Researchers at the Second Institute of Oceanography report that the season of a marine heatwave can strongly shape the ocean’s productivity response. Their study, published July 9 in Communications Earth & Environment, maps a sharper split across the Southern Hemisphere during austral summer. Warm spells were linked with lower net primary production in low and middle latitudes. South of about 50 degrees south, the team found higher production during heatwaves. The analysis measures phytoplankton productivity, the rate at which microscopic marine plants turn carbon dioxide into organic matter. Separate ecosystem observations are needed to evaluate fish, long-term carbon storage and wider food-web effects.
The largest high-latitude productivity anomalies reached 0.1 to 0.2 times 10 to the 11th milligrams of carbon. This value represents cumulative carbon fixed by phytoplankton in the study’s analysis. The researchers say the signal was strongest from December through February, the Southern Hemisphere summer. That rhythm matters in waters whose food webs are built around seasonal blooms. Their results put timing alongside location as a key part of assessing an ocean heat extreme. A heatwave that arrives during the bright growing season encounters a very different ocean from one that arrives during the dark winter.
Summer reveals a sharper latitudinal split
Marine heatwaves are periods of unusually warm seawater that persist for at least several days compared with the local seasonal norm. For this study, the team defined an event as sea-surface temperature above the local 90th-percentile threshold for at least five consecutive days. They examined conditions from 2007 through 2022 across Southern Hemisphere waters. In the 0 to 30 degrees south band, the average production anomaly during heatwaves was negative. The same was true from 30 to 50 degrees south. The Antarctic zone, from 50 to 90 degrees south, showed a positive average anomaly.
During austral summer, the divide strengthened. The paper reports heatwave mean intensity as high as 2.5 degrees Celsius in Antarctic marginal seas during summer composites. Winter values there were at or below 1.7 degrees Celsius. Summer heatwaves also brought more days of exposure and more events in the high southern latitudes. In the Antarctic zone, the study found three to four events per year in summer composites, compared with one to two in winter. Those physical changes lined up with much larger summer productivity anomalies.
The pattern describes broad regional averages and anomalies. Individual heatwaves can unfold differently as currents and winds shift, clouds pass, sea ice changes and nutrient supplies vary. The authors found a rapid change in the zonal average around 50 degrees south during summer. Waters from roughly 30 to 50 degrees south had strongly negative anomalies. Waters farther south had positive anomalies. This geographic boundary reflects a change in the conditions that limit phytoplankton growth. It also shows why a single global average can hide the ecological importance of where and when an extreme event occurs.
A laser satellite filled part of the polar gap
Satellite maps of ocean color are powerful tools for tracking phytoplankton, yet they need sunlight and clear skies. That leaves large gaps during the polar night. The researchers used CALIOP lidar observations from the NASA and CNES CALIPSO mission. Lidar sends out laser pulses and measures the returning light. Signals from the ocean surface and just below it can be used to estimate optical properties related to phytoplankton carbon. The mission’s record allowed the team to include winter observations that conventional ocean-color sensors often miss.
The study analyzed 16,236 CALIOP overpasses from 2006 to 2023. A machine-learning retrieval translated quality-controlled lidar signals into variables used to estimate productivity. The researchers checked those estimates against 5,131 profiles from 125 BGC-Argo floats collected between 2012 and 2023 in the seasonal sea-ice zone. These robotic instruments measure ocean chemistry and biology as they move through the water column. The wider BGC-Argo program carries sensors for nitrate, chlorophyll and oxygen. It also measures pH, suspended particles and incoming light.
To make the lidar results useful, the team first trained the retrieval with open-water data from NASA’s MODIS-Aqua ocean-color products. It then estimated chlorophyll, particle backscattering and light attenuation from the CALIOP record. Those quantities fed a carbon-based productivity model. Water and particles scatter the laser light in ways that provide a remote proxy for properties connected to phytoplankton carbon. Matching the lidar product with float profiles gave the researchers a way to test the seasonal signal against measurements in the water.
Nutrients and light set the seasonal response
Warm water can strengthen stratification, a layering of the upper ocean that reduces mixing. At low and middle southern latitudes, that layering can curb the upward supply of nutrients to the sunlit surface. Phytoplankton need nutrients to grow. The study links this setting to lower net primary production during marine heatwaves. In the 30 to 50 degrees south band, summer mixed layers are already shallow, so another push toward stratification can further reduce nutrient replenishment.
Farther south, phytoplankton growth commonly faces an iron limitation, while light also changes sharply with the seasons. During summer heatwaves, shifts in stratification, mixed-layer depth and sea-ice concentration may change the balance of light and nutrient or iron supply. The authors describe a coupled light, mixing and iron framework for the higher productivity signal. Their calculations suggest that higher light could reduce cellular iron demand by about 3.81 percent on average in the Antarctic zone. The paper treats that result as one plausible contributor. Direct dissolved-iron measurements, iron-chemistry data and cellular iron physiology remain outside the observational framework.
Winter tells the other half of the story. Low sunlight and deeper mixed layers limit the biological response to a warm event across much of the polar south. Summer has longer daylight and an actively growing phytoplankton community. A warm spell then occurs in a water column that is more sensitive to change. The researchers used an XGBoost analysis and SHAP analysis to identify factors associated with monthly production variability. Sea-ice concentration, available light and nutrient variables were important summer contributors in Antarctic waters. These associations frame the mechanism. Event-by-event field observations are needed to isolate individual causal pathways.
Productivity gains leave important questions open
Net primary production is a useful starting point because it describes new organic matter made by phytoplankton. Long-term carbon storage depends on the later fate of that material. Community composition, grazing, breakdown of organic matter and particle sinking all affect the journey into deeper water. Direct measurements of those processes are required to assess the carbon sink. Food-web observations are likewise needed to evaluate consequences for seabirds, whales, fisheries and other marine life.
The observational coverage has limits as well. Lidar offers year-round sampling, but its tracks are sparse. The float measurements are uneven in Antarctic winter and ice-covered regions. The team also used monthly productivity estimates to capture the longer biological response to daily heatwave events. Its future analysis drew on 32 CMIP6 climate models. Those models carry inter-model uncertainty. Better treatment of seasonally changing light, sea ice, mixing and iron can improve projections. The study’s figure data and code are available in a Zenodo archive, giving other researchers a route to test and refine this picture of seasonal extremes.
That combination of observation and modeling makes the work a useful benchmark for future forecasts. The heatwave definition uses each place’s seasonal temperature history, so an event is an unusual warm period for that location. The productivity estimates also describe a large-scale signal. Fine-scale eddies and brief local blooms can be blurred when data are placed on satellite grids. More winter observations, direct iron measurements and models that resolve seasonal ocean mixing can help scientists test how durable this summer pattern is as marine heatwaves become more common.
