Glaciers that terminate in the ocean (i.e., marine-terminating or tidewater glaciers) lose mass at their marine margins through iceberg calving, meltwater runoff and submarine melting. The stability of a glacier is dependent on a balance between mass entering and exiting a catchment. In contrast with land-terminating glaciers, marine-terminating glaciers are able to undergo rapid changes in mass because changes in ocean forcing can lead to rapid changes in the calving and/or submarine melt rates, which can influence glacier flow dynamics by controlling the location and shape of the glacier terminus (which can be grounded or floating). The location and shape of the terminus influences the balance of stresses controlling ice flow; thus, changes in ice-ocean interactions have the potential to strongly influence a glacier’s mass balance.
The magnitude and timing of iceberg calving can be quantified using remote sensing and in situ observations (e.g., satellite imagery, time lapse photography, scanning lidar, and seismic measurements), however the processes controlling calving are poorly understood. Observations and modeling suggest that calving is a two-stage process: (1) fracture/detachment and (2) seaward transport. Transport of the detached ice can be influenced by both the thickness of the glacier relative to the depth of the neighboring ocean and the rigidity of ice mélange (a mixture of sea ice and icebergs).
Due to the difficulty in predicting the magnitude and timing of calving events, the proglacial environment of a tidewater glacier is not easily accessible and direct measurements of submarine melting and subglacial runoff are difficult to collect. Numerical and physical models used to look at this relationship have found that the enhancement of submarine melting occurs when cold fresh buoyant meltwater from the subglacial system entrains warm saline ocean water as it moves up the glacier terminus towards the ocean surface. As such, the magnitude of submarine melting will vary with both the ocean water temperature and the strength of the rising subglacial meltwater plume. Estimated melt rates suggest that submarine melting can be on the order of meters per day for some glaciers but vary widely between glaciers. It has also been suggested that an increase in submarine melt rates in the 1990s and 2000s may have triggered the recent rapid changes in ice flow at numerous outlet glaciers draining the Greenland and West Antarctic ice sheets. However, the construction of submarine melt rate time series is hindered by the scarcity of in situ hydrographic observations and the limitations of remote sensing techniques, preventing a thorough analysis of temporal changes in submarine melting with respect to changes in glacier behavior.
Tidewater glacier stability and the impact of tidewater glaciers on their environment is complex due to the numerous feedbacks occurring in the glacier ice-ocean system. The impact of changes in the glacier-ocean system is not only limited to sea level rise, but also smaller scale changes such as local biologic communities that rely on calved icebergs for breeding or nutrient-rich meltwater plumes for food. Thus, it is imperative that research efforts continue to focus on developing a better understanding of glacier ice-ocean interactions.
A great overview of the current understanding of glacier ice-ocean interactions, critical knowledge gaps, and recommended research plans/objectives can be found in the report published by the US CLIVAR Greenland Ice Sheet-Ocean Interactions Working Group found by clicking here.
Sea ice is a key indicator of the global climate change. Recent decades have been marked by rapid sea ice decline in the Arctic Ocean. In contrast, no significant decrease in Southern Ocean sea ice has been observed. Although the widespread changes in sea ice are concurrent with changing atmospheric and oceanographic conditions, sea ice-climate models are generally not capable of properly simulating the observed variability of Arctic and Antarctic sea ice cover. The failure of these models is likely due to the poor understanding of processes governing sea ice-ocean interactions. As such, a better understanding of sea ice-ocean interactions and an improved parameterization in climate and sea ice in Earth system models must be developed.
An important aspect of sea ice-ocean interactions that warrants further exploration and model development is the interaction of sea ice and the ocean with the atmospheric boundary layer. The formation and melting of sea ice in the polar regions are critical processes that must be included in Earth system models because the associated heat, moisture, momentum and gas exchanges at the ocean-sea ice-atmosphere interface are strongly influenced by changes in sea ice cover. Additionally, changes in sea ice cover influence the penetration of solar radiation and wind-induced turbulent mixing of the upper ocean layer, which will influence the biogeochemical cycling and ecosystem functioning in the upper ocean layer and lower atmosphere.
It is important to remember that the current state of knowledge regarding sea ice-ocean interactions was developed over the past several decades; a time period marked by a shift from relatively stable sea ice cover to the most recent period of declining sea ice extent and thickness. As such, the recent changes in sea ice extent, thickness, and distribution (particularly in the Arctic Ocean) have revealed knowledge gaps that must be addressed by the scientific community in a timely manner. For example, the recent transition from widespread multi-year sea ice towards predominantly first-year ice in the Arctic may enable the penetration of a more solar radiation beneath the ice than observed in the past. Consequently, the upper-ocean warming associated with increased penetration of solar radiation can contribute to enhanced melting of sea ice, further enhancing absorption of solar radiation by the upper ocean layer (i.e., positive feedback loop). Changes in sea ice cover and associated changes in the upper ocean temperature and salinity can lead to changes in sea ice algae/phytoplankton productivity, which will in turn influence the marine food web and carbon cycling. Interconnections such as these make the study of sea ice-ocean interactions an inherently multidisciplinary task.