Chris Funk, Pete Peterson, Diego Pedreros, Greg Husak and Laura Harrison
Introduction and context
As summarized in a recent UN Office for the Coordination of Human Affairs (UN OCHA) report, the drought and food security situation in East Africa is becoming increasingly fragile. Poor water access has helped lead to outbreaks of Acute Watery Diarrhea in Kenya, Somalia and Ethiopia, and cholera outbreaks in Somalia and Kenya. Two of the region’s biggest cereal exporters – Uganda and Tanzania face serious domestic challenges. In Uganda, thousands of immigrants continue to arrive from South Sudan, and fall armyworms may reduce Uganda’s four million metric ton maize output. According to UN OCHA, Ugandan maize prices have increased by 60%. Tanzania, typically the largest regional exporter has closed its borders for exports. Maize and bean prices have risen substantially in Kenya, but stabilized in Somalia in response to extensive humanitarian aid efforts. The FEWS NET March report for East Africa reports that “FEWS NET and FSNAU released joint statements on deteriorating food security in Somalia and the risk of Famine (IPC Phase 5) in a worst-case scenario in which the April to June 2017 Gu season performs very poorly, purchasing power declines to levels seen in 2010/11, and humanitarian assistance is unable to reach populations in need.” FEWS NET reports that there have been 10,571 cases of AWD/cholera since the beginning of 2017 – with half of these cases happening in Bay Province – which we focus on below. This report also identifies severe potential food insecurity in far eastern Ethiopia (Somali Region) and atypical levels of critical food insecurity across eastern and northern Kenya. More details for these countries can be found in the March reports for Somalia, Kenya, and Ethiopia. In general, these reports highlight high levels of concern for eastern Kenya, eastern Kenya, and southern Somalia, areas we believe likely to receive below normal March-May rainfall this year.
As emphasized by the most recent NOAA Climate Prediction Center Weather Hazards report – “Erratic and below-average rainfall continues in East Africa”. Our objective with this posting is to a) highlight the severity of this drying over Eastern Kenya and Somalia, and b) emphasize the ongoing role that an overturning Walker Circulation intensification appears to be playing in supporting these arid conditions.
Current vegetation and climate conditions
We begin with a brief review of current vegetation and hydrologic conditions over East Africa. Poor October-December rains, warm temperatures, and low March precipitation totals in most areas have conspired to produce extensive vegetation stress across almost all of Kenya, southeastern Ethiopia, and Southern Somalia (Figure 1). Kenya appears to be hardest hit in terms of vegetation deficits.
These poor vegetation conditions are likely due to poor March rains
following a poor 2016 short rainy season. Current water availability, based on estimates of water levels at water holes across arid regions of East Africa (Figure 2), also appears to be very low (near-dry) in most of the locations being monitored. Time series of water availability, which can be explored here, indicate many areas with zero water at present in many areas, with the water holes typically drying up between December and March. These same regions – Kenya, southeastern Ethiopia and Somalia – also appear as warm or very warm in Land Surface Temperature images (Figure 3). These warm conditions are likely due to a combination of surface water stress, low vegetation, lower than normal cloud cover and subsiding air, which increases local air temperatures.
Poor March 2017 rainfall conditions
Due to the severity of the current crisis, the CHG has created a first draft CHIRPS products for March that incorporates a large number of additional gauges provided by FAO/SWALIM and the Ethiopian Meteorological Agency. Sixty-two and forty-five additional gauges were provided, respectively, by our colleagues in Somalia and Ethiopia. While these gauge observations are routinely incorporated in the ‘final’ CHIRPS product, this final product is typically only available three weeks after the end of the month. Figure 4 presents a provisional CHIRPS rainfall for March, superimposed with station values. The station values are presented as colored squares, marked with source identifiers.
What we see in this image is that the station data indicate that parts of Kenya and virtually all of Somalia received no rainfall in March. The background coloring shows the station-adjusted CHIRPS totals for March. These are very similar to the CPC ARC2 totals, except that the station data is more pessimistic over Southern Somalia. Both the ARC2 and satellite-only CHIRP estimates indicate about 25 mm of rain in some parts of this region. The station data, in contrast, indicate that extremely low rainfall totals are more likely. Note also that the gauge observations for typically rainy areas in western and central Kenya also exhibit very low rainfall.
We can also plot the March gauge and CHIRPS rainfall values as standardized anomalies (Figure 5). This map suggests that March has likely been much drier than normal over Somalia and parts of Kenya. While climatologically March is typically still quite dry in Somalia, the country appears to have received little relief in 2017.
Focus – Bay Region of Somalia
We next briefly place the current dryness in historic context for the Bay Region in Southern Somalia – an area facing severe IPC 4 (crisis) food security conditions (http://www.fews.net/east-africa/somalia).
To explore potential outcomes for the coming Gu season, we plot (Figure 6) the observed accumulated rainfall for the Bay region through the 1st dekad of April, and then simulate potential future seasonal performance by completing the season using observed CHIRPS rainfall from previous years. We assume that the SWALIM data correctly indicate zero March rainfall for this region. Furthermore, given that GTS observations and CPC ARC2 satellite estimates around this region report little rainfall over the last 7 days, we also assume that the rainfall in this region for the 1st 10 days of April will be 20 millimeters, about half the long term average value for this region and dekad. The result of these assumptions is that it appears likely that the Bay Region will receive below normal March-May rains, just based on the current seasonal deficit and historic rainfall outcomes.
Focus on Eastern Kenya
We next briefly focus on the Eastern Province of Kenya. Parts of this area are heavily populated, and it shows up consistently as an area of high vegetation stress (Figure 1), high temperature anomalies (Figure 3) and low rainfall totals (Figures 4 and 5). Time series (Figure 7) of vegetation (normalized difference vegetation index values) for Eastern Province appear to be very low, and lower than during the 2010/2011 drought period. Note however, that we are just entering the period when NDVI in this area is expected to increase. Figure 8 shows the corresponding cumulative rainfall totals, based on NOAA CPC RFE2 rainfall estimates. This plot indicates conditions similar to 2010/11. During March, this region typically receives about ~34 mm of rainfall (about a third of the typical ~170 mm March-May total). This year the amount received in March and early April appears to be much lower than that amount (~10 mm).
Current climate conditions likely to indicate continued rainfall suppression
We now examine recent climate conditions, which appear likely to be associated with continued rainfall suppression, despite the fact that La Niña climate conditions have dissipated over the past few months.
What the most recent (April 3rd) NOAA CPC assessment shows is that equatorial Pacific sea surface temperatures (Figure 9) are currently exhibiting a ‘split personality’ with very warm temperatures in both the western and far eastern equatorial Pacific, and near neutral conditions in between, in the central Pacific. Since about July of 2016, we have seen relatively warm temperatures in the west Pacific and relatively cool temperatures in the central Pacific. This contrast in temperatures helps drive the Walker Circulation, producing enhanced rainfall near Indonesia and dry sinking air over eastern East Africa. We recently had a chance to discuss this with Steve Baragona, a science writer with Voice of America, who did a piece on the East African drought and climate change (link here). Steve was kind enough to share VOA’s animation describing the Walker Circulation’s impact on East Africa, which you can access here.
When the western Pacific warms relative to the central Pacific, and rainfall increases around Indonesia and decreases in the central equatorial Pacific, rainfall tends to decrease across eastern East Africa. FEWS NET research has suggested that these changes can help explain a shift towards drier long/Gu/Belg rains over East Africa. To help visualize these changes you can see an animation we prepared here at the CHG. The basic idea behind this animation is that when the west Pacific warms relative to the east Pacific and precipitation around Indonesia increases, the East African long rains tend to decrease. This pattern helps explain the very dry March conditions and may persist into April and May.
Figure 10 shows another plot from the recent CPC ENSO assessment. The top panel shows outgoing longwave radiation (OLR) anomalies for March. Since clouds block OLR, decreased OLR around Indonesia and increased OLR around the dateline (180°E) indicates an enhanced Walker Circulation, with more/less rainfall near Indonesia and the dateline.
Between these areas of enhanced and reduced precipitation we find increased low level winds blowing into the area near Indonesia (Figure 10, bottom). These winds increase moisture convergence in that area while also acting to cool the central equatorial Pacific. Note that these OLR and wind responses are constrained to the western and central Pacific and are quite different than the typical ENSO response. This pattern appears similar to circulation anomalies associated with the West Pacific Warming Mode. This climate pattern is produced by a ‘Western V’ of warm sea surface temperatures beginning near Indonesia and stretching to the northeast and southeast – a pattern that appears very similar to sea surface temperature conditions in March of 2017 (Figure 11). This figure shows that March sea surface temperatures resemble a pattern commonly associated with drying over East Africa during March-June. This pattern could be exacerbated by La Niña conditions, but La Niña conditions are not necessary to produce dry East African rainfall outcomes. This ‘Western V’ pattern of warm sea surface temperatures enhances subsidence over the central Pacific, increases trade winds flowing into the region surrounding Indonesia, increases precipitation in that region, and decreases precipitation over Eastern East Africa.
This set of climate anomalies is occurring now and is likely to persist into April and possibly May.
This far into the long/Gu/Belg season, monitoring the strength of the Walker Circulation can help us anticipate how the rest of the season may play out. Unfortunately, what we are seeing is that we continue to observe a large scale pattern of ascending and descending air that acts to enhance the Walker Circulation and reduce eastern East African rainfall. Figure 12 shows March 26th to April 4th vertical velocity anomalies averaged along the equator. Purple shades show areas of ascent, yellow-orange shades indicate sinking air. While the pattern of rising and sinking air for 2017 (top) does not look quite as intense as it did in 2011 (bottom), it still does not look conducive to good long rains in eastern East Africa.
We can look more closely at the surface wind component of the Walker Circulation enhancement (Figure 10, bottom), by extracting and plotting reanalysis surface winds from the central equatorial Pacific (Figure 13). Negative values in this time series indicate increased wind flow towards Indonesia. Note that this time series shows an abrupt change in 1999 – when East Africa switched to a ‘new normal’ associated with much drier East African March-May rains. In March of 2017, the intensity of these winds were similar to 1999, 2000, 2008, 2011 and 2012 – La Niña-like East African drought years. In April-May of these potential analog years Global Precipitation Climatology Project (GPCP) precipitation estimates (Figure 14) showed drying over East Africa and a Walker Circulation enhancement similar to that observed in March 2017 climate anomalies.
Our analysis of March rainfall anomalies and recent large scale climate fields compel continued concerns for the 2017 Long/Gu rainy season, especially over central-eastern Kenya and southern Somalia, where the season appears to have had a very poor start. Newly available station data, provided by FAO/SWALIM, allow us make this assertion for Somalia with an unprecedented level of confidence. This poor March onset of rains appears related to a La Niña-like atmospheric response pattern, despite the current ENSO-neutral conditions. FEWS NET research has linked such a response to a warm ‘Western V’ SST pattern, and this SST pattern seems likely to have contributed to poor March rainfall, and a probable below-normal March-May outcome, if current conditions persist, which seems likely. For Somalia, this could lead to a third failed or poor growing season. For most the Horn, a poor March-May season will exacerbate the current severe drought crisis.