Marine heat waves—defined as events during which seawater temperatures exceed the 90th percentile of the 30-year historical record for 5 or more days—are becoming more frequent and lasting longer in locations around the globe. Because the populations of many aquatic species such as algae, fish, and marine mammals often have difficulty recovering from such environmental extremes, these events can have devastating ecological impacts and severe socioeconomic consequences.
In a new study, Manta et al. examined marine heat waves in the southwestern Atlantic Ocean, a region near the confluence of the cool Malvinas and warmer Brazil Currents. Although not previously known for such events, the waters over the continental shelf between 32°S and 38°S reached a record-setting 26.8°C during the 2017 austral summer. This extreme temperature was 1.7°C higher than the previous maximum and caused mass fish mortalities and toxic algae blooms.
After analyzing remotely sensed sea surface temperatures and daily climatologic and atmospheric variables, the team concluded that although the 2017 marine heat wave was the most intense of the past 3 decades, it was just 1 of 75 events, lasting a total of 893 days, to occur in the region between 1988 and 2017. The data also show that these events are becoming more frequent in the southwestern Atlantic Ocean, with more than 50% of the marine heat wave days occurring since 2014.
The authors attribute the extreme 2017 event to atmospheric forcing due to an unprecedented combination of extremely high air temperatures and low wind speeds. These, in turn, led to an intense feedback loop of increasing temperatures and stratification in the Río de la Plata river plume waters, which affect water column stratification and nutrient distribution across a broad region of the southwestern Atlantic shelf. The team argues that the most likely driver of this event was the Madden-Julian Oscillation, a large-scale wave pattern in the tropical atmosphere that causes major fluctuations in tropical weather over timescales ranging from weeks to months.
As one of the first studies to examine marine heat waves in the southwestern Atlantic region, this research represents an important contribution to increasing our understanding of both the historical context and the causes of these potentially devastating events. (Geophysical Research Letters, https://doi.org/10.1029/2018GL081070, 2018)
—Terri Cook, Freelance Writer