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United States Department of Agriculture

Agricultural Research Service

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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.

May 2015

1. SOI and SST Indices

The SOI index value is now at -0.7 in May down from normal value of 0.0 in April suggesting the development of El Niño conditions. This is supported by increased positive SST anomalies in NINO 3.4, NINO 4 and NINO1&2 monitoring regions which have increased over the last one month with values of +1.03°C, 1.09°C and 2.43°C respectively in May. The western Indian Ocean basin has continued to warm up substantially, the WIO SST index is at +0.52°C in May from +0.43°C in April indicating significant 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 El Niño conditions are present and persistent. Enhanced convection is amplified over the central and eastern equatorial Pacific and suppressed convection over the Indonesian basin is entrenched. Collectively, these atmospheric and oceanic features reflect an ongoing and strengthening El Niño conditions. Currently a majority of model forecasts predict El Niño conditions (90% chance) will continue through the Northern Hemisphere summer 2015, and a greater than 85% chance that these conditions will strengthen and last through the rest of 2015 with a favorable chance of a strong event (3-month values of the Niño -3.4 index +1.5°C or greater). In some locations, certain impacts often associated with El Niño may appear during the Northern Hemisphere through the summer 2015 season.


The central equatorial Pacific Ocean continues to show pronounced above normal seasonal SSTs (three month values: >+1.0°C to +5.0°C) except for the region from 30°S to 1°S (off the South American coast) with below-normal SSTs during the March 2015 to May 2015. The western Pacific Ocean especially the region of the Indonesian basin shows below normal SSTs indicating a reversal of ocean and atmospheric circulation across the equatorial Pacific ocean. The entire equatorial Indian Ocean is anomalously warm with departures of ~ +1.5°C in western equatorial Indian Ocean and as high as +3.0°C (3-month values) in the southern Indian Ocean off the western Australian coast. Other regions of significant anomalies include the north Pacific Ocean, north Atlantic, equatorial Atlantic off the West African coast, the Pacific Ocean off the California coast, southwest Atlantic Ocean off Argentina and Brazil which show significant positive and negative anomalies on the order of -/+1.0°C to -/+2.0°C. Outgoing Longwave Radiation (OLR) anomalies are used here as a proxy for tropical deep convection (rainfall). Reduced convection is shown in yellow to light brown and brown shades and increased/intense convection is shown by shades of blue. Some impacts from the current SST anomaly patterns can be observed in the pattern of global convective activity illustrated by the OLR departure patterns here. During the March 2015 to May 2015 period, drier-than-average conditions (>+35W/M2) are now enhanced over the western Pacific Ocean covering the Indonesian basin and northern Australia as well as drier the normal conditions are prevailing over Alaska, western Canada and northern Russia. The severe drought in western US (Californian) have eased up as shown by the negative departures in OLR extending from the eastern Pacific Ocean into southwestern and southern US. Enhanced cooler than average conditions (-50W/M2) are observed over central to eastern equatorial Pacific and just east of the Date Line. This band extend northern trough Mexic into southern and eastern US. Convective conditions continue to persist over India between 70°E and 90°E, Sahelian west Africa extending into east Africa, central South America covering most of the Amazon Basin. These patterns of depressed and enhanced convective activity coincide well with the patterns of SST departures and reveal certain impacts often associated with El Niño. Monthly and weekly anomalies can be found here. Rainfall and associated anomalies (below) for Africa from March 2015 to May 2015 show rainfall over the entire sub-Saharan region south of 15°N with a maxima along equator. Areas of above normal rainfall (+50 to 300mm) are limited to coastal West Africa, the entire Congo basin, South Sudan, parts of Kenya and Tanzania.

Cumulative NDVI anomalies for Africa for March 2015 to May 2015 still show positive anomalies concentrated in South Sudan, northwestern Ethiopia and Somalia even though the patterns are not spatially coherent. 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 March 2015 to May 2015, the RVF persistence model does not identify any areas where ecological conditions would support the emergence of RVF vectors. Therefore there is no risk of ecologically coupled RVF activity at present. However the above normal rainfall conditions currently in other regions of the continent could lead to outbreaks of numerous vector and water-borne diseases.

NDVI Anomaly RVF Potential

Last Modified: 6/17/2015