NOTES AND CORRESPONDENCE. Time and Space Variability of Rainfall and Surface Circulation in the Northeast Brazil-Tropical Atlantic Sector

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1 April 1984 P.-S. Chu 363 NOTES AND CORRESPONDENCE Time and Space Variability of Rainfall and Surface Circulation in the Northeast Brazil-Tropical Atlantic Sector Pao-Shin Chu* Department of Meteorology, University of Wisconsin-Madison (Manuscript received 2 October 1982, in revised from 25 February 1984) Abstract Rainfall series in two large sectors of Northeast Brazil (Nordeste) and indicative atmospheric/oceanic fields over the tropical Atlantic are studied in terms of their characteristic time and space variability. Spatial couplings between rainfall and surface circulation are first explored using simple correlation. Spectral analysis of rainfall reveals relative maxima of power in the frequency bands of , , and years. The spatial linkages are further examined by maps of the phase relationships in these three preferred bands. 1. Introduction Rainfall in Northeast Brazil (Nordeste) undergoes a pronounced change from year to year, and conditions can become critical for agriculture which is the main economy for the region. In recent years, the study of climate and circulation of the Nordeste has attracted considerable attention. Namias (1972) proposed an interconnection between rainfall in Ceara (Fig. 1) and cyclonic activity over the subpolar North Atlantic. Hastenrath and Fig. 1 Orientation map. State of Ceara is delineated by broken line. * Present affiliation, Department of Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA. Heller (1977) showed that rainfall at the height of the rainy season (March/April) in the Northern Nordeste is sensitive to the meridional displacement of the equatorial trough zone, and to the sea surface temperature (SST) anomalies in the tropical Atlantic. Markham and McLain (1977) likewise found a high correlation between rainfall in Ceara and South Atlantic SST. The mechanisms of extreme climatic events in the Northern Nordeste were further studied numerically by Moura and Shukla (1981) using the GLAS general circulation model. For the Southern Nordeste, the maximum rainy season is around November/ December which differs somewhat from the Northern Nordeste. It has been suggested that rainfall variations are mainly associated with cold fronts of the Southern Hemisphere (Kousky and Chu, 1978; Kousky, 1979; Chu, 1983). Concerning the periodic variations of rainfall in the Nordeste, Markham (1974) found a significant period near 13 years at Fortaleza,, Ceara. Kousky and Chu (1978) noted the prevalence of spectral peaks with periods of 3-5 years in the Northern Nordeste, 2-3 years in the Southern Nordeste, and years in several stations in the entire region.

2 364 Journal of the Meteorological Society of Japan Vol. 26, No. 2 The present study, motivated by earlier investigations linking regional rainfall anomalies to large-scale circulation, uses time series analysis for a dual purpose : to identify the preferred frequency bands of these variations and to explore the spatial couplings at certain time scales. 2. Data analysis and methods Time series of monthly rainfall totals for 14 stations located in the same climatic regime in the Southern Nordeste during the period were used to construct a regional rainfall index. Mean and standard deviation for each station were first calculated and then departures from the mean of each individual year were determined. Subsequently, departures of each individual year were divided by the standard deviation to normalize the time series. The arithmetic average of the values of normalized departures for all 14 stations forms the regional index. Likewise, time series for the 40 stations in the Northern Nordeste during the period were utilized to obtain another regional rainfall index. Ship observations over the tropical Atlantic between 30N-30S during the period have been processed by 5 degree square areas. Due to the large number of missing ship observations from , the period of analysis in this study is restricted to Monthly mean ship records are expressed as departures from their long-term monthly averages. Missing records are linearly interpolated from the departures of preceding and following months. Zonally averaged values of large blocks over the Atlantic were then compiled for sea level pressure (SLP), and smaller blocks were determined for the zonal and meridional wind components (u and v), and sea surface temperature (SST), so as to account for the narrow intertropical convergence zone and the areas of major oceanic currents. Since the rainfall index was seasonalized according to each region's peak rainy season (Chu, 1983), monthly anomaly data of ship observations were also seasonalized in two different forms : from July to June for the Southern Nordeste and from September to August for the Northern Nordeste. The methods used in this study are the simple correlation analysis and the non-integer spectral techniques of Schickedanz and Bowen (1977). Correlation coefficients between seasonalized rainfall and ship observations were computed based on 51 annual values. The innovative spectral method of Schickedanz and Bowen has the advantage of increasing the resolution of wavelengths. Schickedanz and Bowen (1977) recommend that the spectral estimates be determined on the order of 0.1 data point. Accordingly, spectral estimates of rainfall and surface circulation parameters were calculated at equal increments of wavelength with a resolution of 0.1 year, which is approximately equivalent to one month. 3. Spatial couplings of rainfall and tropical Atlantic circulations Circulation departure patterns in the Atlantic were investigated for composites of extremely dry and wet years in both the Northern and Southern Nordeste (Hastenrath and Heller, 1977; Chu, 1983). Drought in the Northern Nordeste (Hastenrath and Heller, 1977) is marked by an equatorward extension of the South Atlantic high and a poleward retraction of the North Atlantic high, along with the further northward position of the wind confluence axis along equatorial Atlantic and warm/cold waters in the North/South Atlantic, departure patterns during wet years being approximately inverse to the dry years. The dry years in the Southern Nordeste (Chu, 1983) are characterized by positive pressure departures over the South Atlantic, strong on-shore Southeast trades, and anomalously warm waters along the South Brazil coast. During the wet years departure patterns are nearly inverse to the dry years. Complementing these earlier stratification studies, spatial correlations are mapped in Figs. 2A to D and Figs. 3A to D. Concerning the Northern Nordeste rainfall, Fig. 2A shows positive/negative correlations with SLP over the North/South Atlantic. Fig. 2B indicates positive correlation with the westerly wind component and Fig. 2C shown negative correlation with the southerly wind component over the equatorial Atlantic. Fig.

3 April 1984 P.-S. Chu 365 Fig. 2 Correlation coefficients, in hundredths, between rainfall in the Northern Nordeste and (A) SLP departures, (B) u departures, (C) v departures, and (D) SST departures. Fig. 3 Same as Fig. 2, except for rainfall in the Southern Nordeste.

4 366 Journal of the Meteorological Society of Japan Vol. 26, No. 2 2D illustrates negative coupling with SST in the North Atlantic and positive correlation with SST in the South Equatorial Atlantic. In context, these patterns are consistent with the earlier studies (Hastenrath and Heller, 1977; Hastenrath and Kaczmarczyk, 1981), and are indicative of the tendency for a more northerly/southerly position of the near-equatorial low pressure trough and associated circulation features during dry/wet years in the Northern Nordeste. The negative/positive linkages between rainfall and SST over the North/equatorial South Atlantic suggest that rainfall activity may also be modulated by the thermally direct meridional circulation cell, proposed by Moura and Shukla (1981). Deficient rainfall in the Northern Nordeste would be concomitant with a local meridional overturning featuring ascending motion over the North Atlantic and subsidence over Northeast Brazil and the adjacent South Atlantic. Some difference between the current study and Hastenrath and Kaczmarczyk (1981) are apparent, particularly for correlations between Northern Nordeste's rainfall and SLP over the North Atlantic. This seems mainly due to the fact that the present study uses seasonalized, rather than the calendar-year, annual values. Correlations with Southern Nordeste rainfall are depicted in Figs. 3A to D. Although the overall spatial patterns of correlation for SLP, u, v, and SST are similar to the ones discussed above in relation to Figs. 2A to D for the Northern Nordeste, the magnitude of correlation coefficients is generally reduced. Also noted are some positive coupling with the southerly wind component along the Southeast coast of Brazil (50-10W, 20-30S) and negative linkages with the SST over the same area. 4. Results of spectral analysis Fig. 4 illustrates the spectra of rainfall series in both Southern and Northern Nordeste. The spectral characteristics in these two regions are similar. The present discussion is limited to the phase relationships between rainfall and surface circulation in the frequency bands of years, years and years, where rainfall variance in the Fig. 4. Spectra of rainfall series in Northeast Brazil. The timing of the first maximum for major spectral peaks is indicated by the numbers in tenths of years (e. g. 211 signifies ). One, two and three dots denote, respectively, ten, five and one percent significance levels. two regions is concentrated. These three bands are embedded within the broader spectral bands of 10-20, 3-5, and 2-3 years identified earlier by Kousky and Chu (1978). These three rather narrow bands are chosen because there is only one peak presented in each band for both regions. One peak per band facilitates the discussions of phase relationships between rainfall and surface circulation patterns. The phase is here defined as the time of the first maximum of power starting from a. The year frequency band Fig. 5A shows that in this frequency band the maximum rainfall in the Northern Nordeste lags behind that of the Southern Nordeste by about 1.3 years. The extremum of rainfall in the Northern Nordeste varies inversely with SLP on the equatorward side of the South Atlantic high, but approximately in phase with SLP on the equtorward side of the North Atlantic high (Fig. 5A). Rainfall in the Southern Nordeste also tends to vary concurrently with SLP over the North Atlantic (20-10N, 70-15W). In Fig. 5B, Northern Nordeste's rainfall and westerly wind departures along its adjoining ocean and the equatorial North Atlantic reach their extrema simultaneously, a feature consistent with the correlation analysis (Fig. 2B). The most conspicuous feature in Fig. 5C is the near reversal of phase between rainfall in the Northern Nordeste and

5 April 1984 P.-S. Chu 367 Fig. 5 Phase relationships in the frequency band of year between rainfall series and (A) SLP departures, (B) u departures, (C) v departures, and (D) SST departures. Dots have the same meanings as in Fig. 4. southerly wind departures over the equatorial Atlantic (0-5S, 30-10W ; 5N-20S,10W-10E). Fig. 5C reveals an approximate in-phase relation between rainfall in the Southern Nordeste and the southerly wind departures along its adjacent ocean. Fig. 5D displays an inverse relation between rainfall in the Northern Nordeste and SST variations over a large portion of the North Atlantic. Also noted in Fig. 5D is the approximate phase reversal between Southern Nordeste's rainfall and SST fluctuations in the area off southeast Brazil (20-30S, 35-10W). The aforementioned phase relationships between rainfall and surface circulation are consistent with the correlation analyses in Figs. 2 and 3, and the general circulation studies of Hastenrath and Heller (1977) and Chu (1983). Thus, the mechanisms identified earlier appear with some preference to operate at this time scale. These features indicate that rainfall in the Northern Nordeste is modulated by the latitudinal displacement of the equatorial confluence axis, and that it may be influenced by a thermally direct meridional overturning with ascending motion over the North Atlantic and subsidence over Northeast Brazil and the acjacent equatorial Atlantic. b. The year frequency band In this frequency band, rainfall variations in the Southern and Northern Nordeste are approximately out of phase (Fig. 6), suggesting different mechanism of rainfall fluctuations in the two regions. In Fig. 6A, rainfall variations in the Nordeste do not show any distinct phase relationships with SLP over the Atlantic. Fig. 6B shows that the extrema of westerly wind departures along the Brazilian coast, the south equatorial Atlantic and off Northwest Africa coincide with the maximum of rainfall in the Northern Nordeste. Rainfall variations in the Southern Nordeste tend to vary concurrently with westerly wind departures off the coast of Southwest Africa (30-10S, 10W- 10E). Fig. 6C displays that the maximum of Northern Nordeste's rainfall follows the minimum of southerly wind departures over the equatorial South Atlantic (5-10S, 30-10W). Thus, when Northern Nordeste's rainfall

6 368 Journal of the Meteorological Society of Japan Vol. 26, No. 2 Fig. 6 Same as Fig. 5, except for the frequency band of year. reaches its maximum, weak southeasterlies prevail along the equatorial South Atlantic. In contrast to the correlation analysis (Fig. 3C), Southern Nordeste's rainfall shows some notion to vary in phase with southerly wind departures over a large portion of tropical Atlantic. In Fig. 6D, rainfall in the Northern Nordeste reaches its maximum around the same time as the maxima of SST over the south equatorial Atlantic (0-10S), implying that drought/ flood in the Northern Nordeste is related to cold/warm waters in this area. There is also some indication for a phase reversal between rainfall in the Northern Nordeste and SST off west Africa. The aforementioned spatial couplings between Northern Nordeste's rainfall and wind as well as SST variations agree broadly with the discussion in Section 3 and earlier diagnostic studies. However, for the Southern Nordeste, distinct couplings between rainfall and surface circulation are not apparent. c. The year frequency band In this frequency band, rainfall variations in both Southern and Northern Nordeste approximately coincide in phase (Fig. 7). Rainfall in these two large regions reaches its extremum concurrently with SLP over the North Atlantic (30-10N), where the pressure variations are significant at the one percent level (Fig. 7A). This spatial linkage is apparent in Figs. 2A and 3A. However, SLP variations over a large portion of the South Atlantic (5-20S) also occur simultaneously with rainfall in the Nordeste. In Fig. 7B, the maximum rainfall in the Nordeste occurs at nearly the same time as weak easterlies over the Southern Nordeste's coast. Fig. 7C displays an approximate phase reversal between rainfall variations in the Nordeste and southerly wind departures in the bands of 0-5N and 30-10W, and and 30-10W. Fig. 7D shows that rainfall variations display a phase reversal with SST over the North Atlantic bounded by 5-20N and 70-30W, but run almost parallel with SST variations over the South Atlantic (5-20S, 30-10W ; and , 35-10W). Together with the features presented in Figs. 7A-D, drought/flood in the Nordeste corresponds well with low/high SLP over the North Atlantic, strong/weak souther-

7 April 1984 P.-S. Chu 369 Fig. 7 Same as Fig. 5, except for the frequency band of year. lies at the equator, and warm/cold North Atlantic surface waters. The spatial linkages indicated in Fig. 7 are consistent with the discussion in Section 3 and earlier stratification studies, and suggest that at this time scale the Southern Nordeste may have the same general circulation mechanisms of rainfall anomalies as that in the Northern Nordeste. Acknowledgments I am grateful to Professor Stefan Hastenrath for his guidance on this work. Ming-Chin Wu assisted in the computer programming. Donna Bobst helped with graphics and typed the manuscript. This study was supported by the National Science Foundation under Grants ATM and ATM References Chu, P.S., 1983: Diagnostic studies of rainfall anomalies in Northeast Brazil. Mon. Wea. Rev., 111, Hastenrath, S., and L. Heller, 1977: Dynamics of climatic hazards in Northeast Brazil. Quart. J. Roy. Meteor. Soc., 103, Hastenrath, S., and E.B. Kaczmarczyk, 1981: On spectra and coherence of tropical climate anomalies. Tellus, 33, Kousky, V.E., and P.S. Chu, 1978: Fluctuations in annual rainfall for Northeast Brazil. J. Meteor. Soc. Japan, 57, Kousky, V.E., 1979: Frontal influences on Northeast Brazil. Mon. Wea. Rev., 107, Markham, C.G., 1974: Apparent periodicities in rainfall at Foraleza, Ceara, Brazil. J. Appl. Meteor., 13, Markham, C.G., and D.R. McLain, 1977: Sea surface temperature related to rain in Ceara, Northeast Brazil. Nature, 265, Moura, A.D, and J. Shukla, 1981: On the dynamics of droughts in Northeast Brazil: Observations, theory and numerical experiments with a general circulation model. J. Atmos. Sci., 38, Namias, J., 1972: Influence of Northern Hemispheric general circulation on drought in Northeast Brazil. Tellus, 24, Schickedanz, P.T. and E.G. Bowen, 1977: The computation of climatological power spectra. J. Appl. Meteor., 16,

8 370 Journal of the Meteorological Society of Japan Vol. 26, No. 2 Pao-Shin Department of Meteorology, University of Wisconsin-Madison,* U. S. A. Chu