Updated Climate Forecasts Continue to Suggest Below-Normal 2023 March-April-May Rains Likely in Eastern Horn of Africa
This post was originally published on the University of California, Santa Barbara’s Climate Hazards Center (CHC) Blog on January 14, 2023. This post was written by Chris Funk, Gideon Galu, Diriba Korecha, Laura Harrison, Juliet Way-Henthorne, Peter Peterson, Will Turner and Greg Husak.
In this post, we provide analysis very similar to our September forecast post, analysis suggesting that below-normal March-April-May (MAM) rains are the mostly likely outcome for 2023, a concern that is widely shared by climate and food security communities and was communicated in a November multi-agency alert (here). As anticipated in the September post, the October-November-December (OND) rains have been poor, especially in central, eastern and northern Kenya, and most of Somalia. In Ethiopia, detailed agro-meteorological analyses identify a delayed onset and poor Hageya/Deyr short rains (September-December) across southern and southeastern Ethiopia, leading to severe droughts and substantial shortage of pasture and water resources over the pastoral regions. Given that late December rains have been characteristically low, the eastern Horn has experienced an historically unprecedented five dry seasons in a row. The latest report from the International Rescue Committee has also noted that “… East African countries, particularly Ethiopia and Somalia pay the highest price for climate change, as the longest period of failed rains ever recorded has pushed millions to the brink of starvation in East Africa.”
Maps of percent of normal OND and MAM CHIRPS rainfall data (Figure 1) identify an exceptional sequence of poor rainy seasons since late 2020. While the precise spatial pattern of drought shifted from season to season, much of central and eastern Ethiopia and Kenya, as well as southern and central Somalia, has experienced repetitive dry seasons, with rainfall totals often falling below 70 or 60 percent of the long-term mean. The rains in MAM 2022 were particularly poor, the worst in at least 70 years (June multi-agency alert). It is also important to consider the absolute values of rainfall in these seasons, vis-a-vis crop and rangeland water requirements in hot arid lowland areas. Intense solar radiation and very low relative humidity rates mean that plants typically require more than 250 or 300 mm of rain (paper here). This can be contrasted with the average OND 2022 rainfall in Eastern East Africa (EEA, Kenya, Somalia and Ethiopia south and east of 8N and 38E), which we currently estimate at ~127 mm, based on the latest seasonal Early Estimates. This number is clearly far below a total adequate for healthy crop and rangeland production.
Five inches or so of rain, over three months, in a very hot region, is very close to no rainfall. Climate Hazard Center Center crop Water Requirement Satisfaction Index (WRSI) model results for OND 2022, indicate poor, very poor, or failed crop growing conditions over southeastern Ethiopia, central and eastern Kenya, and central and southern Somalia (Figure 2). World Food Programme (WFP) rainfall accumulations for the Shebelli-Juba River basin (here) also indicate one of the five or six driest seasons on record, with totals less than half the long-term average. The current totals are only slightly higher than 2021, the driest season on record, according to the WFP accumulations.
Figure 3 shows EEA Standardized Precipitation Index (SPI) values, back to OND 2016, expressed as a bar plot. Also shown on this plot are La Niña-related FEWS NET forecasts, made in support of the monthly Food Security Outlooks. The MAM 2023 forecast is described below.
According to our rainfall data, which extends reliably back to ~1950, a sequence of five dry seasons has never happened before. We can use simple probability theory to calculate the likelihood of multiple dry seasons in a row. Given that there is a 33 percent chance of a below-normal rainy season, there is a 0.333 x 0.333 x 0.333 x .0333 x 0.333 chance of five below-normal rainy seasons. This is a 0.4 percent probability. Such a sequence would be very unlikely by chance. The same probability (0.4 percent) would describe the probability of correctly predicting, by chance, any sequence of five tercile-based forecasts.
What is also notable in Figure 3 is both the overall high frequency of dry events, as well as the low probability of normal outcomes. Since OND 2016, there have been 13 seasons. Eight have been dry or very dry. Three have been wet or very wet. Only two have been normal.
Since late 2020, climate change and La Niña have conspired to produce three years of exceptionally strong Pacific sea surface temperature gradients. Since late 2020, enhanced Pacific trade winds have pushed warm waters and moist winds to the west, enhancing heat and moisture convergence in the area around Indonesia. In June 2022, a multi-agency alert noted the propensity for OND La Niñas to be followed by dry MAM seasons (Figure 2C here).
Since then, CHC scientists have been working with ICPAC scientists to better understand this phenomenon.
Figure 4 presents results from a scientific study currently under review in Earth’s Future. We share these results here today because the eastern Horn, and, in particular, Somalia, faces extreme levels of food insecurity. Figure 4 speaks to concerns for the MAM 2023 season.
The x and y axis of this figure represent, respectively, atmospheric heating (related to energy released during rainfall) and moisture convergence, over the “Warm Pool” area of the western Pacific and eastern Indian Ocean. Each circle represents a single 1981-2022 MAM season. The two terms are highly correlated, and represent the strength of the “Walker Circulation.” The color of the circles represents eastern Horn of Africa rainfall outcomes. Green crosses on these circles identify the 12 recent post-1997 OND La Niña events.
One important story told by this figure is that the strength of the Walker Circulation varies a great deal from year to year. Another important result shows that when the Walker Circulation is very strong, we see a high frequency of below-normal MAM rains. When moisture and energy converge over the Warm Pool, rains and moisture convergence also tends to diminish over the eastern Horn. To highlight this point, we have drawn a vertical line at 540 Wm-2. When there has been an OND La Niña, it is very common that during the subsequent MAM atmospheric heating will be above this threshold (as indicated by the green crosses). When this occurs, there have also been very frequent below-normal rains.
It is extremely important to realize that when there is an OND La Niña, we almost always see a very strong MAM Walker Circulation. The exception to this pattern is MAM 2018 (the left-most blue and crossed circle on Figure 4), which was associated with very strong MAM rains and a vigorous sub-seasonal Madden-Julian Oscillation. Given the ongoing La Niña conditions (ONI index of -0.9 for OND), and the forecast continuation of La Niña conditions into early 2023 (95-percent chance for December-February and 60 percent in Jan-March, according to the official CPC ENSO forecast), it is likely that in MAM 2023, there will be an amplified Walker Circulation. While the NINO3.4 region is expected to warm, the predicted SST gradient structure in the Pacific is very La Niña-like, with very warm Western V conditions, and moderately cool east Pacific SST. As a result, there are elevated chances of drought in MAM 2023.