Tidal Variability of Chl-a in the Indonesian Seas

Abstract

Abstract. Harmonic analysis of time series from 20 years of MODIS-Aqua ocean color observations (2002–2022) is conducted to identify periodic variability of near-surface chlorophyll (Chl-a) inferred from ocean color. As they are based on satellite imagery, the Chl-a observations are characterized by significant gaps in both spatial and temporal coverage due to the masking of clouds in the images. Results yield a coherent picture of surface Chl-a associated with the time mean, annual and semiannual cycles, and spring-neap tidal variability. Spring-neap variability is heterogeneous and associated with regions of significant baroclinic tides as well as coastal regions with strong tidal currents. The observations provide another line of evidence for the significant contribution of ocean tides to mixing in the Indonesian Seas.

Impact of internal tides on distributions and variability of Chlorophyll-a and Nutrients in the Indonesian Seas

“…Regarding the surface Chl-a variability in relation to internal tides' effects, we show that M2 and S2 semidiurnal tides combine to produce a fortnightly (14.8 days, spring tides-neap tides) modulation (Figure 9 a), that was already documented on the SST field in previous studies [Ffield and Gordon, 1992;1996;Ray and Susanto, 2016;Nugroho et al, 2017], and on the Chl-a in a companion paper of Zaron et al [2022]. Over a fortnight the chlorophyll-a range is between 0.06 and 0.1 in the main regions of intensified mixing induced by internal tides and this distribution well compares with the same estimate from satellite-retrieved observations ( The observed biogeochemical tracers, as the physical ones, at the exit of Halmahera sea, can be reproduced with only the 4 days of vertical diffusivity at station 1 and 3 (2 days each) meaning that tidally -induced diapycnal mixing triggers most of the nutrients' vertical changes in the areas where internal tides are particularly energetic.…”

“…This study extends the findings of Zaron et al [2022] on the tidal low-frequency variability of chlorophyll-a to those regions of the IA known as hotspots of internal wave-driven mixing, as shown in their TPX09 diagnostics (Figure 7 of their paper), but where the harmonic analysis of the gappy satellite data hampered to retrieve a significant signal at the MSf frequency. In fact, using the output of a coupled numerical simulation, forced by explicit tides, we were able to map most of the Chl-a components phase-locked with spring/neap tidal cycle and to also depict their manuscript submitted to Journal of Geophysical Research-Oceans marked seasonality.…”

“…Figure9b) and analyzed through theZaron [2018]'s software, able to treat gappy and noisy data as the one we used from the MODIS case-1 product (Figure9 b, d and f). More detail on this analysis can be found inZaron et al [2022]. A stronger signal in the modeled and observed MSf amplitude of Chl-a was detected during the southeast monsoon (July to September, Fig.9e and f), in correspondence to the more enhanced cooling occurring at the surface in this season (Figure10cofNugroho et al [2018]).…”

“…Also, these maps use spatially-smoothed data, with an averaging scale of about 1/4°. Further details on the technical aspects of the 'averaging' methodology are out of the scope of the present paper and constitute the topic of a companion manuscript in preparation byZaron et al [2022]. What we reckon important to underline for the present study is the tidal signature at the MSf frequency detected on the Chl-a concentration, that we were able to capture both in the modeled and observed estimates and showing as well a strong seasonality in relation to the Indonesian monsoonal system.…”

“…Besides, our results complete the picture of Atmadipoera et al [2022] on the vital role assumed by internal tides' mixing in transforming thermohaline properties and controlling the vertical distribution of oxygen measured in the Halmahera Sea, and demonstrate that nutrients' variability are also driven by such a mechanism in this Indonesian subregion. We additionally suggest that for the Chl-a, these turbulent interactions are at play even at other sites of enhanced mixing and, in particular, over the Java and Arafura plateaus, where barotropic tides on the shelf produce intensified mixing interacting with the bottom [Nugroho, PhD;Zaron et al 2022].…”