Characterizing the diel variability of acoustic scattering layers near the Okinawa Islands using cyclostationary empirical orthogonal function

초록

The behavior of marine organisms is part of the oceanic carbon cycle and closely linked to climate change. To investigate biological behaviors such as vertical migration and aggregation layers near the Okinawa Islands, a year-long mooring observation was conducted from June 2015 to July 2016 using acoustic Doppler current profilers at the Okinawa Trough (KCM1) and the North Pacific side of the Okinawa Islands (RCM1). Acoustic backscattering intensity (Sv) served as an indicator of diel vertical migration (DVM) and aggregation layers. At both stations, nocturnal DVM was consistently observed between similar to 100-400 m, including a shallow scattering layer (SSL: 100-200 m) and a deep scattering layer (DSL: 300-400 m). Vertical migration speed was higher at KCM1 (3.6-5.6 cm s(-)(1)) than at RCM1 (2.2-3.2 cm s(-)(1)), likely reflecting differences in water masses. Using the cyclostationary empirical orthogonal function (CSEOF), 24-hour Sv cycles were decomposed into three principal modes and corresponding amplitude time series, primarily representing DVM, DSL, and SSL. DVM intensity increased with temperature above the SSL and showed a one-month lag with surface net solar radiation (SSR), suggesting surface warming as a potential driver. DSL depth was negatively correlated with surface temperature (0-30 m), while SSL showed a longer lag. During summer, therefore, enhanced SSR and warming could intensify DVM and deepen the scattering layers, potentially enhancing the biological carbon cycle. These results highlight the relationship of biological behaviors to surface warming and their role in carbon export near the Okinawa Islands.

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