Research papers Po River plume pattern variability investigated from model data Francesco Marcello Falcieri a,n , Alvise Benetazzo a , Mauro Sclavo a , Aniello Russo a,b , Sandro Carniel a a Consiglio Nazionale delle Ricerche – Istituto di Scienze Marine (CNR – ISMAR), Arsenale-Tesa 104, Castello 2737/F, 30122 Venezia, Italy b Università Politecnica delle Marche – Dipartimento di Scienze della Vita e dell’Ambiente (UNIVPM-DISVA), Via Brecce Bianche, 60131 Ancona, Italy article info Article history: Received 1 May 2013 Received in revised form 7 October 2013 Accepted 5 November 2013 Available online 13 November 2013 Keywords: Coastal plume Po River Northern Adriatic Sea Self-Organizing Maps Numerical modeling abstract The Adriatic Sea is a semi-enclosed basin located in the NE part of the Mediterranean Sea and it is strongly influenced by riverine inputs. In its northern sub-basin the freshwater plume of the Po River, together with those of numerous smaller rivers, plays a fundamental role in driving the physical and biogeochemical processes of the whole basin. In this paper we characterize the surface plume structure and identify its patterns and temporal variability on seasonal and inter-annual scales relating it to its major forcings (i.e., river discharges and winds). To perform this analysis, a 3D hydrodynamic numerical model was implemented over the whole Adriatic for the period 2003–2010 and the resulting outputs were analyzed through a series of statistical tools. The inter-annual and seasonal averages of Sea Surface Salinity (SSS) fields show that the average patterns are composed of a coastal plume, wider or narrower depending on the season, that flows southward of the Po River mouths. The first two modes of the Empirical Orthogonal Functions (EOF) analysis show a similar distribution with a cumulative explained variance up to 60%; the third mode, instead, presents a plume shape that extends well into the basin. To obtain a more detailed representation of the plume, a 2 Â 3 Self-Organizing Map (SOM) analysis was performed over the surface salinity fields. Two antithetic patterns were depicted: (i) a small plume confined to coastal areas, typical of low discharges and/or Bora wind events and (ii) a wider plume that extends into the basin, typical of high river discharges and/or Sirocco winds. The comparison between wind regimes, riverine inputs and the time series of the SOM's Best Matching Units (BMU) suggested that, on long time scales, river discharges represent the dominant forcing in defining the plume size and surface pattern, while on time scales of few days the plume dynamics are modulated mostly by the wind structure. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Riverine discharges and the freshwater plume that they pro- duce on the continental shelf are important drivers of the coastal dynamics and processes both from a physical and biological point of view. The fluvial plume structure depends on many different factors, some of them are here briefly recalled: outflow angle (Garvine, 1999); Coriolis force (Chao and Boicourt, 1986); ambient background currents and water column structure (Fong and Geyer, 2002; Hickey et al., 2005); bathymetry and seabed morphology (Chao, 1988a, b); tidal forcing for a modulation effect (Guo and Valle-levinson, 2007; MacCready et al., 2009) and local winds (Choi and Wilkin, 2007; Liu et al., 2009). The latter have been generally identified as the leading forcings both on short (Chao, 1988b; Otero et al., 2008; Marques et al., 2009) and longer (Chao, 1988b; Fong and Geyer, 2001; Xia et al., 2007) time scales, due to the narrowing/expanding of the plume offshore extent depending on the downwelling/upwelling direction of the resulting currents. Generally speaking, freshwater that reaches an ocean basin forms a buoyant water mass with low density which, in an idealized condition with negligible external forcing, tends to develop in a recirculating bulge structure located in the proximity of the river mouth (Fong and Geyer, 2001). This formation tends to expand until the occurrence of an external forcing changes its dynamics (Fong and Geyer, 2001). In a more realistic scenario, i.e., in presence of external forces, the bulge usually turns anticyclonically under the effects of the Coriolis force and propagates, accordingly to the winds fields, along the shore as a wind stress and buoyancy driven coastal current. The propagation continues until mixing processes with ambient water dissipate the plume buoyancy (Garvine, 1999). The buoyancy inputs associated with a river plume may affect the whole receiving basin, developing lateral gradients and hence circulation features. Riverine waters are an active way of transport for sediments and nutrients of land origin: the former are relevant for sediment budgets, coastal morphodynamics and for the fate of contaminants interacting with cohesive sediments; the latter can play an important role in many ecological aspects of the con- tinental shelves, like maintenance of fish stocks, sustainability of larval and nursery habitats, eutrophication, hypoxic/anoxic events and harmful algal blooms. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.11.001 n Corresponding author. Tel.: þ39 041 2407995; fax: þ39 041 2407940. E-mail address: francesco.falcieri@ve.ismar.cnr.it (F.M. Falcieri). Continental Shelf Research 87 (2014) 84–95