The role of Amazon river runoff on the multidecadal variability of the Atlantic ITCZ

Abstract

Climate model projections for the 21st century predict an elongated dry season in the Amazon basin, potentially reducing the discharge into the equatorial Atlantic Ocean. In order to understand the climatic role of Amazon runoff into the ocean, sensitivity experiments were carried out using the Community Earth System Model (CESM). Without Amazon runoff, the Atlantic Meridional Overturning Circulation (AMOC) strengthens and the associated increase in northward heat transport induces a positive temperature anomaly in the North Atlantic Ocean with a spatial structure similar to the positive phase of the Atlantic Multidecadal Variability (AMV). A positive phase of AMV developed in the absence of Amazon runoff triggers a bipolar seesaw in SST across the thermal equator with warming to the north and cooling to the south. The boreal summer rainfall in the tropical Atlantic Ocean sector responds to this change in SST by displacing the intertropical convergence zone (ITCZ) to the north of its mean position. An alternate experiment by doubling the Amazon runoff shows a weakening of AMOC and AMV and a southward shift in the summer-time ITCZ. In both the experiments, we find that the largest change in rainfall is exhibited over the region where the AMV-induced decadal variability in rainfall is prominent, confirming the source of rainfall variability. Based on sensitivity experiments by varying the runoff, we propose that the Amazon discharge can affect the multidecadal variabilities, the AMOC and AMV, and thereby the low-frequency variability of rainfall over the tropical North Atlantic Ocean and northwest Africa. We conclude that the freshwater input from the Amazon plays a significant role in the sustained wet and dry climatic phases of rainfall events over the tropical North Atlantic Ocean and West African nations and thus have an impact on the regional hydrological cycle and economy.

Influence of the Amazon‐Orinoco Discharge Interannual Variability on the Western Tropical Atlantic Salinity and Temperature

“…The impact of the Amazon runoff on the tropical Atlantic Ocean has been largely studied. A common way of doing so is by using ocean circulation models in which the Amazon runoff is alternatively included or removed (Coles et al., 2013; Hernandez et al., 2016; Huang & Mehta, 2010; Jahfer et al., 2017, 2020; Masson & Delecluse, 2001; Newinger & Toumi, 2015; Varona et al., 2019). The other usual way is to analyze satellite and in‐situ observations (Ffield, 2007; Fournier et al., 2017; Pailler et al., 1999; Zeng et al., 2008).…”

Amplified seasonal cycle in hydroclimate over the Amazon river basin and its plume region

“…1a , see “Methods” for the box definition), the enhanced near-surface heat storage in the APR associated with the barrier layer dynamics provides favorable surface conditions for hurricane genesis over the broad regions of the tropical western Atlantic 8 , 15 . Moreover, the variability of Amazon freshwater and resultant ocean salinity changes have been suggested to affect tropical Atlantic air–sea interactions 17 , 18 and the variability of the Atlantic intertropical convergence zone (ITCZ) 19 , regional sea-level height changes 20 – 22 , as well as to exert potentially far-reaching impacts on the Atlantic meridional overturning circulation (AMOC) 23 . Thus, understanding the mechanisms for changes in the Amazon river discharge and the associated upper ocean stratification in the APR is important not only for the Atlantic hurricane forecasts but also for improved understanding of basin-scale climate variability.…”

Climatology of Air Temperature in Belterra: Thermal Regulation Ecosystem Services Provided by the Tapajós National Forest in the Amazon

“…Moraes et al (2020) related that the rainy season begins during the monthly quarter from December to February (DJF) in the Amazon. This quarter is modulated by the Intertropical Convergence Zone (ITCZ) and is associated with the effects of the South Atlantic Convergence Zone (SACZ) and the Bolivian High (Jahfer et al, 2020). The ITCZ acts to maintain or reduce the large belt of humidity in the region (Loureiro et al, 2014, Drumond et al, 2014, and beginning in the month of August there is a reduction in rainfall, including in areas that are prone to water stress (Martorano et al, 2017), indicating a reduction in soil water stocks.…”