Climate Change and Dynamics of the ITCZ on the Hydro-Climatic Regime of the Northern Ecosystems of the Gulf of Guinea: A Case of the Grand-Lahou Lagoon System

Atmospheric dynamics and climatology in West Africa are strongly dominated by the latitudinal migration of the ITCZ which imposes directly or indirectly determinism on coastal ecosystems. Thus, the Grand-Lahou lagoon system and its watershed are influenced by the ITCZ, whose seasonal study made it possible to understand its mode of action first on climatic factors, then hydroclimatic, and finally hydrological in the context of global changes. The study of these factors showed a differentiated impact of its migration on the hydrological regime defining a new configuration of the lagoon system and beyond; the coastal ecosystems of the northern coast of the Gulf of Guinea irri-gated by numerous fluvial inputs. Salinity, a hydrological parameter of critical ecosystem importance, combined with transparency and depth, has enabled the spatio-temporal description of hydrology. To do this, a series of monthly mea-surements (in-situ) for spatial coverage of 25 stations was tested. Long before that, the climatic and hydroclimatic parameters were obtained respectively at SODEXAM, the meteorological site earth.nullschool.net and ONADE. This study brought together the dynamics of the ITCZ and the hydrological system of the lagoon.


Introduction
Resulting from the convection of the high pressures of the Saint Helena High Pressure (south Atlantic monsoon) and the harmattan (north-east trade winds), the ITCZ is the fundamental key to climate variability in the intertropical belt and particularly in West Africa [1] [2]. According to [1] [2] [3], it is responsible for rainfall in Ivory Coast with two seasons in the coastal zone including a large (54% of annual precipitations) during its rise between May and July (GRS) and a small (16% of annual precipitation) during its return between October and November/December (SRS). It is characterized in the northernmost part of the country by a single season between May and November [4]. According to [5], the climatology of the Gulf of Guinea and the West African coast is now influenced by global warming. In the same vein, [6] cited by [7], show that the interannual variability of precipitation (1500 to 2000 mm/year) between 1948-1997 in the coastline shows a decreasing trend associated with a temporal shift in the maximum rainfall. Indeed, since the 1970s, there has been a decrease in GRS rainfall compared to an increase in SRS. Confirming the above, [8] shows that the rise in nearshore temperature would be notable with the thermal increase in the tropical ocean at 0.5˚C while [9], relates the changing seasons to rising temperatures. In Grand-Lahou, the years 2009, 2011, 2014, and 2016 recorded exceptional rainfall events in the watershed, causing river floods [10]. Annual maximums are observed in June (29.13 m 3 •s −1 ) and October (15.93 m 3 •s −1 ), respectively. The hydrological regime functions as a system characterized upstream by continental inputs and downstream by ocean inputs. In addition, the ocean is manifested in lagoons by the bi-daily tidal regime (of about 12 h 25 mn) with diurnal inequality with a range less than 1.5 m [11]. The tide, therefore, imposes a semi-diurnal rhythm on a number of parameters such as the direction and speed of currents, temperature, and salinity [11] [12]. In addition to these semi-diurnal variations, there are semi-synodic variations defined by the alternation of live and still-water [11]. The watershed of the lagoon system, with an area of 103,000 km 2 , including 97,500 km 2 for Bandama located in the ITCZ swing zone, is characterized by three different climatic regimes [13]. A two-season tropical transition regime, an attenuated equatorial transition regime in the central part and an equatorial transition regime in the southern (coastal) zone with four seasons [11]. According to [12] [13] [14], Bandama accounts for about 60% of the total annual volume during the flood period (September-October) with an average flow of 1645 m 3 •s −1 for a contribution of 54.90% of inputs compared to 25 m 3 •s −1 in January and 45,10% of intakes during the stretch period. The Boubo, with an area of 5100 km 2 ; the Gô and the Krokoma River represent the forest component of the watershed [11] [12]. With an annual contribution of 6%, the Boubo regime is characterized by the duplication of the annual flood corresponding to the seasonal distribution of precipitation on the coast. The GRS contributes about 47.56% of the year-to-date for an average flow of 21.21 m 3 •s −1 , therefore the highest. The smaller SRS (between September and November) recorded around 16 m 3 •s −1 with a total of 35.73% elapsed [12]. The Grand Lahou Lagoon (Figure 1), located between latitudes 5˚05' -5˚15' and longitudes 4˚55' -5˚25', with a length Atmospheric and Climate Sciences  [15], has benefited from numerous studies [16]- [23]. However, the dynamics of the ITCZ, fundamental in the climate of the  [24] at longitude 28˚W. In this study, the choice was for the 10˚W meridian at the weekly scale where its variability is almost zero.
The Bandama and Boubo rivers debits come from ONADE (Office Nationale de l'Assainissement et du Drainage). These data were recorded with hourly resolution throughout the study period at the hydrological stations of Tiassalé and Babocon respectively. Transparency and salinity were observed over the duration of the campaign by a 25-station spatial coverage using a Secchi disk and a Hach HQ-40 multiparameter calibrated monthly prior to measurements. Specifically, salinity was measured at the surface; 0.50 meter below the air-water interface and 0.20 meter above the water-sediment interface. The depth was measured using an echo sounder. The use of a Garmin GPSMAP-78S GPS enabled the georeferencing of the stations throughout the study period.

Statistical Methods
The excel software allowed the statistical processing of the collected data. The  Figure 2 illustrates the latitudinal evolution of the ITCZ in comparison with climatic parameters on the one hand and hydroclimatic parameters on the other.

Climate Variability
In Figure 2

Hydroclimatic and Hydrological Variability
Flows of Bandama and Boubo (Figure 3

Discussion
Separating two different climate systems, the ITCZ determines by its annual migration, atmospheric dynamics, climatic and hydroclimatic characteristics in the watershed of the Grand-Lahou lagoon system as described by [3] [12] [26] This Thus, the thermal field (ocean, air and lagoon), atmospheric pressure, relative humidity and air velocity; hydroclimatic factors such as rainfall and river flows are strongly influenced by ITCZ dynamics. It therefore gives its seasonal signal on the regime of the rivers that make up the watershed [12].

Conclusion
The intertropical convergence zone is the fundamental key to the climate system in West Africa and more particularly the watershed of the Grand-Lahou lagoon. the key issue for the management of increasingly vulnerable coastal ecosystems.