A very poor East African short rains seems almost certain

Chris Funk

 

Last month on this blog we predicted a poor short rainy season for Eastern Kenya and Southern Somalia. Unfortunately that forecast verified. Strong cool-warm-cool sea surface temperature contrasts across the cool Western Indian Ocean, warm Eastern Indian/Western Pacific, and cool Eastern Indian oceans conspire to produce a very strong precipitation dipole (Figure 1) contrasting very heavy precipitation over the Indo-Pacific Warm Pool to the east and the Western Indian Ocean and East Africa to the west. USGS crop and rangeland models indicate ‘no start’ to the growing season almost everywhere.

Figure 1. October 2016 CHIRP rainfall anomalies
Figure 1. October 2016 CHIRP rainfall anomalies

This is very concerning given that the short rains typically end in November for most places in Eastern Kenya and Southern Somalia in November. There is also a great deal of persistence in the climate system between October and November-December. Thus the correlation between October and October-November-December (OND) E. Kenya/S. Somalia rains (please see last post for the specific region) is extremely high: 0.91. Despite that, the mean of the OND rains (224 mm) is much greater than the mean for October (64 mm) (Figure 2).

Figure 2. Scatterplot showing the strong relationship between October and OND E. Kenya/S. Somalia rains.
Figure 2. Scatterplot showing the strong relationship between October and OND E. Kenya/S. Somalia rains.

This is really a quite stunning predictive relationship. November-December E. Kenya/S. Somalia rains also have a strong correlation with October sea surface temperatures (Figure 3). These correlations are really high, with a negative relationship to the Indo-Pacific Warm Pool of -0.9, and positive relationships to the Western Indian and Eastern Pacific of +0.5 and +0.7.

Figure 3. Correlation between November-December E E Kenya/S Somalia rainfall and November-December sea surface temperatures.
Figure 3. Correlation between November-December E E Kenya/S Somalia rainfall and November-December sea surface temperatures. Based on data from 1996-2015.

Unfortunately, the current (October) Ocean looks very much like the opposite of Figure 3 – warm in the Indo-Pacific Warm Pool and cool in the Eastern Pacific (Figure 4). The warmth in the West Pacific is quite exceptional >+2.5 standardized anomalies, which is equivalent to a very warm 30.3 Celsius.

Figure 4. Observed October 2016 sea surface temperatures expressed as standardized anomalies. Based on a 1981-2010 baseline and NOAA Extended Reconstruction v4 data.
Figure 4. Observed October 2016 sea surface temperatures expressed as standardized anomalies. Based on a 1981-2010 baseline and NOAA Extended Reconstruction v4 data.

These strong SST gradients and warm Western Pacific sea surface temperatures contributed to the strong Precipitation dipole (Figure 1) and dry East African conditions in October. At this point, as shown in a recent Africa Hazards Briefing by Nick Novella at NOAA’s Climate Prediction Center (Figure 5), rainfall is very low and recovery is very unlikely. Nick’s awesome Seasonal Performance Probability Tool uses the long historical record of the ARC2 dataset to explore the potential future outcome of a season at a given time and place. It is not looking good for Eastern Africa.

Figure 5. Past rainfall performance and potential future rainfall performance at Baidoa, Somalia. Each grey line represents a projection based on a previous year of the ARC2 data set.
Figure 5. Past rainfall performance and potential future rainfall performance at Baidoa, Somalia. Each grey line represents a projection based on a previous year of the ARC2 data set.

We next explore this issue using two separate approaches: analog seasons and formal (but simple) cross-validated forecasts of OND rainfall. Our composites are based on similar dry Octobers (Figure 6)  1996, 1998, 2005, and 2010 exhibited similar very low October rains.

Figure 6. 1981-2016 CHIRPS E Kenya/S. Somalia rainfall and analog years.
Figure 6. 1981-2016 CHIRPS E Kenya/S. Somalia rainfall and analog years.

What is interesting about these years is that they also tended to exhibit a strong gradient between the Indo-Pacific Warm Pool and Eastern Pacific Ocean (Figure 7). The Western Indian Ocean and the associated Indian Ocean dipole likely played a role as well, but is not explored here, since recent correlations with E Kenya/S Somali rains seems quite low (Figure 3).

Fig. 7. Indo-Pacific warm pool and Eastern Pacific sea surface temperatures from the boxes identified in Figure 5. Dry October analogs are shown with filled black circles.
Figure 7. Indo-Pacific warm pool and Eastern Pacific sea surface temperatures from the boxes identified in Figure 5. Dry October analogs are shown with filled black circles.

Note that the selection of analog years was based on just dry Octobers, so finding our analogs clustered in Figure 7 helps reinforce the strong sea surface temperature forcing indicated in Figure 3, which in turn helps explain the strong Oct-OND relationship indicated in Figure 2. When I used the excellent GeoCLIM tool to plot rainfall anomalies for these analog seasons (Figure 8) I found that these analogs would indicate very poor rainfall performance in Kenya and Somalia, and a poor performance of the Deyr rains in Ethiopia. The 2016 March-April-May rains were very poor in Kenya and Ethiopia. Southeastern-Central Ethiopia is extremely food insecure, having experienced substantial droughts or dryness in the past two Belg and past two Kiremt rainy seasons.

Figure 8. Analog CHIRPS anomalies, expressed as percentages.
Figure 8. Analog CHIRPS anomalies, expressed as percentages.

The prospect of a poor E. Kenya/S. Somalia season can be quantified formally via a simple cross-validated regression using October CHIRPS observations (Figure 2) and Indo-Pacific and East Pacific sea surface temperatures (Figure 7). I first predicted November-December precipitation and then added in the observed October values. The forecast (Figure 8) predicted 1996-2015 OND rains very well (cross-validated R2 of 0.8) and projects an expected OND total of 83 mm, with a standard error of +/- 53 mm. So it seems very likely that the season will be very poor.

This poor season will follow a very poor 2016 MAM rainy season. I made a MAM+OND ‘forecast’ by combining the observed MAM and October E. Kenya/S. Somalia values with our November-December forecasts (Figure 9). The story that emerges is that this region may experience the worst MAM+OND rains in the past 20 years. Since we can be almost certain that short rains outcomes are going to be poor, especially for farmers in Eastern Kenya and Southern Somalia, it may be possible to engage in proactive response planning. While pasture conditions can change more rapidly if anomalous late season rains arrive, current  estimates of vegetation health look quite low.

Figure 9. Cross-validated forecasts of E Kenya/S Somalia MAM+OND 2016 rains.
Figure 9. Cross-validated forecasts of E Kenya/S Somalia MAM+OND 2016 rains.

FEWS NET continues to develop new tools for monitoring and modeling drought.  For example, support from NASA SERVIR is enabling us to provide CHIRPS-compatible Global Ensemble Forecast Systems (GEFS) forecasts, which can be viewed on our Early Warning eXplorer.  Collaboration with the Desert Research Institute has created the FEWS Engine which leverages the incredible processing power of the Google Earth Engine (GEE). We can use the GEE to produce NDVI composites for our analog years (Figure 10) reinforcing our concerns.

Figure 10. Google Earth Engine composites of OND MODIS NDVI for 2005 and 2010.
Figure 10. Google Earth Engine composites of OND MODIS NDVI for 2005 and 2010.

All indicators converge on a very high probability of a very poor short rains. Looming on the horizon is the potential of La Nina-like conditions in the spring. While current NOAA CPC assessments indicate that this is unlikely, if a La Nina did arrive, and the West Pacific continued to be very warm, we might see another poor season in the spring of 2017. If so, providing an adequate response to a 2016 short rains drought would help increase East African resilience in 2017.

4 thoughts on “A very poor East African short rains seems almost certain”

  1. Thanks Chris. It’s rather ominous news on the ground, obviously, but it’s good news that the predictive ability of the models/techniques are getting so sharp. Now the warnings need to be acted upon.

  2. I found it pretty interesting that just knowing the October CHIRPS provided such a good predictor for OND rains. If we do face the prospect of another poor rainy season this spring, effective mitigation of this short rains drought will really help maintain resilience and help mitigate the impact of a 3rd (Kenya/Somalia) or 5th (SE Ethiopia) poor season.

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