Eko Siswanto

Mapping the low salinity Changjiang Diluted Water using satellite‐retrieved colored dissolved organic matter (CDOM) in the East China Sea during high river flow season

Absorption coefficients of colored dissolved organic matter (CDOM) [a g (λ)] were measured and relationship with salinity was derived in the East China Sea (ECS) during summer when amount of the Changjiang River discharge is large. Low salinity Changjiang Diluted Water (CDW) was observed widely in the shelf region and was considered to be the main origin of CDOM, resulting in a strong relationship between salinity and a g (λ). Error of satellite a g (λ) estimated by the present ocean color algorithm could be corrected by satellite-retrieved chlorophyll data. Satellite-retrieved salinity could be predicted with about ±1.0 accuracy from satellite a g (λ) and the relation between salinity and a g (λ). Our study suggests that satellite-derived a g (λ) can be an indicator of the low salinity CDW during summer.

Enhancement of phytoplankton primary productivity in the southern East China Sea following episodic typhoon passage

The enhancement of primary productivity (PP enh ) in the southern East China Sea (ECS) following 16 typhoon passages was investigated using ocean color data and a primary productivity model. PP enh tended to be higher when typhoons traversed slowly with trajectories that allowed strong southerly winds to prevail over Yonaguni Island. Such long-lasting southerly winds were believed to push the Kuroshio current axis shelfward, enhancing the upwelling of nutrients, hence promoting new productivity (NP). The importance of long-lasting southerly winds as a proxy for physical perturbations underlying PP enh was expressed by an empirical equation by which 88% of PP enh variation could be explained. Applying this equation, we assessed that typhoon passages accounted for a minimum of 0.6― 11.8% of the ECS summer―fall NP, suggesting that typhoon passage over the southern ECS is an important phenomenon supporting NP in the ECS.

Assessing post-tsunami effects on ocean colour at eastern Indian Ocean using MODIS Aqua satellite

The 2004 Indian Ocean tsunami had been reported to cause vast destruction on marine environment, but its effect on the primary production and upwelling event was unknown. Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua ocean colour imageries were used to assess the tsunami effects on the chlorophyll‐a (chl‐a) and turbidity variations. Chlorophyll‐a and normalized water leaving radiance at 551 nm (nLw 551) before and after the tsunami event were processed using SeaWiFS data analysis system (SeaDAS), and the variations were examined. Results showed a sudden increase of chl‐a and nLw 551 at the north‐western Sumatra and middle of Malacca Straits (MS) after the tsunami. At western Sumatra, high sedimentation caused by the backwash resulted in overestimation of the chl‐a values. However, the chl‐a in the upwelling area in the northern MS did not exhibit any significant changes. The chl‐a variation in the northern MS was found to be related to the monsoon wind with about one week time lag. At the seriously eroded coastline along western Sumatra, continuous sedimentation was observed. The sedimentation problem became more severe during higher rainfall periods in April 2005. We strongly suggest a long term monitoring of this sedimentation problem.

Sixteen-year phytoplankton biomass trends in the northwestern Pacific Ocean observed by the SeaWiFS and MODIS ocean color sensors

Using multisensor/platform biophysical data collected from 1997 to 2013, we investigated trends of the concentrations of phytoplankton biomass (Chl) in the northwestern Pacific Ocean (NWPO) and the probable responsible factors. The trend of rising sea surface temperature (SST) was the main factor maintaining phytoplankton positive net growth and resulted in a trend of increasing Chl at high latitudes in all seasons. At latitudes of 36–46°N, east of 160°E, the trend of rising SST was accompanied by a trend of declining Chl, markedly in spring and fall, which could be ascribed to strengthened stratification. The trends of environmental variables in the Oyashio area have modified conditions in a way detrimental to phytoplankton growth, the result being a trend of declining Chl from spring to fall. Chl south of roughly 36°N exhibited different trends in different seasons because of the different trends of vertical stratification. Whereas the observed 16-year Chl trends were not primarily influenced by interannual climate variability, to some degree they were likely modified by decadal variability associated with a weakened Aleutian Low pressure. This work prompts further comprehensive studies to investigate the probable ecological consequences of the observed Chl trend for high-trophic-level marine organisms in the NWPO.

Comparison of carbon cycle between the western Pacific subarctic and subtropical time-series stations: highlights of the K2S1 project

A comparative study of ecosystems and biogeochemistry at time-series stations in the subarctic gyre (K2) and subtropical region (S1) of the western North Pacific Ocean (K2S1 project) was conducted between 2010 and 2013 to collect essential data about the ecosystem and biological pump in each area and to provide a baseline of information for predicting changes in biologically mediated material cycles in the future. From seasonal chemical and biological observations, general oceanographic settings were verified and annual carbon budgets at both stations were determined. Annual mean of phytoplankton biomass and primary productivity at the oligotrophic station S1 were comparable to that at the eutrophic station K2. Based on chemical/physical observations and numerical simulations, the likely “missing nutrient source” was suggested to include regeneration, meso-scale eddy driven upwelling, meteorological events, and eolian inputs in addition to winter vertical mixing. Time-series observation of carbonate chemistry revealed that ocean acidification (OA) was ongoing at both stations, and that the rate of OA was faster at S1 than at K2 although OA at K2 is more critical for calcifying organisms.

Storm-induced changes in pCO2 at the sea surface over the northern South China Sea during Typhoon Wutip

In-situ oceanographic measurements were made before and after the passage of Typhoon Wutip in September 2013 over the northern South China Sea. The surface geostrophic circulation over this region inferred from satellite altimetry data features a large-size anti-cyclonic eddy, a small-size cyclonic eddy, and smaller-size eddies during this period. Significant typhoon-induced changes occurred in the partial pressure of CO2 at the sea surface (pCO2sea) during Wutip. Before the passage of Wutip, pCO2sea was about 392.92±1.83, 390.31±0.50, and 393.04±4.31 μatm over the cyclonic eddy water, the anti-cyclonic eddy water, and areas outside two eddies, respectively. The entire study region showed a carbon source (1.31±0.46 mmol CO2 m−2 d−1) before Wutip. In the cyclonic eddy water after Wutip, high sea surface salinity (SSS), low sea surface temperature (SST), and high pCO2sea (413.05±7.56 μatm) made this area to be a carbon source (3.30±0.75 mmol CO2 m−2 d−1). In the anti-cyclonic eddy water after Wutip, both the SSS and SST were lower, pCO2sea was also lower (383.03±3.72 μatm), and this area became a carbon sink (-0.11±0.55 mmol CO2 m−2 d−1), in comparison with the pre-typhoon conditions. The typhoon-induced air-sea CO2 flux reached about 0.03 mmol CO2 m−2 d−1. Noticeable spatial variations in pCO2sea were affected mainly by the typhoon-induced mixing/upwelling and vertical stratifications. This study suggests that the local air-sea CO2 flux in the study region was affected significantly by oceanographic conditions during the typhoon.

Phytoplankton Biomass Dynamics in the Strait of Malacca within the Period of the SeaWiFS Full Mission: Seasonal Cycles, Interannual Variations and Decadal-Scale Trends

Seasonal cycles, interannual variations and decadal trends of Sea-viewing Wide Field-of-view Sensor (SeaWiFS)-retrieved chlorophyll-a concentration (Chl-a) in the Strait of Malacca (SM) were investigated with reconstructed, cloud-free SeaWiFS Chl-a during the period of the SeaWiFS full mission (September 1997 to December 2010). Pixel-based non-parametric correlations of SeaWiFS Chl-a on environmental variables were used to identify the probable causes of the observed spatio-temporal variations of SeaWiFS Chl-a in northern, middle and southern regions of the SM. Chl-a was high (low) during the northeast (southwest) monsoon. The principal causes of the seasonality were wind-driven vertical mixing in the northern region and wind-driven coastal upwelling and possibly river discharges in the middle region. Among the three regions, the southern region showed the largest interannual variations of Chl-a. These variations were associated with the El Nino/Southern Oscillation (ENSO) and river runoff. Interannual variations of Chl-a in the middle and northern regions were more responsive to the Indian Ocean Dipole and ENSO, respectively, with atmospheric deposition being the most important driver. The most significant decadal-scale trend of increasing Chl-a was in the southern region; the trend was moderate in the middle region. This increasing trend was probably caused by environmental changes unrelated to the variables investigated in this study.

An improved MODIS standard chlorophyll-a algorithm for Malacca Straits Water

The Malacca Straits has high productivity of nutrients as a result to potential primary production. Yet, the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua has shown an overestimation of Chl-a retrieval in the case-2 water of Malacca Straits. In an update to the previous study, this paper presents the second validation exercise of MODIS OC3M algorithm using the reprocessed MODIS data (R2013) and locally tuned the algorithm with respect to two in-sit stations located at northern and southern part of Malacca Straits. The result shows the OC3M retrieved in the case-2 (south station) water remarkably overestimated in-situ Chl-a, but it is underestimated in the case-1 (north station). Local tuning was employed by iterative regression at the fourth-order polynomial to improve the accuracy of Chl-a retrieval. As a result, locally tuned OC3M algorithm give robust statistical performance and can be applied best for both case-1 and case-2 water in Malacca Straits.

Detailed spatiotemporal impacts of El Niño on phytoplankton biomass in the South China Sea

The lagging and leading correlations among satellite observations, reanalyzed biogeophysical data, and the Nino3.4 El Nino index were investigated to reveal the impacts of El Nino on the phytoplankton biomass (chlorophyll a [Chl a]) in the South China Sea (SCS), in an attempt to identify the probable responsible factors in greater spatiotemporal detail. A basin-scale high Chl a concentration during the developing phase of El Nino changed to basin-scale low Chl a during the weakening phase. Cyclonic wind circulation in the northern basin, increased wind speed in the southern basin, and strengthened upwelling off the Vietnamese coast likely caused a basin-scale nutrient increase during the developing phase of an El Nino event; the opposite conditions led to low nutrient levels during the weakening phase. Decreases in Chl a east of the Vietnamese coast and northwest of Borneo Island were due to decreases in nutrients supplied by rivers. These spatiotemporal changes are considered biogeophysical responses to a variety of types of El Nino. Regardless of the El Nino type, reanalyzing biogeophysical data sets during central Pacific warming separately from those during eastern Pacific warming is recommended for a more robust understanding of the detailed spatiotemporal impacts of different El Nino types on the biogeophysical environment of the SCS.

Determining temporal and spatial variabilities of biological production in sulu sea using multi-remotely sensed data

This study was conducted to investigate the temporal and spatial variation of the chlorophyll-a concentration over the past 13 years of Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission in the Sulu Sea area. The objectives of this study are to analyze the long term and spatial variability of the Chl-a concentration in Sulu Sea with special emphasize on the relationship between Chl-a concentration with the other oceanographic parameters such as sea surface temperature (SST), wind speed and wind directions. The Chl-a concentration were high during northeast monsoon which experience low SST. The fluctuation of Chl-a and SST are associated by the wind speed variation in this area. This suggests that during northeast monsoon, the increasing of wind speed intensifies water column mixing which entrains submerged cold- and nutrient-rich deep water to the surface layer, resulting in low SST and high nutrient in the surface layer. On the other hand, the southwest monsoon records dramatically decreasing of the Chl-a with regard to lower wind speed and surface water heating occurance. This promotes the downwelling process and weakens the upwelling one in the Sulu Sea [1] and as a result, this phenomenon influence reduction of nutrient upwelling to the surface layer, and thus produces lower Chl-a throughout the seasonal periods.