A43B-0041 1340h
Equatorial Atlantic cold tongue variability and its impact on precipitation
Rank correlation maps of sea-surface temperature anomalies (SSTA) in the equatorial Atlantic cold tongue (ACT) with precipitation anomalies indicate a strong relationship of precipitation at the margin of the Atlantic ITCZ to ACT SSTA in all seasons. In May, the region of significant rain correlation with ACT reaches into equatorial South America, during July and August Upper Guinea. Using both models and observations we provide evidence for active involvement of equatorial ocean dynamics in ACT variability, with antecedent equatorial wind anomalies in the western Atlantic the likely forcing. A special role for the equatorial region appears to hold despite the strong correlation of equatorial SSTA with SSTA in the southern tropical Atlantic originating from local trade wind variations. An ocean dynamical connection between ACT variability and SSTA variability along the south-eastern Atlantic coast may also exist. Ocean model experiments indicate that ocean dynamics introduces a lag of about one month into the system, which may offer some prospects for forecasting. This must, however, be held in perspective with other known influences on precipitation over the Atlantic ITCZ and tropical South America, including El Nino/Southern Oscillation.
A43B-0042 1340h
Winds, Nonlinearity, and the North Equatorial Countercurrent
The currents of the northern tropical Atlantic are dominated by the seasonal North Equatorial Countercurrent, whose strength and position plays an important role in regulating the tongue of cool surface temperatures on and south of the equator. Here we examine the year-to-year variability of this current based on a combination of satellite and in situ observations October 1992 - May 2004. The causes of this variability are examined using a hierarchy of dynamical models driven by scatterometer wind stress products and will be contrasted with a similar reconstruction of currents in the tropical Pacific.
A43B-0043 1340h
The Causes of Tropical Atlantic Precipitation Biases in AGCMs: in Search of a Comprehensive Diagnosis.
This study focuses on the biases in monthly mean Atlantic marine ITCZ (AMI) in reanalyses and AMIP-like GCM simulations. We establish a set of diagnostics that provides a comprehensive, multi-variable description of the thermodynamical and dynamical characteristics of precipitation and its response to changes in local and remote conditions. Maps, vertical profiles, and budget analysis of key variables provide a description of the large scale environment; joint probability density functions and correlation analysis document the relationship between precipitation and its local environment. Comparisons between the Atlantic and the Pacific ITCZs in each model, among different models, and between models and observations allow us to hypothesize explanations for biases in the simulation of AMI. For example, convection in CCM3 responds more strongly to surface humidity than to the integrated humidity in a deeper boundary layer. In turn, surface humidity is determined to a large extent by SST and evaporation, and not mostly by convergence. In both cases, CCM3 is at odds with observations, reanalysis products, and other models. Thus, we hypothesize that biases in the CCM3 simulation of AMI are due both to the way the convection scheme responds to its environment (i.e., the weak dependance on lower tropospheric humidity), and to the way the environment is simulated (what determines the surface humidity).
A43B-0044 1340h
Variability of vertical shear within the tropical North Atlantic
Past studies have shown that vertical wind shear is a key environmental factor that influences Atlantic tropical cyclone activity. Thus, there is the need to better understand the factors that contribute to the variability of the vertical shear in order to the improve the prediction of tropical cyclone activity in this region. In the present study, we consider the interannual to decadal variability of vertical shear over the tropical North Atlantic. We examine an ensemble of 6 simulations performed using the HadAM3 GCM that is forced by reconstructed historical SST for the period 1871-1999. The spatial and temporal variability of vertical shear simulated by the GCM is evaluated against the ECMWF and NCEP/NCAR reanalysis data. The relationship between Atlantic shear and forcings such as SST and Sahelian precipitation are investigated using the GCM simulations as well as the reanalysis datasets. The results to be presented at the conference will emphasize the role of local processes and teleconnections that influence the vertical wind shear over the tropical North Atlantic.
A43B-0045 1340h
West African Rainfall and the Tropical Atlantic Ocean: Interactions in the NCEP Coupled System
We present results from an examination of a four member set of 33 year-long integrations using the NCEP atmosphere/ocean coupled general circulation model. Our diagnostic study focuses on the seasonal and interannual fluctations of rainfall over west Africa and the associated changes in winds, heat flux, SST, and ocean heat storage. We begin by presenting the seasonal climatology. We then remove the seasonal cycle from all variables and present an examination of subseasonal variability. The most prominent feature of the subseasonal rainfall variability occurs in the northern tropics and is associated with meridional perturbations of the seasonal ITCZ. Although 1C too warm on average the model's equatorial cold tongue undergoes strong temperature fluctuations from year-to-year. Warm years are associated with enhanced rainfall in the coastal Gulf of Guinea during boreal summer, a feature that appears in the observational record as well. Thus, the model should be a useful tool for understanding the coupled dynamics of west African rainfall and its interaction with the Atlantic cold tongue.
A43B-0046 1340h
The Atmospheric Dynamics of the West African Monsoon Onset and its Ocean Counterpart
Precipitation in the Sahel is produced by one rainy season during the northern summer monsoon over West Africa. The onset of these rains, linked to the northward migration of the Inter-Tropical Convergence Zone (ITCZ), is an important parameter for a large community of users like meteorologists, farmers, water resources managers... By using combined daily rainfall and OLR data on the period 1968-2002, we show that this migration is characterized by an abrupt latitudinal shift of the ITCZ in late June from a quasi-stationary location at 5N in May-June to another quasi-stationary location at 10N in July-August. A composite analysis based on NCEP reanalyses shows that this northward shift is associated with an enhanced Saharan heat low dynamics, increasing inland zonal moisture advection. The same composite analysis is applied on Reynolds and TMI SST data to investigate the potential surface temperature changes around the monsoon onset. Preliminary results will also be shown on the response to this onset of the oceanic model OPA/ORCA of LODYC/IPSL forced by ERS and QuikSCAT fields.
A43B-0047 1340h
Near surface atmospheric and oceanic fields associated with the West Sahel Rainfall anomalies.
We exploit the availability of recent satellite products to investigate statistical links between year-to-year variability of rainfall in the West African Sahel (expressed in terms of the P. Lamb index), SST, mean sea level pressure, and wind velocity in the Atlantic sector. Wet years in the Sahel are associated with strengthening of the meridional SST gradient (e.g. warmer north Atlantic), strengthening of both trade wind systems in the subtropics, and strengthening westerly winds in the ITCZ. These changes in winds are accompanied by the development of a pressure low off the West African coast. Year-to-year changes of mean sea level pressure in this low pressure region display a tremendous correlation to the variability of West Sahel rainfall.
A43B-0048 1340h
Evaluations of West African Surface Water and Energy Balances from 1982 to 1990 Based on Ground & Satellite Observations and SSIB Vegetation Model
The Sahel is a key area for studying the climate change because of a unique severe drought and a marked land cover changes. However, the studies of surface hydrology at continental scale over this area are spare, mainly due to the lack of data. Here, a comprehensive study of seasonal and inter-annual variability of surface hydrology has been conducted in order to highlight the specific characteristics of different sub-regions in the Sahel. Total 217 rain gauge stations have been selected from the Institut de Recherche pour le Developpement (IRD-France) daily rainfall database and other sources. The choice of the stations is based on three different criteria: the maximization of the spatial coverage within 2°N-20°N and 18°W-25°E, the maximum period length without any gap (1982-90), and the maximum variation of different associated vegetation types (7 types are included). The stations data have been interpolated into 1°x1° grid boxes. Leaf Area Index (LAI) and vegetation cover parameters were derived from AVHRR NDVI satellite data (Los et al., 2001). We also obtain the near surface temperature, humity, wind, and radiation from the NCEP/NCAR Reanalysis data (Kalnay, 1996). The above forcing data are used to drive an offline version of the model SSiB (Xue et al., 1991) to produce surface meteorological and hydrological variables. The spatial and temporal patters of the SSIB's outputs (such as latent and sensible heat fluxes, surface temperature, and runoff are analyzed. Some of them (such as surface temperature) are compared with observations. The results show important roles played by the vegetation properties and/or the soil moisture in different regions of West Africa. In the studied regions, the relationship between soil moisture and evaporation is systematically higher than the relationship between LAI and latent heat flux. In Sahel, the soil moisture is very dominant. The Sudanian region has the lowest relationship among these regions. The soil moisture hence contributes significantly in all the regions than LAI in the water and energy balances, even if in the west part of Central Africa despite the presence of tall vegetation and more precipitation. A study has been conducted to investigate the characteristics in 3 sub-regions' water and energy balances between the 3 driest and the 3 wettest years. The first one, located in the Sahel region, has a regional response to the inter-annual rainfall variability. The anomaly surface variables, such as evaporation, are highly correlated to precipitation and net radiation, which indicates a dominant regional surface-atmosphere feedback. The second one corresponds of the area along the Guinean coast and the west Central Africa. This large area is controlled by large scale circulation. There is absence in coherent signal between major variables, such as net radiation, precipitation, sensible heat flux, and evaporation. In between these two areas, the Sudanian region show different atmosphere-surface feedback. It can be considered as a transition area, with a mixed signal partly due to the cloud feedback.
A43B-0049 1340h
Oceanic Forcing Of The Interdecadal Trend In Sahel Rainfall
Sahel rainfall underwent a pronounced interdecadal drying trend since the 1960s. An ensemble of runs with AM2, an AGCM recently developed at GFDL, driven by the observed record of SST and sea ice from 1950 to 2003, can successfully capture the interdecadal trend as well as some of the interannual variability of Sahelian rainfall. An idealized experiment with only the trend component of the SST forcing reproduces the pattern of the rainfall trend and the associated circulation structure. Decomposing the SST trend into tropical Atlantic and Indian oceans seems able to differentiate the contributions from respective oceans. The oceanic forcing over the tropical Atlantic is characteristic of an inter-hemispheric dipole with warm SST anomalies to the south and cold to the north, which, in the model, generates anomalous wetness centered at 7 deg N, thus moving the center of the tropical Atlantic ITCZ slightly towards the equator. On the other hand, a warming Indian ocean SST drives a subsidence over most of the sub-Sahara, leading to an overall weakening of the tropical Atlantic-African ITCZ. The tropical circulation and mechanisms associated with these oceanic forcing are also discussed.
A43B-0050 1340h
The Impacts of African Deforestation on the Regional and Global Hydroclimate
Using a global climate model (GCM), we simulate the effects of deforestation of West Africa, looking specifically at the local and remote precipitation changes caused by such a land use change. We observe a strong local effect, with a large reduction in African precipitation during the dry season, and little change during either of the two rainy seasons. The effects of African deforestation extend throughout the Tropics, and also reach into the mid-latitudes. The remote effect is caused by the African geopotential changes due to the alterations in sensible and latent heat fluxes released in the atmosphere at the ground surface as a result of the massive land-cover change.
A43B-0051 1340h
Evolution of the ENSO Signal over the Pacific-Atlantic Domain
Space-time evolution of the band-passed El-Nino Southern Oscillation (ENSO) signal in the tropical Pacific-Atlantic domain is investigated (1979-2003). A Multi-Taper-Method (MTM) complex singular Value Decomposition (SVD) is applied to four variables: sea surface temperature (SST), sea level pressure (SLP), ocean upper-level heat storage (HST), and surface wind (SW). Anomalous evolution for all variables finds the ENSO signal composed of mixed standing modes and propagating modes. The propagating modes are such that from peak ENSO phases in the eastern Pacific Ocean, eastward evolution into the tropical Atlantic is evidenced with an ~20 cm s-1 phase velocity. Four large equatorial Atlantic warm (cold) events occurred during the period, 12 to 18 months after the four ENSO peak phases in the eastern Pacific. The presence of a slow ENSO SST/SLP coupled wave propagating over the domain through the Gulf of Mexico, corroborates results by White and Tourre (2003).
A43B-0052 1340h
Intra-Seasonal Convective Modulations Of The West-African Monsoon System: Active Phases And Pauses
This study comes within the research of the characterization of the intra-seasonal modulations of the West Africa monsoon system during its annual cycle. Indeed, these fluctuations are not yet well understood and documented, particularly with regard to the main periods of installation of the various Western African rainy seasons and the phases of transition separating them. Also very few studies were interested in these onsets. Objective diagnostic analyses based on PCA decomposition, Varimax rotation and low-pass filters (1 month) are performed on the 5-day CMAP rainfall data over West Africa. The northward excursion of the rainbelt can be divided into 4 basic subperiods each composed of an equilibrium period (pause) followed by a rapid increase (active phase) of the system. We suggest easy criteria to automatically detect these events and propose a calendar for the 23 years. The pauses are centered around the 27th-28th March (pause #1), between the 29th April and 2nd May (pause #2), the 11th June (pause #3) and the 24th July (pause # 4) with standard deviations of 7-11 days. The end of the two main pauses (#1 and #3) are respectively associated with the onset of the first Guinean rainy season and the sahelian rainy season. In addition, it appears that these pluviometric modulations are not specific to the west-african monsoon system. Indeed, these can be detected over a larger scale and be linked to deep convective modulations propagating over the whole tropical region. For example, the deep convective anomalies associated with the pause #1 (pause #3) spread eastward (westward) and come from the Eastern Tropical Atlantic. Finally, regarding the daily Outgoing Longwave Radiation NOAA data, it appears that the amplitude of these convective modulations over West Africa is related to the activity of the Madden-Julian oscillation.
A43B-0053 1340h
The first Guinean rainy season and role of the Subtropical Jet of the Northern hemisphere
The results presented come from studies relating to the mechanisms associated with the annual cycle of the InterTropical Convergence Zone (ITCZ) over West Africa. We have looked at the determination of the onset of the first rainy season over the Guinean coast and at understanding the mechanisms which are associated to this onset, with an aim of improving its forecast. By analysing the fluctuations of OLR and rainfall indexes, it is shown that the mean date (called T0) of this onset is the April 10 over the period 1979-2000 . A wavelet analysis highlights that OLR indexes have strong periodic fluctuations, between 25 and 70 days, around the date of onset, in connection with the installation of the first rainy season. By using composite analyses of mean atmospheric circulation at 925 hPa and 300 hPa, following and preceding T0, we point out an interaction between the variability of the convection over the Guinean coast and the undulations of the Subtropical Jet over Northern Africa. Thus, it is suggested that this onset would occur from an interaction between the meridian shift of the ITZC and an eastward propagation of the upper tropospheric trough in the Northern Subtropical Jet over the Maghreb region. The study of the year 1979 suggests interesting potentialities for the forecast of the date of the onset.