A warm patch of ocean with no bloom
An ocean mooring at 45°S records a marine heatwave in austral summer 2019. Sea-surface temperature is 3.2°C above the 1991–2020 climatology for 45 days. Before the event the mixed layer is 45 m deep and surface nitrate is 2.5 µmol/L; during the event the mixed layer shoals to 20 m, surface nitrate falls to 0.2 µmol/L, and chlorophyll-a falls from 1.2 to 0.3 mg/m3. Wind stress is 40% below normal, net shortwave radiation is 25 W/m2 above normal, and downwelling longwave is 20 W/m2 above normal. A storm in early autumn deepens the mixed layer to 100 m; within a week chlorophyll-a rises to 2.4 mg/m3 and surface nitrate to 3.0 µmol/L. A sediment trap at 200 m records low particulate organic carbon flux during the heatwave and a pulse after the storm.
1. Explain why chlorophyll-a remained low during the heatwave despite above-normal light and temperature. Evaluate nutrient limitation versus grazing as the controlling mechanism.
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Nutrient limitation is the better explanation. During the heatwave the mixed layer shoals from 45 to 20 m, isolating the surface from the deep nitrate reservoir. Surface nitrate falls to 0.2 µmol/L while chlorophyll falls to 0.3 mg/m3. More light and warmer water cannot produce a bloom if nitrogen is exhausted. Grazing can keep biomass low, but it would not explain the simultaneous collapse in nitrate; grazers recycle nitrogen, so we would expect elevated ammonium and sustained chlorophyll, not both low. The storm is the clincher: within a week of deepening to 100 m, nitrate rises to 3.0 µmol/L and chlorophyll jumps to 2.4 mg/m3, showing that the surface community was poised to respond to new nutrients. The low wind stress and positive heat fluxes caused the shoaling and stratification.
2. Did the heatwave increase or decrease carbon export from the surface ocean? Use the sediment-trap and mixed-layer data to justify your answer, and identify one uncertainty.
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During the heatwave, export likely decreased because production was low; the sediment trap records low particulate organic carbon flux. After the storm, the bloom produced a pulse of export. The net effect on annual export is uncertain: a short intense bloom can sink more carbon than a long low-production period, but if the bloom is grazed or recycled in the surface layer, export efficiency may be low. One key uncertainty is whether the trap at 200 m measures true sinking flux or is affected by lateral advection and zooplankton swimming; sediment-trap blanks and current-meter data would help.
3. How would you test whether this event was made more likely by anthropogenic climate change rather than internal variability?
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Use a large ensemble of climate models with and without anthropogenic forcing to estimate how the probability of a 45-day, 3.2°C anomaly at this location has changed; this gives a probability ratio rather than a yes/no attribution. Check that the models reproduce the observed SST trend and variability, and evaluate internal modes (ENSO, SAM, Indian Ocean Dipole) that can produce warm anomalies. A single event cannot be attributed with certainty, but a significant increase in likelihood, consistent with observed ocean warming, would support a human influence. Longer buoy and reanalysis records would test whether the event is exceptional.