The tropical Brazil does not only encompasses the Earths’s largest rain forest in the Amazon basin, but also the “Caatinga”, which is the dry forest in Northeast Brazil (NEB), occupying an area of around 800,000 Km² and covering approximately 11% of the national territory, as illustrated in Figure 1. The Caatinga is a fragile and unique ecosystem with a great variety of endemic species. Brazil’s Northeast is the third largest, the third most- populous, and also one of the poorest regions in the country. The region is characterized by marked interannual precipitation variability, and has a long history of severe recurrent droughts, which seriously affect its agricul- tural production and water resources.

Given NEB’s unique climatic characteristics and land degradation, the United Nations Convention to Combat Desertification (UNCCD, www.unccd.int) has recognized NEB as particularly vulnerable to desertification [1] . In this context, an area known as Seridó, located in the semiarid interior of Rio Grande do Norte state (RN), has been assessed to be one of the most affected by desertification in this state (see Figure 1 for location). This is due not only to its climatic conditions, but also to the human activities associated with inappropriate land management in the region. Land use patterns and practices in the Seridó have increased land degradation, associated with deforestation of the Caatinga to create fields and pastures, and to supply the demand of fuel- wood for households, and for the production of wood charcoal for industries (e.g., ceramic, gypsum, plaster, limekilns). Salinization of soils and local water resources is another important factor of environmental degra- dation over the area [1] . Water supply in the Seridó is basically provided through dams, small reservoirs, wells and cisterns that collect water during the rainy season.

Precipitation over the Seridó is concentrated in a short wet season, which comprises several spells of heavy rains, each lasting on the order of a few days to a week or more, separated by breaks of reduced rainfall as illustrated in Figure 2. The Seridó experiences large fluctuations in annual rainfall: the year 2000 not only was wetter than 2001, but also showed spells of comparatively enhanced rainfall. Comparing with the annual climatology (see Table 1), the year 2000 showed wetter-than-normal conditions, whereas the year 2001 was drier-than-normal.

All these environmental impacts have contributed to increase social and economic impoverishment, and migration of people from rural areas. Given the importance of intraseasonal rainfall variations for agricultural production and water management in semiarid regions, this study focuses on the influence of the Madden-Julian Oscillation (MJO) over the environmental fragile Seridó/RN region.

The MJO has been documented as a major source of variability in the tropics. The MJO consists of a large-scale region of enhanced convection and precipitation that originates over the tropical Indian Ocean and propagates slowly eastward to the western-central Pacific [2] - [5] . Many studies have revealed that the propagation and evolution of the MJO events have great influence on precipitation not only in the tropical region, but also in the extratropics. For instance, over South America [6] - [11] , Africa [12] [13] , Mexico [14] , United States [15] - [18] , Canada [19] , Australia [20] , Indonesia [21] [22] , and China [23] [24] . The MJO also affects monsoon systems (e.g., [25] - [30] ).

Previous studies using gridded station rainfall over tropical South America showed that the MJO plays an important role in modulating its convection and precipitation. For example, Carvalho et al. [7] found evidence of an increase on extreme rainfall events over north-northeastern Brazil (based on the December-February 95th daily rainfall percentile) during the MJO phase characterized by enhanced convective activity over the central

Pacific and suppression over the Maritime Continent. Opposite features were observed when the enhanced convection was over the Maritime Continent and suppression over the central Pacific. Souza et al. [31] examined precipitation variability over eastern Amazon and NEB during the autumn rainy season (March-May), com- bining the effects of the tropical Pacific and the Atlantic sea surface temperature (SST) anomalies in two extreme and contrasting scenarios. Their favorable (unfavorable) scenario was characterized by simultaneous occurrence of southward (northward) SST gradient in the intertropical Atlantic and La Niña (El Niño). They found evidence that, on intraseasonal timescale, anomalously deficient and abundant precipitation events occurred alternately in both extreme climatic scenarios. They suggested that the MJO played a key role in modulating deep convection and precipitation over the region during years under extreme climatic conditions. Souza and Ambrizzi [11] also documented that, on intraseasonal timescale, the MJO effectively contributes to a considerable fraction of the total rainfall during January-May season over southeastern Amazon and NEB. More recently, Mo et al. [32] examined the influence of the MJO on Pan America during December-March and June- September for the period 1979-2001. Analyzing the evolution of the MJO cycle, they showed that in South America, the largest influence was found over northeastern Brazil during December-March.

Although these studies greatly contributed to improve the understanding of the MJO’s influence over the South America, none of them focused specifically over NEB, or evaluated the MJO impact at individual station level. Even though a MJO signal may be difficult to detect at local scale, such information could be important for municipalities under water stress. Here, we introduce a study of the MJO’s influence on the Seridó region, and we based our investigation on daily rainfall from 17 raingauge data over 30-year period. More specifically, we investigate the MJO signal at individual stations located in Seridó for each phase of the MJO’s life cycle. This article is organized as follows. Data and methods are described in Section 2. The MJO signal on intraseasonal rainfall for each station in Seridó is discussed in Section 3. Then, a summary and final remarks of our work are provided in the final section.

To examine the influence of the MJO on rainfall in the semiarid Seridó/RN we averaged intraseasonal rainfall anomalies at each station for the eight phases of the MJO. During FMAM 1981-2010, we found between 49 (phase 1) and 55 (phase 6) blocks of days (or “events”). For instance, for MJO phase 2 there were 50 events (24- 28 March 1981, 8-11 May 1981 …, 18-20 March 2010, 4-6 May 2010). The composite MJO rainfall signals for each station are shown in Figure 3. If the rainfall anomaly at a particular station was statistically significant in a particular MJO phase, then a larger symbol was plotted on that phase.

During phases 1 - 3 the majority of stations showed enhanced rainfall. Maximum positive anomalies occurred in phase 2, where 13 out of the 17 stations showed statistically significant anomalies (between 0.9 - 1.9 mm/day). Significant positive anomalies are also found at 7 stations in phase 3 (between 1.1 - 1.7 mm/day). During phases 5 - 6 all stations showed suppressed rainfall. Statistically significant negative anomalies occurred at 12 stations in phase 5 (between 0.8 - 1.7 mm/day), and 10 stations in phase 6 (between 0.9 - 1.8 mm/h). Comparatively, stations located in the Western Seridó showed the absolute magnitude of rainfall anomalies slightly greater than those in the Eastern Seridó. Three stations did not show any statistically significant rainfall anomalies: Equador, Jardim de Piranhas and Santana do Seridó. Possibly, this may be related to raingauge exposure and local micrometeorological conditions, since raingauge measurements are influenced by factors such as instrument placement, and the topographic (ridge top, valley, etc.) and environmental (urban, suburban, rural, etc.) settings.

The results showed that the stations exhibited a similar anomaly pattern. Notice that there is a clear MJO signal in all the individual stations throughout the MJO cycle. The rainfall increased by about 30% in phase 2 and decreased by about 25% in phase 5 relative to the climatological mean for the whole Seridó/RN. Combining the MJO signals for wet phases 2 - 3 and dry phases 5 - 6, the difference represented about 50% - 150% modulation of the mean rainfall considering all stations.

Previous work on the MJO influence over tropical Brazil has been based only on gridded station rainfall data. Here, we hypothesize that the MJO effect on daily rainfall over the semiarid Seridó/RN in NEB could be felt at the local station level. We composited the MJO rainfall signal for each station during the region’s main rainy season (FMAM). Each station composite was obtained averaging the rainfall anomalies over the contemporaneous (i.e., with zero lag) days that fell in each of the eight MJO phases, considering a 30-year period (1981-2010). Our results showed that the rainfall patterns in the Seridó undergo substantial changes (from enhancement to suppression) as the convective center of the MJO propagates eastward. Considering a 5% significance level, the maximum positive rainfall anomalies occurred in phase 3 (or occasionally in phase 3)

when the MJO’s active convection was over the Indian Ocean, whereas the maximum negative anomalies occurred in phase 5 (or occasionally in phase 6), when its convection was passing through the Maritime Con- tinent/Western Pacific Ocean. The typical difference between the wet phase 2 and dry phase 5 represented at least 50% modulation of the daily mean rainfall in 13 out of the 17 stations.

As stated in the introduction, in vulnerable semiarid regions, such as NEB, land degradation and droughts have negative impact on the quantity, quality and availability of water resources resulting in water scarcity. Moreover, prolonged breaks or extreme rainfall events during the rainy season can disrupt several sectors of society, such as agriculture, leading to devastating socio-economic consequences. Hence, knowing when wet and dry MJO phases may occur over the region could complement regional weather and short-term seasonal forecasts by combining this information with the MJO monitoring and forecast provided by some agencies (e.g., www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/mjo.shtml). These preliminary results for a small- scale study area have motivated our ongoing investigation of the MJO’s modulation of rainfall over the entire NEB. We are combining multiple raingauge datasets in the analysis to evaluate the spatial extent of the rainfall signal at station level, whether there is a seasonal variation, and also the MJO-related changes in convection and circulation over the region.

The rainfall data were provided by the EMPARN. We thank Dr. Charles Jones for providing the MJO index, and Dr. Rodrigo J. Bombardi for providing some programming routines. We also thank the two anonymous re- viewers for their comments and suggestions. Cati E. A. Valadão was supported by the Coordenação de Aper- feiçoamento de Pessoal de Nível Superior (CAPES) PhD scholarships (REUNI, DS and BEX 9108/12-2).

Cati E. A.Valadão,Paulo S.Lucio,Rosane R.Chaves,Leila M. V.Carvalho, (2015) MJO Modulation of Station Rainfall in the Semiarid Seridó, Northeast Brazil. Atmospheric and Climate Sciences,05,408-417. doi: 10.4236/acs.2015.54032