Role Of Sea Surface Temperature Research Articles

Underestimated role of sea surface temperature in sea spray aerosol formation and climate effects

The uncertainty regarding the correlation between sea spray aerosol (SSA) formation and sea surface temperature (SST) hinders the accurate estimation of SSA’s impact on global climate. Here, we developed a temperature-controlled plunging SSA simulation tank to investigate the impact of SST on SSA formation from two perspectives: SSA particle size distribution and organic enrichment. Our findings show that SSA particle size decreases with decreasing SST, as exhibited by an increase in SSA within Aitken mode and a decrease in SSA within accumulation and coarse modes. SST can significantly enhance organic enrichment in SSA particles, while the multiplicative increases vary from 2 to 10 times depending on the organic matter species and the SSA particle size. Based on our experimental results, it is predicted that SST reduction may lead to a significantly higher contribution of Aitken modal SSA-derived CCN in cold waters (0 °C) than in warm waters (30 °C). Additionally, we incorporate SST for the first time in estimating the global flux of dissolved organic carbon (DOC) emitted via SSA, yielding a value ranging from 23.45 to 55.78 Tg C yr−1. Compared to previous works, our study reveals the crucial role of SST in influencing both cloud formation and the atmospheric organic burden of SSA.

A Physical‐Informed Neural Network for Improving Air‐Sea Turbulent Heat Flux Parameterization

AbstractThe parameterizations of air‐sea turbulent heat flux are one of the major bottlenecks in atmosphere‐ocean coupled model development, which play a crucial role in sea surface temperature (SST) prediction. Recently, neural networks start to be applied for the development of parameterizations of interface turbulent heat flux. However, these new parameterizations are primairily developed for specific regions and have not been tested in real atmosphere‐ocean coupled models. In this study, we propose a new air‐sea heat flux parameterization using a physical‐informed neural network (PINN) based on multiple observational data sets worldwide. Evaluated with an independent observation data set, it is shown that the PINN can significantly reduce the RMSE of latent heat flux by at least about 48.6% compared to three traditional bulk formulas. Moreover, the PINN can be flexibly updated with new observational data by transfer learning. To test the performance of the new parameterization in realistic application, we implement the PINN into a global ocean‐atmosphere coupled model and make seasonal forecasts for the first time. The PINN markedly reduce the errors of equatorial SST forecast, indicating a good performance of the PINN‐based air‐sea turbulent heat flux scheme. Noticeably, due to limited observational data, the NN‐based parameterizations tend to underestimate heat flux at high wind speeds compared with bulk formula‐based parameterizations. With more data available at extreme conditions, the PINN can be improved via transfer learning and need to be futher evaluated. This study suggests that PINN‐based air‐sea heat flux parameterization is promising to improve SST simulation.

Ocean feedback on 50–80-day boreal winter intraseasonal oscillation in the Indian Ocean: comparison of atmospheric and coupled CESM2 simulations

Abstract The role of sea surface temperature (SST) variability in the boreal winter intraseasonal oscillation (ISO) over the tropical Indian Ocean (TIO) is investigated in this study using the Community Earth System Model Version 2 (CESM2). An atmospheric general coupled model (AGCM) run forced by daily SSTs derived from a parallel coupled circulation model (CGCM) run is compared with reanalysis and the original coupled simulation. The comparison of atmospheric-coupled GCMs reveals the frequency and spatial tendency of the SST effect on the atmospheric ISO. The lower-frequency component of the ISO over the TIO is significantly intensified in the AGCM run. The SST-to-atmosphere effect is most prominent over the southeastern TIO. In the AGCM run, low-level wind anomalies related to SST appear west of the ISO convection center, contrasting with the previous findings that the SST effect appears east of the convection center.

Role of SST in Seasonal Western North Pacific Anomalous Anticyclone: Insights From AMIP Simulations in CMIP6

AbstractThe western North Pacific anticyclone (WNPAC) significantly influences the East Asian climate and is modulated by tropical sea surface temperature (SST). This study uses 142 AMIP simulations from 33 Coupled Model Intercomparison Project (CMIP6) models to quantify the contributions of SST to the interannual variability of the WNPAC. SST forcing accounts for 66%, 77%, and 49% of the WNPAC variance in winter, spring, and summer, respectively. The persistence of the WNPAC depends on the relaying effects of SST in three tropical oceans. CMIP6 models exhibit excessive precipitation response to the Pacific SST, leading to an overestimated (underestimated) Pacific (Indian Ocean) effect in modulating the summer WNPAC. Sensitivity experiments with an atmospheric model confirm the crucial role of the Pacific in regulating the WNPAC interannual variation and the contribution from the tropical North Atlantic in spring. The tropical Indian Ocean only exerts a minor impact on the WNPAC when excluding the interactions with other oceans.

Anomalous sea temperatures can impair coral reef fish recruitment

Understanding how temperature affects coral reef fish recruitment success is crucial for assessing impacts of ocean warming on coral reef resilience. We utilized a long-term fish survey dataset along the west coast of Hawai‘i Island to investigate the role of sea surface temperature (SST) in influencing recruitment timing and density. The dataset consisted of 17 years of surveys, with 25 sites annually surveyed in the months of May, July, September and November. We found that peak recruitment, i.e. the maximum number of recruits recorded across all surveys per year, usually occurred during July surveys. For sites where peak recruitment for that year occurred outside July, there were significantly fewer fish recruits than for sites whose peak recruitment occurred in July. In addition, the timing of peak recruitment is influenced by anomalously warm or cool years prior to spawning. The decrease in recruit density outside these times is likely influenced by recruits being exposed to temperatures warmer and cooler than their optimum. Our results show that climate variability is having an impact on the timing of peak recruitment, creating a mismatch between the thermal optimum of developing recruits and the thermal environment they develop in, negatively affecting recruit density in critical coral reef habitats. Altered and reduced recruitment has the potential to disrupt reef community structure and long-term fisheries sustainability in Hawai‘i, with important management implications for coral reefs in the future.

The role of sea surface temperature in shaping the characteristics of future convective afternoon rainfall in Taiwan

Convective afternoon rainfall (CAR) is a significant summer rainfall feature in Taiwan. This study investigates the projected uncertainties in summer CAR in Taiwan, using a dynamical downscaling approach with the Weather Research and Forecasting Model (WRF) and the High-Resolution Atmospheric Model (HiRAM). The projections were driven by four different sea surface temperature (SST) categories derived from CMIP5 model simulations under the RCP8.5 scenario. All projections indicate a reduced frequency but increased intensity of CAR over Taiwan by the end of the 21st century. However, notable differences in the degree of CAR changes were observed among simulations with varying degrees of Pacific SST warming. These variations in potential future changes in CAR frequency and intensity can be attributed to the variations in daytime thermal instability, local inland wind convergence, and moisture flux convergence over Taiwan. Furthermore, this study explored the linkage between local thermodynamic conditions and projected large-scale circulation patterns.

Understanding the Role of Sea Surface Temperature and Urbanization on Severe Thunderstorms Dynamics: A Case Study in Surabaya, Indonesia

AbstractWithin the period 2014–2017, five hail events were reported in the city of Surabaya in Indonesia. Although deep convection commonly develops over the Maritime Continent, severe thunderstorms triggering hail events develop less frequently as specific atmospheric conditions are required. The rapid urbanization in Surabaya might have led to increased heat release to the atmosphere and to the deepening of convection, which raises the question of whether urbanization is the culprit of the recent hail events in Surabaya. Hence, for a selected hail event, we used the high‐resolution Weather Research and Forecasting model to understand the storm dynamics and to explore the role of urbanization, sea surface temperature, and aerosol concentration on the storm dynamics with a total of 11 scenarios. The control simulation reveals that low‐level convergence induced by a sea breeze creates instability. At the same time, the urban heat release enhances the energy supply to induce hail formation and retain the storm's lifetime over the city. A factor separation method revealed that the urbanization (added anthropogenic heat flux, urban aerosol, and the rise in building height) and the sea surface temperature increase contribute to the storm enhancement over Surabaya, producing two times higher updraft velocity, doubling the maximum graupel mass mixing ratio and finally resulting in 15%–30% higher accumulated precipitation over Surabaya, compared to the control simulation.

On the Role of Sea Surface Temperature in the Clustering of Global Tropical Cyclone Formation

Abstract This study examines the potential impacts of large-scale atmospheric circulations that are forced by sea surface temperatures (SST) on global tropical cyclone (TC) formation. Using the Geophysical Fluid Dynamics Laboratory (GFDL) global atmosphere and land surface model, version 4 (AM4), under different SST distributions, it is found that the east–west clustering of global TC formation is mainly governed by large-scale circulations in response to given SSTs, instead of direct ocean surface fluxes associated with zonal SST anomalies. Our zonally homogeneous SST simulations in the presence of realistic surface coverage show that TC clusters still emerge as a result of the breakdown of zonal circulations related to land–sea distribution, which produce specific “hotspots” for global TC formation. Sensitivity experiments with different climate warming scenarios and model physics confirm the persistence of these TC clusters in the absence of all zonal SST variations. These robust results offer new insights into the effects of large-scale circulation and terrain forcing on TC clusters beyond the traditional view of direct SST impacts, which are based on the direct alignment of the warmest SST regions and TC clusters. In addition, our experiments also capture internal variability of the global TC frequency, with an average fluctuation of 6–8 TCs at several dominant frequencies of ∼3, 6, and 9 years, even in the absence of all SST interannual variability and ocean coupling. This finding reveals an intrinsic “noise” level of the global TC frequency that one has to take into account when examining the past and future trends in TC activity and their related significance or detectability. Significance Statement In this study, the clustering of global tropical cyclone (TC) formation is investigated, using global simulations under different idealized sea surface temperature (SST) distributions. Our results show that it is the response of the large-scale tropical circulations to SST anomalies that is mostly responsible for the clustering of global TC formation rather than surface flux differences. It is also found that the tropical atmosphere contains inherent fluctuations in the global TC frequency of 6–8 TCs every 3–9 years, even in the absence of all SST interannual and zonal variability. These results offer new insight into the role of tropical dynamics in governing TC climatology and suggest possible mechanisms underlying the clustering of global TC formation under different climate conditions.

Functional diversity patterns of reef fish, corals and algae in the Brazilian biogeographical province

AbstractAimFunctional diversity encapsulates whole‐community responses to environmental gradients mediated by species traits. Under trait convergence, similar responses may cause distantly related taxa to exhibit spatially correlated functional diversity. We investigated whether similar responses of reef fish, coral and algal functional richness and disparity to the environment produce spatially correlated functional diversity patterns.LocationBrazilian marine biogeographical province.TaxonReef fish, corals, algae.MethodsWe analysed data from 40 coastal and oceanic sites distributed across 27 degrees of latitude in the Brazilian province. Using traits, we measured functional richness (FRic) and disparity (Rao's Q) and calculated Pearson's correlation () between pairs of metrics and taxa. We used Bayesian multivariate linear models to model taxa functional richness and disparity relative to sea surface temperature (SST), turbidity, salinity, species richness and region, and to estimate the residual correlation () between metrics after accounting for these variables.ResultsThe best fitted model contained SST, species richness and region, and explained about 56% of the variation in FRic and Rao's Q across taxa. Yet, FRic and Rao's Q of fish, algae and corals responded differently to environmental variables. Functional diversity metrics were less correlated between algae and corals than compared to fish. Observed correlations of FRic and Rao's Q were low to intermediate across taxa (average = 0.14), and residual correlations were even lower (average = 0.02).Main conclusionsSST, species richness and region had a widespread role in determining spatially congruent functional diversity offish, algae and corals across Brazilian reefs, despite their fundamentally different evolutionary histories. Low residual spatial correlations suggest that other mechanisms might also contribute to functional diversity patterns of reef taxa independently. Given the role of SST, species richness and region, the functional structure of these reefs might be compromised by climate change, pollution and overfishing.

Understanding the role of sea surface temperature and urbanization on severe thunderstorms dynamics: a case study in Surabaya, Indonesia

Understanding the role of sea surface temperature and urbanization on severe thunderstorms dynamics: a case study in Surabaya, Indonesia

Surface Bacterioplankton Community Structure Crossing the Antarctic Circumpolar Current Fronts.

The Antarctic Circumpolar Current (ACC) is the major current in the Southern Ocean, isolating the warm stratified subtropical waters from the more homogeneous cold polar waters. The ACC flows from west to east around Antarctica and generates an overturning circulation by fostering deep-cold water upwelling and the formation of new water masses, thus affecting the Earth's heat balance and the global distribution of carbon. The ACC is characterized by several water mass boundaries or fronts, known as the Subtropical Front (STF), Subantarctic Front (SAF), Polar Front (PF), and South Antarctic Circumpolar Current Front (SACCF), identified by typical physical and chemical properties. While the physical characteristics of these fronts have been characterized, there is still poor information regarding the microbial diversity of this area. Here we present the surface water bacterioplankton community structure based on 16S rRNA sequencing from 13 stations sampled in 2017 between New Zealand to the Ross Sea crossing the ACC Fronts. Our results show a distinct succession in the dominant bacterial phylotypes present in the different water masses and suggest a strong role of sea surface temperatures and the availability of Carbon and Nitrogen in controlling community composition. This work represents an important baseline for future studies on the response of Southern Ocean epipelagic microbial communities to climate change.

Spatio-temporal changes of winter and spring phytoplankton blooms in Arabian sea during the period 1997–2020

Arabian Sea (AS) experiences Chlorophyll-a (Chl-a) blooms during winter and early spring (November–March) mainly due to the changes induced by seasonally reversing monsoon winds and associated processes. The seasonal blooms exhibit distinct regional patterns in their onset, duration, intensity and peak period. Recent changes in ocean dynamics and plankton composition have inflicted adverse effects in the distribution of Chl-a concentration in AS. Here, we analyse the long-term spatio-temporal changes in winter and early spring bloom events during the period 1997–2020, and evaluate the role of sea surface temperature (SST), mixed layer depth (MLD), sea surface salinity, winds, mesoscale eddies and surface currents on these bloom occurrences. We observe a significant reduction in these blooms, which started in the early 2000s and intensified in the last decade (2010–2020), with a notable drop in the adjacent gulfs (Gulf of Aden: 1.38 ± 0.7 × 10−5 mg m−3 yr−1, Gulf of Oman: 4.71 ± 1.35 × 10−6 mg m−3 yr−1) and West coast of India (−6.71 ± 2.85 × 10−6 mg m−3 yr−1). The MLD and ocean temperature are the major factors that govern bloom in Gulf of Oman and open waters. Conversely, the coastal upwelling and eddies drive blooms in Gulf of Aden. The winter cooling trigger the bloom in the northern Indian west coast, but the inter-basin exchange of surface waters through the West Indian Coastal Current inhibits its southward spread. This study, therefore, reveals unique processes that initiate and control the winter and early spring blooms in different regions of AS. The ongoing warming of AS could contribute to further decline in these seasonal blooms, which would be a great concern for regional marine productivity and associated regional food security.

Offshore wind energy forecasting sensitivity to sea surface temperature input in the Mid-Atlantic

Abstract. As offshore wind farm development expands, accurate wind resource forecasting over the ocean is needed. One important yet relatively unexplored aspect of offshore wind resource assessment is the role of sea surface temperature (SST). Models are generally forced with reanalysis data sets, which employ daily SST products. Compared with observations, significant variations in SSTs that occur on finer timescales are often not captured. Consequently, shorter-lived events such as sea breezes and low-level jets (among others), which are influenced by SSTs, may not be correctly represented in model results. The use of hourly SST products may improve the forecasting of these events. In this study, we examine the sensitivity of model output from the Weather Research and Forecasting model (WRF) 4.2.1 to different SST products. We first evaluate three different data sets: the Multiscale Ultrahigh Resolution (MUR25) SST analysis, a daily, 0.25∘ × 0.25∘ resolution product; the Operational Sea Surface Temperature and Ice Analysis (OSTIA), a daily, 0.054∘ × 0.054∘ resolution product; and SSTs from the Geostationary Operational Environmental Satellite 16 (GOES-16), an hourly, 0.02∘ × 0.02∘ resolution product. GOES-16 is not processed at the same level as OSTIA and MUR25; therefore, the product requires gap-filling using an interpolation method to create a complete map with no missing data points. OSTIA and GOES-16 SSTs validate markedly better against buoy observations than MUR25, so these two products are selected for use with model simulations, while MUR25 is at this point removed from consideration. We run the model for June and July of 2020 and find that for this time period, in the Mid-Atlantic, although OSTIA SSTs overall validate better against in situ observations taken via a buoy array in the area, the two products result in comparable hub-height (140 m) wind characterization performance on monthly timescales. Additionally, during hours-long flagged events (< 30 h each) that show statistically significant wind speed deviations between the two simulations, both simulations once again demonstrate similar validation performance (differences in bias, earth mover's distance, correlation, and root mean square error on the order of 10−1 or less), with GOES-16 winds validating nominally better than OSTIA winds. With a more refined GOES-16 product, which has been not only gap-filled but also assimilated with in situ SST measurements in the region, it is likely that hub-height winds characterized by GOES-16-informed simulations would definitively validate better than those informed by OSTIA SSTs.

What Controls the Interannual Variability of the Boreal Winter Atmospheric River Activities over the Northern Hemisphere?

Abstract Interannual variability of the winter atmospheric river (AR) activities over the Northern Hemisphere is investigated. The leading modes of AR variability over the North Pacific and North Atlantic are first identified and characterized. Over the Pacific, the first mode is characterized by a dipole structure with enhanced AR frequency along the AR peak region at about 30°N and reduced AR frequency farther north. The second mode exhibits a tripole structure with a narrow band of positive AR anomalies at about 30°N and sandwiched by negative anomalies. Over the Atlantic, the first mode exhibits an equatorward shift of the ARs with positive anomalies and negative anomalies located on the equatorward and poleward side of the AR peak region at about 40°N, respectively. The second mode is associated with the strengthening and eastward extension of the AR peak region, which is sandwiched by negative anomalies. A large ensemble of atmospheric global climate models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), which shows high skill in simulating these modes, is then used to quantify the roles of sea surface temperature (SST) forcing versus internal atmospheric variability in driving the formation of these modes. Results show that SST forcing explains about half of the variance for the Pacific leading modes, while that number drops to about a quarter for the Atlantic leading modes, suggesting higher predictability for the Pacific AR variability. An additional ensemble driven only by observed tropical SST is further utilized to demonstrate the more important role that tropical SST plays in controlling the Pacific AR variability while both tropical and extratropical SST exert comparable influences on the Atlantic AR variability.

Impact of Meteorological Factors on the Mesoscale Morphology of Cloud Streets during a Cold-Air Outbreak over the Western North Atlantic

Abstract Postfrontal clouds (PFC) are ubiquitous in the marine boundary layer, and their morphology is essential to estimating the radiation budget in weather and climate models. Here we examine the roles of sea surface temperature (SST) and meteorological factors in controlling the mesoscale morphology and evolution of shallow clouds associated with a cold-air outbreak that occurred on 1 March 2020 during phase I of the Aerosol Cloud Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE). Our results show that the simulated PFC structure and ambient conditions by the Weather Research and Forecasting (WRF) Model are generally consistent with observations from GOES-16 and dropsonde measurements. We also examine the thermodynamical and dynamical influences in the cloud mesoscale morphology using WRF sensitivity experiments driven by two meteorological forcing datasets with different domain-mean SST and spatial gradients, which lead to dissimilar values of hydrometeor water path and cloud core fraction. The SST from ERA5 leads to weaker stability and higher inversion height than the SST from FNL does. In addition, the use of large-scale meteorological forcings from ERA5 yields a distinctive time evolution of wind direction shear in the inner domain, which favors the formation and persistence of longer cloud rolls. Both factors contribute to a change in the time evolution of domain-mean water path and cloud core fraction of cloud streets. Our study takes advantage of the simulation driven by the differences between two large-scale forcing datasets to illustrate the importance of SST and wind direction shear in the cloud street morphology in a realistic scenario.

Linkage Between Preceding August Asian–Pacific Oscillation and September Precipitation Over Southeast China

Understanding the spatiotemporal variation of autumn precipitation and its relationship with the large-scale circulation is important for planning industrial and agricultural production, economic development, and ecological protection. This study investigated the relationship between the August Asian–Pacific Oscillation (APO) and September precipitation over Southeast China (SC) during the period 1961–2020. Results showed that the August APO can exert considerable control on September precipitation over SC and that a significant positive correlation exists between them. With a strong (weak) August APO, the anomalous southerly (northerly) winds are observed in the north (south) of SC at the upper level, and the deep trough over East Asia and ridge over North Pacific at 500hPa are both reinforced (weakened). This leads to anomalous northward (southward) lower-tropospheric winds over the East Asian coast, accompanied by enhanced convergence (divergence) of warm and cold air masses and anomalous ascent (descent) motion which results in more (less) precipitation over SC. The underlying mechanism can be explained as thermal anomalies induced by the APO that can persist from August to September, which modulates the atmospheric circulation anomalies in September and eventually causes more (less) precipitation over SC. Therefore, in addition to the role of sea surface temperature forcing on precipitation variations, our analyses suggest that the preceding temperature anomalies at the middle and upper troposphere also should be considered as an important precursory factor for the following precipitation over SC.

On Investigating the Dynamical Factors Modulating Surface Chlorophyll-a Variability along the South Java Coast

Twelve years of remotely sensed all-sat merged chlorophyll-a concentration unveils strong signatures of chlorophyll-a blooms along the south Java coast. An unprecedented three-times increase in chlorophyll-a concentration is significantly observed along the south Java coast during the southeast monsoon (June–October) than the northwest monsoon (December–April). The multiple regression analysis of dynamic factors evidently indicates that seasonal upwelling is predominantly controlled by the seasonally evolving coastal eddies associated with the seasonally reversing south Java coastal currents (SJCC) and Ekman mass transport (EMT), followed by the relative roles of sea surface temperature (SST) and wind stress curl. The eddy-induced upwelling and EMT-induced coastal upwelling lead to chlorophyll-a blooms during southeast monsoon, well-supported by the entrainment of cold and saline waters (thermocline doming) with low spiciness. On the other hand, the coastal eddies associated with SJCC and SST anomalies play a significant role in modulating the interannual surface chlorophyll-a variability in the domain. Intense chlorophyll-a blooms are observed during the positive IOD years, whereas the least chlorophyll-a concentration is observed during the negative IOD years. The unprecedentedly least chlorophyll-a concentrations during 2010 and 2016 are attributed to the intense and prolonged surface marine heatwaves.

A spatial model for predicting North Indian Ocean tropical cyclone intensity: Role of sea surface temperature and tropical cyclone heat potential

Tropical cyclones (TCs) are extreme weather events that may result in enormous loss of life and property especially for countries surrounding the North Indian Ocean (NIO) rim. In this study, a regional scale spatial statistical model has been developed to define the relationship between TCs and sea surface temperature (SST)/tropical cyclone heat potential (TCHP) for the period 1979 to 2017 over the NIO region. The spatial model employed here tessellates the NIO basin with hexagonal grids (area of each hexagon is approximately 213,961 km2) in order to analyze the relationship between cyclone intensity and the two predictors-SST and TCHP. The role of SST and TCHP contribution to the NIO cyclone intensity is determined by using a geographically weighted regression (GWR) method. This study postulates that a hexagon with positive coefficient signifies a direct relationship between the cyclone intensity and the predictors, for example, between SST (TCHP) and the Bay of Bengal (Arabian Sea) TCs. Based on robust model verification and test of significance, it is attributed that the spatial model using GWR has the potential to model NIO cyclone intensity, particularly using SST as the predictor for Bay of Bengal and TCHP for the Arabian Sea.

Active Role of Sea Surface Temperature Changes Over the Kuroshio in the Development of Distant Tropical Cyclones in Boreal Fall

AbstractThis study examined the active role of sea surface temperature (SST) changes over the Kuroshio in the development of distant tropical cyclones (TCs) approaching Japan in October, using a cloud‐resolving model. We designed a real SST experiment and SST sensitivity runs by adding and subtracting SST anomalies equivalent to a spread of year‐to‐year October‐mean Kuroshio's SST changes. Under a synoptic environment that low‐level northeasterlies prevailed over the Kuroshio, systematic differences in TC intensity between the real and modified SST runs were simulated despite the developing TCs being far away from the modified SST areas. In the warm (cool) SST runs, active (inactive) moisture import from the Kuroshio increased (decreased) the equivalent potential temperature within the planetary boundary layer (PBL) in the inner core, thereby enhancing (weakening) the latent eyewall heating and the associated secondary circulation. The differences in the secondary circulation led to those in the inward advection of absolute angular momentum in the PBL, affecting a vortex spin‐up in the inner core. We also found that the surface heat and moisture supply from the Kuroshio facilitated the warming and moistening processes of midlatitude air parcels traveling over the underlying current, causing further reinforcement of the moisture import into the distant TC. These thermodynamic processes were more and less apparent in the warm and cool SST runs, respectively. Since the synoptic environment in this study often arises in boreal fall, the Kuroshio's SST warming in recent decades may influence the intensities of TCs approaching Japan in fall.

Improvement of Heavy Rainfall Simulated with SST Adjustment Associated with Mesoscale Convective Complexes Related to Severe Flash Flood in Luwu, Sulawesi, Indonesia

Flash flooding is an important issue as it has a devastating impact over a short time and in a limited area. However, predicting flash floods is challenging because they are connected to convection systems that rapidly evolve and require a high-resolution forecasting system. In addition, modeling a case study of a mesoscale convective complex (MCC) is the key to improving our understanding of the heavy rainfall systems that trigger flash floods. In this study, we aim at improving modeling skills to simulate a heavy rainfall system related to flash-flood-producing MCCs. We simulated a heavy rainfall event related to a severe flash flood in Luwu, Sulawesi, Indonesia, on 13 July 2020. This flood was preceded by persistent heavy rainfall from 11 to 13 July 2020. In this case, we investigated the role of sea surface temperature (SST) in producing the persistent heavy rainfall over the region. Therefore, we explore the physical and dynamic processes that caused the heavy rainfall using a convection-permitting model with 1 km resolution and an experiment comparing the situation with and without updated SST. The results show that the heavy rainfall was modulated by the development of a pair of MCCs during the night. The pair of MCCs was triggered by a meso-low-pressure system with an anti-cyclonic circulation anomaly over the Makassar Strait and was maintained by the warm front passing between the sea and land over central Sulawesi. This front was characterized by moist–warm and cold–dry low-level air, which may have helped extend the lifetime of the MCCs. The north-westward propagation of the MCCs was due to the interaction between predominantly a south-easterly monsoon and SST anomalies. This study suggested that the long-lived (>10 h) MCCs (>80,000 km2 cloud shield) and persistent precipitation are reproduced well in the updated SST scenario in the WRF model. This relatively simple technique in the running model provides a new strategy for improving flash flood forecasting by better predicting rainfall as an input in the hydrological model. Our findings also indicated a long-lived MCC maintained by back-building mechanisms from night to morning inland as an exceptional MCC, which does not correspond to a previous study.