Ocean colour remote sensing in high latitude environments (2)

Ocean colour remote sensing has often been used to study polar seas, especially in Antarctica where the optical properties of the upper ocean are not as complex as they are in the Arctic (Comiso et al., 1990, Comiso et al., 1993, Sullivan et al., 1993, Arrigo et al., 1998, Stramski et al., 1999, Arrigo et al., 2008b). It was shown based on OC data that primary production in Antarctic waters has changed little over the last 14 years (Arrigo et al., 2008b). In contrast, the few studies that have been conducted to date in the Arctic Ocean suggest that pan‐Arctic primary production, as well as photooxidation of coloured dissolved organic matter have been increasing (Belanger et al., 2006, Pabi et al., 2008, Arrigo et al., 2008a) as a consequence of receding perennial ice. The annual maximum phytoplankton biomass is now reached earlier in several Arctic seas (Kahru et al., 2010).
As the extent of the seasonal ice zone increases (difference between the annual maximum and minimum extents), ice‐edge blooms may play a heightened role (Perrette et al., 2011).
The on‐going changes within the context of accelerating climate change necessitate a vastly improved understanding of the polar ecosystems based on an intensive observation program. The use of ocean colour remote sensing in polar regions is, however, impeded by a number of difficulties and intrinsic limitations including:
The prevailing low solar elevations. At high latitudes, the Sun zenith angle is often larger than the maximum (generally 70°) for which atmospheric correction algorithms have been developed based on plane‐parallel radiative transfer calculations. Consequently, at high latitudes, a large fraction of the ocean surface is undocumented for a large part of the year even though primary production may be significant.
The impact of ice on remotely sensed reflectance. Belanger et al (2007) and Wang et Shi (2009), used radiative transfer simulations to examine the effects of the sea ice adjacency and sub‐pixel ice contamination on retrieved seawater reflectance and level‐2 ocean products. They found significant impacts within the first several kilometres from the ice‐edge and for concentrations of sub‐pixel ice floes exceeding a few percent.
The deep chlorophyll maximum (DCM). A DCM is very often observed both in the Antarctic and Arctic Oceans. In the Arctic Ocean, the freeze‐thaw cycle of sea ice and the large export of freshwater to the ocean by large Arctic rivers create pronounced haline stratification within the surface layer. In post‐bloom conditions, a deep‐chlorophyll maximum is associated with such vertical stratification.
Contrary to the DCM observed at lower latitudes (Cullen, 1982), the Arctic DCM often corresponds to a maximum in particulate carbon and primary production (Martin et al., 2010). The statistical relationships between surface chlorophyll and chlorophyll concentration at depth developed for lower latitudes (Morel et Berthon, 1989) are most probably not valid for the polar seas (Martin et al., 2010).
Ignoring the vertical structure of the chlorophyll profile in the Arctic Ocean leads to significant errors in the estimation of the areal primary production (Pabi et al., 2008, Hill et Zimmerman, 2010).
The peculiar phytoplankton photosynthetic parameters. The low irradiance and seawater temperature prevailing in polar seas are associated with unique biooptical and photosynthetic parameters characteristic of extreme environments (Rey, 1991) that must be accounted for in primary production models. To date, only a few studies have attempted to do so in the Arctic Ocean (Arrigo et al., 2008b).
The optical complexity of seawater, especially over the Arctic shelves. Because of the important freshwater inputs, the Arctic continental shelves, which occupy 50% of the area, are characterized by high concentrations of CDOM (Matsuoka et al., 2007, Belanger et al., 2008). Also, as a consequence of photoacclimation to low irradiances, phytoplankton cells often contain large amounts of pigments. The chlorophyll‐specific absorption coefficient is therefore particularly low due to pronounced pigment packaging (Cota et al., 2003, Wang et al., 2005). Because of these optical peculiarities, standard ocean colour algorithms do not work in the Arctic Ocean (Cota et al., 2004, Matsuoka et al., 2007).
The persistence of clouds and fog. High latitudes are known to present a heavy cloud cover. In addition, as soon as sea ice melts and opens waters come in direct contact with the atmosphere, fog develops near the sea surface. These features limit the usage of ocean colour data.
This lecture will cover all of the topics mentioned above and will be organized into two parts (90’ each) as detailed below :
1 Ocean colour remote sensing in polar seas
Ocean, sea ice and atmosphere in Arctic and Antarctic: relevant features
Seawater optical properties
Retrieval of ocean properties from ocean colour:
Atmospheric corrections
Contamination of the signal by sea ice
Retrieval of IOPs and AOPs, and biogeochemically relevant variables
Availability of data as favoured by polar orbits and limited by elevated Cloudiness
2 Primary production estimates from OC in polar seas
General features of Arctic and Antarctic Oceans related to PP (phytoplankton species, annual cycle of PP, nutrients, DCM)
PP models and their validation
Results from PP models
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