Why did sea surface temperatures go up when summer air temperatures did not? This perplexing question arose after last summer’s unusual findings.

Ocean surface water temperatures off Boston, Portsmouth and Portland were nearly 2 degrees Fahrenheit warmer than normal, aligning with the long-term trend of increasingly warm conditions primarily driven by climate change.

To understand this, we must dive into the unique geography and ecosystem of the Gulf of Maine. This body of water is nestled between Nova Scotia to the north and Cape Cod to the south. Its connection to the Atlantic Ocean is only through a 60-mile-wide deep channel. This distinct geographical feature greatly influences the circulation and temperature of its waters.

The Gulf of Maine is nourished by several rivers, including the Neponset, Charles, Mystic, Merrimack, Piscataqua, Saco, Kennebec, Penobscot, and Saint John. These rivers flow into this 36,000-square-mile sea, making it less salty than the Atlantic Ocean.

Freshwater from these rivers spreads across the sea’s surface during the summer because of its lower density, forming a distinct layer on top rather than mixing with the saltwater below.

River water input creates a counter-clockwise circulation due to the “coriolis effect,” bringing nutrient-rich deep waters to the surface and fostering a rich ecosystem for marine life. However, during the winter months, rivers discharge less water, the circulation halts, and surface waters mix with continental shelf water.

In 2023, the Gulf of Maine experienced the hottest sea surface temperatures ever recorded despite the average summer air temperature being only 70 degrees. This was a stark contrast to 2021, when the average summer air temperature was almost 4 degrees warmer, yet the sea surface temperatures were cooler.

So, how can we explain this paradox? The answer lies in the rainfall. More than 20 inches of rain fell in 2023, the largest volume since 1955. This rain, falling on hot, impervious surfaces, resulted in warm water flowing into the sea, raising the surface ocean waters by 2 degrees and adding energy to the sea.

It’s like trying to warm a cup of coffee with a hair dryer. It won’t work. But place that cup on a hot plate, and you’ll see a difference.

The same principle applies here. We need to slow and retain rainwater on the land to cool the ocean with more vegetation, soil, and groundwater infiltration. More water on the land will enable more photosynthesis to draw more carbon dioxide and further moderate climate change.

This paradox of rising sea surface temperatures despite stable summer air temperatures is a wake-up call. It reminds us that every action has a reaction, and every choice we make can have far-reaching impacts on our environment.

Rob Moir is a nationally recognized and award-winning environmentalist. He is president and executive director of Cambridge-based Ocean River Institute, a nonprofit providing expertise, services, resources, and information unavailable on a localized level to support the efforts of environmental organizations. Visit www.oceanriver.org for more information.