The long-term North Atlantic Cold Anomaly (Cold Blob) was largely the consequence of three major episodes of low sea surface temperature (SST) in the subpolar North Atlantic in 1972-74, 1984-85 and 1991-94. Each of these episodes correlated with unusually low SST at Flemish Cap (a subsurface island of the Canadian continental shelf 600km east of Newfoundland) and with periods of high sea-ice cover over the deep basin of the Labrador Sea a year earlier. These cold periods at Flemish Cap and the Cold Blob were associated with the advance of sea-ice and icebergs to Flemish Cap, high iceberg counts off the coast of Newfoundland and the appearance of icebergs along the path of the North Atlantic Current (NAC) east of Flemish Cap. Studies of SST anomalies provided evidence for surface connections between Flemish Cap and the CB which utilize part of the NAC pathway. We propose that in the cold periods, residual meltwater from sea-ice and icebergs conveyed in the Labrador Current to Flemish Cap was relayed via the NAC to the subpolar North Atlantic to form the Cold Blob. After 1995, anomalous ice expansion in the Labrador Sea basin greatly diminished, sea-ice and icebergs did not reach Flemish Cap and cold meltwater was no longer transmitted to the subpolar North Atlantic to sustain the Cold Blob. This improved understanding of 20th century meltwater pathways in the North Atlantic may relate to changes in the Atlantic Meridional Overturning Circulation and associated impacts on regional climate in the 21st century.

In each of the last three decades of the 20 th century there were unprecedented expansions of sea-ice over the Labrador Sea basin and influxes of cold fresh water into the subpolar gyre which have been described as the Great Salinity Anomalies (GSAs). Employing data for sea surface temperature, salinity and sea ice cover, we propose that these events were downstream consequences of the expansion and subsequent melting of so-called 'Odden' ice formed over the deep basin of the Greenland-Iceland-Norway (GIN) Sea in the 1960s, 1970s and1980s and additional to the normal East Greenland shelf sea-ice. We expand previous findings that Odden ice expansions were linked to winter episodes of high atmospheric pressure north of Greenland that directed freezing Arctic winds across the GIN Sea and may also have been associated with increased Arctic sea-ice volume leading to enhanced ice export through Fram Strait. We show that cold water and ice derived from Odden melting in the summer passed through Denmark Strait and along the East Greenland shelf, and accumulated in the Labrador Sea, creating favourable conditions for winter ice formation during particularly cold years in southwest Greenland. Meltwater from Odden and Labrador Sea ice appeared to break out into the subpolar gyre in the fall of 1982 and 1984 respectively and this cold water represents the likely source of the 1982-1985 GSA. These findings further our understanding of the physical processes linking ice formation and melt with ocean circulation in this key component of the climate system.

This study presents a new dataset of gauged streamflow (N=3,224) for Europe spanning the period 1962 to 2017. The Monthly Streamflow of Europe Dataset (MSED) is freely available at http://msed.csic.es/. Based on this dataset, changes in the characteristics of hydrological drought (i.e. frequency, duration, and severity) were assessed for different regions of Europe. Due to the density of the database, it is possible to delimit spatial patterns in hydrological droughts trend with the greatest detail available to date. Results reveal bidirectional changes in monthly streamflow, with negative changes predominating over central and southern Europe, while positive trends dominate over northern Europe. Temporally, two dominant patterns were noted. The first pattern corresponds to a consistent downward trend in all months, evident for southern Europe. A second pattern was noted over central and northern Europe and western France, with a predominant negative trend during warm months and a positive trend in cold months. For hydrological drought events, results suggest a positive trend toward more frequent and severe droughts in southern and central Europe and conversely a negative trend over northern Europe. This study emphasizes that hydrological droughts show complex spatial patterns across Europe over the past six decades, implying that hydrological drought behaviour in Europe has a regional character. Accordingly it is challenging to adopt “efficient” strategies and policies to monitor and mitigate drought impacts at the continental level.

Globally, thermodynamics explains an increase in atmospheric water vapor with warming of around 7%/°C near to the surface. In contrast, global precipitation and evaporation are constrained by the Earth’s energy balance to increase at ∼2–3%/°C. However, this rate of increase is suppressed by rapid atmospheric adjustments in response to greenhouse gases and absorbing aerosols that directly alter the atmospheric energy budget. Rapid adjustments to forcings, cooling effects from scattering aerosol, and observational uncertainty can explain why observed global precipitation responses are currently difficult to detect but are expected to emerge and accelerate as warming increases and aerosol forcing diminishes. Precipitation increases with warming are expected to be smaller over land than ocean due to limitations on moisture convergence, exacerbated by feedbacks and affected by rapid adjustments. However, these temperature-dependent changes offset rapid atmospheric adjustments to radiative forcings which tend to increase precipitation over land relative to the oceans. These factors therefore drive complex changes in the regional water cycle in time and space, some examples of which will be discussed. Thermodynamic increases in atmospheric moisture fluxes amplify wet and dry events, driving an intensification of precipitation extremes. The rate of intensification can deviate from a simple thermodynamic response due to in‐storm and larger‐scale feedback processes, while changes in large‐scale dynamics and catchment characteristics further modulate the frequency of flooding in response to precipitation increases. Changes in atmospheric circulation in response to radiative forcing and evolving surface temperature patterns are capable of dominating water cycle changes in some regions. Moreover, the direct impact of human activities on the water cycle through water abstraction, irrigation, and land use change is already a significant component of regional water cycle change and is expected to further increase in importance as water demand grows with global population. This talk will summarize recent advances in understanding past and future large-scale responses in the water cycle.

Global-scale changes in water vapor and responses to surface temperature variability since 1979 are evaluated across a range of satellite and ground-based observations, a reanalysis (ERA5) and coupled and atmosphere-only CMIP6 climate model simulations. Global-mean column integrated water vapor increased by 1\%/decade during 1988-2014 in observations and atmosphere-only simulations but coupled simulations overestimate trends because internal climate variability suppressed observed warming in this period. Decreases in low-altitude tropical water vapor in ERA5 and ground-based observations before around 1993 are inconsistent with simulations and increased column integrated water vapor in a satellite dataset since 1987. AIRS satellite data does not capture the increased tropospheric water vapor since 2002 in other satellite, reanalysis and model products. However, global water vapor responses to interannual temperature variability is consistent across datasets with increases of $\sim$4-5\% per K near the surface and 10-15\%/K at 300 hPa. Global water vapor responses are explained by thermodynamic amplification of upper tropospheric temperature changes and the Clausius Clapeyron temperature dependence of saturation vapor pressure that are dominated by the tropical ocean responses. Upper tropospheric moistening is larger in climate model simulations with greater upper tropospheric warming.