The short version
Ocean waters off New England are warming faster than most of the world’s oceans, but older satellite records were too coarse to show what was happening right around Cape Cod. Two scientists from Woods Hole Oceanographic Institution and Boston University used a sharper satellite record, covering 2006 to 2020, to map sea-surface temperature around the Cape. They found a “warm spot” in the shallow, sheltered waters of Nantucket Sound, along the Cape’s south shore, where the hottest summer temperatures rose about 3.1 °C (about 5.6 °F) in 15 years, roughly three times the rise elsewhere on the shelf. Summer there also got about 20 days longer. Just to the southeast, over Nantucket Shoals, they found the opposite: a “cold spot” where summers got cooler and shorter.
What the researchers did
Most earlier studies of New England ocean warming used satellite data on grids about 21 km (13 miles) across or larger, which blurs small bays and sounds together. This study used a UK Met Office product on a grid about 4 km across, five times finer.
- They analyzed daily surface water temperatures from 2006 to 2020 for the waters from Long Island to Cape Ann, including Cape Cod Bay, Buzzards Bay and Vineyard and Nantucket Sounds.
- They defined seasons by water temperature (summer starts when the water passes 16 °C, about 61 °F, and winter when it drops below 8 °C, about 46 °F) and tracked how the start, length and peak of each season changed year to year.
- They compared three areas: Nantucket Sound (a box along the Cape’s south shore), Nantucket Shoals, and the rest of the shelf. They checked the results against four NOAA buoys, including one in Nantucket Sound, and against the coarser satellite product most earlier studies used.
What they found
- A warm spot south of the Cape. In Nantucket Sound, the year’s peak summer temperature rose by 3.1 ± 1.0 °C from 2006 to 2020. On the broad shelf away from the Cape it rose by 1.1 ± 0.9 °C. At the NOAA buoy in Nantucket Sound, whose readings closely matched the satellite record, the satellite data showed a rise of about 3.1 °C as well.
- Longer summers, shorter springs and falls. In Nantucket Sound, summer arrived about 12 days earlier and lasted about 20 days longer. Spring and fall got shorter, and the water warmed and cooled faster through those in-between seasons, by about 20% in spring and 46% in fall.
- Bigger swings as well as warmer summers. Winter low temperatures in Nantucket Sound did not rise. They dipped by about 1 °C, though that change was not statistically meaningful. The authors describe the Sound’s yearly temperature cycle as getting stronger in both directions.
- A cold spot next door. Over Nantucket Shoals, where strong tides mix cold deep water to the surface, peak summer temperatures fell by about 2.5 °C and summer got about 32 days shorter.
- Coarse data misses it. The widely used 0.25° satellite product did not show the cold spot at all and blurred the warm spot. It also showed nearly twice as much overall warming across the region as the finer product did.
- Short record, honest uncertainty. Fifteen years is short for climate trends, and the authors tested their trends at a 90% confidence level. They say better local monitoring and finer-scale models are needed to explain the causes.
What it means for Waquoit Bay today
Waquoit Bay opens onto the sounds south of Falmouth and Mashpee, within the “warm spot” area mapped in this study. The satellite grid is too coarse to see inside the bay itself, so the study tells us about the open water at our doorstep, outside the bay. Still, it fits a pattern: the Waquoit Bay Reserve’s own monitoring shows the bay’s water warming at all four of its stations (see the Reserve’s 2000–2022 trends), and EPA researchers found that very shallow estuary water may warm faster than deeper water (see our summary of Oczkowski et al., 2015).
Longer, hotter summers matter because warm water holds less oxygen and speeds up the algae blooms that nitrogen pollution feeds. The water that flushes Waquoit Bay on every tide is itself getting warmer in summer, so the bay may get less relief from the ocean during the hottest weeks. We can’t change sea temperatures locally, but we can cut the nitrogen that makes warm summers so damaging. This study adds to the case for meeting the bay’s nitrogen limits sooner rather than later.
Read the paper
Yu, L., & Yang, K. T. (2022). A warm and a cold spot in Cape Cod waters amid the recent New England shelf warming. Frontiers in Marine Science, 9, 922046. https://doi.org/10.3389/fmars.2022.922046
License: © 2022 Yu and Yang. This is an open-access article distributed under the Creative Commons Attribution License (CC BY 4.0). The full PDF below is reproduced unmodified from the publisher, Frontiers. The study was funded by NOAA’s Global Ocean Monitoring and Observation Program. The card image is the article’s first page (CC BY).
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