Stalled Rossby waves leave the Great Barrier Reef becalmed and bleaching

For centuries, the doldrums were the dread of sailors: stretches of the tropical ocean where the trade winds collapse and ships drift for days under a punishing sun. A new study published in the journal Weather and Climate Dynamics shows that when these eerie calm periods settle over the Great Barrier Reef, they are not random accidents of the tropics alone. Instead, they emerge from a precise and unsettling alignment of weather systems spanning the entire South Pacific, from the mid-latitude jet stream to the equatorial wave guides. And when that alignment stalls, the result can be the kind of sustained ocean heating that has driven mass coral bleaching on the reef in 1998, 2016, 2020, 2022 and 2024.

The research team, led by Lara Richards of Monash University and the University of Reading, set out to answer a question that coral scientists have largely taken for granted. Reports of calm, clear conditions during bleaching events stretch back to at least the 1960s, and the physical consequences are well documented: when the trade winds fail, evaporative cooling collapses, the shallow waters of the reef stop mixing, and shallow cumulus clouds replace deeper cloud cover, letting more solar radiation pour into the upper ocean. Yet the meteorological machinery that creates and, crucially, prolongs these doldrum episodes had never been systematically dissected. Given that the reef’s corals live in relatively shallow water whose temperature is dictated by the weather overhead, that gap in understanding mattered.

To isolate the phenomenon, the team analysed nearly three decades of austral summer data, from December to April between 1996 and 2024, using a K-means cluster analysis of temperature, humidity and wind profiles at Davies Reef in the central Great Barrier Reef. Out of five distinct synoptic regimes, one stood out: a doldrums cluster comprising 835 days, defined by weak winds through the boundary layer. These days were grouped into events and split into short episodes lasting two days or less, with 155 samples, and long episodes of five days or more, with 57 samples. The researchers then performed a lead-lag analysis centred on the first day of each event, using the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis to reconstruct the atmospheric circulation before, during and after the calm.

The extratropical story begins with Rossby waves, the vast undulations of the upper-level jet stream that circle the Southern Hemisphere. In both short and long doldrums, a Rossby wave packet sends a cyclonic anomaly, essentially a pocket of low geopotential height, over eastern Australia right around the time the doldrums begin. The circulation around this anomaly generates westerly winds over the eastern seaboard, winds that run directly against the climatological south-east trade easterlies that normally ventilate the reef. When the trades collapse, the latent heat flux that normally cools the ocean surface vanishes almost overnight, and the heating begins.

What separates a two-day calm from a week-long one is the fate of that cyclonic anomaly. In short events, the Rossby wave packet races eastward at roughly 9.6 metres per second, sweeping the anomaly across the Australian coast within about three days and dissipating in the central Pacific, after which the trades quickly reassert themselves. In long events, the packet moves far more slowly, at about 6.8 metres per second, and the cyclonic anomaly, likely a cut-off low, parks itself over eastern Australia. Downstream, the wave packet amplifies and stalls over New Zealand and the central Pacific, accompanied by a marked increase in atmospheric blocking, the phenomenon in which a stubborn anticyclone locks the flow pattern in place.

The study links this stalling to repeated cyclonic Rossby wave breaking over Australia, the process by which the upper-level potential vorticity contours overturn irreversibly. In the long-duration composites, enhanced cyclonic wave breaking persists over the continent for up to five days after the doldrums form, apparently re-capturing the cut-off low and feeding the downstream block near New Zealand. Anticyclonic wave breaking upstream, present on 40 to 44 per cent of days in the long cases, may help create the cyclonic anomaly in the first place. The result is a self-reinforcing traffic jam in the jet stream, reminiscent of the blocking dynamics described in recent theoretical work, that keeps the anomalous westerlies blowing over the reef day after day.

The tropics, it turns out, must cooperate as well. The researchers examined two slow-moving tropical oscillations: the Madden-Julian Oscillation, an eastward-propagating pulse of convection with a 40-to-50-day cycle, and Equatorial Rossby waves, which travel westward. During long doldrums, the suppressed phases of both waves, marked by reduced cloud and rainfall, align over the Great Barrier Reef far more often than chance would suggest. The co-occurrence of suppressed phases rises to 18 per cent of long events by day four, and suppression of at least one wave covers 72 per cent of them, compared with just 50 per cent of short events. Critically, the position of the MJO matters: in short events it sits over north-west Australia, promoting easterlies over the reef, while in long events it moves into the western Pacific, its circulation reinforcing the anomalous westerlies that fight the trades.

Perhaps the most striking finding concerns the air itself. Above the doldrums sits a column of dry, descending air, capped by an anomalously warm and moist surface layer near the ocean. Using Lagrangian trajectory analysis with the LAGRANTO tool, the team launched nearly 3,900 back trajectories per day from around Davies Reef and traced where this dry air came from. The answer: the mid-latitudes. Air parcels originating over the South Atlantic or southern Indian Ocean near 200 to 400 hectopascals ride the jet stream towards Australia, then sweep equatorwards across the arid interior of the continent, warming near-adiabatically by as much as 40 kelvin in 48 hours while staying bone dry, before descending over the reef. Parcels descending into the boundary layer even cross the Coral Sea, picking up moisture and cooling evaporatively by up to 10 kelvin, more than previously documented for tropical dry intrusions.

This dry subsiding air acts as a lid. With weak winds preventing the mixing of the moist surface layer into the drier air above, and with the descending column suppressing deep convection, heat and humidity accumulate at the surface for days on end. The synoptic fingerprint, a cyclonic anomaly over eastern Australia or the Tasman Sea with anomalous descent and suppressed rainfall over the reef, closely mirrors the patterns documented for terrestrial heatwaves in south-east Queensland and north-east Australia. The authors argue that the doldrums and their meteorology are, in effect, the same beast as the heatwaves that scorch the adjacent coastline, meaning the same stalled wave dynamics can cook the reef and the land simultaneously.

The implications reach beyond atmospheric curiosity. Because the duration of a doldrums episode hinges on whether a Rossby wave packet stalls or sails through, the findings offer a potential pathway to earlier warnings of marine heatwave conditions, a question of growing urgency as background ocean temperatures climb. The researchers caution that their classification, based on a single daily profile at one reef, may underestimate event durations, and that interannual players such as the El Niño-Southern Oscillation and the Interdecadal Pacific Oscillation, along with the Australian monsoon and tropical cyclones, remain to be woven into the picture. How the frequency and persistence of these becalmed, bleaching-favourable episodes will shift in a warming climate is, for now, an open and troubling question.

Subject of Research: Tropical-extratropical weather dynamics driving the formation and persistence of doldrums over the Great Barrier Reef

Article Title: The development and persistence of doldrums over the Great Barrier Reef

Article References: Richards, L. S., Huang, Y., Barnes, M. A., Jin, C., Muhammad, F. R., Harrison, D. P., & Siems, S. T. (2026). The development and persistence of doldrums over the Great Barrier Reef. Weather and Climate Dynamics, 7(4), 1977-1993. https://doi.org/10.5194/wcd-7-1977-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1977-2026

Keywords: doldrums, Great Barrier Reef, coral bleaching, Rossby waves, marine heatwaves, trade winds, Madden-Julian Oscillation, Equatorial Rossby waves, atmospheric blocking, dry intrusions, ERA5 reanalysis, tropical-extratropical interactions