Sea Surface Heat Flux Research Articles

Seasonality of Submesoscale Vertical Heat Transport Modulated by Oceanic Mesoscale Eddies in the Kuroshio Extension

AbstractEnergetic mesoscale eddies are often accompanied by strong submesoscale variability, which plays a significant role in connecting mesoscale and turbulent motions in the ocean and leads to strong vertical motions. The product of a high‐resolution (1/48°) oceanic numerical model, the LLC4320, is employed to investigate the seasonal variations of vertical heat transport induced by submesoscale processes within multiple mesoscale eddies in the Kuroshio Extension (KE) region. In different seasons, the submesoscale vertical heat transport exhibits a consistent upward pattern, with notably higher magnitudes observed during winter. In winter, the maxima value of submesoscale vertical heat flux (SVHF) can account for approximately 60% of the total vertical heat flux (VHF). This is equivalent to the average net sea surface heat flux in a single eddy region. In summer and autumn, the maxima absolute value of submesoscale vertical heat flux can account for approximately 30% of the total VHF. Energy analysis reveals that baroclinic instability associated with vertical buoyancy flux has a crucial effect on generating submesoscale processes within the eddy region. The submesoscale motions are influenced by the mixed layer instability, strain‐induced frontogenesis, turbulent thermal wind and turbulent thermal wind‐induced frontogenesis within the upper mixed layer, while they are largely associated with the strain‐induced frontogenesis in the ocean interior. Furthermore, the upward low‐frequency submesoscale vertical heat transport is generated by submesoscale secondary circulation at eddy peripheries.

Oceanic response to tropical cyclone in the northern South China Sea observed by underwater gliders during 2018 and 2020

In this study, we mainly used in-situ observations from underwater gliders to analyze the ocean response in the northern South China Sea affected by Son-tinh (2018), Mandal et al. (2018) Mangkhut (2018)and Noul (2020). The results showed that these TCs caused 0.6 °C, 1.1 °C and 1.7 °C maximum sea surface temperature cooling respectively, which were weaker than general conditions because of long distance, weak intensity and fast movement speed. Net solar radiation, precipitation, 10-m wind and sea surface heat flux also made contribution in changes of SST. The mixed layer depth (MLD) became shallower after Son-Tinh and Noul passed through, while during Mangkhut it did not change significantly. After TCs passed through, the stratification around MLD became more obvious, with a banded distribution and stronger high-value areas of buoyancy frequency. Within 1 week after the shortest distance, the temperature and salinity responses in the upper ocean were stronger than those at the sea surface, and the gradients of temperature and salinity and their anomalies were more evident in the subsurface layer. The results of this study show that underwater glider observations are important for understanding oceanic responses to tropical cyclones and are useful for studying tropical cyclone activities.

Assessing Climate Forcing From the Sea Surface Temperature‐Surface Heat Flux Relation for SST‐Coupled Oscillatory Variability

AbstractThe interaction between sea surface temperatures (SST) and surface heat flux (SHF) is vital for atmospheric and oceanic variabilities. This study investigates SST‐SHF relationship in the framework of a coupled oscillatory model, extending beyond previous research that predominantly used AR‐1 type simple stochastic climate models. In contrast to the AR‐1 type model, we reveal distinct features of SST‐SHF relationships in the oscillatory model: sign reversals occur in the imaginary part of SST‐SHF coherence and the low‐pass SST tendency‐SHF correlation. However, these sign reversals are absent in the real part of SST‐SHF coherence and in the low‐pass SST‐SHF correlation. We find these features are robust across both the twentieth Century Reanalysis and GFDL SPEAR model for El Niño‐Southern Oscillation (ENSO) variability. Furthermore, we develop a new scheme to assess ENSO's climate forcing magnitude and natural frequency. Our findings thus provide novel insights into understanding ENSO dynamics from the perspective of heat flux.

The Atlantic Niño Mode: A Thermodynamic or a Dynamic Phenomenon?

AbstractThe Atlantic Niño is an important source of the year‐to‐year variability of the tropical Atlantic, consisting in an irregular oscillation of the Sea Surface Temperature (SST) in the eastern tropical Atlantic. The physical mechanism underlying this oscillation is topic of debate. Some theories, known as dynamical, suggest that the Atlantic Niño is driven by internal wave dynamics. Other theories, termed thermodynamic, propose that the eastern tropical Atlantic SST variability is caused by thermodynamic processes associated with heat fluxes at the ocean surface and/or heat transport by oceanic currents. Here, we examine the SST variability from 1940 to 2022 and find that it can be described in terms of two spatial modes: one in the central and the other in eastern tropical Atlantic. However, focusing on the period from 1993 to 2022, these patterns behave differently. In 1993–2009 they concur in determining the eastern tropical Atlantic SST variability, while in 2010–2022 they act in opposite direction. Therefore, we use Sea Surface Height data and heat fluxes advected by oceanic currents to explore the relative contribution of wave dynamics and heat advection to the eastern tropical Atlantic SST fluctuation. We show that, between 1993 and 2010, the eastern tropical Atlantic SST variability is mainly determined by heat advected from the south by horizontal currents, while between 2010 and 2022 it can be explained in terms of wave dynamics. This finding suggests that the two spatial patterns determining the eastern tropical Atlantic SST variability, are two distinct physical phenomena forced by different physical mechanisms.

Spatial and Temporal Characteristics and Mechanisms of Marine Heatwaves in the Changjiang River Estuary and Its Surrounding Coastal Regions

Marine heatwave (MHW) events have significant consequences for marine ecosystems and human society. This paper investigates a MHW’s spatial–temporal characteristics in the Changjiang River Estuary and its surrounding coastal regions (CRESs), as well as analyzes the drivers, using satellite and reanalysis data spanning from 1982–2021. The findings show that, during the last 40 years, all four of the MHW indicators have increased. The summer MHW is more severe than other seasons, showing a rising pattern from southeast to northwest. The rise of MHWs can be attributed to the increase in sea surface heat flux, weak wind speed, and powerful El Niño events. Additionally, two special MHW events were detected during the entire study period: Event A lasted for 191 days from 9 October 2006 to 17 April 2007; Event B had an average intensity of 4.93 °C from 5 July 1994 to 1 August 1994. For locations so close to each other, the characteristics of MHWs can also vary, and the mechanisms behind them are highly complex.

Impact of Arctic sea ice on the boreal summer intraseasonal oscillation

This study investigates the relationship between sea ice concentration (SIC) in the Arctic Ocean and the Boreal Summer Intraseasonal Oscillation (BSISO) from 1991 to 2020 and its underlying mechanism. A significantly positive (negative) correlation was found between the frequency of phase 7 (3) of BSISO1 (30–60 d) and the preceding winter SIC, which is located the north of the East Siberian-Beaufort Sea (ESBS). Compared with low-SIC years, the conditions including northeasterly vertical wind shear, an enhanced ascending branch of the anomalous Walker circulation, an eastward water vapour transport channel, and an increased humidity gradient induce active convection over the Philippine Sea in high-SIC years, which benefits (hinders) to phase 7 (3) of BSISO1. The positive SIC anomaly during the transition from winter to spring influences local temperature and pressure through anomalous local sensible heat flux. This anomaly induces wave activity flux from the ESBS, which converges over the Bering Sea, enhancing the Aleutian Low (AL). Subsequently, the AL triggers an anomalous subtropical anticyclone through wave-mean flow interaction in the North Pacific. Due to southerly wind stress and increased sea surface heat flux, positive sea surface temperature anomalies near Japan persist in the summer, heating the lower troposphere and increasing baroclinicity. Significant positive geopotential heights and anticyclone anomalies occur over Japan, accompanied by a negative vorticity anomaly. The enhanced ascending motion over the Philippine Sea, facilitated by Ekman pumping, favours convection and influences the frequency of phases 7 and 3.

Larger wind turbines as a solution to reduce environmental impacts

The EU aims for carbon neutrality by 2050, focusing on offshore wind energy. Investments in North Sea wind farms, with optimal wind resources, play a crucial role. We employed a high-resolution regional climate model, which incorporates a wind farm parametrization, to investigate and address potential mitigating impacts of large wind farms on power generation and air-sea fluxes. Specifically, we examined the effects of replacing 5 MW turbines with larger 15 MW turbines while maintaining total capacity. Our study found that substituting 15 MW turbines increases the capacity factor by 2–3%, enhancing efficiency. However, these turbines exhibit a slightly smaller impact on 10 m wind speed (1.2–1.5%) and near-surface kinetic energy (0.1–0.2%), leading to reduced effects on sea surface heat fluxes compared to 5 MW turbines. This was confirmed by a stronger reduction in net heat flux of about 0.6–1.3% in simulations with 5 MW compared to 15 MW wind turbines. Air-sea fluxes influence ocean dynamics and marine ecosystems; therefore, minimizing these impacts is crucial. Overall, deploying 15 MW turbines in offshore wind farms may offer advantages for ocean dynamics and marine ecosystems, supporting the EU's carbon–neutral objectives.

Marine heatwaves and cold spells in the Brazil Overshoot show distinct sea surface temperature patterns depending on the forcing

The Brazil Current Overshoot regions feature the strongest sea surface temperature variations over the global ocean but forecast skill for these extreme variations is limited. Here we investigate the drivers of marine heatwaves and cold spells in this region and find that ocean flows and sea surface heat fluxes are the primary drivers. We also find that sea surface temperature extremes driven by air-sea heat fluxes produce consistent changes in sea surface temperature and related atmospheric variables over a wide horizontal area, whereas ocean flow-driven sea surface temperature extremes exhibit low consistency in air variables and limited consistency in the spatial extent of sea variables across most sea surface temperature extreme events. These differing air-sea characteristics associated with extremes driven by different drivers offer potential avenues for improving prediction skills and advancing the understanding of three-dimensional air-sea impacts associated with extreme events.

Mixed Convection in an Idealized Coastal Urban Environment With Momentum and Thermal Surface Heterogeneities

AbstractCoastal marine heatwaves (MHWs) modulate coastal climate through ocean‐land‐atmosphere interactions, but little is known about how coastal MHWs interact with coastal cities and modify urban thermal environment. In this study, a representative urban coastal environment under MHWs is simplified to a mixed convection problem. Fourteen large‐eddy simulations (LESs) are conducted to investigate how coastal cities interact with MHWs. We consider the simulations by simple urban roughness setup (Set A) as well as explicit urban roughness representation (Set B). Besides, different MHW intensities, synoptic wind speeds, surface fluxes of urban and sea patches are considered. Results suggest that increasing MHW intensity alters streamwise potential temperature gradient and vertical velocity direction. The magnitude of vertical velocity and urban heat island (UHI) intensity decrease with increasing synoptic wind speed. Changing urban or sea surface heat flux also leads to important differences in flow and temperature fields. Comparison between Set A and B reveals a significant increase of vertical velocity magnitude and UHI intensity. To further understand this phenomenon, a canopy layer UHI model is proposed to show the relationship between UHI intensity and urban canopy, thermal heterogeneity and mean advection. The effect of urban canopy is considered in terms of an additional vertical velocity scale that facilitates heat transport from the heated surface and therefore increases UHI intensity. The model can well explain the trend of the simulated results and implies that overlooking the effect of urban canopy underestimates canopy UHI in urban coastal environment.

Seasonal Prediction of Regional Arctic Sea Ice Using the High‐Resolution Climate Prediction System CMA‐CPSv3

AbstractSea ice is a central part of the Arctic climate system, and its changes have a significant impact on the Earth's climate. Yet, its state, especially in summer, is not fully understood and correctly predicted in dynamical forecast systems. In this study, the seasonal prediction skill of Arctic sea ice is investigated in a high‐resolution dynamical forecast system, the China Meteorological Administration Climate Prediction System (CMA‐CPSv3). A 7‐month‐long retrospective forecast is made every other month from 2001 to 2021. Employing the anomaly correlation coefficient as the metric of the prediction skill, we show that CMA‐CPSv3 can predict regional Arctic sea ice extent and sea ice thickness up to 7 lead months. The Bering Sea exhibits the highest prediction skill among the 14 Arctic subregions. CMA‐CPSv3 outperforms the anomaly persistence forecast in the Bering Sea, Sea of Okhotsk, Laptev Sea, and East Siberian Sea. The sources of the sea ice prediction skill partly come from the good performance of upper ocean temperature in CMA‐CPSv3. This holds true not only for winter sea ice in the Arctic marginal seas but also for summer sea ice in several Arctic central seas. Furthermore, CMA‐CPSv3 exhibits a strong relationship between the variability of sea ice and surface heat fluxes. This underscores the importance of enhancing the representation of air‐sea heat exchanges in dynamical forecast systems to improve the prediction skill of sea ice.

The Investigation of the Response Mechanism of SST and Chlorophyll to Super Typhoon “Rey” in the South China Sea

As one of the most significant disturbance sources in the upper marine environment of the South China Sea, tropical cyclones (typhoons) serve as a typical research subject for investigating the energy transfer process between the ocean and atmosphere. Utilizing satellite remote sensing data and focusing on Typhoon Rey No. 22’s transit event in 2021, this study quantitatively analyzes typhoon-induced energy input through heat pumping and cold suction at both surface and subsurface levels of the ocean. Additionally, it explores the response characteristics and feedback mechanisms of sea surface temperature (SST) and chlorophyll-a concentration (Chl-a) in the South China Sea to typhoon events. The research results show that the SST variation along the typhoon track displayed an asymmetric pattern, with a more pronounced warming on the right side and a cold anomaly lasting for 3–5 days on the left side. The subsurface warm anomaly dominated on the right side, showing a maximum temperature difference of 1.54 °C, whereas Ekman suction-induced upwelling led to cooling effects both at the subsurface and surface level on the left side, resulting in a maximum temperature difference of −3.28 °C. During the typhoon event, there was a significant decrease in sea surface heat flux, reaching 323.36 W/m2, accompanied by corresponding changes in SST due to processes such as upwelling, seawater mixing, and air–sea heat transfer dynamics where anomalies arising from oceanic dynamic processes and exchange through sea surface heat flux contributed equally. Furthermore, strong suction-induced upwelling during the typhoon influenced chlorophyll concentration within the central and western regions of the South China Sea (13.5° N–16.5° N, 111° E–112.5° E), resulting in significant enhancement and reaching its peak value at approximately 0.65 mg/L. The average chlorophyll concentration increased by approximately 0.31 mg/L.

Far-Reaching Effects of Okhotsk Sea Ice Area on Sea Surface Heat Flux, Lower Atmosphere, and Ocean Mixed Layer

Abstract The impact of interannual variations in sea ice area in the Okhotsk Sea was investigated through a composite analysis of years with extensive and limited sea ice areas (referred to as heavy and light ice years, respectively), using atmospheric and oceanic reanalysis data. The comparison of heavy and light ice-year composites in February revealed a substantial decrease in upward surface turbulent heat flux in the Okhotsk Sea (∼−250 W m−2) and a notable increase in a surprisingly extensive region in the western North Pacific (30–120 W m−2), spanning 2300 km from the ice edge. These differences were consistent with the decrease in surface air temperature and specific humidity, suggesting that during heavy ice years, cold and dry air blowing from Siberia to the North Pacific via the Okhotsk Sea undergoes less modification over larger sea ice areas, remaining colder and drier in the North Pacific and thereby enhancing the heat flux. Such advection can be associated with the Asian winter monsoon and migratory cyclones. Cloud cover and surface radiation flux altered consistently with these differences, although longwave and shortwave radiation largely counterbalanced each other. Additionally, the Pacific storm track exhibited variation. In accordance with the heat flux difference, sea surface temperature decreased, and the ocean mixed layer deepened around the subarctic during heavy ice years. These findings suggest that sea ice area in the Okhotsk Sea influences the lower atmosphere and surface ocean in the North Pacific. Such impacts could further affect ocean nutrient circulation, ecosystems, and atmospheric teleconnections. Significance Statement Sea ice cover influences heat exchange between the atmosphere and ocean. Advection of less heated air over larger sea ice areas can enhance heat exchange downwind, and vice versa. We found that this was effective in the Okhotsk Sea, situated along the path of the Asian winter monsoon. Upward surface heat flux increased significantly across a vast region of the North Pacific associated with colder and drier air during years with larger sea ice areas. Additionally, cloud cover, Pacific storm track, sea surface temperature, and ocean mixed layer depth exhibited significant differences associated with sea ice area. These variations could impact ocean heat content, nutrient circulation, and primary production in the North Pacific, and atmospheric circulation in North America and Europe.

Intraseasonal and interannual variability of sea temperature in the Arabian Sea Warm Pool

Abstract. The Arabian Sea Warm Pool (ASWP) is a part of the Indian Ocean Warm Pool, formed in the Arabian Sea before the onset of the Indian summer monsoon. The ASWP has a significant impact on climate change in the Indian Peninsula and globally. In this study, we examined the intraseasonal and interannual variability of sea temperature in the ASWP using the latest Simple Ocean Data Assimilation (SODA) reanalysis dataset. We quantified the contributions of sea surface heat flux forcing, horizontal advection, and vertical entrainment to the sea temperature using the mixed-layer heat budget analysis method. We also used a lead–lag correlation method to examine the relationship between the interannual variability of the ASWP and various large-scale modes in the Indo-Pacific Ocean. We found that the ASWP formed in April and decayed in June; its formation and decay processes were asymmetrical, with the decay rate being twice as fast as the formation rate. During the ASWP development phase, the sea surface heat flux forcing had the largest impact on the mixed-layer temperature with a contribution of up to 85 %. Its impact was divided into the net surface heat flux (0.41–0.50 ∘C per 5 d) and the shortwave radiation loss penetrating the mixed layer (from −0.08 ∘C per 5 d to −0.17 ∘C per 5 d). During the decay phase, the cooling effect of the vertical entrainment on the temperature variation increased (from −0.05 ∘C per 5 d to −0.18 ∘C per 5 d) and dominated the temperature variation jointly with the sea surface heat flux forcing. We also found that the ASWP has strong interannual variability related to the basin warming of the Indian Ocean. The lead–lag correlation indicated that the ASWP had a good synchronous correlation with the Indian Ocean Dipole. The ASWP had the largest correlation coefficient at a lag of 5–7 months of the Niño3.4 index, showing the characteristics of modulation by the El Niño–Southern Oscillation (ENSO). When the El Niño (La Niña) event peaked in the winter of the previous year, the ASWP that occurred before the summer monsoon was more significant (insignificant) in the following year.

Impact of lateral melting on Arctic sea ice simulation in a coupled climate model

Lateral melting is an important process driving the sea ice decay, yet it is not well represented in many Coupled Model Intercomparison Project Phase 6 (CMIP6) models. This study explores the impact of lateral melting on Arctic sea ice simulation by implementing lateral melting and floe size parameterization schemes in the medium resolution version of the Beijing Climate Center Climate System Model. Results from a series of CMIP6 historical-type experiments indicate that inclusion of lateral melting results in a reduction in both the Arctic sea ice concentration and thickness, thus improving the sea ice extent and volume simulation. Lateral melting increases open waters, leading to an enhanced net sea surface heat flux into the ocean and further increased lateral and bottom melting. This positive feedback is intensified from 1982 to 2014, particularly when the floe size parameterization scheme is introduced. This accelerates the Arctic sea ice decline from 1982 to 2014 in the model, which is more consistent with observations. Further analysis indicates that the enhancement of this feedback is associated with accelerated lateral melting due to the increased (decreased) trend of the sea surface temperature (floe size) from 1982 to 2014. This study highlights that sea ice lateral melting is an important factor affecting the simulation of Arctic sea ice decline and needs to be better represented in current climate models.

Cross-shelf transport off the northern Taiwan, East China Sea

Cross-shelf transport is an important material exchange process between the shelf seas and open seas. The distribution characteristics of phytoplankton, surface sediment particle size, minerals, etc., all indicate the possible existence of a cross-shelf transport conduit from the inner shelf to the outer shelf off northern Taiwan. This study confirmed cross-shelf transport off northern Taiwan based on remote sensing sea surface temperature (SST) data and heat flux data, which can be divided into two patterns: cold eddy and cold water tongue. Furthermore, combined with AVISO sea surface level data, CCMP wind field data, the dynamic formation factors of different morphological characteristics of cross-shelf transport are studied.Topographic changes mainly induce cross-shelf transport off northern Taiwan and occur at the curve of isobath to deep water. When there is a strong thermal front along the coast, cross-shelf transport is more likely to occur. Against the steady, strong northerly wind, the Zhe-Min coastal current's southward flow and Kuroshio's invasion strengthened the temperature front and facilitated cross-shelf transport. Typhoon strengthened cross-shelf transport over northern Taiwan after passing through the southern part of the study area. This research has important implications to find the channel and transporting pattern for material exchange between the East China Sea and the West Pacific, which will contribute to the study of ecological environmental protection and seabed muddy area formation.

Annual Cycle in Upper-Ocean Heat Content and the Global Energy Budget

Abstract As a major component of Earth’s energy budget, ocean heat content (OHC) plays a vital role in buffering climate change. The annual cycle is the most prominent change in OHC but has always been removed to study variations and changes in Earth’s energy budget. Here, we investigate the annual cycle of the upper-2000-m OHC at regional to global scales and assess the robustness of the signals using the spread of multiple observational products. The potential drivers are also investigated by comparing the annual OHC signal with the corresponding change in top-of-atmosphere radiation, surface heat flux, ocean heat divergence, and meridional heat transport. Results show that the robust signal of annual OHC change is significant down to a 1000-m depth globally and can reach down to 1500 m in some areas such as the tropical ocean. The global OHC (0–1500 m) changes from positive anomalies within September–February to negative anomalies within March–August, mainly because of the larger ocean area in the Southern Hemisphere and the seasonal migration of solar irradiance. Owing to the huge ocean heat capacity, the annual cycle of OHC dominates that of the global energy budget. The difference among the OHC annual cycles in the three major ocean basins is mainly attributed to ocean heat transport, especially in the tropics. In the upper 1500 m at mid- and high latitudes and in the upper 50 m of the tropics, the net sea surface heat flux dominates the OHC annual cycle, while in the tropics below 50 m, wind-driven Ekman heat transport associated with the geostrophic flow is the main driver.

Four-Dimensional Variational Data Assimilation and Sensitivity of Ocean Model State Variables to Observation Errors

The use of Four-Dimensional variational (4D-Var) data assimilation technology in the context of sea dynamics problems, with a sensitivity analysis of model results to observation errors, is presented. The technology is applied to a numerical model of ocean circulation developed at the Marchuk Institute of Numerical Mathematics, Russian Academy of Sciences (INM RAS), with the use of the splitting method and complemented by 4D-Var data assimilation with covariance matrices of background and observation errors. The variational data assimilation involves iterative procedures to solve inverse problems so as to correct sea surface heat fluxes for the model under consideration. An algorithm is formulated to study the sensitivity of the model outputs, considered as output functions after assimilation, to the observation errors. The algorithm reveals the regions where the output function gradient is the largest for the average sea surface temperature (SST) in a selected area, obtained by assimilation. In the numerical experiments, a 4D variational problem of SST assimilation for the Baltic Sea area is solved.

Subsurface Anticyclonic Eddy Transited from Kuroshio Shedding Eddy in the Northern South China Sea

Abstract Subsurface eddies are a special type of oceanic eddy that display the maximum velocity in the subsurface layer. Based on field observations, a lens-shaped subsurface anticyclonic eddy (SAE) was detected in the northern South China Sea (SCS) in May 2021. The SAE was located between 20 and 200 m, with a shoaling of the seasonal thermocline and deepening of the main thermocline. Satellite images showed that the SAE exhibited positive sea level anomaly (SLA) and negative sea surface temperature (SST) anomaly. Eddy track indicated that this SAE originated from the Luzon Strait and was generated in the Kuroshio Loop Current (KLC) last winter. The evolution of the SAE was related to the anomalous water properties inside the eddy and the seasonal change of sea surface heat flux. In winter, the continuous surface cooling and Kuroshio intrusion led to a cold, salty core in the upper part of the anticyclonic eddy, which resulted in a subsurface-intensified structure through geostrophic adjustment. As the season changed from winter to spring, sea surface temperature increased. The lens-shaped structure was formed when the seasonal thermocline appeared near the surface that capped the winter well-mixed water inside the eddy. From 1993 to 2021, nearly half of the winter KLC shedding eddies (12/25) survived to late spring and evolved into subsurface lens-shaped structures. This result indicates that the transition of KLC shedding eddy to SAE is a common phenomenon in the northern SCS, which is potentially important for local air–sea interaction, heat–salt balance, and biogeochemical processes. Significance Statement Subsurface eddies are lens-shaped eddies with anomalous water properties in the subsurface layer. While such eddies have been reported in many regions of the World Ocean, they are poorly investigated in the SCS, especially the periodic subsurface eddies that appear in a fixed time frame with similar patterns and trajectories. This study reported a subsurface anticyclonic eddy (SAE) in the northern SCS and elucidated its generation and evolution processes. Statistical results confirm that this is a periodic SAE, which occurs nearly annually in late spring and evolves from the Kuroshio shedding eddy with seasonal changes. This study provides a new perspective on the evolution of subsurface eddies in the SCS and will benefit targeted observations in the future.

Heat balance characteristics in the South China Sea and surrounding areas simulated using the TRAMS model—a case study of a summer heavy rain and a winter cold spell

Introduction: This study is first to apply d iagnostic analysis of the forecast tendencies to evaluate the simulation of equilibrium features in the South China Sea and its surrounding areas using The Tropical Regional Atmosphere (TRAMS) model, and further identify the sources of simulation biases of the model.Methods: On the basis of the quasi equilibrium between dynamic and physical processes, the deviation of net forecast tendencies, which reflects the overall equilibrium of the model, serves as a good indicator of the model simulations bias. The sources of the forecast error can be further inferred by decomposing the net forecast tendencies into the dynamic and each physical process.Results: Focusing on moisture and temperature tendencies, the results show that the TRAMS model generally captures the thermal equilibrium characteristics and the contributions of each physical process, which are markedly different between summer and winter and are affected by the Northwest Pacific Subtropical High (WPSH) and deep trough of East Asia respectively. Furthermore, the underestimation of vapour consumption by cloud microphysical parameterisation and the overestimation of sea surface heat flux by boundary layer parameterisation near the surface contribute the most to systematic model bias. The temperature bias from 900 to 300 hPa during winter mainly originates from the responses of radiation, cumulus convection, and cloud microphysical parameterisations because water vapour absorbs long wave radiation, heating the atmosphere, and clouds reduce short wave radi ation absorption, cooling it.Discussion: The presented analyses provide a reference for further optimisation and improvement of the model.

Modulation of Upper Ocean Vertical Temperature Structure and Heat Content by a Fast-Moving Tropical Cyclone

Abstract The ocean temperature response to tropical cyclones (TCs) is important for TC development, local air–sea interactions, and the global air–sea heat budget and transport. The modulation of the upper ocean vertical temperature structure after a fast-moving TC was studied at the observation stations in the northern South China Sea, including TCs Kalmaegi (2014), Rammasun (2014), Sarika (2016), and Haima (2016). The upper ocean temperature and heat response to the TCs mainly depended on the combined effect of mixing and vertical advection. Mixing cooled the sea surface and warmed the subsurface, while upwelling (downwelling) reduced (increased) the subsurface warm anomaly and cooled (warmed) the deeper ocean. An ideal parameterization that depends on only the nondimensional mixing depth (HE), nondimensional transition layer thickness (HT), and nondimensional upwelling depth (HU) was able to roughly reproduce sea surface temperature (SST) and upper ocean heat change. After TCs, the subsurface heat anomalies moved into the deeper ocean. The air–sea surface heat flux contributed little to the upper ocean temperature anomaly during the TC forcing stage and did not recover the surface ocean back to pre-TC conditions more than one and a half months after the TC. This work shows how upper ocean temperature and heat content varies by a TC, indicating that TC-induced mixing modulates the warm surface water into the subsurface, and TC-induced advection further modulates the warm water into the deeper ocean and influences the ocean heat budget. Significance Statement Tropical cyclones can cause a strong ocean response that modulates the upper ocean temperature structure and contributes to the local heat budget and transport. This manuscript shows how mixing and vertical advection modulate upper ocean temperature after four fast-moving tropical cyclones, and then gives a parameterization of how sea surface temperature and upper ocean heat change depend on the two mechanisms. The temperature anomalies can propagate into deeper ocean after the tropical cyclones, and sea surface heat flux is not important for upper ocean temperature response during a tropical cyclone. These results show how the upper ocean temperature responses to a tropical cyclone, and influences the local heat budget.