Conductivity, temperature and depth （CTD） data from 1993 2010 are used to study water tempera- ture in the upper Canada Basin. There are four kinds of water temperature structures： The remains of the winter convective mixed layer, the near-surface temperature maximum （NSTM）, the wind-driven mixed layer, and the advected water under sea ice. The NSTM mainly appears within the conductive mixed layer that forms in winter. Solar heating and surface cooling are two basic factors in the formation of the NSTM. The NSTM can also appear in undisturbed open water, as long as there is surface cooling. Water in open water areas may advect beneath the sea ice. The overlying sea ice cools the surface of the advected water, and a temperature maximum could appear similar to the NSTM. The NSTM mostly occurred at depths 10-30 m because of its deepening and strengthening during smnmer, with highest frequency at 20 m. Two clear stages of interannual variation are identified. Before 2003, most NSTMs were observed in marginal ice zones and open waters, so temperature maxima were usually warmer than 0~C. After 2004, most NSTMs occurred in ice-covered areas, with nmch colder temperature maxima. Average depths of the temperature maxima in most years were about 20 m, except for about 16 m in 2007, which was related to the extreme minimum of ice cover. Average temperatures were around 0.8~C to 1.1~C, but increased to around 0.5~C in 2004, 2007 and 2009, corresponding to reduced sea ice. As a no-ice summer in the Arctic is expected, the NSTM will be warmer with sea ice decline. Most energy absorbed by seawater has been transported to sea ice and the atmosphere. The heat near the NSTM is only the remains of total absorption, and the energy stored in the NSTM is not considerable. However, the NSTM is an important sign of the increasing absorption of solar energy in seawater.
Photosynthetically Available Radiation（PAR） is an important bio-optical parameter related to marine primary production.PAR is usually measured by a broadband sensor and can also be calculated by multispectral data.When the PAR is calculated by multispectral data in polar region,four factors are possible error sources.PAR could be overestimated as the wavelengths of multispectral instrument are usually chosen to evade main absorption zones of atmosphere. However,both PARs calculated by hyperspectral and multispectral data are consistent with an error less than 1%.By the fitting function proposed here,the PAR calculated by multispectral data could attain the same accuracy with that by hyperspectral data.To calculate the attenuation rate of the PAR needs PAR_0, the PAR just under the surface.Here,an approach is proposed to calculate PAR_0 by the best fit of the irradiance profile of 1-5 m with a content attenuation coefficient under surface.It is demonstrated by theory and observed data in different time at same location that the attenuation coefficient of PAR is independent of the intensity of radiation.But under sea ice,the attenuation coefficient of PAR is a little bit different,as the spectrum of the light has been changed by selective absorption by the sea ice.Therefore,the difference of inclusions inside the sea ice will result in different PAR,and impact on the attenuation of PAR.By the results of this paper,PAR can be calculated reliably by multispectral data.
