Analyzing climate variations at multiple timescales can guide Zika virus response measures
The 20th century decomposition for annual rainfall totals (Fig. 1a–c) and annual mean temperature (Fig. 1d–f) signals in LAC show sharp differences in the variability explained by each timescale. The black box overlaid onto Fig. 1 shows the area in which the highest number of reports associated with typical arbovirus vectors [22] and Zika cases [3] have been made, thus this region was selected for further analysis. On average, results for the selected region indicate that the portion of variance in rainfall associated with the climate change signal is nil (Fig. 1a), whereas that for the inter-annual component is about 60–90 % throughout the region (Fig. 1c). The decomposition also reveals that all three timescale components for surface air temperature are important (Fig. 1d–f).
The temperature long-term trend signal is particularly important in the southeastern regions of Brazil (Fig. 1d). The decadal signal is, in general, more important for temperature than for rainfall in the region, the contribution to precipitation being higher along the coast (20–30 %, Fig. 1b). For surface air temperature, however, the highest decadal component is found in the Amazon (~50 %, Fig. 1e). Inter-annual variations for surface air temperature show values over 30 % of the explained variance in most locations, with a local maximum in northeastern Brazil that explains at least 60 % of the variability (Fig. 1f). The lowest values of the explained variance at the inter-annual scale tend to correspond with the highest values of the long-term trend signal (see Fig. 1f and d).
Results are similar for the region of interest when particular seasons are considered [19, 21]: for rainfall, inter-annual and decadal scales are the most important, while for surface air temperature the three timescales share similar importance, although locally one timescale may exhibit greater importance than the others.
Complementary analysis was performed for the average climate over the boxed region of interest (Fig. 2). When summed, the specific contributions explain the observed anomalies for each particular year. These results show that a positive superposition between the rainfall inter-annual and decadal signals and all three temperature components (climate change, decadal and inter-annual) is key to understand the recent climate behavior in the region. This collection of drivers was responsible for the particularly warmer and drier than normal conditions present in the region during the last few years. The unprecedented positive temperature anomalies that started in the 1990s are consistent with the positive sign of the decadal component for that period, combined with the contributions of the long-term trend and inter-annual variability.
The spatial distribution patterns of temperature and rainfall anomalies in LAC were fairly similar in 2014 and 2015 (Fig. 3), which were, at their respective termini, the hottest years on record [23, 24]. The pattern correlations between these years are 0.81 for temperature and 0.73 for rainfall, both statistically significant (P??0.05) according to a Student’s t-test. The year 2015 also marked the start of one of the three most intense El Niño events on record. In terms of temperature anomalies, 2013 was normal in most parts of LAC, although the warming pattern in the Amazon extending through the study region in the following years was already present. A similar claim can be made for the annual rainfall anomalies in the region under study (see black box in Fig. 3): the progressive drier than normal signal exhibited during 2014 and 2015 was already evolving in 2013. Similar anomaly patterns were present in other countries too; for example, warmer and drier than normal conditions were observed in regions of Colombia, Venezuela, Ecuador, and Puerto Rico, which have also been affected by the ZIKV epidemic.