The loss of Arctic sea ice has emerged as a leading signal of global warming. This, together with acknowledged impacts on other components of the Earth system, has led to the term “the new Arctic.” Global coupled climate models predict that ice loss will continue through the twenty-first century, with implications for governance, economics, security, and global weather. A wide range in model projections reflects the complex, highly coupled interactions between the polar atmosphere, ocean, and cryosphere, including teleconnections to lower latitudes. This paper summarizes our present understanding of how heat reaches the ice base from the original sources—inflows of Atlantic and Pacific Water, river discharge, and summer sensible heat and shortwave radiative fluxes at the ocean/ice surface—and speculates on how such processes may change in the new Arctic. The complexity of the coupled Arctic system, and the logistic and technological challenges of working in the Arctic Ocean, require a coordinated interdisciplinary and international program that will not only improve understanding of this critical component of global climate but will also provide opportunities to develop human resources with the skills required to tackle related problems in complex climate systems. We propose a research strategy with components that include 1) improved mapping of the upper- and middepth Arctic Ocean, 2) enhanced quantification of important process, 3) expanded long-term monitoring at key heat-flux locations, and 4) development of numerical capabilities that focus on parameterization of heat-flux mechanisms and their interactions.

The authors propose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates may be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profile ρ(z) and an imaginary profile ˆρ(z) constructed by reordering ρ(z) to make. it gravitationally stable. The resulting “reordering regions” are subjected to two tests. • The “Thorpe fluctuation” profile ρ′(z) = ρ(z) − ˆρ(z) is examined for “runs” of adjacent positive or negative values. The probability density function (PDF) of the run length is compared with the corresponding PDF of random noise. This yields a threshold value for rms run length within individual reordering regions that must be exceeded for adequate resolution of overturns, taking into account both CTD characteristics and local hydrographic properties. • Temperature and salinity covariations with respect to density are screened for systematic CTD errors ... Abstract The authors propose a scheme to test whether inversions in CTD density profiles are caused by overturning motions (from which mixing rates may be inferred) or by measurement noise. Following a common practice, possible overturning regions are found by comparing the observed profile ρ(z) and an imaginary profile ˆρ(z) constructed by reordering ρ(z) to make. it gravitationally stable. The resulting “reordering regions” are subjected to two tests. • The “Thorpe fluctuation” profile ρ′(z) = ρ(z) − ˆρ(z) is examined for “runs” of adjacent positive or negative values. The probability density function (PDF) of the run length is compared with the corresponding PDF of random noise. This yields a threshold value for rms run length within individual reordering regions that must be exceeded for adequate resolution of overturns, taking into account both CTD characteristics and local hydrographic properties. • Temperature and salinity covariations with respect to density are screened for systematic CTD errors ...

Convection occurs in ice-covered lakes if solar radiation warms near-surface water from the freezing point towards the temperature of maximal density. One effect of convective mixing may be to suspend non-motile phytoplankton in the upper water column, providing cells with enough light for growth during ice-covered periods. Observations of the diatom Aulacoseira baicalensis under the ice cover of Lake Baikal, Siberia, support the hypothesis that convective mixing causes net suspension of cells. This paper presents a theoretical examination of the conditions under which convective flow fields can suspend algae in the photic zone of the upper water column. It is shown that the efficiency of algal suspension depends on the ratio of the still-water algal sinking rate, W_p, to convective updraft speed, W_u. The suspension efficiency is also shown to be affected by asymmetries in the flow field and night-time cessation of convection, but only if W_p and W_u are comparable in value. It is concluded that convection in Lake Baikal should be vigorous enough to increase the mixed-layer residence time of A.baicalensis from a few days to over a month, at least during years with thin snow cover.