Effect of sea‐ice melt on inherent optical properties and vertical distribution of solar radiant heating in Arctic surface waters

Abstract

The inherent optical properties (IOPs) of Polar Waters (PW) exiting the Arctic Ocean in the East Greenland Current (EGC) and of the inflowing Atlantic waters (AW) in the West Spitsbergen Current (WSC) were studied in late summer when surface freshening due to sea‐ice melt was widespread. The absorption and attenuation coefficients in PW were significantly higher than previous observations from the western Arctic. High concentrations of colored dissolved organic matter (CDOM) resulted in 50–60% more heat deposition in the upper meters relative to clearest natural waters. This demonstrates the influence of terrigenous organic material inputs on the optical properties of waters in the Eurasian basin. Sea‐ice melt in CDOM‐rich PW decreased CDOM absorption, but an increase in scattering nearly compensated for lower absorption, and total attenuation was nearly identical in the sea‐ice meltwater layer. This suggests a source of scattering material associated with sea‐ice melt, relative to the PW. In the AW, melting sea‐ice forms a stratified surface layer with lower absorption and attenuation, than well‐mixed AW waters in late summer. It is likely that phytoplankton in the surface layer influenced by sea‐ice melt are nutrient limited. The presence of a more transparent surface layer changes the vertical radiant heat absorption profile to greater depths in late summer both in EGC and WSC waters, shifting accumulation of solar heat to greater depths and thus this heat is not directly available for ice melt during periods of stratification.

Characteristics of chromophoric and fluorescent dissolved organic matter in the Nordic Seas

“…Recorded chlorophyll a fluorescence intensity signals, I FChla , were reported as analog voltage output in the range 0-5 V DC. The instrument setup is described in detail in Granskog et al (2015b).…”

“…= 450 nm) distribution presented by Granskog et al (2015b). The humic-like fraction of DOM in the eastern Fram Strait is more than 10 times lower compared to PW in the EGC (Granskog et al, 2015b). A 20 m layer of less saline water diluted by sea ice melt characterized by significantly lower humic-like FDOM intensity overlaid the PW water with high FDOM intensity in the EGC (Granskog et al, 2015b).…”

“…= 280 nm and em. = 450 nm) distribution presented by Granskog et al (2015b). The humic-like fraction of DOM in the eastern Fram Strait is more than 10 times lower compared to PW in the EGC (Granskog et al, 2015b).…”

“…Once entered or produced in surface waters of the Arctic Ocean, CDOM has a significant influence on heating of the uppermost ocean layer and its stratification (Pegau, 2002;Hill, 2008;Granskog et al, 2007Granskog et al, , 2015b. Particularly in the absence of sea ice, light absorbed by CDOM in the visible part of the spectrum limits the light available for photosynthetic organisms (Arrigo and Brown, 1996) but also shields marine ecosystems from potentially harmful ultraviolet radiation by strongly absorbing electromagnetic radiation in the UVB and UVA bands (Erickson III et al, 2015).…”

The spatial and interannual dynamics of the surface water carbonate system and air–sea CO<sub>2</sub> fluxes in the outer shelf and slope of the Eurasian Arctic Ocean

“…Increasing ice-free water area leads to lower albedo and increased surface layer heating. Moreover, the supply of large quantities of optically active dissolved OM and suspended material (Dudarev et al, 2006;Sánchez-García et al, 2014;Vonk et al, 2014;Charkin et al, 2015) also promotes the accumulation of solar radiation in the surface layer, which increases the heat content, leading to further lengthening of the ice-free season (Granskog et al, 2007(Granskog et al, , 2015Hill, 2008;Logvinova et al, 2016). Thus, water heating leads to additional increasing pCO 2 in the surface water.…”