remote sensing Article Satellite-Observed Multi-Scale Variability of Sea Surface Chlorophyll-a Concentration along the South Coast of the Sumatra-Java Islands Tengfei Xu 1,2,3 , Zexun Wei 1,2,3,* , Shujiang Li 1,2,3, Raden Dwi Susanto 4,5 , Nyoman Radiarta 6, Chao Yuan 1,2,3, Agus Setiawan 6, Anastasia Kuswardani 6, Teguh Agustiadi 6 and Mukti Trenggono 7 1 Key Laboratory of Marine Science and Numerical Modeling, First Institute of Oceanography, Ministry of Natural Resources, Qingdao 266061, China; xutengfei@fio.org.cn (T.X.); lisj@fio.org.cn (S.L.); yuanchao@fio.org.cn (C.Y.) 2 Laboratory for Regional Oceanography and Numerical Modeling, Pilot National Laboratory for Marine Science and Technology, Qingdao 266237, China 3 Shandong Key Laboratory of Marine Science and Numerical Modeling, Qingdao 266061, China 4 Department of Atmospheric and Oceanic Science, University of Maryland, College Park, MD 20742, USA; [email protected] 5 Bandung Institute of Technology, Faculty of Earth Sciences and Technology, Bandung 40116, Indonesia 6 Agency for Marine and Fisheries Research and Human Resources, Ministry of Marine Affairs and Fisheries, Jakarta 14430, Indonesia; [email protected] (N.R.); [email protected] (A.S.); [email protected] (A.K.); [email protected] (T.A.) 7 Department of Marine Science, Faculty of Fisheries and Marine Science, Jenderal Soedirman University, Purwokerto 53121, Indonesia; [email protected] * Correspondence: weizx@fio.org.cn; Tel.: +86-532-88966225 Citation: Xu, T.; Wei, Z.; Li, S.; Susanto, R.D.; Radiarta, N.; Yuan, C.; Abstract: The southern coast of Java is known as one of the most productive fishing grounds for Setiawan, A.; Kuswardani, A.; tuna, feeding by nutrient-rich water along the coast caused by the subsurface water upwelling. This Agustiadi, T.; Trenggono, M. primary productivity can be evidenced by the high sea surface chlorophyll-a concentration (SSC). Satellite-Observed Multi-Scale Based on satellite remote sensing products, we investigate the multi-scale variability in SSC along the Variability of Sea Surface Chlorophyll-a Concentration along Sumatra-Java coast. The results show that seasonal variability of SSCs is primarily due to monsoon- the South Coast of the Sumatra-Java driven upwelling and rainfall in the Indian Ocean and Indonesian seas sides of the Sumatra and Islands. Remote Sens. 2021, 13, 2817. Java Islands, respectively. Local Ekman pumping plays a secondary role, while rainfall input to the https://doi.org/10.3390/rs13142817 ocean has little effect. Coastally trapped Kelvin waves and mesoscale eddies are responsible for the intraseasonal SSC anomalies in regions along the south coast of Java and off the Sunda and Lombok Academic Editor: Jorge Vazquez Straits, respectively. The interannual variability in SSC is caused by the anomalous upwelling related to the Indian Ocean Dipole. There was a weak increasing trend of ~0.1–0.2 mg/m3 per decade, above Received: 22 May 2021 the global averaged trend, which may be related to enhanced local Ekman pumping. These analyses Accepted: 8 July 2021 provide an overall description of SSC variations based on satellite observations; however, further Published: 17 July 2021 investigations based on in situ observations are needed to achieve better quantification. Publisher’s Note: MDPI stays neutral Keywords: chlorophyll-a; Java coastal upwelling; southeastern tropical Indian Ocean; seasonal with regard to jurisdictional claims in variability; intraseasonal variability; interannual variability; trend; Kelvin waves; mesoscale eddy published maps and institutional affil- iations. 1. Introduction Marine primary productivity is the fundamental factor that feeds the marine ecosys- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. tems and fisheries. So far, only satellite ocean color sensors can provide coverage of global This article is an open access article ocean chlorophyll, which in turn indicating the phytoplankton biomass and marine pri- distributed under the terms and mary productivity [1–3]. High sea surface chlorophyll-a concentration (SSC) areas generally conditions of the Creative Commons are identified in the upwelling and coastal areas feeding by nutrient-rich water from deeper Attribution (CC BY) license (https:// layer and runoff, respectively (Figure1a). The global SSC is found subject to multi-scale creativecommons.org/licenses/by/ variability associated with ocean dynamical processes such as upwelling, mesoscale eddies, 4.0/). and sea ice melting [4–7]. Previous investigations show no significant trend in global Remote Sens. 2021, 13, 2817. https://doi.org/10.3390/rs13142817 https://www.mdpi.com/journal/remotesensing Remote Sens. 2021, 13, x FOR PEER REVIEW 2 of 20 Remote Sens. 2021, 13, 2817 deeper layer and runoff, respectively (Figure 1a). The global SSC is found subject to multi-2 of 19 scale variability associated with ocean dynamical processes such as upwelling, mesoscale eddies, and sea ice melting [4–7]. Previous investigations show no significant trend in global annual median chlorophyll-a from 1998 to 2015 [8]. Among these high SSC regions, annual median chlorophyll-a from 1998 to 2015 [8]. Among these high SSC regions, the the southern coast of Java is known as one of the most productive fishing grounds for tuna southern coast of Java is known as one of the most productive fishing grounds for tuna as as a result of the nutrient-rich water along the coast, which is caused by subsurface water a result of the nutrient-rich water along the coast, which is caused by subsurface water up- upwelling (Figure 1b) [9–12]. Java coastal upwelling can be captured well by satellite im- welling (Figure1b) [ 9–12]. Java coastal upwelling can be captured well by satellite images agesof of cold cold sea sea surface surface temperature temperature (SST), (SST), low low sea sea surface surface height height (SSH), (SSH), and highand high SSC [SSC13–15 ]. [13–15].From From the viewpoint the viewpoint of dynamics, of dynamics, the seasonal the seasonal Java coastal Java coastal upwelling upwelling is caused is caused by winds by windsdriven driven offshore offshore Ekman Ekman transport transport and corresponding and corresponding vertical vertical pumping pumping [12]. In [12]. addition, In addition,the eastward the eastward propagation propagation of upwelling of upwelling Kelvin Kelvin waves waves also generates also generates thermocline thermocline shoaling shoalingalong along the Sumatra-Java the Sumatra-Java coast oncoast an on intraseasonal an intraseasonal timescale timescale [16–19 [16–19].]. 3 FigureFigure 1. Distributions 1. Distributions of annual of annual mean mean SSC SSC (mg/m (mg/m3): (a)): global; (a) global; (b) Indonesian (b) Indonesian surrounding surrounding waters. waters. During the southeast monsoon seasons, the southeasterly wind along the southern During the southeast monsoon seasons, the southeasterly wind along the southern coast of Java forces offshore Ekman transport, which induces compensation upwelling off coast of Java forces offshore Ekman transport, which induces compensation upwelling off the coast [20]. The upwelling brings cold and nutrient-rich water from the subsurface to the the coast [20]. The upwelling brings cold and nutrient-rich water from the subsurface to upper layer, resulting in increased phytoplankton biomass, which is consumed by higher- the upper layer, resulting in increased phytoplankton biomass, which is consumed by trophic-level organisms such as fish [21,22]. Consequently, the abundant season of fisheries higher-trophic-level organisms such as fish [21,22]. Consequently, the abundant season of production generally coincides with the monsoon-driven coastal upwelling [11,23]. An- fisheries production generally coincides with the monsoon-driven coastal upwelling other factor responsible for the increase in SSC relies on nutrients conveyed by rainfall and [11,23]. Another factor responsible for the increase in SSC relies on nutrients conveyed by river discharge during the rainy season, when the northwesterly monsoon prevails [13,24]. rainfallSince and few river rivers discharge discharge during into the the Indian rainy Ocean,season, therewhen is the a collective northwesterly water monsoon volume flux prevailsroughly [13,24]. two ordersSince few smaller rivers than discharge that into into the the Indonesian Indian Ocean, seas [there25]. As is a result,collective the wa- role of ter rivervolume discharge flux roughly is assumed two orders to be important smaller than only that within into thethe Indonesian Indonesian seas seas [ 26[25].], which As a re- result,sults the in role fish catchesof river peakingdischarge during is assumed the northwest to be importan monsoont only seasons within in the the Java Indonesian Sea [27,28 ]. seasMarine [26], which primary results production in fish catches can be peak retrieveding during from satellitethe northwest observations monsoon of solar-inducedseasons in the chlorophyllJava Sea [27,28]. fluorescence Marine primary [29–31]. production Thus, satellite-derived can be retrieved SSC hasfrom been satellite suggested observa- as an tionseffective of solar-induced indicator for chloro the investigationphyll fluorescence of marine [29–31]. productivity Thus, satellite-derived and the identification SSC has and beenassessment suggested of as fish an abundanceeffective indicator [11,32,33 for]. the investigation of marine productivity and the identificationIn addition and to theassessment seasonal of cycle fish that abundance