Coastal hydrographic signatures of heat waves and extreme events of dense water formation during 2002-2012 (Barcelona, NW Mediterranean)

Abstract

In a global context of climate change affecting the marine environment, increasing importance is given to the effect of extreme events in driving ecological change and to get a better understanding of what are the conditions to be expected under future climate scenarios. The identification of such events in shallow waters should overcome the difficulties of the high range of variability of the hydrographic parameters (daily, monthly, seasonal, interannual). In this study we focus in the period 2002-2012. Temperature, salinity, turbidity and fluorescence, measured monthly off Barcelona city during the study period, are analysed in order to identify the signature of extreme heat waves and the largest episodes of dense water formation in the Western Mediterranean. The hydrographic parameters measured in the coastal zone, being a >proxy> to recognize such large scale events, provides an additional tool for the monitoring and forecasting of oceanographic events and to improve our understanding of these processes.

The spatial (across-shelf and water column) and temporal (seasonal and interannual) variability of oceanographic parameters is described focusing on periods when extreme meteorological or oceanographic events were previously identified. Extreme sea water temperatures were recorded during heat waves in 2003 and 2006. However, only the heat wave occurred in 2003 caused a serious damage on shallow communities of the NW Mediterranean. It is suggested that the duration of the period of extreme warm waters in August 2003 together with the coldest observed waters in winter 2003 can be additional factors (to the maximum temperature) to be considered to explain the 2003 event impact on shallow benthic communities. On the other hand, the strongest episodes of sinking dense waters (both cascading and convection processes) described in the Western Mediterranean were noticed in Barcelona coastal waters by water densities anomalies higher than 29 kg/m3. The presence of these high density coastal waters weakens the oceanographic front between shallow and oceanic waters, favours the arrival of deeper and shallow oceanic waters to the coast and has implications into the nutrients and phytoplankton characteristics in coastal waters