Simulated impacts of the South Atlantic Ocean Dipole on summer precipitation at the Guinea Coast

An intermediate complexity atmospheric general circulation model has been used to investigate the influence of the South Atlantic Ocean (SAO) dipole (SAOD) on summer precipitation over the Guinea Coast of West Africa. Two ensemble integrations in which idealized but realistic SAOD-type sea surface temperature (SST) anomaly is prescribed only in the SAO, and then globally are performed and inter-compared. Consistently, above (below) the average precipitation is simulated over the Guinea Coast during the positive (negative) phase of the SAOD. Comparison of the two set of experiments reveal that in its active years, the SAOD is a dominant mechanism that shapes the spatial character of summer precipitation at the Guinea coast, the global SST variability merely slightly moderate its effects. During the SAOD, cool SST anomaly in the extra-tropical SAO off the Brazil–Uruguay–Argentina coast gives rise to suppressed convection and mass divergence. In turn, the subsidence tends to amplify the sub-tropical arm of anomalous Hadley-type circulation and consequently large scale convection and mass flux convergence in the equatorial Atlantic Ocean/Gulf of Guinea region bordering on the coastal fringes of West Africa. Precipitation is therefore increased at the Guinea Coast.     

Numerical Simulation of Fluxes Generated by Inhomogeneities of the Underlying Surface over the Jinta Oasisin Northwestern China

Using land-use types derived from satellite remote sensing data collected by the EOS Moderate Resolution Imaging Spectroradiometer (EOS/MODIS), the mesoscale and turbulent fluxes generated by inhomogeneities of the underlying surface over the Jinta Oasis, northwestern China, were simulated using the Regional Atmospheric Modeling System (RAMS4.4). The results indicate that mesoscale circulation generated by land-surface inhomogeneities over the Jinta Oasis is more important than turbulence. Vertical heat fluxes and water vapor are transported to higher levels by mesoscale circulation. Mesoscale circulation also produces mesoscale synoptic systems and prevents water vapor over the oasis from running off. Mesoscale circulation transports moisture to higher atmospheric levels as the land-surface moisture over the oasis increases, favoring the formation of clouds, which sometimes leads to rainfall. Large-scale wind speed has a significant impact on mesoscale heat fluxes. During the active phase of mesoscale circulation, the stronger large-scale winds are associated with small mesoscale fluxes; however, background wind seems to intensify the turbulent sensible heat flux and turbulent latent heat flux. If the area of oasis is enlarged properly, mesoscale circulation will be able to transport moisture to higher levels, favoring the formation of rainfall in the oasis and protecting its "cold island" effect. The impact of irrigation on rainfall is important, and increasing irrigation across the oasis is necessary to protect the oasis.     

Observed trends in diurnal temperature range over Nigeria

本文基于站点观测及再分析资料,考察了1979–2013年期间尼日利亚不同地区温度日较差(DTR)的变化趋势及其可能原因。结果表明,尼日利亚北部萨赫勒地区(NSR)日最低温的增温趋势显著大于日最高温增加的趋势,导致该区域DTR呈显著下降趋势(–0.34°C/10年);而在南部几内亚湾地区(NGC),区域平均日最高温度增加趋势略大于最低温度的增加趋势,导致该区域平均DTR呈微弱上升趋势(0.01°C/10年)。NSR区域显著的DTR下降趋势与该地区降水量呈长期增加趋势密切相关,而NGC区域DTR的增加趋势则可归因于该地区云量存在的长期减少趋势。