Seasonal distribution of mono-modal, monsoonal rainfall across the semi-arid ecotone of sub-Saharan of West Africa is highly variable and unpredictable. The ever-present risk of drought and crop failure in this enviro...Seasonal distribution of mono-modal, monsoonal rainfall across the semi-arid ecotone of sub-Saharan of West Africa is highly variable and unpredictable. The ever-present risk of drought and crop failure in this environment often results in food shortages that are met by emergency food aid. Humanitarian assistance planners would be better prepared for such interventions in a timely manner if they have reliable indicators that forewarn the impending failure of the rains. A good indicator would be a characteristic of the seasonal rainfall distribution that can be shown to be reasonably invariant over time and space. The objective of this study is to investigate whether such invariance existed for the seasonal median date (meaning the date when 50% of the seasonal total occurs). Such invariance is expected since the sun’s cyclic declination forces the advance and retreat of the Inter-tropical Front over West Africa. We examined the statistical properties of the seasonal median date for 1349 station-years of rainfall records for 30 rainfall stations in Burkina Faso and Niger with coordinates ranging from 9.88° to 18.5° north latitude and -4.77° to 13.2° longitude. The results showed that the median date was quite narrowly distributed over years with rather weak dependence on geographical coordinates. It can therefore be used as a reasonable ex-ante indicator of the success or failure of the rains as the rainy season progress.展开更多
This study examines the inter-annual variability of rainfall and Mean Sea Level Pressure (</span><span style="font-family:Verdana;">M</span><span style="font-family:Verdana;"&g...This study examines the inter-annual variability of rainfall and Mean Sea Level Pressure (</span><span style="font-family:Verdana;">M</span><span style="font-family:Verdana;">SLP) over west Africa based on analysis of the Global Precipitation</span><span style="font-family:""><span style="font-family:Verdana;"> Climatology Project (GPCP) and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis respectively. An interconnection is found in this region, between Mean Sea Level Pressure (MSLP) anomaly (over Azores and St. Helena High) and monthly mean precipitation during summer (June to September: JJAS). We also found that over northern Senegal (15</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">N</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">-</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">17</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">N;17</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">W</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">-</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">13</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;"></span><span style="font-family:Verdana;">W) the SLP to the north is strong;the wind converges at 200</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">hPa corresponding to the position of the African Easterly Jet (AEJ) the rotational wind 700</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">hPa (corresponding to the position of the African Easterly Jet (AEJ) coming from the north-east is negative. In this region, th展开更多
Monsoon seasons, occasionally also known as wet seasons or trade-wind littoral seasons, are found in the regions where there is a complete seasonal reversal of the prevailing surface winds. Accompanying these shifts i...Monsoon seasons, occasionally also known as wet seasons or trade-wind littoral seasons, are found in the regions where there is a complete seasonal reversal of the prevailing surface winds. Accompanying these shifts in the prevailing surface winds are modulations in rainfall activity. Given the fact that our knowledge of the monsoons is mainly based on the interpretation of the mean values of precipitation, cloudiness and winds;relationships between earth’s rotation or revolution and geographical extent of the global surface monsoons deserve to be highlighted. In the abundant literary and audiovisual production devoted to monsoons worldwide and despite the fact that everyone agrees with physical law which shows that Coriolis force acts to the right in the northern hemisphere (to the left in the southern hemisphere), there is no reference to the relationship between Coriolis force (due to earth’s rotation) effects on troposphere general circulation and geographical extent of the global surface monsoons. Furthermore knowing that the ITCZ oscillations on either side of the equators (due to earth’s revolution) determine the seasons (mainly winter and summer), it is clear that earth’s revolution also plays a crucial role in the seasonal reversal of the prevailing surface winds observed in the regions where monsoons are found. Our main objective is to provide a rational answer to the question: what is a monsoon?展开更多
Research on the Asian-Pacific monsoon has a long history. This paper starts by summarizing field experiments investigating the Asian-Pacific monsoon. Since the 1960s, a number of international and regional monsoon pro...Research on the Asian-Pacific monsoon has a long history. This paper starts by summarizing field experiments investigating the Asian-Pacific monsoon. Since the 1960s, a number of international and regional monsoon projects and field experiments have been carried out, and substantial progress regarding research on the Asian-Pacific monsoon has been made. Second, the onset and the seasonal march of the Asian summer monsoon and the annual cycle of active and break periods of the monsoon, which are characterized by precipitation maxima and minima, are studied. Since the inter-tropical convergence zone (ITCZ or TCZ) is the dominating weather system and is the major birthplace of typhoons and tropical convective systems, the monsoonal rainfall and ITCZ are analyzed after the onset of the Asian mon- soon. Finally, because the ITCZ has a close relationship with tropical convective systems and rainfall events in monsoon regions, analyses of the developments of deep convection and rainfall events are briefly introduced.展开更多
An evaluation of Radar Precipitation Feature (RPF) characteristics and distribution of convective intensity is performed across 12 regions in West Africa. Results presented in this study have shown that these characte...An evaluation of Radar Precipitation Feature (RPF) characteristics and distribution of convective intensity is performed across 12 regions in West Africa. Results presented in this study have shown that these characteristics over West Africa revealed interesting results which were not observed on a larger spatial scale. The ice scattering characteristics and heights attained by the 15, 20, 30, and 40 dBZ echoes show patterns that agree with the season and movement of the Inter-Tropical Discontinuity (ITD). Some locations in the Western-coast rainforest, Nigeria/Cameroon rainforest and South Sudan savannah had strong potential for convective intensity during MAM, JJA, and SON as shown by their 37-GHz and 85-GHz PCT which fell below 250 K and 225 K respectively while the maximum height attained by their 20 dBZ, 30 dBZ and 40 dBZ are well above the freezing level in those locations. One result revealed a location on the eastern part of south-central Sahel (SC Sahel) where the maximum height attained by the 30 dBZ reflectivity is above 12 km and the maximum height attained by the 40 dBZ reflectivity is above 10 km during SON. The 37-GHz and 85-GHz PCT for this particular location are below 215 K and 150 K respectively indicating a very strong potential for intense convection and hence destructive storms. The distribution of convective intensity, considering only the 85-GHz PCT ice scattering signature, revealed that the percentage of convective intensity increases, especially in the rainforest and savannah, as the ITD shifts northwards during MAM, JJA and during its retreat in SON.展开更多
Understanding hydroclimatic patterns and their possible driving mechanisms during distinct climate periods over the last 1500 years—such as the Medieval Warm Period(MWP),the Little Ice Age(LIA),and the Current Warm P...Understanding hydroclimatic patterns and their possible driving mechanisms during distinct climate periods over the last 1500 years—such as the Medieval Warm Period(MWP),the Little Ice Age(LIA),and the Current Warm Period—is crucial for predicting future changes to monsoon precipitation in southwest China under global warming scenarios.In this study,based on ^(210)Pb and ^(137)Cs dating of surface sediments and AMS ^(14)C dating of terrestrial plant residues,we establish a robust age model that covers the last~1500 years(AD 439–2012)at Lake Yihai in southwest China.We use analyses of multiple geochemical proxy indices,including loss on ignition at 550℃,total organic carbon,total nitrogen,C/N ratios,and stable carbon isotopes of organic matter to reconstruct changes in summer monsoon precipitation at Lake Yihai during the last~1500 years.The results show that,over southwest China,warm and dry climate conditions prevailed during the MWP(AD 1000–1400)and the past 200 years,whereas conditions during the LIA(AD 1400–1800)were cold and wet.This is consistent with evidence from other geological records over southwest China,such as stalagmite and lake sediment data.Similar hydroclimatic patterns have occurred over the last 1500 years in adjacent tropical/subtropical monsoon regions where the climate is similarly dominated by the Indian summer monsoon(e.g.,South China,the South China Sea,Southeast Asia,Northeast India).We suggest that the meridional migration of the mean position of the Inter-tropical Convergence Zone,and El Niño/Southern Oscillation conditions which are linked to tropical Pacific sea surface temperature,are responsible for centennial-scale hydroclimatic patterns over southwest China and adjacent areas during the last 1500 years.展开更多
Interannual variability of the precipitation over West Africa Sahel is analyzed based on 32 years (1979-2010) from monthly and daily database of the Global Precipitation Climatology Project (GPCP). In this region, we ...Interannual variability of the precipitation over West Africa Sahel is analyzed based on 32 years (1979-2010) from monthly and daily database of the Global Precipitation Climatology Project (GPCP). In this region, we found that there is a link between the West Africa Monsoon (WAM) and the daily means of the precipitation in the summer, unseasonal rains can occur in the transition seasons and even in the heart of the dry season. Rainfall is the most important element for agro-pastoral activities in this region. The 850-hPa wind and wind divergence structure show a maximum convection over Mountain region (Fouta-Djalon and Mont-Cameroon) which corresponds to the high precipitation and OLR observed in these regions. The trend and empirical orthogonal function (EOF) of the precipitation are presented, including the mid-July variability of the precipitation. The dominant EOF of GPCP precipitation accounts for around 25.3% of the variance with slightly large amplitude in the north while relatively small in the equatorial band respectively. The second and third EOF which accounts for 20.5% and 14%, describes a longitudinal contrast with a zonal gradient.展开更多
文摘Seasonal distribution of mono-modal, monsoonal rainfall across the semi-arid ecotone of sub-Saharan of West Africa is highly variable and unpredictable. The ever-present risk of drought and crop failure in this environment often results in food shortages that are met by emergency food aid. Humanitarian assistance planners would be better prepared for such interventions in a timely manner if they have reliable indicators that forewarn the impending failure of the rains. A good indicator would be a characteristic of the seasonal rainfall distribution that can be shown to be reasonably invariant over time and space. The objective of this study is to investigate whether such invariance existed for the seasonal median date (meaning the date when 50% of the seasonal total occurs). Such invariance is expected since the sun’s cyclic declination forces the advance and retreat of the Inter-tropical Front over West Africa. We examined the statistical properties of the seasonal median date for 1349 station-years of rainfall records for 30 rainfall stations in Burkina Faso and Niger with coordinates ranging from 9.88° to 18.5° north latitude and -4.77° to 13.2° longitude. The results showed that the median date was quite narrowly distributed over years with rather weak dependence on geographical coordinates. It can therefore be used as a reasonable ex-ante indicator of the success or failure of the rains as the rainy season progress.
文摘This study examines the inter-annual variability of rainfall and Mean Sea Level Pressure (</span><span style="font-family:Verdana;">M</span><span style="font-family:Verdana;">SLP) over west Africa based on analysis of the Global Precipitation</span><span style="font-family:""><span style="font-family:Verdana;"> Climatology Project (GPCP) and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis respectively. An interconnection is found in this region, between Mean Sea Level Pressure (MSLP) anomaly (over Azores and St. Helena High) and monthly mean precipitation during summer (June to September: JJAS). We also found that over northern Senegal (15</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">N</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">-</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">17</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">N;17</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;">W</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">-</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">13</span><span style="white-space:nowrap;font-family:Verdana;">°</span><span style="font-family:Verdana;"></span><span style="font-family:Verdana;">W) the SLP to the north is strong;the wind converges at 200</span></span><span style="font-family:""> </span><span style="font-family:Verdana;">hPa corresponding to the position of the African Easterly Jet (AEJ) the rotational wind 700</span><span style="font-family:""> </span><span style="font-family:""><span style="font-family:Verdana;">hPa (corresponding to the position of the African Easterly Jet (AEJ) coming from the north-east is negative. In this region, th
文摘Monsoon seasons, occasionally also known as wet seasons or trade-wind littoral seasons, are found in the regions where there is a complete seasonal reversal of the prevailing surface winds. Accompanying these shifts in the prevailing surface winds are modulations in rainfall activity. Given the fact that our knowledge of the monsoons is mainly based on the interpretation of the mean values of precipitation, cloudiness and winds;relationships between earth’s rotation or revolution and geographical extent of the global surface monsoons deserve to be highlighted. In the abundant literary and audiovisual production devoted to monsoons worldwide and despite the fact that everyone agrees with physical law which shows that Coriolis force acts to the right in the northern hemisphere (to the left in the southern hemisphere), there is no reference to the relationship between Coriolis force (due to earth’s rotation) effects on troposphere general circulation and geographical extent of the global surface monsoons. Furthermore knowing that the ITCZ oscillations on either side of the equators (due to earth’s revolution) determine the seasons (mainly winter and summer), it is clear that earth’s revolution also plays a crucial role in the seasonal reversal of the prevailing surface winds observed in the regions where monsoons are found. Our main objective is to provide a rational answer to the question: what is a monsoon?
文摘Research on the Asian-Pacific monsoon has a long history. This paper starts by summarizing field experiments investigating the Asian-Pacific monsoon. Since the 1960s, a number of international and regional monsoon projects and field experiments have been carried out, and substantial progress regarding research on the Asian-Pacific monsoon has been made. Second, the onset and the seasonal march of the Asian summer monsoon and the annual cycle of active and break periods of the monsoon, which are characterized by precipitation maxima and minima, are studied. Since the inter-tropical convergence zone (ITCZ or TCZ) is the dominating weather system and is the major birthplace of typhoons and tropical convective systems, the monsoonal rainfall and ITCZ are analyzed after the onset of the Asian mon- soon. Finally, because the ITCZ has a close relationship with tropical convective systems and rainfall events in monsoon regions, analyses of the developments of deep convection and rainfall events are briefly introduced.
文摘An evaluation of Radar Precipitation Feature (RPF) characteristics and distribution of convective intensity is performed across 12 regions in West Africa. Results presented in this study have shown that these characteristics over West Africa revealed interesting results which were not observed on a larger spatial scale. The ice scattering characteristics and heights attained by the 15, 20, 30, and 40 dBZ echoes show patterns that agree with the season and movement of the Inter-Tropical Discontinuity (ITD). Some locations in the Western-coast rainforest, Nigeria/Cameroon rainforest and South Sudan savannah had strong potential for convective intensity during MAM, JJA, and SON as shown by their 37-GHz and 85-GHz PCT which fell below 250 K and 225 K respectively while the maximum height attained by their 20 dBZ, 30 dBZ and 40 dBZ are well above the freezing level in those locations. One result revealed a location on the eastern part of south-central Sahel (SC Sahel) where the maximum height attained by the 30 dBZ reflectivity is above 12 km and the maximum height attained by the 40 dBZ reflectivity is above 10 km during SON. The 37-GHz and 85-GHz PCT for this particular location are below 215 K and 150 K respectively indicating a very strong potential for intense convection and hence destructive storms. The distribution of convective intensity, considering only the 85-GHz PCT ice scattering signature, revealed that the percentage of convective intensity increases, especially in the rainforest and savannah, as the ITD shifts northwards during MAM, JJA and during its retreat in SON.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41807429 & 41702190)the Open Fund Project of State Key Laboratory of Loess and Quaternary Geology, IEECAS (Grant No. SKLLQG1517)
文摘Understanding hydroclimatic patterns and their possible driving mechanisms during distinct climate periods over the last 1500 years—such as the Medieval Warm Period(MWP),the Little Ice Age(LIA),and the Current Warm Period—is crucial for predicting future changes to monsoon precipitation in southwest China under global warming scenarios.In this study,based on ^(210)Pb and ^(137)Cs dating of surface sediments and AMS ^(14)C dating of terrestrial plant residues,we establish a robust age model that covers the last~1500 years(AD 439–2012)at Lake Yihai in southwest China.We use analyses of multiple geochemical proxy indices,including loss on ignition at 550℃,total organic carbon,total nitrogen,C/N ratios,and stable carbon isotopes of organic matter to reconstruct changes in summer monsoon precipitation at Lake Yihai during the last~1500 years.The results show that,over southwest China,warm and dry climate conditions prevailed during the MWP(AD 1000–1400)and the past 200 years,whereas conditions during the LIA(AD 1400–1800)were cold and wet.This is consistent with evidence from other geological records over southwest China,such as stalagmite and lake sediment data.Similar hydroclimatic patterns have occurred over the last 1500 years in adjacent tropical/subtropical monsoon regions where the climate is similarly dominated by the Indian summer monsoon(e.g.,South China,the South China Sea,Southeast Asia,Northeast India).We suggest that the meridional migration of the mean position of the Inter-tropical Convergence Zone,and El Niño/Southern Oscillation conditions which are linked to tropical Pacific sea surface temperature,are responsible for centennial-scale hydroclimatic patterns over southwest China and adjacent areas during the last 1500 years.
文摘Interannual variability of the precipitation over West Africa Sahel is analyzed based on 32 years (1979-2010) from monthly and daily database of the Global Precipitation Climatology Project (GPCP). In this region, we found that there is a link between the West Africa Monsoon (WAM) and the daily means of the precipitation in the summer, unseasonal rains can occur in the transition seasons and even in the heart of the dry season. Rainfall is the most important element for agro-pastoral activities in this region. The 850-hPa wind and wind divergence structure show a maximum convection over Mountain region (Fouta-Djalon and Mont-Cameroon) which corresponds to the high precipitation and OLR observed in these regions. The trend and empirical orthogonal function (EOF) of the precipitation are presented, including the mid-July variability of the precipitation. The dominant EOF of GPCP precipitation accounts for around 25.3% of the variance with slightly large amplitude in the north while relatively small in the equatorial band respectively. The second and third EOF which accounts for 20.5% and 14%, describes a longitudinal contrast with a zonal gradient.
