Oceanographic observations in supercooled water: Protocols for mitigation of measurement errors in profiling and moored sampling

摘要

Supercooled water is liquid water that is colder than its pressure- and salinity-dependent freezing temperature. As supercooling is a metastable state, it is sensitive to small environmental changes that may force switches between solid and liquid phases, including disruption from instruments introduced to the environment. Here we present a thoroughly-tested set of observational protocols that have been developed over 15 years working in supercooled Ice Shelf Water in McMurdo Sound, Antarctica. We identify the four issues most commonly encountered in observations of supercooled ocean water as (i) the requirement for an extended process of thermal equilibration, (ii) contamination by ice within conductivity cells, (iii) anchor ice accumulation on suspended equipment, and (iv) ice coating and resultant sensing of artificially-produced latent heat. We present protocols for identifying and managing these potential modes of corruption, which include determining and allowing appropriate time for thermal equilibration, identifying and eliminating data that have been corrupted by ice capture and growth, and practical steps that can be taken to minimise accumulation of anchor ice on suspended equipment. We stress the need for planned data redundancy to enable identification and elimination of data contaminated by ice capture and phase change. We also include a complete procedure for CTD profiling which includes a 10-min soak (for an instrument initially warm relative to the ocean) and exercises data redundancy in the form of multiple up- and down-casts. Finally, we discuss how these protocols may be extended to other platforms since there is increasing scope for ice-related contamination of data as access to ice-covered oceans, and the use of autonomous vehicles in the polar oceans, increases.