Nonlinear response of iceberg side melting to ocean currents

Abstract

Icebergs calving into Greenlandic Fjords frequently experience strongly sheared flows over their draft, but the impact of this flow past the iceberg is not fully captured by existing parameterizations. We present a series of novel laboratory experiments to determine the dependence of submarine melting along iceberg sides on a background flow. We show, for the first time, that two distinct regimes of melting exist depending on the flow magnitude and consequent behavior of melt plumes (side‐attached or side‐detached), with correspondingly different meltwater spreading characteristics. When this velocity dependence is included in melt parameterizations, melt rates estimated for observed icebergs in the attached regime increase, consistent with observed iceberg submarine melt rates. We show that both attached and detached plume regimes are relevant to icebergs observed in a Greenland fjord. Further, depending on the regime, iceberg meltwater may either be confined to a surface layer or distributed over the iceberg draft.

Greenland iceberg melt variability from high-resolution satellite observations

“…The apparent decrease in the melt rate below 350 m depth reflects one observation from March 2011, when melt rates were particularly low, as discussed more below. Although the dip in melt rates at ∼ 200 m depth is not significant (i.e., does not exceed the uncertainty of neighboring bins), it coincides with the approximate depth of the interface between the colder near-surface waters and warmer subsurface waters observed in Jakobshavn's fjord (Ilulissat Icefjord) (Gladish et al, 2015) and the Upernavik fjord system (Fenty et al, 2016), where water velocities should be relatively slow and turbulent melting should reach a local minimum (Moon et al, 2017). These observations suggest that our remote sensing method may be capable of resolving the depth of the near-and subsurface water interface where hydrographic observations are difficult or impossible to acquire, such as near the termini of calving glaciers.…”

“…Although detailed in situ hydrographic analyses of Greenland's glacial fjords are limited in space and time, existing observations indicate that there are much steeper gradients in water temperature and velocity in the vertical plane (i.e., with depth) than in the horizontal plane (i.e., along fjord) (Sutherland et al, 2014;Bendtsen et al, 2015;Gladish et al, 2015;Jackson and Straneo, 2016). As such, we expect to find pronounced variations in melt rates for icebergs that do and do not penetrate into the relatively warm and salty water masses found below ∼ 100-200 m depth around the ice sheet periphery (Straneo et al, 2012;Moon et al, 2017) but no discernible variations in melt rates with distance from the parent glacier.…”

“…Where iceberg residence times can be estimated from, for example, iceberg tracking (Sulak et al, 2017), these data can be paired with remotely sensed iceberg size and area distributions Sulak et al, 2017) and empirical iceberg melt rates to estimate iceberg freshwater fluxes. However, there are only a handful of locations around Greenland where there are sufficient water temperature and velocity records to constrain empirical iceberg melt rate estimates in iceberg-congested fjords (e.g., Bendtsen et al, 2015;Gladish et al, 2015;Jackson and Straneo, 2016). To address the dearth of iceberg melt rate estimates in Greenland's fjords, here we use a satellite remote sensing method to construct time series of submarine melt rates and meltwater fluxes for icebergs calved from seven large outlet glaciers spanning the periphery of the Greenland Ice Sheet (Fig.…”

Sensitivity of ocean biogeochemistry to the iron supply from the Antarctic Ice Sheet explored with a biogeochemical model

“…The vertical grid follows a partialstep z-coordinate scheme and has 75 levels with 25 levels in the upper 100 m. Lateral mixing is computed along isoneutral surfaces (Madec, 2008). Mesoscale eddy-induced turbulence follows the Gent and McWilliams (1990) parameterization, and vertical mixing is parameterized using the turbulent kinetic energy scheme (Blanke and Delecluse, 1993) as modified by Madec (2008). The biogeochemical model PISCES simulates two phytoplankton functional types (diatoms and nanophytoplankton), two zooplankton size classes (microzooplankton and mesozooplankton), the biogeochemical cycles of five limiting nutrients (NO 3 , PO 4 , NH 4 , Si(OH) 4 , and Fe), dissolved oxygen, dissolved inorganic carbon, total alkalinity, dissolved organic matter, and small and large organic particles.…”

Prospects for seasonal forecasting of iceberg distributions in the North Atlantic

“…Vertical shear in ocean currents and temperature stratification may substantially alter both the dynamics and melting rates of icebergs in baroclinic and stratified environments. This is most evident in fjords (FitzMaurice et al 2016), for which a new parameterisation has recently been proposed (FitzMaurice et al 2017). However, in the current study, we largely address advection and melting of icebergs in the strongly barotropic (low shear) Labrador Current, where temperature stratification is limited.…”