“This year [in the Gulf] was exceptionally warm given how many days we were seeing at heat wave status,” said Kathy Mills, who runs the Integrated Systems Ecology lab at the Gulf of Maine Research Institute. “But I had not thought about the aggregation of this effect into having this year come in with the warmest fall on record … I was very surprised.”
The Gulf of Maine is warming faster than 96 percent of the world’s oceans — increasing at a rate of 0.09 degrees per year over the past four decades. In the past 10 years or so, many of the increases can be tied to marine heat waves.
A marine heat wave is a period of five or more days where sea surface temperatures are in the 90th percentile compared to average. The gulf experienced marine heat wave conditions for most of 2021, with only brief breaks in March, April and the end of July. Heat wave conditions returned in August and remained through the rest of the year. The largest fall temperature anomaly occurred on Oct. 16 when the daily temperature peaked at 6.3 degrees above the long-term average (1982-2011).
Heat waves in the region initially caught researchers’ attention in 2012, when warmer-than-normal temperatures swept over the region for nearly the entire year. Since 2012, the warm spells have only increased, especially in the later part of the year. Mills said summer and fall in the gulf are warming nearly twice as fast as winter and spring.
Since 2008, fall sea surface temperatures have warmed around 1.1 degree each decade. The average fall temperature hasn’t dipped below 57 degrees since 2010. Four of the warmest fall seasons have occurred in the past seven years.
Even so, fall sea surface temperatures in 2021 stood out.
Mills said last fall’s exceptional warmth can be explained starting with a change in a broader ocean circulation pattern, which has been present in the region over the past decade.
She explained the region is currently in a phase where the Gulf Stream, a strong ocean current from the Gulf of Mexico that brings warm water to the Gulf of Maine, has been pushing farther north. Mills said Maine is “getting more spinoff of that warm water.” As the Gulf Stream pushes further north, it also constrains the flow of the Labrador current, which brings cold water from the Labrador Sea to the Gulf of Maine.
“You’re turning up the dial on the hot water, you’re turning down the dial on the cold water, and that’s really contributing to the large-scale pattern of warming that we’re seeing in the Gulf of Maine,” she said.
On top of this ocean circulation pattern, the gulf’s exceptional summer — the second warmest on record — also kept temperatures elevated into the fall season. Warmer-than-normal atmospheric conditions also hindered sea surface temperatures from cooling down.
“Our fall temperatures are staying high longer, and that’s being affected by how warm we get during the summer,” said Mills. “If you’re starting from a higher point in the summer and your temperatures are cooling down more slowly, it makes these fall temperatures look even that much more different from prior fall temperatures.”
The temperature changes have not gone unnoticed in the gulf’s waters. In the past year, the Gulf of Maine Research Institute received reports of unusual animals in the area — including blue crabs, black sea bass and a smooth hammerhead shark. Other offshore marine species, such as river herring and striped bass, are also sticking around longer instead of migrating.
Lobster industries have also adjusted to the temperature shifts and marine heat waves. Mills said the 2012 heat wave brought a high volume of lobsters closer to shore earlier than normal, causing a lot of disruption to typical practices.
“The industry has really adjusted to that, and we don’t see those types of situations really causing the same alarm and problems that they caused in 2012,” Mills said. She said some fishery boats are also stocked with equipment to chill or oxygenate waters during stressful, high temperature days, especially during the summer.
As species change in the region, Mills said, fisheries will need to adapt and plan. New species coming from the south could provide new opportunities.
“We can draw from the Gulf of Maine experiences to provide insights that hopefully will enable other ocean regions to prepare for change and to be ready to adapt to that change,” she said. “We’re sort of at the front edge of that with a lot of things happening very quickly here.”