A Biological‐Parameter‐Optimized Modeling Study of Physical Drivers Controlling Seasonal Chlorophyll Blooms off the Southern Coast of Java Island

Abstract

A physical‐biogeochemical model consists of many key parameters; theoretically, 10,000 or more numerical experiments are required to obtain a set of optimal biological parameters. The corresponding computing resources are too expensive when running a three‐dimensional and high‐resolution physical‐biogeochemical model. In this study, we improve the performance of a three‐dimensional physical‐biogeochemical model via the conditional nonlinear optimal perturbation method for southern Java Island, where chlorophyll blooms occur seasonally. Based on the improved model results, we calculated the mixed‐layer nutrient budget, thereby characterizing how each physical process transports nutrients into the surface oligotrophic layer and then stimulates the chlorophyll blooms during the southeast monsoon season. Nutrients that upwell along the submarine slope are found in narrow regions along the coast and spread into most of the study area by local wind‐induced offshore transport. The contribution of nonlinearity (advection) is essential to modulating the upslope movement of nutrients off the coast. If local winds are neglected in the model, onshore transport induced by the pressure gradient force in the zonal direction (alongshore) and subsequent downwelling would be dominant, and chlorophyll blooms would diminish in most regions. The role of the South Java Coastal Current varies under the joint influence of remote and local wind forcings. The Indonesian Throughflow from the Lombok Strait also contributes to surface phytoplankton growth along its path off the south of Java Island.

Water sources of the Lombok, Ombai and Timor outflows of the Indonesian throughflow

“…The simulated amplitude and phases of the major tidal constituents M2 and K1 by ROMS are well matched with the TOPEX/POSEIDON crossover data from Robertson and Ffield (2008), with a high correlation (Figure S1). The passage transports have been verified in our previous study (Gao et al, 2022), and further validations relevant to the critical straits are here. The along-strait velocity observations of the LS and TP are well reproduced by ROMS, although the OS overflow from July to August and the southward Makassar flow from 200 to 600 m are slightly underestimated (Figures S2, S3).…”

“…A spin-up integration of 15 years was conducted under monthly climatology forcing fields and open boundary conditions, in which the climatological threedimensional velocity fields in the last year, stored as a 5-day average, are to be used to track the ITF water. Detailed descriptions of the model configuration have been provided by Gao et al (2022).…”

“…The ROMS simulation analyzed here is updated from that of Gao et al (2022) by slightly modifying the land-sea geometry to improve the representation of ITF. Intense tidal mixing in the Indonesian Seas is shown to modify the temperature-salinity properties of the Pacific water (Iskandar et al, 2023).…”

Understanding the compound marine heatwave and low-chlorophyll extremes in the western Pacific Ocean

“…On the inter-annual timescale, the higher total MHW-LChl days are observed during the negative DMI years (e.g., 2010 and 2016), which is well supported by the deepening thermocline, positive sea surface height anomalies, and comparatively warm SST anomalies at the surface, all indicating reduced upwelling (Mandal et al, 2022). However, further in-depth studies are needed to explore the exact physical processes of individual climate modes in modifying MHW-LChl occurrences by incorporating additional approaches, such as empirical copula, logistic regression model (Hao et al, 2018;Mukherjee et al, 2022;Zhang et al, 2023), and mixed layer heat (nutrient) budget analysis (Gao et al, 2022;Liu et al, 2022).…”