2. DATA AND METHODS
2.1 Study area
This research focuses on atmospheric moisture transport over central and northern Colombia. The target region (Fig. 1) corresponds to the largest hydrological system in the country, draining the Andes Mountain chain towards the Caribbean Sea and encompasses two regions with marked orographic differences: the Magdalena-Cauca River basin (Andean), and the Caribbean regions. Due to their location, the oceanic influences in the regional climate are characterized by the transport of moisture from the Tropical and Subtropical Atlantic and Pacific Oceans, and terrestrial sources from the Orinoco and Amazon basins, as well as local recycling (Hoyos, 2017; Hoyos and Rodriguez, 2020).
[Insert Figure 1]
The Magdalena-Cauca River basin system is the major Andean catchment in the country. It is characterized by a complex orography defined by the division of the Andes into three branches, i) the western branch crosses Colombia from south to north with approximately 1200 km in length, parallel to the pacific coast, ii) the central branch between the Cauca and Magdalena River valleys and iii) the eastern branch with an extension of approximately 1500 km and heights up to 4500 meters above sea level, towards the Amazon and Orinoco basins. The Magdalena River flows in a Valley between the Eastern and central branches, and the Cauca flows between the Central and Western branches. Their confluence occurs at the lowlands that mark the transition to the Caribbean region. The interaction between the large-scale circulation systems, the trade winds, and the orographic systems results in differentiated climatic regions over the territory.
The rainiest areas generally occur on the eastern slopes of the eastern cordillera and the western slopes of the western cordillera, while rainfall is lower (and more variable) in the inter-Andean valleys (Snow, 1975; Hoyos, Baquero-Bernal, & Hagemann, 2013). Likewise, the Amazon region exports water vapor to the Andean and inter-Andean zones generating high rainfall intensity, due to the ascent of trade winds and the orographic effect of the Andes (Espinoza et al., 2020). The distribution of precipitation in this region has significant spatial and temporal variability (Poveda, Waylen, & Pulwarty, 2006; Hoyos et al., 2013; Poveda et al., 2014; Espinoza et al., 2020). The most influential seasonal event in the region is the migration of ITCZ that determines the annual distribution of rainfall, characterized by a bimodal regime with two wet seasons between March to May and September to November (Giannini, Kushnir, & Cane, 2000; Alvarez-Villa et al., 2011; Espinoza et al., 2020) and its interaction with the Atlantic and the Pacific Ocean, the moisture contribution from the Caribbean Sea, and the Amazon and Orinoco basins (Hoyos, 2017).
The Caribbean region, located in the northernmost portion of Colombia (and South America), is low and flat in the north (except for the occurrence of the Sierra Nevada de Santa Marta complex, a coastline isolated mountain with a height of 5.7 km that influences local atmospheric circulation in the North), in contrast with the southern part of this zone, which is framed by the foothills of the Andes Mountain range. The annual cycle of rainfall in this zone has a different pattern from the Andes: a dry period from November to April and a rainy period between May to October, explained by the ITCZ migration, the occurrence of synoptic disturbances associated with the Tropical Easterly Waves (TEWs), and the Caribbean Low-Level Jet (Poveda et al., 2006; Arias, Martínez, & Vieira, 2015; Cárdenas, Arias, & Vieira, 2017).