[Abstract]

There has been a long history of interest on how (if) the absorption coefficient of “pure” fresh water (a_fw(λ)) and “pure” seawater (a_sw(λ)) changes with temperature (T), yet the impact of T reported in the literature differs significantly in the blue domain. Unlike the previous studies based on laboratory measurements, we took an approach based on ~18 years (2002–2020) of MODIS ocean color and temperature measurements in the oligotrophic oceans, along with field measured chlorophyll concentration and phytoplankton absorption coefficient, to examine the relationship between T and the total absorption coefficient (a(λ)) at 412 and 443 nm. We found that the values of a(412) and a(443) in summer is nearly flat (slightly decreasing) for the observed T range of ~19–27 ^oC. Since there is no detectable change of chlorophyll during this period, the results suggest that T has a negligible impact on a_sw(412) and a_sw(443) in this T range. As a complement, the impact of salinity on a_fw(λ) was also evaluated using two independent determinations of a_sw(λ) and a_fw(λ), where good agreements were found from these observations.

For the oligotrophic oceans (the white and red box in Figure 1), a_sw(λ) play a significant role in the total absorption coefficient (a(λ)) at the blue wavelengths. There are ~18 years of MODIS ocean color measurements in this region, we thus obtained the long-term, high-quality (a(λ)) (derived by QAA) at the blue wavelengths, as well as the concurrent MODIS T. We analyze the relationship between a(412), a(443) and T obtained from satellites and combining field measurements, to examine the relationship between T and a_sw(λ).

Figure 1. Location of data used in this study. The Hawaii Ocean Time-series (HOT; white star symbol);Center of the South Pacific gyre (cSPG, the white box) and the entire SPG (the red box).

For the summer months (December–February) at the “center” of the South Pacific Gyre (cSPG, figure 1), although T varied in a range of ~19–27 ^oC, values of a(412), a(443) and Chl in this period are generally stable. For oceanic “Case 1” waters, 412 nm and 443 nm phytoplankton absorption coefficient (a_ph) and combined absorption by detrital particulate matter and gelbstoff (a_dg) co-vary with Chl. Since a_ph and a_dg are expected to be stable, we infer that stable values of a_sw(λ) for T in a range of ~19–27 ^oC (figure 2). Such a stable pattern is further confirmed when analyzing the entire SPG (figure 3).

Figure 2. Climatology of 8-day a(412) (blue), a(443) (green), and T (orange) for waters of cSPG during the period of July 2002 to July 2020.

Figure 3. The occurrence frequency of slope between T and Chl (a), slope between T and a(412) (b), and T difference (c) in December–February for the entire SPG.

Wei G, Lee Z, Wu X, Yu X, Shang S, Letelier R. Impact of Temperature on Absorption Coefficient of Pure Seawater in the Blue Wavelengths Inferred from Satellite and In Situ Measurements[J]. Journal of Remote Sensing, 2021, 2021: 1-13.