zonal flow in mid- and high 1987; Castañeda and Barros 1994, 2001;. latitudes (maps associated with direct and Penalba and Vargas 1996, 2004). inverse modes of the second PC pattern) mostly in the second half of the record. This result may 40 1300 be related to the variability of baroclinic AV RAINFALL 35 FR_PC1 D 1200 FR_PC2 I situations which are associated with the intensity FR_PC4 I of the zonal flow. The opposite behavior (major 30 1100 standard deviation in the first half of the record) 25 1000 is observed in the interannual variability of situations with intense S-SW flow over the 20 900 southern part of the continent (inverse mode of 15 800 Absolute Frequencies the third PC pattern) (not shown). 10 700 RainfallAveraged (mm)

5 600 20 40 FR_PC1 I 0 500 FR_PC3 D 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 FR_PC4 I 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 FR_PC5 I 15 30 Years

10 20 Figure 4. Time series of the areal average of seasonal precipitation in the Pampas (bars) and time series of seasonal frequencies of daily fields Absolute Frequencies 5 10 associated with the inverse mode of the PC2 and PC4 and with the direct mode of the PC1.

0 0 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 Years 3.3 Soybean Yield Figure 3. Time series of seasonal frequencies of daily fields associated with the inverse mode of Finally, the annual frequency of provincial the PC1, PC4 and PC5 and with the direct mode departments in the Pampas region where the of the PC3. soybean yield was classified as a minimum was related to the averaged total precipitation series 3.2 Seasonal rainfall and the circulation patterns. It was found that higher frequencies of minimum soybean yield The seasonal frequency of daily maps for are well associated with minimum averaged the 12 synoptic patterns associated with the seasonal precipitation (significant correlation with areal average of the total precipitation in α=5%). Moreover, higher (lower) frequencies of Argentine Pampas was also analyzed. It was circulation types associated with the direct found that the maximum total precipitation in (inverse) mode of the first (third) PC can be central-eastern Argentina is mainly associated linked to higher frequencies of minimum with the occurrence of inverse modes of the (maximum) soybean yield condition (Figure 5). second and fourth PCs (Figure 4). These These synoptic types induce, on one hand, warm patterns favor N-NE flow (warm and humid air and humid anomalous advection without advection in the lower troposphere) over the ascending air motions resulting in high air Pampas and weaker zonal flow situations. Total temperature and no rainfall and on the other minimum precipitation is linked to high hand, cold southwestesterly flow anomalies in frequencies of circulation patterns associated the Pampas region resulting in below normal air with the direct mode of the first PC. Even tough temperature or probably in late frosts. Both these synoptic types cause a warm and humid effects impact negatively on soybean yield. N-NE advection over the whole region, the is strengthened inhibiting ascending air motions and obstructing disturbances migration toward lower latitudes. These results show that areal precipitation variability in the Argentine Pampas can be linked to temporal variations of specific synoptic situations. This is also observed in the increase

frequencies synoptic situations where the South 40 1300 Atlantic High is strengthened inhibiting Av rainfall 35 Minimum Yield 1200 ascending air motions and obstructing FR_PC1 D disturbances migration toward lower latitudes. 30 1100 The interactions of climatic system with 25 1000 agriculture was studied by means of soybean 20 900 yield as an indicator of a crop’s vulnerability to

15 800 climate variability. Higher frequencies of minimum soybean yield are well associated with Absolute Frequencies 10 700 Averaged Rainfall (mm) minimum averaged seasonal precipitation and 5 600 with synoptic types that induce, on one hand,

0 500 warm and humid anomalous advection without 1979 1981 1983 1985 1987 1989 1991 1993 1995 1997 1999 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 ascending air motions resulting in high air Years temperature and no rainfall and on the other hand, cold southwestesterly flow anomalies in Figure 5. Time series of frequencies of the Pampas region resulting in below normal air provincial departments with minimum soybean temperature or probably in late frosts. Both yield condition (bars), of areal average of total effects impact negatively on soybean yield. precipitation in the Pampas and of seasonal This work was intended as a first approach frequencies of daily fields associated with the to identify characteristics of the atmospheric direct mode of the PC1. circulation over southern South America that can be related to precipitation amounts and soybean yields in the Pampas region. The study of the 4. CONCLUSIONS distribution and temporal series of the circulations patterns in temporal scales lower This paper dealt with synoptic situations than seasonal as well as others geopotential represented by daily 1000 hPa geopotential levels are recommended in further studies. fields in order to find synoptic patterns in southern South America. Using an unrotated Acknowledgments PCA with T-mode approach, the outline spatial This work was supported by grants from structures of the PCs obtained fit very well with Universidad de Buenos Aires X234 and X135 the main synoptic fields recognized by and from AGENCIA BID 1207/OC-AR PICT 99 forecasters. However, it could be possible to find N° 07-06921 other synoptic fields resulting from the combination of two or more PCs. REFERENCES Even though the analyzed period in this work (1979-2001) is relatively short to infer Barry, RG and Perry, AH, 1973: Synoptic temporal variabilities it was found a progressive Climatology: Methods and applications. increase in the seasonal frequency of N-NE flow Methuen, London, 555 pp. (warm and humid) situations over central Bischoff, SA and Vargas, WM, 2003: The 500 Argentina and a decrease in the seasonal and 1000 hPa weather type circulations and their frequencies of situations with cold air advection relationship with some extreme climatic over this area. Further studies are needed but it conditions over southern South America. Int. J. can be suggested that an increase of favorable Climatol, 23, 541-556 situations for generating rain (warm and moist Castañeda, E. and Barros, V., 1994: Las advection) is coincident with the gradual rise in tendencias de la precipitación en el Cono Sur de annual precipitation observed in recent decades América de los . Meteorologica, 19, over this area. On the other hand, a progressive 23-32. diminution of cold air advection situations is Castañeda, E. y Barros, V., 2001: Tendencias coincident with the increase of temperature de la precipitación en el Oeste de Argentina. values over this area (Easterling et al. 1997; Meteorologica, 26, 5-24 Rusticucci and Barrucand 2004). Compagnucci, RH, Fornero, L. and Vargas, Areal precipitation variability in the W.M., 1985:Algunos métodos estadísticos para Argentine Pampas can be linked to temporal tipificación de situaciones sinópticas: Discusión variations of specific synoptic situations. metodológica. Geoacta, 13, 43-55. Maximum total precipitation in central-eastern Compagnucci, RH and Salles, MA, 1997: Argentina is mainly associated with patterns that Surface pressure patterns during the year over favor N-NE flow generating warm and humid air southern South America. Int. J. Climatol, 17, advection in the lower troposphere over the 635-653. Pampas and weaker zonal flow situations. Total minimum precipitation is linked to high

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