What Happened Under the Surface of the 2023 North Sea Heatwave? A Glider Study, Explained

In June 2023, a marine heatwave hit the North Sea off eastern Scotland, the most severe there in 30 years of records. Satellites saw surface chlorophyll, a stand-in for plankton, spike past its normal range, and within two days the spike was gone. Underneath, a Slocum glider was diving from near the surface to near the seabed, again and again, for researchers at the UK's National Oceanography Centre and five partner institutions.

It found that the chlorophyll in a hidden layer near the thermocline rose and stayed up for about a week. On average it stayed about normal for June, but a few short bursts went beyond anything in the June record. The total chlorophyll from top to bottom rose too. The authors can't yet say why.

In this video

• The heatwave: at least five days of unusually warm sea, graded in categories like storms. East of the Firth of Forth it began on June 2, reached category IV (extreme) on June 16, stayed there for about a week and lasted the rest of the month
• The hidden layer: chlorophyll as a stand-in for plankton, the thermocline as a lid between warm and cold water, and why plankton gather near it in summer
• The glider: a shallow-water Slocum flying a 16 km line, on most dives from within 5 m of the surface to within 10 m of the seabed, about 20 minutes a dive
• The view from above: surface chlorophyll from under 1 to more than 5 mg/m³ on June 14, up to 40% above the usual June high. The glider saw the spike too
• The lid: the top passed 17 °C while the bottom stayed at 8–9 °C. The warm top layer thinned to 3.4 m on June 13, the lid rose and fell every 12–13 hours, and the bottom water warmed a third faster than in May
• A greener hidden layer: from 1–2 to about 3 mg/m³ from June 11 to 18, with spikes past 9. More chlorophyll overall, peaking on June 17, likely biological material rather than stirred-up sediment
• Against the June record: 29 June profiles from 2007 to 2024 rarely went above 3. On average the layer was about normal; the short extremes are missing from the record
• What caused it: currents, new nutrients, more sunlight under clear skies, a change in which plankton grew, or a mix. Underwater light, nutrients during the event, mixing and currents weren't measured
• Why it matters: food for grazers and young fish, possible oxygen loss later in summer, and why weeks of watching show what snapshots miss

For engineers

A Teledyne Webb Slocum with a Sea-Bird GPCTD and a WET Labs ECO Puck (chlorophyll fluorescence, 700 nm backscatter), deployed in late April and recovered August 10, 2023, flew repeats of a 16 km E–W transect at about 56.6°N, 1.1–1.4°W. Fluorescence was calibrated against the ship's bottle-calibrated CTD (100 matched points, R² = 0.948) and corrected for non-photochemical quenching with night profiles. Heatwave categories follow Hobday et al. against a fixed 30-year baseline.

Along-track bottom-layer gradients doubled (0.004 → 0.008 °C/km), about 10× what an assumed 0.1 m/s residual current would need to carry the extra heat, so currents may have contributed. Thermocline oscillations near the M2 tidal period may be partly aliased by the glider's motion.

The paper

"Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea"
Charlotte A. J. Williams, Beth E. Scott, Tom P. Rippeth, Zoe L. Jacobs, Juliane U. Wihsgott, Matthew R. Palmer, Rory O'Hara Murray, Joanne E. Hopkins
National Oceanography Centre, University of Aberdeen, Bangor University, Plymouth Marine Laboratory, University of Plymouth, Scottish Government Marine Directorate
Communications Earth & Environment, published online October 8, 2026, doi:10.1038/s43247-026-04118-2. Open access, CC BY 4.0.

Read the paper

Part of the PELAgIO project in the ECOWind programme, co-funded by NERC and the Offshore Wind Evidence and Change (OWEC) Programme (grant NE/X008835/1), with NERC grant NE/Y005589/1. NOC's Marine Autonomous Robotic Systems team deployed and piloted the glider.

This is an independent explainer. I'm not affiliated with or endorsed by the authors or their institutions, and the research is all theirs. The animations are my own drawings, not the glider's data; the curves are schematic, drawn from the paper's figures. Any mistakes are mine.

I'm Bora Celik from Piccard. I explain new research in ocean science and robotics.