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  1. Abstract Prediction of ice formation in clouds presents one of the grand challenges in the atmospheric sciences. Immersion freezing initiated by ice-nucleating particles (INPs) is the dominant pathway of primary ice crystal formation in mixed-phase clouds, where supercooled water droplets and ice crystals coexist, with important implications for the hydrological cycle and climate. However, derivation of INP number concentrations from an ambient aerosol population in cloud-resolving and climate models remains highly uncertain. We conducted an aerosol–ice formation closure pilot study using a field-observational approach to evaluate the predictive capability of immersion freezing INPs. The closure study relies on collocated measurements of the ambient size-resolved and single-particle composition and INP number concentrations. The acquired particle data serve as input in several immersion freezing parameterizations, which are employed in cloud-resolving and climate models, for prediction of INP number concentrations. We discuss in detail one closure case study in which a front passed through the measurement site, resulting in a change of ambient particle and INP populations. We achieved closure in some circumstances within uncertainties, but we emphasize the need for freezing parameterization of potentially missing INP types and evaluation of the choice of parameterization to be employed. Overall, this closure pilot studymore »aims to assess the level of parameter details and measurement strategies needed to achieve aerosol–ice formation closure. The closure approach is designed to accurately guide immersion freezing schemes in models, and ultimately identify the leading causes for climate model bias in INP predictions.« less
  2. Stratocumulus clouds over the Southern Ocean have fewer droplets and are more likely to exist in the predominately supercooled phase than clouds at similar temperatures over northern oceans. One likely reason is that this region has few continental and anthropogenic sources of cloud-nucleating particles that can form droplets and ice. In this work, we present an overview of aerosol particle types over the Southern Ocean, including new measurements made below, in and above clouds in this region. These measurements and others indicate that biogenic sulfur-based particles >0.1 μm diameter contribute the majority of cloud condensation nuclei number concentrations in summer. Ice nucleating particles tend to have more organic components, likely from sea-spray. Both types of cloud nucleating particles may increase in a warming climate likely to have less sea ice, more phytoplankton activity, and stronger winds over the Southern Ocean near Antarctica. Taken together, clouds over the Southern Ocean may become more reflective and partially counter the region’s expected albedo decrease due to diminishing sea ice. However, detailed modeling studies are needed to test this hypothesis due to the complexity of ocean-cloud-climate feedbacks in the region.
  3. Dense water masses formed in the Nordic Seas flow across the Greenland-Scotland Ridge and provide a major contribution to the lower limb of the Atlantic Meridional Overturning Circulation. Originally considered an important source of dense water, the Iceland Sea regained focus when the North Icelandic Jet - a current transporting dense water from the Iceland Sea into Denmark Strait - was discovered in the early 2000s. Here we use recent hydrographic data to quantify water mass transformation in the Iceland Sea and contrast present conditions with measurements from hydrographic surveys conducted four decades earlier. We demonstrate that substantial changes in the large-scale hydrographic structure and in the properties of the locally formed dense waters have taken place over this period in concert with a retreating ice edge and diminished ocean-to-atmosphere heat fluxes. This development has impacted the properties of the dense water masses available to supply the North Icelandic Jet.
  4. The dense outflow through Denmark Strait is the largest contributor to the lower limb of the Atlantic Meridional Overturning Circulation. While its hydrographic structure is well documented, a full description of the velocity field across the strait remains incomplete. Here we analyze a set of 22 shipboard hydrographic and velocity sections occupied along the Látrabjarg transect at the Denmark Strait sill, obtained over the time period 1993-2018. The sections provide the first complete view of the kinematic components at the sill: the shelfbreak East Greenland Current (EGC), the combined flow of the Separated EGC and the North Icelandic Jet (NIJ), and the northward flowing North Icelandic Irminger Current (NIIC). We deconstruct the dense overflow in terms of water mass constituents and flow components, demonstrating that the combined EGC branches and NIJ transport comparable amounts. A strong cyclonic structure was present in two-thirds of the occupations, which is thought to be due to the combined effect of eddies and wind. Strong negative wind stress curl north of the strait intensifies the separated EGC, while the enhanced northerly winds under these conditions strengthen the NIIC and cause it to shift the west. Both the cyclonic and non-cyclonic flow states can be super-criticalmore »in different parts of the strait, leading to symmetric instability and enhanced mixing. A proxy is used to assess this condition in a larger set of shipboard crossings with hydrography only, elucidating the degree to which mesoscale features drive such mixing.« less