Global mean sea level rise is one of the most important consequences of the ongoing global warming. Satellite altimetry has revealed a linear increase of 3.1 mm/year since 1993. This global rise is explained by both global ocean warming and freshwater incomes from continental ice melt (mountain glaciers melting and ice-sheets mass loss from Greenland and Antarctica) and land water storage (rivers, lakes, aquifers) [1
]. At regional scale, sea level trends present large variations around its global average. In addition to processes explaining the global mean sea level rise, this regional variability is a consequence of temperature and salinity variations, ocean circulation and heat and freshwater air-sea fluxes [2
]. If the evolution of the ocean heat content and its contribution to sea level change has been largely investigated for years, the evolution of the salt content and its impact on sea level variations has been poorly considered, mainly because of the lack of in situ salinity data. Since the 2000s, the Argo program provided an unprecedented amount of salinity measurements revealing the importance of salinity contribution to basin scale and regional sea level changes [3
River discharge is one of the causes of regional salinity variability along with precipitation, evaporation and ocean circulation. River discharge can influence sea level through two dynamic responses: First, through the mass input propagation over the global ocean by fast barotropic waves [5
]; secondly, through regional density changes caused by modulation of the salt content by the river. This contribution is known as halosteric sea level changes. The discharge results in a low-salinity plume that remains at the ocean surface because of its low density and contributes locally to sea level [6
]. The modification of the upper ocean heat content by the plume [7
] could also lead to a thermosteric contribution to the regional sea level.
The total world river discharge impacts global sea level through the first process described above (global mass propagation). Its variability is directly linked to world water storage variability. Over 1948–2000, global water storage contribution to global mean sea level change has been investigated with ORCHIDEE Land Surface Model outputs [8
]. No significant multi-decadal trend has been detected but a strong interannual variability. The interannual variability of the global water storage explains between −4 to 2 mm in equivalent global mean sea level. This study shows that the interannual variability of global water storage is driven by precipitation principally in the tropics and especially over the south America and northern tropical Africa basins.
Global continental water storage has also been investigated by remote sensing data. Gravity Recovery And Climate Experiment (GRACE) data have been considered over a 7-year time period (i.e., 2002–2009) to investigate global continental water storage from the 33 world’s major river basins. GRACE data confirm the significant contribution of global land water variations to the interannual variability of global mean sea level [9
]. This freshwater transfer between the oceans and the continents at interannual time scales is partly due to El Nino Southern Oscillation climate variability.
The contribution of continental waters, based on GRACE data, to global mean sea level trend reaches −0.22 ± 0.05 mm/year for the period 2002–2009 [9
] and is largely explained by an evolution of the tropical rivers outflows [10
]. However, another study founds a positive contribution of +0.45 ± 0.16 mm/year for the period 1992–2013 [11
]. While these different studies highlight the importance of tropical continental river basins, they also shed light on the large trend uncertainties of global continental water storage contribution to the global mean sea level budget [1
Also, a few studies investigated the regional impact of rivers on sea level, focusing mainly on the coastal impact. Based on tide gauge data, Meade and Emery [12
] observed that river discharge explains between 20% and 31% of detrended interannual sea level variance during 1930–1970 along the United States East and Gulf Coasts. Gough and Robinson [13
] suggest that discharge from the Churchill River explains 43% of the monthly sea level variance in the Churchill tide gauge records in Hudson Bay (Canada) from 1974 to 1994. Piecuch et al. [14
] studied river-discharge effects on United States Atlantic and Gulf coast sea-level changes. They found that sea level rises between 0.01 and 0.08 cm for a 1 km3
annual river-discharge increase, depending on the region. Other studies found no correlation between river discharge and sea level variation, such as Blaha [15
] for nonseasonal monthly records in the Chesapeake Bay (from tide gauge) or Han and Webster [16
] for interannual sea level variability in the Bay of Bengal (based on simulations). Investigations on the importance of river discharges to coastal sea level variations have been made from hours to seasonal and interannual time scales [17
]. Less attention has been brought on open ocean variations caused by river runoffs.
In this study, we will focus on the regional contribution of the Amazon River to regional sea level. The Amazon basin is the largest river in term of volume of freshwater in the world, discharging a mean value of 209,000 m
to the ocean [18
], corresponding to 17% of the total world river discharge [19
]. Its runoff is almost three times greater during the rainy season (May-June) than during the dry season (December-January) and the standard deviation of its seasonal and interannual time series reach 70,000 m
and 25,000 m
, respectively (standard deviation of monthly time series obtained from the hydrological reanalysis ISBA-CTRIP ) [20
]. The Amazon river discharges into the Atlantic ocean at 0° N, forming a plume of freshwater whose propagation in the Tropical Atlantic varies seasonally. During boreal winter and spring the plume is transported by the North Brazilian Current (NBC) toward the Caribbean Sea. The remainder of the year, the NBC connects with the North Equatorial Countercurrent (NECC) through a retroflection zone between 5° N and 10° N, transporting the Amazon plume to the east [21
]. Durand et al. [6
] suggest that the impact of the Amazon River discharge on sea level is potentially important at global and regional scales. The authors converted the Amazon discharge into units of equivalent GMSL and show that the standard deviation of its interannual variability is 0.4 mm. Such value is of the same order of magnitude as the interannual standard deviation of 1.0 mm of the GMSL. This confirms the significant contribution of continental water storage and more specifically of tropical rivers basins to interannual variability of global mean sea level.
The impact of river runoff on coastal sea level has received more attention than the impact on open ocean sea level. In the present study, we will focus our investigations on both coastal and open ocean of sea level change to the Amazon freshwater runoff. The main objectives of this analysis are (1) to quantify the Amazon discharge contribution to the mean-state of sea level; (2) to disentangle the processes at play by analyzing changes of temperature and salinity conditions and identifying where and when these changes are generated; (3) to evaluate the impact of the Amazon discharge and variability on regional sea level variability.
This study highlights large river discharges affecting sea level not only at the coasts but also for the open ocean. We find that the sea level influence of the Amazon river is noticeable 3000 km away from the river mouth. For this reason, attention has been given to both remote and coastal sea level changes. We quantified the Amazon contribution to the mean-state of sea level and we shed light on the mechanisms associated with this contribution. We also investigated sea level variability through sensitivity simulations with constant, seasonally varying and inter-annually varying Amazon runoff. Main results are summarized below.
The Mean Dynamic Topography (MDT) is up to 11 cm higher at the Amazon mouth with Amazon runoff and 3.3 cm higher around the Caribbean Archipelago.
The MDT anomaly is mostly explained by the halosteric response to freshwater input coming from the Amazon river discharge, located in the upper 250 m of the ocean, while the thermosteric response is weak and tends to counterbalance this effect.
Regional mass redistributions are observed with a decrease at the river mouth equivalent to 8 cm of mean sea level and increases on continental shelves of the Gulf of Mexico and Caribbean Sea equivalent to 4.5 cm of mean sea level.
The Amazon low-salinity plume is mixed with subsurface waters in two remote areas: The first one at (55° W, 10° N); and the second one near (60° W, 23° N). This results in a time-mean salinity anomaly of 0.5 PSU located under 30 m depth over a large region encompassing the Caribbean Archipelago. This subsurface anomaly partly explains the regional sea level anomaly.
The Amazon discharge contributes to 23% and 12% of the seasonal and interannual sea level variances in a large area around the Caribbean Archipelago. We showed that the Amazon mean discharge largely explains this sea level variability increase. This suggests that most of the sea level variability associated with the Amazon runoff is the result of the variability of the river plume in response to seasonal and interannual variability of the upper ocean regional circulation. The Amazon seasonal cycle introduces a 6% and 2% additional sea level seasonal and interannual variances, compared to the case with constant Amazon. Finally, interannual variations of the Amazon discharge do not clearly impact sea level variability.
This study highlights the importance of the Amazon river discharge to regional sea level variations in the tropical Atlantic Ocean. Radical changes in river discharge variability and trends are expected due to anthropogenic climate change and modification of the Amazon basin water cycle. Projections indicate that seasonal variability of precipitations in the Amazonia will increase and that extreme weather events like droughts and floods will become more frequent and severe, resulting in an increase in discharge variability and so to a probable increase in sea level variability [2
]. Also, more than a hundred hydropower dams have already been built over the Amazon tributaries [38
] and more than 200 are planned for the next decades [39
]. Dams can reduce river discharge as water evaporates in reservoirs or is diverted for irrigation and it decreases seasonal flow variability, mostly by attenuating flood maxima [40
]. Therefore, it is difficult to know whether or not river discharge variability will increase. We leave for future investigations the potential anthropogenic contribution of river discharge to regional sea level changes.