Decadal Rain Patterns Along East African Coast Controlled by Eastern Indian Ocean Sea Surface Temperatures

The eastern African Coast has been experiencing a long, steady decrease in annual rainfall over the past few decades, including an especially dry period between 2010–2011.  After a failure of the seasonal rainy season in 2011, Tierney et al. (2013) investigated the link between regional precipitation and sea surface temperatures (SSTs) in the Indian and Pacific Oceans, hoping to determine if natural climate variance or anthropogenic climate change were responsible for the drought.  The authors collected water level data from lakes in eastern Africa and used a series of simulations to compare the water levels to both atmosphere-ocean climate models and approximated SSTs in the Indian Ocean over the last 700 years to determine if oceanic climate in the Indian or equatorial Pacific Oceans affected precipitation along the eastern coast of Africa.  A significant relationship was found between the eastern Indian Ocean SSTs and rainfall in the Cape Horn region.  The authors speculated that the SSTs in the eastern Indian Ocean were able to affect a large atmospheric circulation pattern that has a localized anomaly over the Indian Ocean, the Walker circulation.  Under this model, the current homogenous SSTs across the Indian Ocean could be suppressing the Walker circulation anomaly over the Indian Ocean and causing the current drought conditions in Africa.  Definite conclusions as to the cause of current SSTs in the Indian Ocean could not be reached;  the resulting drought conditions could be a part of the natural cycle of the region or a result of anthropogenic climate change.—Alison Marks

Tierney, Jessica E., Jason E. Smerdon, Kevin J. Anchukaitis, and Richard Seager. Multidecadal Variability in East African Hydroclimate Controlled by the Indian Ocean. Nature 493, 389–392.

Tierney et al. averaged water level records from seven lakes in Eastern Africa and calculated water level variance over the last 700 years to determine spatial and temporal trends of water availability in the area.  Because the lake records aren’t uniform by year in the same way as tree rings might be, they used a Monte Carlo empirical orthogonal function (MCEOF).  The MCEOF compared the datasets from the lakes against one another to align the data sets by year.  From that data, Tierney et al. were able to separate the region into two areas with opposing trends:  the eastern coast/Horn of Africa and the Rift Valley area, located south and inland from Cape Horn. The eastern coast/Horn of Africa area was identified as the target region and all following analysis was done on the data collected from that region.  The data from the lakes in the eastern coast region were then analyzed with a simulation using a 1,300 and 3000 year atmosphere-ocean climate circulation control models to determine if the changes in water level overtime could be attributed to changes in climate.  The data were analyzed in increments of 50 years.  The authors found that the region was wetter when the models predicted warm sea surface temperatures in the west Indian Ocean and cooler sea surface temperatures in the east Indian Ocean.  No significance was found between the El Niño–Southern Oscillation (ENSO), which is mainly impacted by Pacific Ocean SSTs, and the water levels along the African coast.  From this point on in the study, the possibility of the conditions of the Pacific Ocean being a contributing factor to the precipitation in eastern Africa was disregarded.  Finally, the scientists used a recent sediment reconstruction of the Makassar Strait, a set of consistent SSTs in the western Pacific Ocean over the last millennium, to approximate the SSTs of the Indian Ocean over the same period of time.  This approximation was done by using the known influence of the Indonesian Throughflow from the Pacific Ocean to the Indian Ocean to approximate the temperatures present in both the eastern and western Indian Ocean at the time. 
The relationship between the SSTs in the Indian Ocean and the rainfall levels in the Cape Horn region can be viewed through an understanding of the general trends found in the datasets.  The MCEOF test found that in the target region there was a period of drought between 1300 and 1400 CE, followed by a transition to a wetter climate until 1700 to 1750 CE, when peak rainfall levels were reached.  After 1750 CE, there was a gradual decrease in precipitation that has continued into current day.  When these SSTs were compared to the water level trends, Tierney et al. found that the highest SSTs in the eastern Indian Ocean during this time correlated with the wettest periods along the eastern African coast.  This is probably due to the Walker circulation anomaly above the Indian Ocean.  The Walker circulation is an atmospheric climate system that interacts mostly over the equatorial Pacific Ocean, but can be affected by stratified temperatures between the eastern and western Indian Oceans.  When the temperature in the western Indian Ocean becomes warmer than the eastern side, the warmer air along the western side moves up, and then cycles down along into the east.  This creates a front along the eastern coast of Africa and leads to cloud formation and eventually precipitation. 

Despite these correlations, the researchers were not able to make any definite conclusions about their findings.  The Indian Ocean SST data set was created through inference and approximation.  Although the trends suggest that the more stratified the temperatures of the Indian Ocean are, the stronger the Walker circulation affects the African coast, there is not enough certainty that the SSTs determined over the last one thousand years for the Indian Ocean are accurate.  Tierney et al. were also not able to form any definite conclusions on the cause of the current drought along the eastern coast of Africa.  Although the results suggest that the current drought could just be a continuation of the current climate regime, the last major climate peak in the area, between 1770 and 1750 CE, occurred during the Little Ice Age.  Because that wet period can be correlated with another major climate event that was caused by radiative forcing, the current climate situation cannot be disregarded as a possible cause for the droughts being experienced along Cape Horn.  More certain data on historical SSTs needs to be investigated before a significant conclusion can be made. 

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