Recently, a number of studies suggested that increased SSTs in the STIO may cause large-scale changes in atmospheric circulation and divert important moisture transports away from the GHA, altering previous relationships between regional circulation and precipitation . In an analysis of boreal spring , Williams and Funk found that higher SSTs in the STIO have driven large increases in convection and precipitation over the STIO. The resulting increase in the amount of diabatic energy released during precipitation has led to increased diver gence of dry static energy in the mid- and upper troposphere. During MAMJ, this intensified outflow of mid- and upper-tropospheric DSE from the STIO has increased subsidence over northern Africa and decreased moisture transport into the GHA for at least the past 30 years. The decrease in the amount of moisture trans ported into the GHA region appears to have caused a reduction in what MAMJ precipitation totals would have otherwise been in the absence of increased SSTs in the STIO. Therefore, increased SSTs in the STIO and subsequent changes in moisture transports may signify a long-term alteration in how interannual variability in atmospheric circulation impacts interannual variability in precipitation totals throughout much of the GHA. While no existing data sources detail the true sources of GHA precipitation,hydroponic grow table the composition of stable isotopes in Ethiopian rainwater suggests that most of the moisture is derived from the rainforests of the Congo Basin.
Rain water molecules in the Ethiopian Highlands and sur rounding areas have exceptionally high levels of the heavy stable isotope of oxygen . The high level of 18O relative to 16O is evidence that the Congo Basin is a primary source of moisture to the GHA because moisture transpired by the wet forests of the Congo Basin does not undergo isotopic fractionation . Unfortunately, isotopic records of precipitation throughout the GHA are inadequate to determine whether there is a trend in the proportion of precipitation coming from the humid continental interior. Conveniently, trees use rainwater to create cellulose during photosyn thesis, and d18O in precipitation is partially conserved in cellulose . In trees that produce annual growth rings, the isotopic composition of cellulose in tree rings may reflect an annual record of d18O in precipitation . We will investigate whether this is the case for trees in the Ethiopian Highlands. In this study we use a unique network of over 1,200 publicly and privately available gauge records from northern and eastern Africa to calculate a best-estimate record of JJAS precipitation for the GHA through 2009. Where sufficient precipitation data are available, we quantify the degree to which the GHA has recovered rel ative to the 1970–1989 mean. We then identify the atmospheric mechanisms that governed interannual variability in GHA precipitation during the period of 1948–1989 and determine whether the relationships between GHA precipitation and these mechanisms remained stable from 1990 to 2009. We utilize reanalysis climate data and a tree ring isotope record from the Ethiopian Highlands to test the possibility that rapidly rising SSTs in the Indian Ocean have acted to suppress JJAS precipitation in the GHA following the peak of the Sahel drought in the 1980s.
As the primary basis for evaluating precipitation trends throughout the Sahel and GHA, we use 0.25 spatial inter polation of a dense set of observational gauge precipitation records . This dataset was developed by the Climate Hazards Group at the University of California, Santa Barbara. The gridded product was constructed using qual ity-controlled rain-gauge data from 109 stations in Sudan, 210 stations in Ethiopia, 57 stations in Uganda, 144 stations in Kenya, and 817 stations throughout other Sahel and east African countries. Because the CHG is an active member of the Famine Early Warning Systems Network and routinely obtains and analyzes up-to-date meteorolog ical records to support decision-making processes for food security, the CHG archive has substantially more recent observations than those found in standard global station archives. Figure 1 shows maps of mean gridded JJAS precipitation totals, gridded mean standard-error estimates, station locations, each station’s percent of reported monthly JJAS data from 1948 to 2009, and a plot of the number of reporting stations in the Greater Horn region during each JJAS season. The CHG-CLIM dataset and underlying methodologies are described in detail in Funk et al. and summarized in the ‘‘Appendix’’.We limit our region of focus to areas within Sudan, Ethi opia, Uganda, and Kenya where the following criteria are met: mean JJAS precipitation during 1990–2009 is less than that of 1970–1989, mean JJAS precipitation exceeds 75% of mean MAMJ precipitation, which is the dominant rainy season immediately south of our region of interest, and mean JJAS precipitation exceeds 150 mm. The study area includes southern Sudan, western Ethiopia, northern Uganda, and parts of western Kenya. While the term ‘‘GHA’’ is generally used to refer to a quite larger region, we use the term to refer only to our study area defined here. Our GHA region is outlined in red in all map figures beginning with Fig. 2. To calculate a time series of JJAS precipitation for the GHA, we first standardized each grid cell’s time series by subtracting its mean and dividing by its standard deviation.
We then calculated the region’s average standardized anomaly from each JJAS season during 1948–2009. The resulting time series of mean seasonal anomalies for the region accounts for 59% of the interseasonal variability throughout the region. This meanprecipitation time series is most representative of precipitation in the central portion of the GHA and less representative toward the boundaries . Similar to what has already been established by several studies focusing on the Sahel ,flood tray precipitation varies fairly ubiquitously across the GHA as a whole . Superimposed upon this regional mode of variability is a dipole-like pattern that causes the north and south to experience anomalies of opposite sign . We explore sub regional variability later in the drought-diagnostics portion of this paper. It is crucial to this study that the CHG-CLIM data accurately reflect the direction of precipitation trends in the GHA following the Sahel precipitation decline that ended in the 1980s. Gaps in publicly available data and the cost of privately held data from eastern Africa, especially from Ethiopia, have caused previous studies to exclude parts of the GHA . A recent study that employed data purchased from the Ethiopian Meteorological Agency found good agreement with global gridded precipitation estimates for boreal summer months. The authors concluded that the purchased dataset is more accurate for Ethiopia than the global products, although they did not compare multi decade trends among the various products . The Ethiopian station data used in our study were also purchased from the Ethiopian Meteorological Agency, and are similar in quality and quantity to those examined by Dinku et al. . Given a shortage of literature on JJAS precipitation trends in the GHA following the Sahel drought, we com pared our calculations of post-drought trends in the GHA to those of 10 alternate global precipitation products . We used CHG-CLIM and each of the 10 alternate products to compare the direction and rate of change in JJAS precipitation in the GHA during the 1970–1989 period to that of the 1990–2009 period. For a wider regional perspective, we repeated this procedure for the Sahel region to the west . We also evaluated precipitation trends in the GHA and Sahel from 1998 to 2009 using the satellite-derived Tropical Rainfall Measurement Mission 3B43 merged product . While TRMM 3B43 covers a shorter time period than the other data sets used here, it is useful for evaluating precipitation trends over the last decade because the amount of missing gauge data over the GHA increases during the same period.The sub-regional precipitation records accounted for 75% of variability in the north and 68% in the south. In order to evaluate Indian Monsoon-GHA precipitation relationships we compared JJAS Bombay SLP to the north and south sub-regional CHG CLIM precipitation time series using monthly station SLP data from the Global Historical Climate Network . If Bombay SLP did not adequately represent a regional precipitation record, we considered monthly SLP data from 1,107 alternate stations in Africa, southern Eurasia, and the Maritime Continent . We calculated the correlation between precipitation and SLP at every pair of stations from 1953 to 1988. We identified the pair of regions where SLP records could either be summed or subtracted to most accurately estimate JJAS precipitation. We used three different reanalysis data sets to analyze how sub-regional precipitation variability is related to large-scale atmospheric circulation and vapor transports.
For the period 1948–1988 we used the NCEP/NCAR and ECMWF ERA-40 reanalysis climate datasets. To test whether relationships established with SLP during 1948–1988 were stationary after the peak of the Sahel drought in 1988, we analyzed trends in both the original NCEP/NCAR reanalysis and NCEP-DOE Reanalysis 2 data . NCEP2 uses an updated forecast model and data-assimilation system from the NCEP/NCAR reanalysis and it includes satellite data. We visualized trends in, and correlations with, wind-vector data using quiver-plot maps. In these maps, the x-axis of an arrow refers to the zonal wind component and the y-axis refers to the meridional wind component. In vertical profile maps depicting trends in, or correlations with, both horizontal and vertical wind velocity along a horizontal transect, the x-axis refers to the horizontal velocity component parallel to the transect and the y-axis refers to the vertical velocity component. Informed by previous research , we investigated whether trends in Indian Ocean SSTs have impacted atmospheric circulation and moisture transports in ways that would influence GHA precipitation. We focused on the STIO region, defined here as 15 S–0 S, 55 E–90 E. We evaluated correlations between average JJAS SST in the STIO and spatial fields of four variables: total cloud cover , net upward surface energy flux , dry static energy , and precipitation . We calculated energy fluxes using definitions provided by Trenberth and Stepaniak . Net surface energy flux comprises the net upward radiation flux from the ocean’s surface plus the upward fluxes of sensible and latent heat from the ocean’s surface. DSE is the sum of the energy contained in the temperature and geopotential height of an air column. We evaluated how higher SSTs in the STIO during recent decades have impacted interannual relationships between SLP and JJAS precipitation in the GHA. For each of the two GHA sub-regions, we classified JJAS seasons from 1948 to 2009 based upon whether SLP conditions were favorable for precipitation , and whether SSTs in the STIO were warm or cool. We designated all seasons with neutral SLP or SST conditions to a fifth ‘‘neutral’’ class. We evaluated mean JJAS precipitation in the GHA during each of the five classes of seasons.At the compound of the Debrebirkan Selassie church in Gondar, Northwest Ethiopia , 5 mm diameter cores were obtained from Juniperus procera trees in May 2007 as part of a larger sampling campaign . Successful cross-dating between 32 trees from five sites in the North Gondar zone was achieved by comparison of the wood anatomy directly on the surface of the samples and skeleton plotting . Cross-dating was evaluated using the computer program COFECHA and the annual nature of the tree rings was confirmed by AMS radiocarbon dating . Oxygen isotope ratios were measured on one tree core in a pilot study to obtain a preliminary insight into the potential environmental sensitivity of this isotopic variable in tree rings. Slivers were cut from absolutely dated annual growth rings representing years 1905–2003. Lignin was oxidized by the in situ generation of chlorine dioxide and hemicelluloses were hydrolyzed from the resulting holocellulose to yield a-cellulose . After sample homogenization, 0.30–0.35 mg of dry a-cellulose was weighted into silver vessels and pyrolysed over glassy carbon at 1,090C. Oxygen isotope ratios were measured using a PDZ Europe 20-20 mass spectrometer interfaced to a Europa ANCA GSL elemental analyzer at Swansea University . Oxygen isotope ratios are expressed as permille deviations relative to the VSMOW standard . Analytical precision was typically 0.3% as was illustrated by a repeat analysis of samples from 1910 . Two influential variables on tree-ring d18O are the d18O of precipitation and the amount of isotopic enrichment of leaf water that occurs during transpiration .