Rapid surface water warming and impact of the recent (2013-2016) temperature anomaly in shallow coastal waters at the eastern entrance of the Gulf of California

Anthropogenically-induced global warming is increasing surface seawater temperatures (SST) worldwide. Furthermore, the Northeastern Tropical Pacific is affected by seasonal, interannual (El Nino - Southern Oscillation, marine heatwaves), and decadal variabilities (Pacific Decadal Oscillation). Here we analyzed a long-term (1973-2016) SST time-series from the Mazatlan coastal zone, at the eastern entrance of the Gulf of California (Mexico) to record the warming trend and to examine the effect of the 2013-2016 North Pacific SST positive anomaly. This SST time-series can be useful to better understand the interplay of surface water masses in the region (Tropical Surface Water, Gulf of California Water, and California Current Water) and their transport mechanisms. Variability analysis was focused on 2007-2016, for which a 10-year climatology was established. During this period, two La Nina and El Nino events, and a regional marine heatwave occurred in the Northeast Pacific. There was a significant positive correlation between the Oceanic Nino Index (ONI) and SST anomalies. La Nina events caused large negative anomalies (up to 4 degrees C) in maximum SST, and El Nino events caused large positive anomalies (up to 7 degrees C) in the minimum SST. The 2013-2014 regional marine heatwave was preceded by a previously unreported anomalous regional warming at the entrance of the Gulf of California (2012-2013), which caused positive anomalies of 4 degrees C in minimum SST. Since then, each winter showed increasing minimum SST anomalies until reaching a 7 degrees C maximum in 2016. Minimum SST positive anomalies were caused by i) the inhibition of coastal upwelling of cold waters, and ii) a limited penetration of California Current cold waters. For the past 40 years, we estimated a mean warming trend of 0.57 +/- 0.01 degrees C per decade in the study area, which is larger than the global ocean increasing trend. This relatively fast warming rate could be caused by i) local effects caused by shallowness and coastal dynamics, ii) an increasing influence of warmer Tropical Surface Water transported by the Mexican Coastal Current, iii) the transport and upwelling of warmer waters from the Western Pacific Warm Pool during El Nino events, and iv) global warming, as increasing SST can induce stronger and longer-lasting thermoclines, especially in regions commonly affected by upwelling events. If this trend persists, a simple linear extrapolation would indicate that the mean SST from 1977 to 2100 would increase by 7 degrees C. This extrapolation should be taken with caution, but such an increase would have profound effects on the coastal ecosystems, so possible adaptation strategies merit urgent attention by scientists and coastal zone managers to dynamically adapt to coastal global warming and marine heatwaves. This methodology can be used in other coastal zones worldwide where enough field and satellite data are available.