Profound knowledge of sea surface temperature (SST) anomalies distribution over the continental shelves is essential to study oceanic processes such as frontal dynamics, upwelling and downwelling events, as well as eddies and plumes evolutions, which are also important in biological applications. The knowledge of SST anomalies in the shelves is also useful for climate monitoring and prediction, since they respond to a multiple-scale organization of atmospheric circulation anomalies occurring over distant parts of the globe, commonly called teleconnections. Climate teleconnections can exhibit variability on intraseasonal (IS) time scales, which are those between 10 and 100 days (Feldstein, 2003; Murakami, 1981; Wang et al., 2016). Moreover, in recent years there has been a greater appreciation of the importance of the two-way interactions between the tropics and the mid-latitudes and high latitudes on IS time scales (Stan et al., 2017). Whereas mid-latitudes are typically characterized by being energetic areas where eastward-moving atmospheric baroclinic waves dominate surface energy flux variations on synoptic timescales (3 to 10 days), quasi-stationary large-scale Rossby waves, blocking anticyclones as well as slowly moving cutoff lows contribute significantly to extratropical atmospheric variability on timescales longer than approximately 10 days (Alexander and Scott, 1997; Deser et al., 2010). Berbery and Nogués-Paegle (1993), among other early studies, showed that austral summer atmospheric convection variability over the Indonesia region contributes to induce atmospheric Rossby wave trains that propagate poleward from this region towards South America and the Southwestern Atlantic Ocean (SWAO). More recently, the description and understanding of the atmospheric IS variability between 10 and 90 days has received considerable attention at both global and regional scales. Results show that atmospheric IS variability is essentially internally generated by atmospheric instability (Stan et al., 2017). The Madden-Julian Oscillation (MJO) is recognized as the leading global pattern on IS timescales, with maximum amplitude at the tropics and dominant activity between 30 and 60 days (e.g., Madden and Julian, 1972). The MJO is characterized by a wavenumber 1 spatial structure propagating eastwards influencing not only atmospheric variables such as air temperature, pressure, winds, cloudiness and rainfall, but also ocean conditions like SST, ocean surface evaporation, and ocean chlorophyll (Madden and Julian, 1972; Hendon and Salby, 1994; Zhang, 2005).
The IS atmospheric variability over South America and the surrounding oceans has received some attention from the scientific community during the last twenty years. Vera et al. (2017), and references therein, show that it exhibits considerable amplitude all year round, with large and significant influence on regional climate conditions. The South Atlantic Convergence Zone (SACZ), which is a band of active atmospheric convection that develops during the warm season (Carvalho et al., 2004), plays a key role in determining the leading patterns of regional atmospheric IS variability (e.g., Vera et al., 2017). Based on observations and reanalysis (Alvarez et al., 2016, 2017) and numerical models (Barreiro et al., 2018) it has also been confirmed that the regional IS atmospheric variability is in part influenced by the MJO activity, where the Atlantic air-sea interaction was found to be important to enhance the rainfall within the SACZ through a local warm SST anomaly forced by heat flux anomalies associated with the direct MJO impact. Air-sea interaction over the SACZ region can be directed from ocean to atmosphere, from the atmosphere to the ocean, be mutual or neutral, and each interaction is associated with distinct atmospheric anomalies (Tirabassi et al., 2014).
The study of SST variability in the Southwestern Atlantic has received considerable attention on annual (e.g., Podestá et al., 1991; Lentini et al., 2000; Rivas, 2010; Simionato et al., 2010; Delgado et al., 2014; Luz Clara et al., 2019), interannual and decadal timescales (e.g., Venegas et al., 1997; Paegle and Mo, 2002; Sterl and Hazeleger, 2003). Performing numerical experiments with both atmospheric and ocean global models, Rodrigues Chaves and Nobre (2004) showed the predominance of a negative thermodynamic feedback between the atmosphere and the ocean over the southwest tropical Atlantic during strong SACZ events. They conclude that negative SST anomalies often observed underlying the SACZ represent an oceanic response to atmospheric forcing.
The surface ocean conditions in the Southwestern Atlantic Northern Argentinean Continental Shelf (SWACS NACS) present a very marked SST annual cycle, explaining over 90% of the total variance (Luz Clara et al., 2019). It is characterized by SST amplitude diminishing offshore and southward, and with maximum amplitudes in the inner Río de la Plata and El Rincón areas (Podestá et al., 1991; Rivas, 2010; Simionato et al., 2010; Delgado et al., 2014; Luz Clara et al., 2019). Regarding the SST variability on shorter time scales (synoptic to intra-seasonal), it is known that the wind variability has a large impact on coastal dynamics (Guerrero et al., 1997; Framiñan, 2005; Pimenta et al., 2008; Simionato et al., 2006, 2007, 2010; Saraceno et al., 2014). Dominant winds, that vary from the northeast to the southwest in a scale of a few days (Simionato et al., 2005, 2007), produce alternating events of downwelling and upwelling that cool and heat surface waters along the Uruguayan coast (e.g., Framiñan, 2005; Pimenta et al., 2008; Simionato et al., 2010; Trinchin et al., 2019). In addition, other atmosphere-ocean interaction processes such as marine heat waves have been lately studied in the SWACS. Marine heat waves in the Atlantic ocean are caused by persistent anticyclonic circulation associated with atmospheric blocking, which is in turned triggered by tropical convection in the Indian and Pacific oceans (Rodrigues et al., 2019), probably associated with the MJO (Manta et al., 2018), being this mode of variability a key source of long-term variability in subtropical South America blocking frequency (Rodrigues and Woollings, 2017).
However, SST variability on IS timescales on the SWACS NACS and its relationship with atmospheric circulation anomalous patterns have not been fully studied yet.
The aim of this paper is, therefore, to describe the IS variability of the SST anomalies in the SWACS NACS and to explore the associated atmosphere-ocean interaction mechanisms. The study focuses particularly on the warm season, that is when regional atmospheric IS variability is strong in association with the SACZ activity (Vera et al., 2017) and when IS variability of SST in the SWACS presents higher activity, as will be shown below.