On the evening of 15 January 2022, the Hunga Tonga-Hunga Ha’apai volcano 1 unleashed a violent underwater eruption, blanketing the surrounding land masses in ash and debris 2 , 3 . The eruption generated tsunamis observed around the world. An event of this type last occurred in 1883 during the eruption of Krakatau 4 , and thus we have the first observations of a tsunami from a large emergent volcanic eruption captured with modern instrumentation. Here we show that the explosive eruption generated waves through multiple mechanisms, including: (1) air–sea coupling with the initial and powerful shock wave radiating out from the explosion in the immediate vicinity of the eruption; (2) collapse of the water cavity created by the underwater explosion; and (3) air–sea coupling with the air-pressure pulse that circled the Earth several times, leading to a global tsunami. In the near field, tsunami impacts are strongly controlled by the water-cavity source whereas the far-field tsunami, which was unusually persistent, can be largely described by the air-pressure pulse mechanism. Catastrophic damage in some harbours in the far field was averted by just tens of centimetres, implying that a modest sea level rise combined with a future, similar event would lead to a step-function increase in impacts on infrastructure. Piecing together the complexity of this event has broad implications for coastal hazards in similar geophysical settings, suggesting a currently neglected source of global tsunamis.

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Aim To examine the population genetic structure in Posidonia australis meadows, a marine foundation species capable of long distance dispersal (LDD), and the role of historical versus contemporary processes in shaping post Last Glacial Maximum (LGM) re‐colonization. Location Southeastern Australia including the Bass Strait Islands. Methods We generated multilocus genotypes and assessed spatial patterns of genetic diversity. Relationships among meadows were assessed in terms of historical sea level changes, oceanic boundary currents and contemporary seed dispersal based on a hydrodynamic model. Results There was strong regional spatial genetic structuring among P. australis meadows in south‐eastern Australia, which was congruent with three recognized marine biogeographical provinces [Peronian (eastern), Flindersian (western and southern), and Maugean (south‐eastern)]. The genetic data suggest Maugean meadows persisted in isolation during the LGM, with evidence for admixture and contemporary gene flow. Simulated dispersal events identified high rates of local and regional demographic connectivity, with evidence for occasional LDD events. Main conclusions The strong regional differentiation is consistent with long‐term barriers to dispersal persisting in the marine environment through many sea level fluctuations. Bass Strait Island meadows all have strong signals of genetic admixture. A weak but significant isolation by distance relationship is consistent with a historical signal and contemporary seed dispersal mostly within the Bass Strait.

Observations of a large temperate embayment in Victoria, Australia, reveal a sustained climatic shift that occurred in response to a prolonged drought in the region during 1997-2009. Historically, the bay is fresher than the ocean with fresh outflow to the sea. However, the drought has caused substantially elevated salinity and temperatures above adjacent oceanic waters. The bay's capacity to dilute and flush waste discharges to the ocean was also changed. Observed conditions have been numerically modelled with hydrodynamic and coupled lagrangian particle dispersion models to test differences in dispersion and exchange during historically fresher conditions and hypersaline bay scenarios. Further scenarios were tested for projected climate conditions which were similar to the recent drought responses in the bay. The models identified the effects on the circulation of the climatic shift including regions of increased vulnerability to extreme salinity in the bay, with some existing discharges concentrating in these regions of heightened vulnerability. Absolute salinity in the bay could reach critical levels of over 38 g kg −1 , in places, which may compromise bay ecology.

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