Oren, I was thinking the creation of white ice as surface ponds drained and the reduction of visible and UVR resulted in less heat to surface water below the ice via a reduction in insolation. Here is an excerpt from an article I posted month ago.
Stage I Prior to Melt Pond Onset on 15 June
Only 0.02 ± 0.01 of incoming PAR was transmitted through the snow-covered ice and spatial variability of light transmission did not change noticeably.
Stage II From 15 to 22 June
Once melt water became visible in large stretches at the ice surface, T¯(PAR)
increased by an order of magnitude to 0.31 on 22 June, while under-ice irradiance became increasingly variable.
Stage III From 23 June to 2 July
A short snowfall event followed by an enhanced surface melt resulted in discrete areas of white ice and melt pond, defining stage III. PAR transmittance and its spatial variability did not increase further during this stage. In fact, T¯(PAR)
measured along the ND transect decreased from 0.23 to 0.16.
The observed large drop in T¯(PAR)
measured along the ND transect on 28 June was attributed to the snowfall event. Unfortunately, surface albedo was not measured that day. Repeated measurements along this transects also showed more pronounced transmittance peaks beneath melt ponds while PAR transmittance below white ice became less variable over time (Supplementary Figure S3). These high transmittance values of discrete surface ponds became pronounced as outliers in the boxplots after the surface flooding in stage III. The larger areas of white ice transmitting less PAR compared to ponded ice also resulted in a skewed distribution and the median to be less than the calculated mean for most of the days within stage III. On the last sampling day, the variability in measured under-ice PAR levels decreased while T¯(PAR)
remained unchanged at 0.20. As shown in the aerial drone image of the sampling area on 2 July (Figure 4E), more white ice had emerged at the surface due to ongoing drainage of melt ponds, leading to a drop in the melt pond coverage and a more uniform sea ice surface. It should be noted that the proposed stages of changes in T¯(PAR)
are different from the stages of melt pond evolution described elsewhere (Eicken et al., 2002).
For the comparison of measured mean PAR transmittance and length-weighted average transmittance, T¯LW(PAR)
was calculated for all D transects. To do so, T(PAR) values of 0.16 to 0.24 beneath white ice and 0.25 to 0.40 beneath ponded ice, measured along four destructive transects, were used. As shown in Figure 5B, T¯(PAR)
and T¯LW(PAR)
were not significantly different (t(12) = 0.005, p = 0.996) over the sampling period.
The increase in the transmission of one wavelength (305 nm) in the UVB spectrum and three wavelengths (325, 340, and 379 nm) in the UVA spectrum at 2 m is shown for all transects over the sampling period (Figure 5C). Beneath snow-covered sea ice in stage I, T¯(UVA)
, ranged from 0.01 to 0.02, while UVB radiation was not detectable. It is noted that surface and transmitted irradiance were integrated over the UVA wavelength spectrum (320–400 nm) prior to estimating T¯(UVA)
. With melt pond onset, T¯(UVA)
increased to 0.26 by the end of stage II on 22 June. Also, UVB radiation was detectable beneath the ice cover with a T¯(305nm)
of 0.01. In stage III, transmission of UVA radiation did not increase further, displaying a mean of 0.21 ± 0.05 for D and ND transects. However, T¯(305nm)
was on average greater during stage III than stage II, reaching a mean value of 0.07 ± 0.06. During stage III, UVR transmittance remained relatively consistent, while the variability in measured under-ice UVR levels decreased. Furthermore, UVR transmission through melt ponds was twice as high than through white ice. TWI(305nm) and TMP(305nm) ranged from 0.03 to 0.08 and 0.11 to 0.14, respectively
https://www.frontiersin.org/articles/10.3389/fmars.2020.00183/full