The near-bottom water on the U.S. Northeast continental shelf provides a critical cold-water habitat for the rich regional marine ecosystem. This “cold pool” preserves winter temperatures, even when waters become too warm or salty elsewhere during the summer.
However, the U.S. Northeast coastal ocean has experienced accelerated warming in recent years, compared to the global average. Now, scientists using salt as a tracer are investigating how much the influx of warm, salty offshore water onto the continental shelf contributes to the observed seasonal “erosion” of the cold pool.
Lukas Taenzer, a recent Ph.D. graduate from the MIT-WHOI Joint Program between the Massachusetts Institute of Technology (MIT) and the Woods Hole Oceanographic Institution (WHOI), is the lead author of the paper published in the Journal of Geophysical Research: Oceans.
“This paper provides first evidence for a seasonal salinification of the cold pool on the US Northeast continental shelf, as consistently observed in the multi-year mooring record of the [Ocean Observatories Initiative] Coastal Pioneer Array,” said Taenzer.
“We follow the signatures of the ocean’s salinity, rather than temperature, to identify the physical processes that are responsible for the observed changes of coastal ecosystem conditions. Our results demonstrate the value of salinity measurements to highlight how the interplay between air-sea interactions, offshore forcing, and upstream conditions influence coastal ecosystem conditions in the sheltered cold pool on the timescales of weeks to years. This understanding is important to help NOAA Fisheries to manage U.S. fish stocks sustainably.”
To explain the seasonal erosion of the cold pool, the authors set up a salinity budget, which acts like a census taker and counts the influx and exit of ocean water into and out of the cold pool from different directions and over different timescales. Using salinity as a tracer can show why the cold pool erodes during the summer season when stratification (or, ocean layering) is high and where these changes come from.
Novel observations – such as those from the Coastal Pioneer Array (which was located off the coast of New England from 2016-2022) – have motivated new research questions, said Svenja Ryan, an assistant scientist in the physical oceanography department at WHOI. “We didn’t see the salinification of the cold pool previously because we didn’t have continuous subsurface measurements. The fact that salinity is a valuable tracer enables us to advance our dynamic understanding of the seasonal cycle and the processes in the ocean,” she said.
Ryan is the lead author of a recently published companion journal article in JGR: Oceans in 2024. That study provides insights into seasonal and year-to-year salinity variations in the Northwest Atlantic and examines how salinity influences stratification (water layering) on the continental shelf using historical data. While the current study focuses on the ocean depth, the 2024 study focuses mostly on surface values.
“Continuous, global sea surface salinity observations from satellites, such as the NASA Soil Moisture Active Passive satellite mission, are critical for tracking surface ocean conditions in near-real time. They enabled our novel findings about the seasonal evolution of freshwater signals across the Northeast U.S. shelf over the past decade. Also, they are increasingly used by the fishing community, for example, in their decision-making,” said Caroline Ummenhofer, a senior scientist in the physical oceanography department at WHOI and lead-PI on the NASA project that co-funded both papers.
“We have known for a while that the cold pool changes throughout the year, and that it gets slightly warmer. What we didn’t know was why,” said Taenzer. “That’s really hard to determine when using temperature as a tracer, because with temperature you can’t really distinguish why the pool gets warmer. By tracking salt, we can actually pinpoint why the cold pool changes throughout the year.”
