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1.
In order to test the sensitivity of the transitional phase of the 2006 West African monsoon (WAM) onset to different mechanisms, weather research and forecasting (WRF) model simulations have been carried out addressing the role of the Saharan heat low (SHL) and its sensitivity to the albedo field and to the northern Africa orography, and the role of the sea surface temperature (SST) in the eastern tropical Atlantic and Mediterranean. Lowering albedo over the desert region induces a northward location of the inter-tropical convergence zone (ITCZ), while removing mountains in North Africa reduces rainfall over West Africa. Shifting SST forward by 15?days leads to a northward location of the ITCZ before the WAM onset. However none of these factors modifies the timing of the WAM onset in 2006. The transitional phase of the 2006 WAM onset has been examined in more detail. The enhancement of SHL intensity, combined with the development of the oceanic cold tongue in the Guinea gulf, leads to low-level moisture flux divergence in the ITCZ reducing rainfall and increasing low-level humidity over the Sahel. However, weakening of convection can be clearly attributed to dry-air intrusions in mid-levels, originating from the subtropical westerly jet and associated with Rossby wave pattern over North Africa. Sensitivity tests on the synoptic scale forcing outside of the WRF model domain confirm the dominating role of large-scale dynamics to control the transitional phase of the WAM onset and its timing. However it is shown that the regional factors can modulate this larger scale forcing. 相似文献
2.
Ismaïla Diallo Caroline L. Bain Amadou T. Gaye Wilfran Moufouma-Okia Coumba Niang Mame D. B. Dieng Richard Graham 《Climate Dynamics》2014,43(3-4):575-594
The performance of the Hadley Centre Global Environmental Model version 3 regional climate model (HadGEM3-RA) in simulating the West African monsoon (WAM) is investigated. We focus on performance for monsoon onset timing and for rainfall totals over the June–July–August (JJA) season and on the model’s representation of the underlying dynamical processes. Experiments are driven by the ERA-Interim reanalysis and follow the CORDEX experimental protocol. Simulations with the HadGEM3 global model, which shares a common physical formulation with HadGEM3-RA, are used to gain insight into the causes of HadGEM3-RA simulation errors. It is found that HadGEM3-RA simulations of monsoon onset timing are realistic, with an error in mean onset date of two pentads. However, the model has a dry bias over the Sahel during JJA of 15–20 %. Analysis suggests that this is related to errors in the positioning of the Saharan heat low, which is too far south in HadGEM3-RA and associated with an insufficient northward reach of the south-westerly low-level monsoon flow and weaker moisture convergence over the Sahel. Despite these biases HadGEM3-RA’s representation of the general rainfall distribution during the WAM appears superior to that of ERA-Interim when using Global Precipitation Climatology Project or Tropical Rain Measurement Mission data as reference. This suggests that the associated dynamical features seen in HadGEM3-RA can complement the physical picture available from ERA-Interim. This approach is supported by the fact that the global HadGEM3 model generates realistic simulations of the WAM without the benefit of pseudo-observational forcing at the lateral boundaries; suggesting that the physical formulation shared with HadGEM3-RA, is able to represent the driving processes. HadGEM3-RA simulations confirm previous findings that the main rainfall peak near 10°N during June–August is maintained by a region of mid-tropospheric ascent located, latitudinally, between the cores of the African Easterly Jet and Tropical Easterly Jet that intensifies around the time of onset. This region of ascent is weaker and located further south near 5°N in the driving ERA-Interim reanalysis, for reasons that may be related to the coarser resolution or the physics of the underlying model, and this is consistent with a less realistic latitudinal rainfall profile than found in the HadGEM3-RA simulations. 相似文献
3.
The Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981?C1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032 and 2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10°N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards deflection of moist air and contributing to deep moist convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water resources vulnerability locally. 相似文献
4.
Regional climate modelling of the 2006 West African monsoon: sensitivity to convection and planetary boundary layer parameterisation using WRF 总被引：3，自引：3，他引：0
Regional climate model (RCM) is a valuable scientific tool to address the mechanisms of regional atmospheric systems such as the West African monsoon (WAM). This study aims to improve our understanding of the impact of some physical schemes of RCM on the WAM representation. The weather research and forecasting model has been used by performing six simulations of the 2006 summer WAM season. These simulations use all combinations of three convective parameterization schemes (CPSs) and two planetary boundary layer schemes (PBLSs). By comparing the simulations to a large set of observations and analysis products, we have evaluated the ability of these RCM parameterizations to reproduce different aspects of the regional atmospheric circulation of the WAM. This study focuses in particular on the WAM onset and the rainfall variability simulated over this domain. According to the different parameterizations tested, the PBLSs seem to have the strongest effect on temperature, humidity vertical distribution and rainfall amount. On the other hand, dynamics and precipitation variability are strongly influenced by CPSs. In particular, the Mellor?CYamada?CJanjic PBLS attributes more realistic values of humidity and temperature. Combined with the Kain?CFritsch CPS, the WAM onset is well represented. The different schemes combination tested also reveal the role of different regional climate features on WAM dynamics, namely the low level circulation, the land?Catmosphere interactions and the meridional temperature gradient between the Guinean coast and the Sahel. 相似文献
5.
The reproducibility and future changes of the onset of the Asian summer monsoon were analyzed based on the simulations and projections under the Representative Concentration Pathways(RCP) scenario in which anthropogenic emissions continue to rise throughout the 21 st century(i.e. RCP8.5) by all realizations from four Chinese models that participated in the Coupled Model Intercomparison Project Phase 5(CMIP5). Delayed onset of the monsoon over the Arabian Sea was evident in all simulations for present-day climate, which was associated with a too weak simulation of the low-level Somali jet in May.A consistent advanced onset of the monsoon was found only over the Arabian Sea in the projections, where the advanced onset of the monsoon was accompanied by an increase of rainfall and an anomalous anticyclone over the northern Indian Ocean. In all the models except FGOALS-g2, the enhanced low-level Somali jet transported more water vapor to the Arabian Sea, whereas in FGOALS-g2 the enhanced rainfall was determined more by the increased wind convergence. Furthermore,and again in all models except FGOALS-g2, the equatorial SST warming, with maximum increase over the eastern Pacific,enhanced convection in the central West Pacific and reduced convection over the eastern Indian Ocean and Maritime Continent region, which drove the anomalous anticyclonic circulation over the western Indian Ocean. In contrast, in FGOALS-g2, there was minimal(near-zero) warming of projected SST in the central equatorial Pacific, with decreased convection in the central West Pacific and enhanced convection over the Maritime Continent. The broader-scale differences among the models across the Pacific were related to both the differences in the projected SST pattern and in the present-day simulations. 相似文献
6.
The West African monsoon has over the years proven difficult to represent in global coupled models. The current operational seasonal forecasting system of the UK Met Office (GloSea4) has a good representation of monsoon rainfall over West Africa. It reproduces the various stages of the monsoon: a coastal phase in May and June, followed by onset of the Sahelian phase in July when rainfall maxima shift northward of 10N until September; and a secondary coastal rainfall maximum in October. We explore the dynamics of monsoon onset in GloSea4 and compare it to reanalyses. An important difference is the change in the Saharan heat low around the time of Sahelian onset. In Glosea4 the deepening heat low introduces moisture convergence across an east-west Sahelian band, whereas in the reanalyses such an east-west organisation of moisture does not occur and moisture is transported northwards to the Sahara. Lack of observations in the southern Sahara makes it difficult to verify this process in GloSea4 and also suggests that reanalyses may not be strongly constrained by station observations in an area key to Sahelian onset. Timing of monsoon onset has socio-economic importance for many countries in West Africa and we explore onset predictability in GloSea4. We use tercile categories to calculate probabilities for onset occurring before, near and after average in four different onset indicators. Glosea4 has modest skill at 2–3 months’ lead time, with ROC scores of 0.6–0.8. Similar skill is seen in hindcasts with models from the ENSEMBLES project, even in models with large rainfall biases over the Sahel. Forecast skill derives from tropical SST in June and many models capture at least the influence of the tropical Atlantic. This suggests that long-range skill for onset could be present in other seasonal forecasting systems in spite of mean rainfall biases. 相似文献
7.
Babatunde J. Abiodun Zachariah D. Adeyewa Philip G. Oguntunde Ayobami T. Salami Vincent O. Ajayi 《Theoretical and Applied Climatology》2012,110(1-2):77-96
This study investigates the potential impacts of reforestation in West Africa on the projected regional climate in the near two decades (2031–2050) under the SRES A1B scenario. A regional climate model (RegCM3) forced with a global circulation model (ECHAM5) simulations was used for the study. The study evaluates the capability of the regional model in simulating the present-day climate over West Africa, projects the future climate over the region and investigates impacts of seven hypothetical reforestation options on the projected future climate. Three of these reforestation options assume zonal reforestation over West Africa (i.e., over the Sahel, Savanna and Guinea), while the other four assume random reforestation over Nigeria. With the elevated GHGs (A1B scenario), a warmer and drier climate is projected over West Africa in 2031–2050. The maximum warming (+2.5°C) and drying (?2?mm?day?1) occur in the western part of the Sahel because the West Africa Monsoon (WAM) flow is stronger and deflects the cool moist air more eastward, thereby lowering the warming and drying in the eastern part. In the simulations, reforestation reduces the projected warming and drying over the reforested zones but increases them outside the zones because it influences the northward progression of WAM in summer. It reduces the speed of the flow by weakening the temperature gradient that drives the flow and by increasing the surface drag on the flow over the reforested zone. Hence, in summer, the reforestation delays the onset of monsoon flow in transporting cool moist air over the area located downwind of the reforested zone, consequently enhancing the projected warming and drying over the area. The impact of reforesting Nigeria is not limited to the country; while it lowers the warming over part of the country (and over Togo), it increases the warming over Chad and Cameroon. This study, therefore, suggests that using reforestation to mitigate the projected future climate change in West Africa could have both positive and negative impacts on the regional climate, reducing temperature in some places and increasing it in others. Hence, reforestation in West Africa requires a mutual agreement among the West African nations because the impacts of reforestation do not recognize political boundaries. 相似文献
8.
Elsa Mohino Serge Janicot Hervé Douville Laurent Z. X. Li 《Climate Dynamics》2012,38(11-12):2319-2334
Observational evidence suggests a link between the summer Madden Julian Oscillation (MJO) and anomalous convection over West Africa. This link is further studied with the help of the LMDZ atmospheric general circulation model. The approach is based on nudging the model towards the reanalysis in the Asian monsoon region. The simulation successfully captures the convection associated with the summer MJO in the nudging region. Outside this region the model is free to evolve. Over West Africa it simulates convection anomalies that are similar in magnitude, structure, and timing to the observed ones. In accordance with the observations, the simulation shows that 15–20?days after the maximum increase (decrease) of convection in the Indian Ocean there is a significant reduction (increase) in West African convection. The simulation strongly suggests that in addition to the eastward-moving MJO signal, the westward propagation of a convectively coupled equatorial Rossby wave is needed to explain the overall impact of the MJO on convection over West Africa. These results highlight the use of MJO events to potentially predict regional-scale anomalous convection and rainfall spells over West Africa with a time lag of approximately 15–20?days. 相似文献
9.
We have developed a hydrological prognostic index, HOWI (hydrological onset and withdrawal index), for the onset and the withdrawal of the West African monsoon (WAM), based on the vertically integrated moisture transport (VIMT). The regions of West Africa with the same climatological onset (withdrawal) date are characterized by a large change of the VIMT around the onset (withdrawal) date. By analyzing the variability of the VIMT, we determine the extension and the geographical position of these regions, which we take sufficiently large to filter out the fast weather variability. It turns out that the regions with the same climatological onset date do not usually coincide with the regions with the same climatological withdrawal date, the areas with the maximum variability of the VIMT during the onset phase are usually a fraction of the area where the variability of the VIMT is large during the withdrawal phase. This is because the onset has active phases and pauses in time and it is fragmented in space, while the withdrawal is rather rapid and almost uniformly distributed through the entire monsoonal region. When the monsoon moves inland, the rainfall slightly trails behind the arrival of the moisture, and, when the monsoon moves back towards the gulf of Guinea, the moisture slightly precedes the retreating rainfall. In a specific region, we say that the onset (withdrawal) of the monsoon occurs when the moisture reaches (declines to) half of its climatological value. The level of the moisture relatively to its climatological value is evaluated through the HOWI, i.e., at the onset (withdrawal) the HOWI is zero with a positive (negative) tendency. We find that the dates of the onset of the monsoon determined using the HOWI, computed in the region where the VIMT has its maximum variability during the onset phase of WAM, well agree with the dates of the sudden transition of the ITCZ (Intertropical Convergence Zone) from 5 to 10°N. The uncertainty on the onset date is of the order of 2 pentads, which is comparable to the uncertainty on the date of the sudden transition of the ITCZ. We, then, use the HOWI to determine the onset and the withdrawal dates of the monsoon for the period 1979–2004, finding that an early (late) onset usually preludes to a longer (shorter) monsoonal season with more (less) cumulated rain. Finally, we compare the onset dates in the Sahelian region, for the period 1979–2004, with those determined using methods based on rainfall. 相似文献
10.
This paper introduces an objective definition of local onset and demise of the Indian summer monsoon (ISM) at the native grid of the Indian Meteorological Department’s rainfall analysis based on more than 100 years of rain gauge observations. The variability of the local onset/demise of the ISM is shown to be closely associated with the All India averaged rainfall onset/demise. This association is consistent with the corresponding evolution of the slow large-scale reversals of upper air and ocean variables that raise the hope of predictability of local onset and demise of the ISM. The local onset/demise of the ISM also show robust internannual variations associated with El Nino and the Southern Oscillation and Indian Ocean dipole mode. It is also shown that the early monsoon rains over northeast India has a predictive potential for the following seasonal anomalies of rainfall and seasonal length of the monsoon over rest of India.
相似文献11.
West African monsoon response to greenhouse gas and sulphate aerosol forcing under two emission scenarios 总被引：1，自引：0，他引：1
The impact of increased greenhouse gases (GHG) and aerosols concentrations upon the West African monsoon (WAM) is investigated
for the late twenty-first century period using the Météo-France ARPEGE-IFS high-resolution atmospheric model. Perturbed (2070–2100)
and current (1961–2000) climates are compared using the model in time-slice mode. The model is forced by global sea surface
temperatures provided by two transient scenarios performed with low-resolution coupled models and by two GHG evolution scenarios,
SRES-A2 and SRES-B2. Comparing to reanalysis and observed data sets, the model is able to reproduce a realistic seasonal cycle
of WAM despite a clear underestimation of the African Easterly Jet (AEJ) during the boreal summer. Mean temperature change
indicates a global warming over the continent (stronger over North and South Africa). Simulated precipitation change at the
end of the twenty-first century shows an increase in precipitation over Sudan-Sahel linked to a strong positive feedback with
surface evaporation. Along Guinea Gulf coast, rainfall regimes are driven by large-scale moisture advection. Moreover, results
show a mean precipitation decrease (increase) in the most (less) enhanced GHG atmosphere over this region. Modification of
the seasonal hydrological cycle consists in a rain increase during the monsoon onset. There is a significant increase in rainfall
variance over the Sahel, which extends over the Guinea coast region in the moderate emission scenario. Enhanced precipitation
over Sahel is linked to large-scale circulation changes, namely a weakening of the AEJ and an intensification of the Tropical
Easterly Jet. 相似文献
12.
Convective mixing in the monsoon boundary layer 总被引：1，自引：0，他引：1
Surendra S. Parasnis 《Boundary-Layer Meteorology》1991,56(4):395-400
Aerological observations over a tropical inland station collected during different phases of the Indian summer monsoon have been used to study the convective mixing in the monsoon boundary layer. The mixing line (ML) model for the thermodynamic structure of a partially cloudy boundary layer has been used to estimate the mixing by convection. The ML model together with the saturation point approach is able to depict different convective mixing regimes in the monsoon boundary layer. 相似文献
13.
Besides sea surface temperature (SST), soil moisture (SM) exhibits a significant memory and is likely to contribute to atmospheric
predictability at the seasonal timescale. In this respect, West Africa was recently highlighted as a “hot spot” where the
land–atmosphere coupling could play an important role, through the recycling of precipitation and the modulation of the meridional
gradient of moist static energy. Particularly intriguing is the observed relationship between summer monsoon rainfall over
Sahel and the previous second rainy season over the Guinean Coast, suggesting the possibility of a soil moisture memory beyond
the seasonal timescale. The present study is aimed at revisiting this question through a detailed analysis of the instrumental
record and a set of numerical sensitivity experiments. Three ensembles of global atmospheric simulations have been designed
to assess the relative influence of SST and SM boundary conditions on the West African monsoon predictability over the 1986–1995
period. On the one hand, the results indicate that SM contributes to rainfall predictability at the end and just after the
rainy season over the Sahel, through a positive soil-precipitation feedback that is consistent with the “hot spot” hypothesis.
On the other hand, SM memory decreases very rapidly during the dry season and does not contribute to the predictability of
the all-summer monsoon rainfall. Though possibly model dependent, this conclusion is reinforced by the statistical analysis
of the summer monsoon rainfall variability over the Sahel and its link with tropical SSTs. Our results indeed suggest that
the apparent relationship with the previous second rainy season over the Guinean Coast is mainly an artefact of rainfall teleconnections
with tropical modes of SST variability both at interannual and multi-decadal timescales. 相似文献
14.
Bidyut B. Goswami Medha Deshpande P. Mukhopadhyay Subodh K. Saha Suryachandra A. Rao Raghu Murthugudde B. N. Goswami 《Climate Dynamics》2014,43(9-10):2725-2745
We have evaluated the simulation of Indian summer monsoon and its intraseasonal oscillations in the National Centers for Environmental Prediction climate forecast system model version 2 (CFSv2). The dry bias over the Indian landmass in the mean monsoon rainfall is one of the major concerns. In spite of this dry bias, CFSv2 shows a reasonable northward propagation of convection at intraseasonal (30–60 day) time scale. In order to document and understand this dry bias over the Indian landmass in CFSv2 simulations, a two pronged investigation is carried out on the two major facets of Indian summer monsoon: one, the air–sea interactions and two, the large scale vertical heating structure in the model. Our analysis shows a possible bias in the co-evolution of convection and sea surface temperature in CFSv2 over the equatorial Indian Ocean. It is also found that the simulated large scale vertical heat source (Q1) and moisture sink (Q2) over the Indian region are biased relative to observational estimates. Finally, this study provides a possible explanation for the dry precipitation bias over the Indian landmass in the simulated mean monsoon on the basis of the biases associated with the simulated ocean–atmospheric processes and the vertical heating structure. This study also throws some light on the puzzle of CFSv2 exhibiting a reasonable northward propagation at the intraseasonal time scale (30–60 day) despite a drier monsoon over the Indian land mass. 相似文献
15.
Summary During most El-Ni?o events the Indian summer monsoon rainfall has been below normal. El-Ni?o that occurred during 1997 was
one of the strongest in the 20th century, but did not have an adverse impact on the Indian summer monsoon rainfall in 1997.
This is despite the fact that most parameters observed in May 1997 suggested that the Indian summer monsoon rainfall may be
below normal. This intriguing feature of the 1997 Indian summer monsoon rainfall has been examined by studying the evolution
of various parameters from May to August. The behavior of the 1997 monsoon is related to its evolution during June and July,
with westward migration of cloudbands from West Pacific that increased convection over Bay of Bengal. We find that there exists
a significant correlation between convective activity over Bay of Bengal and winds over the Arabian Sea with the latter lagging
convection over Bay of Bengal by about three days. The convective activity over Bay of Bengal induces stronger winds over
the Arabian Sea and this in turn enhances advection of moisture into the Indian landmass and leads to increased precipitable
water and strength of the monsoon. Using a simple thermodynamic model we show that increased precipitable water during July
leads to increased rainfall. A similar behavior has also been noticed during the 1983 monsoon, with precursors indicating
a possible poor monsoon but subsequent events changed the course of the monsoon.
Received May 21, 2001 Revised October 10, 2001 相似文献
16.
Seasonal and intraseasonal changes of African monsoon climates in 21st century CORDEX projections 总被引：1，自引：0，他引：1
We analyze a mini ensemble of regional climate projections over the CORDEX Africa domain carried out with RegCM4 model as part of the Phase I CREMA experiment (Giorgi 2013). RegCM4 is driven by the HadGEM2-ES and MPI-ESM global models for the RCP8.5 and RCP4.5 greenhouse gas and aerosol concentration scenarios. The focus of the analysis is on seasonal and intraseasonal monsoon characteristics. We find two prominent change signals. Over West Africa and the Sahel MPI produces a forward shift in the monsoon season in line with previous findings, and this shift is also simulated by the RegCM4. Furthermore, the regional model produces a widespread decrease of monsoon precipitation (when driven by both MPI and HadGEM) associated with decreased easterly wave activity in the 6–9 days regime and with soil moisture-precipitation interactions. South of the equator we find an extension of the dry season with delayed onset and anticipated recession of the monsoon and a narrowing and strengthening of the ITCZ precipitation band. This signal is consistent in all global and regional model projections, although with different spatial detail. We plan to enlarge this mini-ensemble as a further contribution to the CORDEX project to better assess the robustness of the signals found in this paper. 相似文献
17.
This work addresses the mechanisms that leads to an early onset of monsoon over Andaman Sea but advances further rapidly (slowly) to the Indian mainland resulting in the early (delayed) onset over Kerala. The upper tropospheric temperature, production of kinetic energy (KE) and outgoing long wave radiation (OLR) from the month of May till onset over Kerala are analysed for two delayed onset years (1997, 1995) and two early onset years (2004, 1990). It is observed that the maximum temperature over Tibetan plateau (TP), an increase in the production of KE and strong equatorial convection in early May, is associated with early onset over Andaman Sea. However, when there is a lull in advance of monsoon after the early onset over Andaman Sea, shifting of the warm region south of TP, weak production of KE in the lower troposphere and convective region shifting to Western Pacific resulted in the delayed onset over Kerala in 1997 and 1995. During the early onset years viz. 2004 and 1990, the warm region moving westwards, high production of KE extending to mid troposphere and deep convection moving westwards in the north Indian Ocean (10–15°N) is noticed. 相似文献
18.
S. P. Ghanekar P. V. Puranik V. R. Mujumdar 《Theoretical and Applied Climatology》2010,99(3-4):457-468
Outgoing Longwave Radiation (OLR) data, obtained from NOAA polar orbiting satellites for the months March–June, for 31 years is utilized to investigate low-frequency oscillation in pre-monsoon convection over southwest peninsular Indian region. The analysis reveals a characteristic fall in OLR over the region, about 5–9 weeks prior to the onset of Indian summer monsoon in almost all the years under study. Such fall is shown to be related with a characteristic peak in convective activity associated with the movement of Inter-Tropical Convergence Zone over the region. The results suggest an indirect non-linear relationship between the time of occurrence of the pre-monsoon convective peak and the onset date. A regression equation is developed to predict the onset date. The results of estimation of present method and that of an earlier method (based on conventional synoptic observations) are compared. The performance of both these methods is validated for two independent years (2007 and 2008). The study highlights the potential application of the developed method for the prediction of onset of Indian summer monsoon well in advance using the remotely sensed satellite-derived OLR data. 相似文献
19.
NCEP/GFS analysis is used to investigate the scale dependence and the interplay between the terms of the atmospheric water
budget over West Africa using a dedicated decomposition methodology. The focus is on a 2-month period within the active monsoon
period of 2006. Results show that the dominant scales of seasonal mean precipitation and moisture flux divergence over West
Africa during the monsoon period are large scales (greater than 1,400 km) except over topography, where mean values of small
scales (smaller than 900 km) are strong. Correlations between moisture flux divergences in monsoon and African Easterly Jet
layers and precipitation indicate that precipitation is strongly correlated to moisture flux divergence via both large-scale
and small-scale processes, but the correlation signal is quite different depending on the region and vertical layer considered.
The analysis of the scales associated with the rainfall and the local evaporation over 3 different regions shows that positive
correlation exists over the ocean between precipitation and evaporation especially at large scale. Over the continent south
of the Sahel, the correlation is negative and driven by large scale. Over the northern part of Sahel, positive correlation
is found, only at small scales during the active monsoon period. Lag correlation reveals that the maximum evaporation over
the Sahel occurs 1–3 days after the maximum precipitation with maximum contribution from small-scale processes during the
first day. This study shows that NCEP/GFS reproduces well the known atmospheric water budget features. It also reveals a new
scale dependence of the relative role of each term of the atmospheric water budget. This indicates that such scale decomposition
approach is helpful to clarify the functioning of the water cycle embedded in the monsoon system. 相似文献
20.
Observations of the South China Sea summer monsoon （SCSSM） demonstrate the different features between the early and late onsets of the monsoon. The determining factor related to the onset and the resultant monsoon rainfall might be the off-equatorial ITCZ besides the land-sea thermal contrast. The northward-propagating cumulus convection over the northern Indian Ocean could enhance the monsoon trough so that the effect of the horizontal advection of moisture and heat is substantially increased, thus westerlies can eventually penetrate and prevail over the South China Sea （SCS） region. 相似文献