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 1,090 C. Oxygen isotope ratios were measured using a PDZ Europe 20-20 mass spectrometer interfaced to a Europa ANCA GSL elemental analyzer at Swansea University.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 . Leaf-water enrichment is driven by the water vapor pressure deficit of the atmosphere under a wide range of environmental conditions.

To test the impact of evaporative enrichment from leaves, we statistically regressed the tree-ring d18O record against monthly and multi-month VPD,rolling bench as estimated by NCEP2 reanalysis. A positive relationship between treering d18O and VPD would suggest evaporative enrichment is an important driver of inter annual tree-ring d18O variability. A non-positive relationship would suggest tree-ring d18O variability is dominated by variations in d18O of precipitation. We compared monthly NCEP2 reanalysis data to treering d18O to test relationships between tree-ring d18O and variables suspected of influencing the source of precipitation in the GHA. Variables chosen were anti-cyclonic circulation of atmospheric moisture over the Gulf of Guinea and STIO. A good inverse proxy for anti-cyclonic moisture transports is cloud cover. For each month and range of months, we calculated the mean cloud frequency over the southern Gulf of Guinea and western STIO . We then found the range of months when this cloud-cover correlated most strongly with tree-ring d18O. Exploratory analysis revealed that another good anti-cyclonic moisture transport index is the sum of meridional moisture transports over southern Africa to the east of the Gulf of Guinea and zonal moisture transports in the western STIO . We repeated the analysis described above for this alternate index. For visualization of the more general circulation features associated with variability of tree-ring d18O, we also compared tree-ring d18O to the circulation fields evaluated throughout this study.JJAS precipitation declined substantially throughout much of the GHA during the 1948–2009 period. Figure 2a is a time series of standardized JJAS precipitation totals in the GHA region considered in this study, outlined in red in Fig. 2b, c. Declines during the 1960s–1980s occurred in concert with the well-known reductions throughout the Sahel . GHA precipitation decreased by approximately 1 standard deviation during the 1950–1989 period , corresponding to decreases of over 30 mm per decade throughout much of the Ethiopian Highlands, over 60 mm per decade in western Sudan, and little change in southern Sudan, northern Uganda, and western Kenya.

Following the 1980s, JJAS precipitation continued to decline in the GHA while there was no decline in precipitation throughout much of the Sahel region west of Sudan . During 1990–2009, mean GHA precipitation was reduced by more than 0.5 standard deviation units compared to 1970–1989. Exceptions were observed in some areas in northeastern Ethiopia and northern Sudan, where post-1989 precipitation increased by over 0.5 standard deviation units. Notably, Fig. 14 in ‘‘Appendix’’ shows a similar spatial structure in recent precipitation trends for the MAMJ season. The similarity in spatial patterns of precipitation trends during JJAS and MAMJ suggests the same mechanism may be suppressing precipitation during both seasons despite the fact that the circulation patterns that cause precipitation during these two season are quite different. Since station coverage has decreased over the past 20 years , we considered 10 alternate gridded estimates of JJAS precipitation in addition to our CHGCLIM data. For the GHA and Sahel regions, Table 1 lists each additional dataset’s rate of change in mean JJAS precipitation total comparing the 1970–1989 to the 1990–2009 period. Eight of the 10 alternate datasets agree with CHG-CLIM that JJAS precipitation did not increase in the GHA after the 1970–1989 period. For the Sahel region, 7 of 10 alternate datasets agree that 1990–2009 totals exceeded 1970–1989 totals. In addition, 9 out of 10 alternate datasets agree with CHG-CLIM that precipitation in the GHA either declined more or recovered less than in the Sahel. Notably, not all precipitation datasets cover the entire 1970–2009 period. If the negative post-1970 precipitation trend indicated by most datasets is real, datasets beginning in 1979 may indicate artificially low precipitation totals for 1970–1989 and datasets ending before 2009 may indicate artificially high totals for 1990–2009. Despite this bias, 5 of 6 datasets not covering the entire 1970–2009 still agree on reductions in GHA precipitation. Finally, while the TRMM 3B43 data have limited temporal coverage compared to the other data sets, they indicate a substantial decline in precipitation over the GHA and no change in the Sahel from 1998 to 2009 .

Although missing data surely hinder the accuracyof interannual GHA precipitation estimates for all gauge based datasets, particularly for the southern Sudan region , the broad consensus from the data considered here is that recovery from the drought in the 1970s and 1980s has been at least partially suppressed in the GHA relative to the Sahel.NCEP/NCAR and ECMWF reanalysis datasets suggest that JJAS vapor transports into the GHA primarily originate from the Atlantic Ocean . Transports into the GHA are almost entirely confined to the lower troposphere below 700 hPa and are likely recycled through the rainforests of the Congo Basin on their way from the Atlantic . Moisture in the Congo Basin appears to mainly originate from the Gulf of Guinea, driven by strong subtropical surface highs over the southern Atlantic and Indian Oceans and a strong low over northern Africa. Moisture is transported from the Congo Basin toward the GHA by low level westerly and southwesterly winds, drawn into the region by thermally driven low surface pressures within the area of the Inter-Tropical Convergence Zone to the north and east into the GHA. There is substantial inter annual variability in the amount of moisture entering the GHA via this pathway. South of the center of the North African surface low, deep convection and enhanced moisture transports from the south and west drive monsoonal precipitation in the Sahel and GHA . For the Sahel,grow table hydroponic the primary driver of inter annual and longer-term variability of JJAS precipitation is the north-south displacement of upper-level jet features, zone of maximum convection, and southern boundary of the thermal low over northern Africa . Wetter years occur when these features are displaced to the north. This rule was apparent in the northern GHA. From 1953 to 1988, northern GHA precipitation correlated very well with the SLP gradient between Sudan and the southeastern coast of the Mediterranean Sea . The rainiest seasons occurred when SLP was anomalously high in Sudan and anomalously low to the north in Egypt and surrounding countries . Variability in this SLP gradient explains inter annual swings in JJAS precipitation as well as the Sahel-like decline in JJAS precipitation from the mid-1950s through mid- 1980s over the GHA. For the southern GHA, JJAS precipitation correlated much more strongly with Bombay SLP during 1953–1988 . This corroborates the findings of Camberlin , who, along with Hoskins and Rodwell , hypothesized that diabatic heating associated with the Indian monsoon supported an intensified tropical easterly jet across Africa and a low-level response across Africa that involved ridging across the Sahara at *25 N, surface lows around 12 N, and westerly wind toward the GHA. NCEP/NCAR and ECMWF reanalyses support the idea that enhanced westerly transports across tropical Africa contribute to enhanced GHA precipitation . Figures 6 and 7 show correlation between various aspects of NCEP/NCAR atmospheric circulation and JJAS precipitation in the northern and southern GHA, respectively . Northern GHA precipitation is correlated with a strong North African surface low and cyclonic southwesterly winds toward the GHA . Correlation with southwesterly wind velocity is not confined to the lower troposphere where the vast majority of water vapor is transported, but instead extends throughout all altitudes below the TEJ . Northern GHA precipitation is positively correlated with an enhanced TEJ . Both of these results are consistent with observations of a relatively weak African Easterly Jet and strong TEJ during wet monsoon seasons in the Sahel region . While correlations between reanalysis atmospheric circulation and JJAS precipitation in the southern GHA were weaker than for the northern GHA , precipitation records for both regions correlate with moisture transport traveling from the Gulf of Guinea and across the Congo Basin .

A main difference between drivers of north versus south GHA precipitation is that southwesterly cyclonic circulation is not necessary to deliver moist air to the southern GHA. Instead, southern GHA precipitation correlates with purely westerly flow across Africa between the surface and 500 hPa. This westerly flow is associated with enhanced surface low-pressure over western India and the western Indian Ocean north of Madagascar , consistent with the results of the SLP analysis. Southern GHA precipitation also correlated with ascending motion over western Africa and on the eastern boundary of the STIO . Correlations with ascending motion were particularly strong over the STIO and weaker over the GHA in the ECMWF analysis. In both reanalyses, correlation with ascending motion was generally confined to the mid- and upper-troposphere between 600 and 200 hPa. The correlation fields indicate that enhanced convection in rainy seasons corresponds with an enhanced TEJ, as was the case for northern GHA precipitation. Previous work has shown that an enhanced TEJ contributes to enhanced instability throughout the atmospheric column during the north African summer monsoon .From 1989 to 2009, the SLP gradient between Sudan and the southern Mediterranean coast strengthened and Bombay SLP declined . According to the strong statistical relationships between SLP and GHA precipitation, these trends should be associated with increasing JJAS precipitation throughout the GHA region . The fact that GHA precipitation did not rebound following the 1980s suggests that a secondary factor worked to suppress it and alter the previously established relationships with SLP. To diagnose these altered relationships, we evaluate trends in atmospheric circulation and moisture transports in recent decades. Figure 8 shows linear trends in atmospheric circulation and moisture transports from 1979 to 2009, according to the NCEP2 reanalysis. Alternatively, see Fig. 18 in ‘‘Appendix’’ for differences between post- and pre-1988 periods rather than trends . Striking features in both figures are strong negative trend in convection and atmospheric water vapor content within and around the GHA. These negative trends appear to be linked to changes in SSTs and convection over the STIO that have been previously associated with negative precipitation trends over the GHA during boreal spring . In an analysis of the MAMJ season, Williams and Funk found that a westward extension of the Indian-Pacific tropical warm pool has led to a westward extension of the convective branch of the tropical Walker circulation over the STIO, and consequently, a westward migration of the western descending branch of the Walker system into northern Africa. Dynamically, increased SST in the STIO has driven large increases in convection and precipitation over the STIO, and the increase in the amount of diabatic energy released during precipitation has led to increased divergence of DSE in the overlying mid- and upper-troposphere. This intensified outflow of mid- and upper-tropospheric DSE from the STIO has caused increased subsidence over northern Africa and decreased moisture transports into the GHA for at least the past 30 years.