AbstractUnderstanding how El Niño‐Southern Oscillation (ENSO) responds to natural variability is of key importance for future climate projections under a warming climate. However, there is no clear consensus on what drives ENSO's variability on centennial timescales. Here, we find that the epikarst in southeastern Alaska is effective at filtering ENSO and solar irradiance signals from the Aleutian Low regional climate, which are subsequently recorded in speleothem proxy data. By applying a correlation test, we find that ENSO was significantly influenced by solar irradiance over the past ∼3,500 years. This relationship dissolved after ∼1970 CE, with ENSO now being dominated by anthropogenic forcing. This implies a new ENSO mean state that will need to be incorporated into future climate projections.

AbstractHyporheic exchange (HE), fine particle deposition and clogging are tightly coupled processes that control ecosystem services in rivers. While HE is assumed to be induced primarily by riverbed topography, surface flow turbulence also drives significant exchange. We show that turbulence‐driven HE produces large interfacial fluxes and drives long‐term feedback between HE and fine suspended particles via bed clogging. Turbulence significantly increases total HE fluxes as it rapidly delivers suspended particles into porewater over the entire interface, whereas advective pumping exchange only delivers particles into focused downwelling regions on the upstream side of bedforms. While turbulence is associated with rapid fluctuations and shallow HE, it is key on longer‐timescale outcomes, namely bed clogging. However, beyond the general effect of clogging in attenuating HE, turbulence‐driven HE will also be important for other river‐borne materials that are retained and transformed within hyporheic zones, such as nutrients and organic pollutants.

AbstractReliable regional temperature projections including heat extremes are essential for climate change adaptation and mitigation. Taking China as an example, simple averages from Coupled Model Intercomparison Project Phase 6 (CMIP6) models project high warming due to sampling many high climate sensitivities in the ensemble. Here, we develop an emergent constraint (EC) framework to obtain constrained mean and daily maximum temperature (TXx) warming over China by using observed global warming and local residual warming. The constrained annual mean and TXx warming over China (2.33°C [1.61–3.05°C] and 2.31°C [1.21–2.99°C]) are 0.65°C [0.04–1.76°C] and 0.63°C [–0.50–2.39°C], respectively, lower than raw projections (2.98°C [1.85–4.22°C] and 2.94°C [2.04–4.39°C]) for 2080–2099 under the intermediate‐emission scenario. Approximately half model uncertainty is reduced after constraint. The land area (population) experiencing temperature extremes in our metric is 78% (85%) of the raw projections. Our results imply a lower impact of extreme heat than implied by current raw CMIP6 projections.

AbstractProxy‐based reconstructions show that the climate during the mid‐Holocene (MH) was warmer and wetter than the present day in most regions of China, characterized by a northwestward extension of the East Asian summer monsoon and an increase in woody cover. However, climate simulations that neglect vegetation shifts yield cooler climates than proxy‐based reconstructions for the MH. Here, we simulate MH vegetation cover, productivity, and terrestrial parameters in China using a state‐of‐the‐art land surface model forced with realistic proxy‐based climate data. Our simulations corroborate MH vegetation cover inferred from 246 pollen sites, and indicate a higher vegetation productivity in China, despite the lower CO2 concentration during the MH compared to the present day. Enhanced vegetation growth during the MH could have affected land surface energy and hydrological budgets through biophysical feedbacks. Our findings highlight the impact of vegetation dynamics on the terrestrial carbon cycle and regional climate in China.

AbstractLow cloud feedback in global warming projections by climate models is characterized by its positive sign, the mechanism of which is not well understood. Here we propose that the positive sign is primarily caused by the increase in upward longwave radiation from the sea surface. We devise numerical experiments that enable separation of the feedback into components coming from physically distinct causes. Results of these experiments with a climate model indicate that increases in upward longwave radiation from the sea surface cause warming and absolute drying in the boundary layer, leading to the positive low cloud feedback. The absolute drying results from decrease in surface evaporation, and also from decrease in inversion strength which enhances vertical mixing of drier free tropospheric air into the boundary layer. This mechanism is different from previously proposed understanding that positive low cloud feedback is caused by increases in surface evaporation or vertical moisture contrast.

AbstractWe present Magnetospheric Multiscale (MMS) observations of a K‐H wave event under southward IMF conditions, accompanied by ongoing magnetic reconnection. The nonlinear K‐H waves are characterized by quasi‐periodic fluctuations, the presence of low‐density and high‐speed ions, and variations in the boundary normal vectors at both the leading and trailing edges. Our observations reveal clear evidence of on‐going magnetic reconnection through the identification of Alfvénic ion jets and the escape of energetic magnetospheric electrons. Among the 36 magnetopause current‐sheet crossings in this event, 19 exhibit unambiguous signatures of reconnection at both the leading (7) and trailing (12) edges. Notably, the estimated current‐sheet thicknesses at both edges are comparable to the ion‐inertial scale, confirming the compression effect resulting from the large‐scale evolution of the K‐H waves. The reconnection jets potentially contribute to the suppression of K‐H growth through boundary‐layer broadening and the development of complex flow and magnetic field patterns.

AbstractThe attenuation of ocean surface waves during seasonal ice cover is an important control on the evolution of Arctic coastlines. The spatial and temporal variations in this process have been challenging to resolve with conventional sampling using sparse arrays of moorings or buoys. We demonstrate a novel method for persistent observation of wave‐ice interactions using distributed acoustic sensing (DAS) along existing seafloor fiber optic telecommunications cables. DAS measurements span a 36‐km cross‐shore cable on the Beaufort Shelf from Oliktok Point, Alaska. DAS optical sensing of fiber strain‐rate provides a proxy for seafloor pressure, which we calibrate with wave buoy measurements during the ice‐free season (August 2022). We apply this calibration during the ice formation season (November 2021) to obtain unprecedented resolution of variable wave attenuation rates in new, partial ice cover. The location and strength of wave attenuation serve as proxies for ice coverage and thickness, especially during rapidly evolving events.

AbstractUsing new continuous geodetic time series, we identify five shallow slow slip events (SSEs) offshore and beneath the Osa peninsula in southern Costa Rica. An early event was detected by one station in 2013, and two events occurring in close succession in both 2018 and 2022 were detected by multiple stations, indicating a preliminary recurrence interval of ∼4–5 years. While SSEs have been observed to the northwest at Nicoya, this is their first documentation in southern Costa Rica. Modeled slip distributions of the 2018 and 2022 events indicate they likely ruptured the same or overlapping patches of the plate interface, near the trench, updip of the 1983 Mw 7.4 Osa event. Immediately offshore, estimated cumulative slip from the 2018 and 2022 events is sufficient to close the slip deficit from tectonic loading over the recurrence interval, potentially limiting the magnitude and spatial slip distribution of future large ruptures.

AbstractEvery austral spring when Antarctic sea ice melts, favorable growing conditions lead to an intense phytoplankton bloom, which supports much of the local marine ecosystem. Recent studies have found that Antarctic sea ice is predictable several years in advance, suggesting that the spring bloom might exhibit similar predictability. Using a suite of perfect model predictability experiments, we find that November net primary production (NPP) is potentially predictable 7 to 10 years in advance in many Southern Ocean regions. Sea ice extent predictability peaks in late winter, followed by absorbed shortwave radiation and NPP with a 2 to 3 months lag. This seasonal progression of predictability supports our hypothesis that sea ice and light limitation control the inherent predictability of the spring bloom. Our results suggest skillful interannual predictions of NPP may be achievable, with implications for managing fisheries and the marine ecosystem, and guiding conservation policy in the Southern Ocean.

AbstractIntense eruptions of the Tonga volcano activated prominent traveling atmospheric disturbances (TADs) at 04:05UT on 15 January 2022. Himawari‐8 satellite images depict that TADs of the tropospheric Lamb wavefront propagate with a speed of 315 m/s and arrive in Taiwan at 11:30UT. Networks of 98 barometers, 28 tide gauges, an ionosonde, and 10 magnetometers are used to study the responses of magnetic fields to TADs in Taiwan. The horizontal components in magnetic field changes of the Taiwan magnetometers all point toward and away from the Tonga volcano at 11:00–12:00UT upon the tropospheric Lamb wavefront arrival and at 22:00–23:00UT when the thermospheric Lamb wavefront with speeds of 487 m/s coming, respectively. Analyses of the raytracing and beamforming techniques on the horizontal components in magnetic field changes of 69 INTERMAGNET magnetometers show that both tropospheric and thermospheric Lamb waves efficiently activate traveling ionospheric disturbances and modify ionospheric currents of the globe.