Future Trends in West African Precipitation

1. Future Trends in West African Precipitation Edward K. Vizy and Kerry H. Cook, Cornell University Introduction When the Gulf of Guinea is anomalously warm, a dry Sahel/ wet Guinean Coast mode of summer monsoon rainfall variability over West Africa has been found to be prominent during the 20th Century. The physical mechanisms responsible for this response have been identified (Vizy and Cook 2001, 2002) and were discussed in the context of the 20th Century (experiment 20C3M) CMEP simulations in our previous poster. Results from this study indicate that some models capture this mode of rainfall variability over West Africa. Based on our examination of the dipole dynamics, the MRI model demonstrated the most realistic representation of this particular mode of variability, and of the boreal summer African climate in general. The dipole rainfall response is a prominent mode of interannual variability over West Africa, but it is not thought to be related to decadal scale variability in the current climate. For example, the observed downward trend in Sahelian rainfall over the latter half of the 20th century is not being forced by Gulf of Guinea warming (since the Gulf of Guinea has not warmed). However, if the Gulf of Guinea starts to warm in the 21st century, the dipole precipitation anomaly could occur more frequently, or more intensely, and contribute to a drying trend in the Sahel. This possibility is investigated here. The three models, under three scenarios each, generate a relatively consistent prediction of future warming in the Gulf of Guinea, but not of whether or not the dipole mode will become more frequent and, perhaps, contribute to a drying trend in the Sahel. To investigate the response more deeply, details of the dynamical response are examined. Each model behaves differently, so the results are presented separately below. Averages over the first 30 years and the last 30 years of the 21st century integrations are examined. MRI_cgcm2_3_2a GFDL Approach We use the model simulations chosen as described in our other poster (Cook and Vizy, Tuesday), namely, gfdl_cm2_0, miroc3_2_medres, and the mri_cgcm2_3_2a. These coupled models each demonstrated an ability to capture a reasonable precipitation climatology for northern Africa. Here we examine 21st century integrations with these models to see if the Gulf of Guinea warms, and to determine whether the dipole rainfall pattern occurs more frequently, or becomes permanently established in the future (e.g., becomes a trend). The dynamics of the response is examined to better understand and build confidence in the simulated dipole response. Figure 7. MRI_cgcm2_3_2a (top row) precipitation (mm/day) differences from the 1949-2000 mean for the 2001-2030 (left) and 2071-2100 (right) averages, and (bottom row) meridional (m/s) – vertical (multiplied by –100 Pa/s) circulation cross-sections along the Greenwich Meridian for the 2001-2030 (left) and 2071-2100 (right) means for the (left) COMMIT, (center) A1B, and (right) A2 scenarios. The MRI 21st century integration produces more frequent dipole events, with the number of events in both the A1B and A2 scenarios greater during the second half of the century than during the first half. As was the case in the 20th century MRI run, the physical mechanism that produces the dipole pattern is realistic. Results from this model suggest that the dipole response will contribute to a modest drying trend over the Sahel in the A1B and A2 scenarios, but not in the COMMIT scenario. Figure 1. GFDL_cm2_0 (top row) precipitation (mm/day) differences from the 1949-2000 mean for the 2001-2030 (left) and 2071-2100 (right) averages, and (bottom row) meridional (m/s) – vertical (multiplied by –100 Pa/s) circulation cross-sections along the Greenwich Meridian for the 2001-2030 (left) and 2071-2100 (right) means for the (left) COMMIT, (center) A1B, and (right) A2 scenarios. Under the COMMIT scenario, rainfall anomalies are similar in both 30 year periods, with wetter conditions along the Guinean Coast and drier conditions over the Sahel. The precipitation anomalies look like the dipole response, but drying over the Sahel is quite strong and the pattern seem uncorrelated with Gulf of Guinea SSTs. As was seen the the examination of the 20th century runs with this model, the circulation associated with the dipole precipitation pattern is not realistic in general. How do Gulf of Guinea SSTAs change in the future simulations? • Discussion • A dipole rainfall response over West Africa, by which warm Gulf of Guinea SSTAs are associated with dry years in the Sahel, was a prominent mode of interannual variability in the 20th century. We investigate the possibility that this dipole response may cause additional Sahel drying during the 21st century. If the Gulf of Guinea warms in the 21st century, the dipole precipitation anomaly could occur more frequently, or more intensely, and contribute to a drying trend in the Sahel. • 21st century simulations from 3 coupled models are examined, namely, the gfdl_cm2_0 (GFDL), miroc3_2_medres (MIROC), and mri_cgcm2_3_2a (MRI) models, for the COMMIT, A1B, and A2 climate scenarios. These models were chosen because they produce a good simulation of the present day summer precipitation climatology over northern Africa (7 other integrations examined do not). The ability of these models to capture the dipole variability mode was investigated in our previous poster (Cook and Vizy – Tuesday). Results from that study indicated that the MIROC and, especially, the MRI models reproduce the dipole dynamics in a reasonable way. • A simple examination of the number of years with the dipole anomaly in the 21st century integrations does not reveal a pattern or consensus among the 9 cases (3 models, 3 scenarios each). For example, under the A1B and A2 scenarios the GFDL model has a modest increase in the number of dipole summers, the MRI a large increase in dipole events, while the MIROC model has a marked reduction in the number of events. However, since the physics of the dipole mode is known, we can focus more closely on the dynamics to evaluate the behavior of the models and choose those which are more reasonable. • The GFDL model has an increase in frequency of dipole events during the first half of the 21st century, from 25% to approximately 35% of the years in the three scenarios. But the dynamics of the dipole response is not realistic. Also, during the second half of the 21st century in the A1B and A2 scenarios, a very strong (unrealistic?) warming trend develops, which drastically reduces the frequency of the dipole mode and dries all of West Africa due to the generation of a very strong African easterly jet. • In the MIROC model, in all three scenarios, the frequency of dipole events decreases from 20% of the years in the 20th century, to 15% in COMMIT, and approximately 2% in the A1B and A2 scenarios. This model invokes a different mode of variability in the 21st century, namely, an enhancement of the low-level westerly flow from the tropical North Atlantic and rainfall over the Sahel in association with strong surface warming over northernmost Africa. This response is similar to the wet Sahel conditions during the African Humid Period. • The MRI model simulates an increase in the frequency of dipole events under the A1B and A2 scenarios. This model reproduces the hypothesized response to future warming in the Gulf of Guinea, including a realistic representation of the physical processes that cause the dipole response. • This analysis suggests that the MRI model is providing the most reliable prediction of the dipole mechanism for the 21st century for West Africa. Predictions from the MRI model indicate that under the…. • COMMIT scenario, the occurrence of dry years in the Sahel due to the dipole mechanisms would increase from 15% of the years in the 20th century to about 30% of the years in the 21st century. • A1B scenario, the occurrence of dry Sahel years due to the dipole mechanism would be about the same (15% of the years) for the first half of the 21st century, but increase over the second half to about 38% of the years. • A2 scenario, the occurrence of dry Sahel years due to the dipole mechanism would increase from 15% of the years in the 20th century, to 32% of the years in the first half of the 21st century, and 52% of the years during the second half of the century. • These changes are forced by warming in the Gulf of Guinea. Other forcing mechanisms may also contribute to trends in Sahelian rainfall and need to be better understood. One example is warming in the Indian Ocean, which is observed in the late 20th century. The MIROC and MRI integrations capture this warming trend during the 20th century, while the GFDL ocean model does not. However, none of these models capture the observed 20th century drying trend in the the Sahel which may be associated with this Indian Ocean warming. In the 21st century integrations, each of the 9 integrations examined indicate a strong warming trend in the Indian Ocean. The implications of this Indian Ocean warming for African precipitation require further investigation. Scenario COMMIT Scenario A1B Scenario A2 In the second half of the 21st century in the A1B and A2 scenarios, the GFDL model produces a circulation that is similar to that observed in today’s climate over the Sahara, with low-level convergence into the thermal low and outflow (mainly northerly and easterly) near 600 hPa. This occurs in the model in association with pronounced surface temperature increases (Fig. 2), and it is not the dipole variability mode. This response, which may indicate a strong land surface feedback, dries not only the Sahel, but nearly all of West Africa. The strong surface temperature gradients generate a strong African easterly jet (Fig.3) , which contributes to the drying by diverging moisture below the level of condensation and feeding it westward. SSTAs in the Gulf of Guinea, calculated as deviations from each model’s 1949-2000 mean over 5W-5E; 5S – 4N. All three models, in each CO2 increase scenario, predict that the Gulf of Guinea will warm in the 21st century. The GFDL ocean model exhibits more interannual variability than the MRI and MIROC models. Gulf of Guinea warming is similar in the A1B and A2 scenarios. Figure 3. GFDL_cm2_0 A2 scenario 600 hPa zonal wind (m/s) for the 2001-2030 (left) and 2071-2100 (middle) averages. Also the difference (right) between the 2071-2100 and 2001-2030 averages. Does the dipole pattern appear more frequently in the 21st century simulations? Figure 2. GFDL_cm2_0 surface temperature (K) differences from the 1949-2000 mean for the 2001-2030 (left) and 2071-2100 (right) averages for the A2 scenario. In the 20th century integrations, the three models capture a reasonable number of dipole events in association with warm SSTAs in the Gulf of Guinea. (See other poster for details.) MIROC3_2_MEDRES The MIROC model simulates very few dipole events in all of the 21st century integrations. The GFDL model does not indicate that there will be an increase in the number of dipole events, despite a simulated warming in the Gulf of Guinea.. Note the slight decrease in events in the second half of the 21st century in each scenario. The MRI model indicates an increase in occurrence of dry Sahel/wet Guinean Coast events for the A1B and A2 scenarios. Figure 4. MIROC3_2_medres (top row) precipitation (mm/day) differences from the 1949-2000 mean for the 2001-2030 (left) and 2071-2100 (right) averages, and (bottom row) meridional (m/s) – vertical (multiplied by –100 Pa/s) circulation cross-sections along the Greenwich Meridian for the 2001-2030 (left) and 2071-2100 (right) means for the (left) COMMIT, (center) A1B, and (right) A2 scenarios. This model predicts wetter conditions over the Sahel, and drier conditions along the Guinean coast in the 21st century. This is opposite to what is expected in association with Gulf of Guinea warming if the dipole mechanism operates. The response is quite pronounced in both the A1B and A2 scenarios. There is no evidence of the dipole mechanism in the circulation in these simulations. Instead, the model seems to have invoked another known mode of variability over West Africa, namely, an enhancement of the low-level westerly flow (Fig. 5). This mode is often associated with wet years in the Sahel, and also with the African Humid Period (AHP). During the AHP, strong surface warming over northernmost Africa due to the stronger summer insolation of that time (6,000 years ago) triggered added precipitation across Sahelian and Saharan Africa. The precipitation system was maintained climatologically when the low-level relative vorticity increases associated with stretching were balanced by importing low relative vorticity air from the Atlantic high. A similar process seems to be operating in this model (Fig.6). Is there a Sahelian rainfall trend (10W-10E;10N-15N) in the future simulations? Scenario COMMIT Scenario A1B Scenario A2 Figure 5. MIROC3_2_medres A2 scenario 850 hPa flow (m/s) for the 1949-2000 (left) and 2071-2100 (middle) climatologies. Also the difference(right) between the 2071-2100 and 1949-2000 averages. Figure 6. MIROC3_2_medres surface temperature (K) differences from the 1949-2000 mean for the 2001-2030 (left) and 2071-2100 (right) averages for the A2 scenario. ACKNOWLEDGEMENTS - This research was supported by the NSF SGER program. The GFDL model simulates a strong drying trend, MRI a weak drying trend. MIROC suggests a positive trend.