This section of the report will provide a rolling three month update on a monthly basis of the state of the climatic and ecological indicators used in monitoring areas at risk to RVF activity. These indicators include, global SST anomalies patterns, Equatorial Western Indian Ocean (WIO) and Eastern Pacific Ocean (EPO: NINO 3.4) SST anomalies, Southern Oscillation Index (SOI) and Outgoing Longwave Radiation (OLR) anomalies, Rainfall and anomalies, Normalized Difference Vegetation index anomalies and RVF risk map for Africa and the Arabian Peninsula.
1. SOI and SST Indices
The SOI index has further decreased with a value of -1.7 in October from -1.6 in September, suggesting a further strengthening of El Niño conditions since early summer. This is supported by continued anomalous increase in positive SSTs in NINO 3.4, NINO 4 and NINO1&2 monitoring regions which have tremendously warmed over the last three months with values of +2.46°C, 1.12°C and 2.52°C respectively in October. The western Indian Ocean basin has continued the warming pattern with the WIO SST index at +0.90°C in October (a further record) from +0.81°C in October indicating continued warmer than normal conditions over this ocean basin. The persistent above-average sea surface temperatures (SST) (below) in the central equatorial Pacific region indicate that strong El Niño conditions are present and significantly strengthening. Enhanced convection is amplified over the central and eastern equatorial Pacific and suppressed convection over the Indonesian basin is fully entrenched. Collectively, these atmospheric and oceanic conditions reflect strong El Niño conditions are present and persisting. According to NOAA, a majority of the model forecasts predict El Niño conditions (95% chance) through Northern Hemisphere winter 2015-2016, gradually weakening through late spring and early summer 2016. The consensus forecast is for a significant El Niño near or in excess of +2.0°C in the Niño-3.4 region(3-month values of the Niño -3.4 index +2.0°C or greater) during the peak period in December - January with a likelihood of this event ranking in the top 3 since 1950. In some locations, certain impacts often associated with El Niño are already manifesting during this Northern Hemisphere 2015 fall season.
Cumulative NDVI anomalies for Africa for August 2015 to October 2015 still show positive anomalies concentrated in parts of western Sahel (Senegal, Mauritania and Mali), Somalia, SE Ethiopia and parts of NE Kenya. The RVF risk map below was derived from thresholding NDVI anomaly data to detect areas persistent of above normal NDVI. Periods of widespread and prolonged heavy rainfall lead to flooding of dambos and anomalous green up in vegetation, creating ideal ecological conditions for the emergence RVF vectors. For the period August 2015 to October 2015, the RVF persistence model identifies areas at risk in central Somalia which have received above normal rainfall over the last two months and on the border of Mauritania and Senegal where RVF activity is occurring. Given the elevated rainfall conditions in many areas East Africa enhanced surveillance is advised in these areas especially in the next three months (November 2015 to January 2016). The current East Africa seasonal rainfall under this El Niño event is evolving in a rather unique way compared to other previous El Niño events, in particular the western parts of the region that don't fall within the potential epizootic/epidemic (PEAM) region are currently receiving the bulk of excess rainfall. We therefore suggest that some surveillance activities be carried out in the western highlands of Kenya and Tanzania that border the Rift Valley system where some RVF activity has occurred sporadically the past. Rainfall in this western sector is approaching and surpassing the 2006/2007 season. The above normal rainfall conditions during this period could lead to outbreaks of other vector and water-borne diseases.