At a Glance
Climate changes involve atmospheric motions, ocean flows, and evolution of ice on land and in the sea. These dynamics are closely interrelated; insights into individual processes can help to illuminate poorly understood aspects of global climate dynamics, such as factors affecting the maintenance of sea ice cover in the Arctic basin. Sea ice cover can impact fresh water fluxes, local ecology and ocean circulation. The Stone group is providing simplified models for understanding the movement and distribution of ice during the formation of polynyas, which refer to localized regions of water surrounded by ice, and through narrow straits, which can affect flow, mixing, and ecology in the ocean. The approach seeks to draw generalizations valid for various geometric and climate conditions.
A polynya is a region of persistent open ocean water surrounded by sea ice and/or rigid boundaries, such as a coastline, land-grounded ice or ice shelves (Figure 5.1); effectively it is a “hole of ice.” Polynyas remain open from a regional balance between the rate of ice production (due to freezing seawater) and the rate of ice depletion, for example, due to flow. Such polynyas may exist either in the open ocean or close to coastal boundaries. The latter are formed when winds sweep ice away from the coast, exposing open seawater that freezes to form new ice. Thus, these coastal polynyas, especially along the Antarctic coast and some regions of the Arctic, are an important source of new sea ice, are crucial to ocean-atmosphere energy, momentum and moisture exchanges, and are thought to regulate thermohaline circulation, i.e., the circulation of temperature and salinity, in the ocean. In addition, phytoplankton and other marine life thrive in polynyas, especially in summer months, and so these water-ice structures are important to ecology.
Although some qualitative mechanisms of polynya formation have been identified, modeling their extent precisely has proved challenging. Previous attempts have relied mostly on high-resolution numerical simulations, where a clear connection between polynya formation dynamics and the mechanical stresses due to the ice motion is lacking. Recently, the Stone group has succeeded in developing simplified descriptions of ice motion due to wind in the context of ice bridge formation in straits, taking into account the frictional stresses in ice. These simplified models represent the mechanical behavior of ice and ice flows, agreeing both with available measurements and with numerical simulations. This approach to seeking physically relevant simplified descriptions forms the basis for our more recent investigation of coastal polynyas.
The Stone group’s current ice-related research efforts are focused on mathematical modeling wind- driven polynya formation in coastal regions, including islands and fjords. As with the group’s previous studies of ice flows, they interacted with Dr. Michael Winton at the Geophysical Fluid Dynamics Laboratory (GFDL). These studies of polynyas quantify the formation of new ice by freezing seawater, while incorporating findings from the group’s previous work to quantify the stresses and motion of the formed ice in response to wind. The study includes a fully resolved numerical model, consistent with more sophisticated models published in the literature, as well as a simplified model, designed to capture important physical characteristics, to predict the roles of freezing (ice production), flow, and ice accumulation in determining the extent of coastal polynyas. The team has demonstrated that the theoretical predictions agree quantitatively with the results of direct numerical simulations with and without considering the curvature of the coastline. The combination of modeling approaches provides clear connections between the mechanics of sea ice motion and the thermodynamics of sea ice production. Consequently, these modeling efforts not only explain a complex geophysical phenomenon but also provide a means to refine the modeling of sea ice in the more general context of Arctic and Antarctic ice flows near land boundaries.
Zhu, L., B. Rallabandi, M. Winton, and H.A. Stone, 2019. An analytical model of wind-driven formation of coastal polynyas. Preprint.