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The 1257 CE eruption of Mt. Samalas in Indonesia is argued to be the largest of the last two millennia in terms of global volcanic aerosol forcing, with a reduction in insolation of more than 30 W/m2 (Sigl et al., 2015, Nature, 523). Large volcanic eruptions are tied to short-term climatic shifts, including changes to monsoon rainfall (Ridley et al., 2015, Nature Geoscience, 8). In order to investigate the impact of this eruption on the Indian summer monsoon in Nepal, we analyzed at ultra-high resolution the carbon and oxygen isotopes of a fast-growing, precisely-dated aragonite stalagmite from Siddha cave in the Pokhara Valley of central Nepal (28.0˚ N, 84.1˚ E; ~850 m.a.s.l.). We micromilled the stalagmite in ~40 µm-wide traverses during the interval through the Mt. Samalas eruption (a total of 261 analyses). Studies near Siddha cave and in Kathmandu, 130 km to the southeast, reveal that amount effects of oxygen isotopes in precipitation in this region are weak, and so we rely on carbon isotopes as a proxy for rainfall. Carbon isotopes define sinusoids that appear to represent annual cycles of rainfall associated with the summer monsoon and winter dry season. The average magnitude of these cycles is ~0.3 to 0.6‰. While some ambiguities exist, the number of seasonal cycles (18-21) is within error of the years of growth for this interval as determined by U/Th dating (26±8 years). To investigate the impact of the eruption on regional hydroclimate, we detrended the carbon isotope data and then calculated anomalies in the wet and dry seasons relative to the mean of those values. The most prominent feature of the time series is two large positive isotope anomalies separated by a moderate negative isotope anomaly. We interpret these to reflect disruptions to both the monsoon and dry season precipitation regimes by aerosol forcing from Mt. Samalas. If true, then these results reveal somewhat surprising an anomalously wet monsoon season in the first year after the eruption and that seasonal sinusoids return to their pre-eruption pattern after only two years following the eruption. In order to better understand these results, we investigate this interval using the Last Millennium Ensemble, a state-of-the-art suite of climate model simulations conducted by the National Center for Atmospheric Research with the Community Earth System Model. 

Abstract The Gulf of Maine, located in the western North Atlantic, has undergone recent, rapid ocean warming but the lack of long-term, instrumental records hampers the ability to put these significant hydrographic changes into context. Here we present multiple 300-year long geochemical records (oxygen, nitrogen, and previously published radiocarbon isotopes) measured in absolutely-datedArctica islandicashells from the western Gulf of Maine. These records, in combination with climate model simulations, suggest that the Gulf of Maine underwent a long-term cooling over most of the last 1000 years, driven primarily by volcanic forcing and North Atlantic ocean dynamics. This cooling trend was reversed by warming beginning in the late 1800s, likely due to increased atmospheric greenhouse gas concentrations and changes in western North Atlantic circulation. The climate model simulations suggest that the warming over the last century was more rapid than almost any other 100-year period in the last 1000 years in the region. 

Stalagmites and climate models reveal ITCZ shifts drove concurrent changes in Australian tropical cyclone and monsoon rainfall. 

Abstract This study uses sea surface salinity (SSS) as an additional precursor for improving the prediction of summer [December–February (DJF)] rainfall over northeastern Australia. From a singular value decomposition between SSS of prior seasons and DJF rainfall, we note that SSS of the Indo-Pacific warm pool region [SSSP (150°E–165°W and 10°S–10°N) and SSSI (50°–95°E and 10°S–10°N)] covaries with Australian rainfall, particularly in the northeast region. Composite analysis that is based on high or low SSS events in the SSSP and SSSI regions is performed to understand the physical links between the SSS and the atmospheric moisture originating from the regions of anomalously high or low, respectively, SSS and precipitation over Australia. The composites show the signature of co-occurring La Niña and negative Indian Ocean dipole with anomalously wet conditions over Australia and conversely show the signature of co-occurring El Niño and positive Indian Ocean dipole with anomalously dry conditions there. During the high SSS events of the SSSP and SSSI regions, the convergence of incoming moisture flux results in anomalously wet conditions over Australia with a positive soil moisture anomaly. Conversely, during the low SSS events of the SSSP and SSSI regions, the divergence of incoming moisture flux results in anomalously dry conditions over Australia with a negative soil moisture anomaly. We show from the random-forest regression analysis that the local soil moisture, El Niño–Southern Oscillation (ENSO), and SSSP are the most important precursors for the northeast Australian rainfall whereas for the Brisbane region ENSO, SSSP, and the Indian Ocean dipole are the most important. The prediction of Australian rainfall using random-forest regression shows an improvement by including SSS from the prior season. This evidence suggests that sustained observations of SSS can improve the monitoring of the Australian regional hydrological cycle. 

The long-term trend of sea surface salinity (SSS) reveals an intensification of the global hydrological cycle due to human-induced climate change. This study demonstrates that SSS variability can also be used as a measure of terrestrial precipitation on inter-seasonal to inter-annual time scales, and to locate the source of moisture. Seasonal composites during El Niño Southern Oscillation/Indian Ocean Dipole (ENSO/IOD) events are used to understand the variations of moisture transport and precipitation over Australia, and their association with SSS variability. As ENSO/IOD events evolve, patterns of positive or negative SSS anomaly emerge in the Indo-Pacific warm pool region and are accompanied by atmospheric moisture transport anomalies towards Australia. During co-occurring La Niña and negative-IOD events, salty anomalies around the maritime continent (north of Australia) indicate freshwater export and are associated with a significant moisture transport that converges over Australia to create anomalous wet conditions. In contrast, during co-occurring El Niño and positive IOD events, there is the moisture transport divergence anomaly over Australia and results in anomalous dry conditions. The relationship between SSS and atmospheric moisture transport also holds for pure ENSO/IOD events but varies in magnitude and spatial pattern. The significant pattern correlation between the moisture flux divergence and SSS anomaly during the ENSO/IOD events highlights the associated ocean-atmosphere coupling. A case study of the extreme hydroclimatic events of Australia (e.g. 2010-11 Brisbane flood) demonstrates that the changes in SSS occur before the peak of ENSO/IOD events. This raises the prospect that tracking of SSS variability could aid the prediction of Australian rainfall.