Sea-level Trends along Freshwater and Seawater Mixing in the Uruguayan Rio de la Plata Estuary and Atlantic Ocean Coast

José E. Verocai<sup>1,2</sup>; Gustavo J. Nagy<sup>1</sup>; Mario Bidegain<sup>3</sup>

Sea-level Trends along Freshwater and Seawater Mixing in the Uruguayan Rio de la Plata Estuary and Atlantic Ocean Coast

José E. Verocai^1,2

, Gustavo J. Nagy¹

, Mario Bidegain³

1. Grupo de Cambio Ambiental y Gestión Costero-Marina, IECA, Facultad de Ciencias, Universidad de la República (UdelaR) and Climate Vulnerability, Impacts and Adaptation Network (CliVIA-N), Montevideo, Uruguay
2. Servicio de Oceanografía, Hidrografía y Meteorología de la Armada (SOHMA), Montevideo, Uruguay
3. Instituto Uruguayo de Meteorología (INUMET) and CliVIA-N, Montevideo, Uruguay

Author

Correspondence author
International Journal of Marine Science, 2016, Vol. 6, No. 7 doi: 10.5376/ijms.2016.06.0007
Received: 14 Dec., 2015 Accepted: 19 Feb., 2016 Published: 23 Feb., 2016

This is an open access article published under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Preferred citation for this article:

Verocai J.E., Nagy G.J., and Bidegain M., 2016, Sea-level trends along freshwater and seawater mixing in the Uruguayan Rio de la Plata estuary and Atlantic Ocean coast, International Journal of Marine Science, 6(7): 1-18 (doi: 10.5376/ijms.2016.06.0007)

Abstract

Sea level is rising worldwide with local differences due to global and regional drivers. This article analyses yearly freshwater and sea level trends and fluctuations during the mixing of fresh- and sea-water along the Uruguayan coast of the Rio de la Plata River estuary and the Atlantic coast from 1961 to 2014. The global and regional drivers as well as local co-variables are described, classified in nine discrete classes and inter-correlated. Despite the observed increasing trends, local sea level rises (SLR) are not well correlated with global SLR except at the estuarine-ocean boundary (Punta del Este station). Freshwater inflow, which variability often coincides with Oceanic El Niño-La Niña (ONI-ENSO) events, is the first descriptor of sea level fluctuations and outliers all along the coast, particularly at Punta del Este. Local SLR roughly follows the overall global trend with periods of acceleration and stabilization often coinciding with ENSO events.

Keywords

Global sea level rise; ENSO; Global and local drivers; Estuaries

1 Introduction

Sea level rise (SLR) is changing the dynamics at play along coasts. The dynamics at play includes (UCS, 2013):

· Amplified wind-storm surge. With rising seas, storm surge occurs on top of an elevated water level (UCS, 2013).

· More intensive shoreline erosion, degradation and coastal destabilization. SLR increases the potential for erosion by allowing waves to penetrate further inland, even during calm weather (Bruun, 1962; Shepard et al., 1962; Zhang et al., 2004; Syvitski et al., 2005).

· Permanent inundation of low-lying coastal lands (Cooper et al., 2008).

Because of tidal and wind-driven changes, sea level is constantly fluctuating. Therefore, it is important to calculate the mean sea level (MSL), which is the average sea level at a given location over several years (Douglas, 2001). According to Chao et al., (2002) sea level change occurs on all time-scales, and with a continuous range of spatial scales, from local (e.g., wind storm surge), to regional (e.g., El Niño Southern Oscillation (ENSO)), and global (eustatic). On decadal to multi centennial time scales sea level fluctuations are mainly driven by climate change in response to natural forcing factors (e.g. solar radiation variations, volcanic eruptions) and to internal variability of the climate system (related for example to atmosphere–ocean perturbations such as El Nino-Southern Oscillation – ENSO, North Atlantic Oscillation – NAO, Pacific Decadal Oscillation – PDO) (Meyssignac and Cazenave, 2012).

As SLR accelerates, it will become increasingly necessary and useful to distinguish coastal “flooding” from “inundation” (Flick et al., 2012). The Rio de la Plata (RdlP) river estuary (Argentina-Uruguay) is the most exposed region of Latin America to coastal inundation due to SLR and storm surges, mainly due to Southeastern winds (Volonté and Nicholls, 1995; Barros et al., 2005; Nagy et al., 2005; Magrin et al., 2007, 2014; Nagy et al., 2007; ECLAC, 2011; Losada et al., 2013; Nagy et al., 2015). Along the Uruguayan coast of the RdlP the impacts of episodic flooding related to storm surges are greater than the permanent inundation related to SLR from 1983 to 2013 (Verocai et al., 2015).

Most estuaries have a series of landscape subcomponents: i) a fresh water source, ii) a tidal-estuarine segment, and iii) a pass to the sea. The interaction of three primary natural forces causes estuaries to be unique and different (Montagna et al., 2013):

· Climate: causing variability in the freshwater runoff, which is fundamental to the functioning of estuaries.

· Continental geology and geomorphology: causing variability in elevation, drainage patterns, landscapes, and seascapes (Measurements of isostatic land uplift or downlift are not available yet and is assumed not to be relevant in coastal Uruguay).

· Tidal regime: causing differences in the degree of mixing and elevation of the mixing zone.

This study review the background and investigates sea level (SL) trend patterns and fluctuations at four stations along the freshwater and ocean mixing along the Uruguayan coasts of RdlP river estuary (Argentina-Uruguay) and the Atlantic Ocean over the last 50 to 60 years. Emphasis is put on the relationships of SL with global and regional drivers of trends, fluctuations and extremes, including a few local co-variables from 1961-2014.

2 Study Area and Background

2.1 Geographical setting

The Uruguayan coast is 670 km in length with 450 km lying within the RdlP estuary and the remaining 220 km on the Atlantic Ocean (Figure 1).

Figure 1 Rio de la Plata basin and river estuary, Southeastern South America. The four tide gauges are shown (red circles), and weather stations Carrasco and Laguna del Sauce airports (violet circles). The turbidity satellite image shows the divide of tidal fresh turbid water and estuarine marine water for a very low La Niña-linked river inflow. Modified from Nagy et al. (2014a).

The RdlP is a large (38 x 10³ km²; 40-30 km wide) river-influenced tidal river and primary estuary (Perillo, 1995a,b), defined by López Laborde and Nagy (1999) as “a funnel-shaped coastal plain tidal river with a semi enclosed shelf area at the mouth and a river paleovalley at the northern coast that favors river discharge and sediment transport to the adjacent continental shelf” (Figure 2).

Figure 2 Río de la Plata bathymetry and morphological units. The most relevant for the hydology and water/sea level are the natural channels along the northern coast (Sistema Fluvial Norte, Canal Norte and Canal Oriental) which canalize fresh- and estuarine discharge, and. the “estuarine delta” (Barra del Indio) which roughly represent the upward limit of salt intrusion. Source: López Laborde and Nagy (1999)

The system may be divided into four regions (Nagy et al., 2002): i) Tidal River (1 to 6 m depth), ii) Estuarine Front (6 to 12 m depth), iii) Marine region (12 to 20 m depth), and iv) "Canal Oriental" (Paleovalley, 7 to 25 m depth), which behaves as an effective highly stratified mass transporting channel to the coastal ocean thus influencing both salinity and water-levels along the Uruguayan coast (Nagy et al., 2003; 2008a; Lappo et al., 2005).

2.2 Hydrological and climatic background

The main system’s forcings are the big tributary discharges (namely Parana river and Uruguay river), the weak tidal currents (amplitude < 0.5 m) and front that propagates from the sea, and the action of winds (Balay, 1961; Guerrero et al., 1997; Simionato et al., 2007; Nagy et al., 2008a; Meccia et al., 2009).

The tide wave comes from the Atlantic Ocean, being deformed within the estuary due to the shape, banks, channels, depth, and Coriolis deflection towards Montevideo, where the cross-sectional area sharply decreases (Figure 3). Therefore, isoamplitudes increase upstream the transverse sections 24-20 (estuarine front). This facilitates the occurrence of “storm surges”, that is to say the positive anomaly between the astronomic tide and the observed water level due to residual effects of winds, waves, sea level pressure (SLP), and freshwater inflow (Balay, 1961; Nagy et al., 1997; López Laborde and Nagy, 1999; Luz Clara, 2014; Verocai et al., 2015).

Figure 3 Southeastern width and average depth increase of Rio de la Plata river estuary. Source: López Laborde and Nagy (1999). The tidal freshwater extends from sections 1 to 18 or 22. The Atlantic coast extends from section 32 northeastward.

The fluctuations of freshwater inflow and axial offshore and onshore winds influence those of sea-level (Balay, 1961; Verocai et al., 2015). For this reason, the observed increase of river flow (García and Vargas, 1998; Nagy et al., 2002; 2014b) and Southeastern (SE) and East-Southeastern (ESE) winds over the last few decades (Escobar et al., 2004), which are the main causes of storm surges in the RdlP (Balay, 1961; D’Onofrio et al., 2008; Verocai et al., 2015), is changing the balance of the physical forcings on the system.

Historically, as a consequence of the permanent South American anticyclone (High Pressure Belt) over southeastern South America (SESA), the predominant wind direction has been from Northern quadrants (N to NE) (Balay, 1961; Nagy et al., 1997). An increase of S, SE and ESE wind directions was reported over the RdlP region since the 1960s (Escobar et al., 2004; Simionato et al., 2005; Nagy et al., 2008a, b; Meccia et al., 2009; Ortega et al., 2013; Pescio, 2015) and SE wind became the predominant one in Montevideo since 1998 (Nagy et al., 2014 b; Verocai et al., 2015). A relationship between stronger E to SE wind waves at the estuarine front was reported by Panario et al., (2008), whereas an increase of ESE winds was found during moderate to strong El Niño years (Gutierrez et al., 2016).

3 Materials and Methods

3.1 Global sea level and Uruguayan tide gauge stations

Global sea level from 1961 to 2014 was taken from EPA (EPA, 2014) and CSIRO (CSIRO, 2016). Yearly averages from four tide gauge stations (Figure 1) were analysed over the last 50-60 years: Colonia (Station 1), Montevideo (Station 2, for which a time-series from 1902 to 2014 is presented), Punta del Este (Station 3) and La Paloma (Station 4) which are part of the global network of tide stations “Permanent Service for Mean Sea Level” (PSMSL), National Oceanography Centre (NOC, UK) (http://www.psmsl.org/about_us/overview/). Station 2 Montevideo ("Punta Lobos") is an international GLOSS program station (number 300) of the Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM) of the World Meteorological Organisation (WMO) and the Intergovernmental Oceanographic Commission (IOC), (http://www.gloss-sealevel.org/). The four stations meet standard equipment, measurement, storage and data communication, and quality control requirements, including the correction for the effect of local sea level pressure (SLP).

Station 1: Colonia from 1954 to 2014, with a few gaps (N = 51), located at the tidal river close to the mouth of the main tributary discharges. Data was available from the Navy Oceanographic, Meteorological and Hydrographic Bureau (SOHMA), and from Ministry of Public Works and Transports (MTOP).

International Journal of Marine Science

• Volume 6

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