Atmospheric moisture transportthe bridge between ocean evaporation and hydrological extremes in major tropical river basins

Resumo

This work aimed to investigate the role of the atmospheric branch of the hydrological cycle in the source-sink of moisture relationships for major tropical river basins, namely, the Congo and Niger in Africa, the Negro and Madeira in the Amazon region in South America, and the Indus, Ganges, and Brahmaputra in Southeast Asia. Besides their location along the tropical region, the atmospheric circulation controlling the climate in these basins is characterised by seasonal wind reversals due to asymmetric heating between the land and oceans causing a monsoonal regime that results in the heavy rainy season with respect to other tropical river basins. The transport of moisture in the atmosphere is considered the bridge between the ocean and land evaporation and later the precipitation. For modelling, the Lagrangian particle dispersion model FLEXPART v9.0 was used with data from the reanalysis product ERA-Interim of the European Centre for Medium-range Weather Forecast (ECMWF). FLEXPART allow us to track backward in time atmospheric air masses over the basins while calculating changes in the specific humidity through the budget of evaporation minus precipitation (E – P). This permitted the identification of the regions where the air masses uptake humidity before arriving at the basins. The spatial pattern of the (E – P) provides the transport distance, but also the most important moisture sources according to the intensity of the (E – P) > 0 values. Once the sources were identified, the air masses over them were tracked forward in time to finally compute moisture loses over the target basin ((E – P) < 0) that were considered to contribute to the precipitation. The results indicated that moisture uptake principally occurs in the basins themselves, surrounding continental regions, and the oceanic regions in the Atlantic for the basins in the Amazon, in the Atlantic and the Indian Ocean for the African basins, and in the Indian Ocean for Asian River Basins. The Vertically Integrated Moisture Flux (VIMF) supports these results. The moisture contribution from the sources and basins themselves revealed that the contribution is generally linked to the transport distance and supports documented information regarding the important role of moisture recycling in tropical-equatorial regions, and specifically in the Congo and Southeast Asia River Basins. The results also revealed the conclusive characteristic scales of the atmospheric moisture transport for the South American, West African, and Indian Summer Monsoons. Dry and wet conditions in the basins were identified through the Standardised Precipitation-Evapotranspiration Index (SPEI). We selected a few years affected by severe and extremely dry and/or wet conditions to investigate the moisture contribution from the sources. The VIMF divergence anomalies were calculated as complementary values to assess the dynamic conditions in the atmosphere. In the Congo River Basin (CRB), the hydrological drought conditions were quantified at the Kinshasa gauging station using the Standardised Streamflow Index (SSI) (Vicente-Serrano et al., 2012). Here was analised the impact of meteorological drought on the hydrological regime. The role of the sources that provided moisture during years with extreme and severe conditions confirmed the key role of the Congo River Basin in modulating the water balance within itself. The results showed that during wet (dry) years, the contribution of moisture ((E−P)i10 < 0) from the CRB to precipitation over itself increased (decreased). On average, the water balance in the atmosphere over the CRB was not homogenous during these years, indicating a distinct role within itself. This result confirmed that research on the hydrological cycle should not be conducted for the entire basin as a whole. In the Niger River Basin (NRB), seasons under severe and extremely dry conditions were randomly affected by the contribution of humidity from the South Atlantic Ocean, which is the most important oceanic source. The position of the Intertropical Convergence Zone and its influence on the residence time of the water vapour in the atmosphere may be responsible for the unequal response of the eastern South Atlantic Ocean for these extreme cases. Anomalies in the VIMF and the Outgoing Longwave Radiation (OLR) for each case support the previous results. The roles of the sources in the moisture contribution to precipitation during severe and extremely dry and wet conditions in the Indus, Ganges, and Brahmaputra River Basins were assessed through the Winter Precipitation Regime (WPR) (November–April) and Monsoonal Precipitation Regime (MPR) (May–October) composites. This confirmed the crucial role of the most important moisture sources (e.g., Indian region (IR); Indian Ocean (IO); Bay of Bengal (BB), and the basins themselves) in providing less (more) humidity during dry (wet) conditions in both the WPR and MPR periods. The IR plays a crucial role on the Monsoon onset, after which the moisture contribution from continental moisture sources commence to play a fundamental role. Dry and wet conditions within the Amazon River Basin did not typically occur simultaneously during the study period. However, the Negro River Basin (NeRB) and the Madeira River Basin (MRB) were simultaneously affected by intense dry conditions in 2015–2016. Here was analised the impact of meteorological drought on the water level of principal rivers. Throughout the five most severe dry episodes in the NeRB, the anomalies in the contribution from the Tropical North Atlantic Ocean (TNA) principally, and the Tropical South Atlantic Ocean (TSA), seemed to be associated with the SPEI temporal evolution. This was also observed in the MRB, where both oceanic and terrestrial sources played an important role. On average, the episodes were associated with a reduction in atmospheric moisture contribution from the sources, and subsidence based on predominantly positive VIMF divergence anomalies over the basins. The moisture contribution from the oceanic sources modulates the rainy season onset and demise. In summary, this study investigated the role of atmospheric moisture transport, as the bridge between evaporation in the oceans and land and finally the precipitation and subsequent steps of the hydrological cycle at the main tropical river basins affected by a monsoonal regime. The Lagrangian approach permitted the establishment of the source-sink of atmospheric moisture relationship. We considered this approach useful to better understand the hydrological cycle, and essentially to diagnose the causes of droughts and floods. Further results and deep explanations for each river basin are in the manuscripts. However, further research must be done to investigate the influences of different modes of climate variability, and the role of the basins themselves as sources of moisture for the surrounding continental regions. Further, new questions arose with respect to the impact of the residence time of the water vapour in the atmosphere along the trajectories of the air parcels and its impact on the quantification of moisture loss over the basins.