Net Surface Heat Flux Research Articles

Insights on Simulating Summer Warming of the Great Lakes: Understanding the Behavior of a Newly Developed Coupled Lake‐Atmosphere Modeling System

AbstractThe Laurentian Great Lakes are the world's largest freshwater system and regulate the climate of the Great Lakes region, which has been increasingly experiencing climatic, hydrological, and ecological changes. An accurate mechanistic representation of the Great Lakes thermal structure in Regional Climate Models (RCMs) is paramount to studying the climate of this region. Currently, RCMs have primarily represented the Great Lakes through coupled one‐dimensional (1D) column lake models; this approach works well for small inland lakes but is unable to resolve the realistic hydrodynamics of the Great Lakes and leads to inaccurate representations of lake surface temperature (LST) that influence regional climate and weather patterns. This work overcomes this limitation by developing a fully two‐way coupled modeling system using the Weather Research and Forecasting model and a three‐dimensional (3D) hydrodynamic model. The coupled model system resolves the interactive physical processes between the atmosphere, lake, and surrounding watersheds; and validated against a range of observational data. The model is then used to investigate the potential impacts of lake‐atmosphere coupling on the simulated summer LST of Lake Superior. By evaluating the difference between our two‐way coupled modeling system and our observation‐driven modeling system, we find that coupled‐lake atmosphere dynamics can lead to a higher LST during June‐September through higher net surface heat flux entering the lake in June and July and a lower net surface heat flux entering the lake in August and September. The unstratified water in June distributes the entering surface heat flux throughout the water column leading to a minor LST increase, while the stratified waters of July create a conducive thermal structure for the water surface to warm rapidly under the higher incoming surface heat flux. This research provides insight into the coupled modeling system behavior, which is critical for enhancing our predictive understanding of the Great Lakes climate system.

Role of Ocean Advections during the Evolution of the Pacific Meridional Modes

Abstract The Pacific meridional modes (PMMs) are the leading ocean–atmosphere coupled modes in the subtropical northeastern (NPMM) and southeastern (SPMM) Pacific, respectively, and have been suggested to be key precursors to equatorial Pacific variability. Previous studies pointed out that both NPMM- and SPMM-related sea surface temperature (SST) anomalies are primarily driven by net surface heat flux variations during their equatorward evolution. However, whether oceanic heat advective processes would play a role during the evolution remains unclear. To address this issue, we perform an ocean mixed layer heat budget analysis based on observations and three ocean reanalysis datasets, and then reveal the effect of ocean advections on the evolution by comparing a fully coupled dynamic ocean model (DOM) to a slab ocean model (SOM). Our results suggest that for the NPMM evolution, ocean advections—primarily by anomalous meridional Ekman heat advections driven by mean and anomalous zonal wind stresses—play a damping role in the south of the NPMM. Thus, the NPMM SST anomalies appear to instead exhibit a poleward shift, although still freely propagating westward from the preceding boreal winter to the following summer. This finding challenges the traditional view that the NPMM propagates equatorward through the wind–evaporation–SST feedback. For the SPMM evolution, ocean advections play a damping role in the center of the SPMM from boreal spring to summer, as well as an intensification role in the southwest Pacific during summer. However, the effect of the intensification on the SPMM evolution is hard to determine due to the strong simulation bias of the SPMM evolution in the DOM. Significance Statement While it is known that both NPMM- and SPMM-associated SST anomalies are primarily driven by net surface heat flux variations during their evolution, whether ocean advections would play a role remains unknown. Here, we show that ocean advections play a role in the evolution of both PMMs. In particular, for the NPMM evolution, ocean advections play a damping role in the south of the NPMM center, causing a tendency for the NPMM to be displaced northward. The role of ocean advection challenges the prevailing notion that the NPMM simply evolves equatorward through the wind–evaporation–SST feedback.

Interdecadal Variation of Spring Extreme High-Temperature Events in the Western Tianshan Mountains and Its Relationship with the Tropical SST

This study performed an observational analysis to examine the interdecadal variation in the frequency of extreme high-temperature events (EHEs) during spring over the western Tianshan mountain, China, which were characterized by relatively fewer (more) EHEs during 1983–1996 (2000–2015). A composite analysis indicated that the interdecadal increase in EHEs is closely related to a deep dynamic anomalous Iranian high. Under the control of this high system, the water vapor content decreased over the western Tianshan mountains, and atmospheric circulation was dominated by a descending motion. Both were attributed to the decreased cloud cover, inducing a cloud-forced net solar radiation increase. The short-wave radiation flux and sensible heat flux reaching the surface increased, and the net surface heat flux increased cumulatively, which was conducive to the surface temperature increase and EHE occurrence. The anomalous Iranian high responsible for ECEs occurrence was related to the air-sea interaction over the Atlantic and Indo-Pacific. The latitudinal sea surface temperature (SST) difference between the tropical western Pacific and the western Indian Ocean directly strengthens the Walker circulation and thus enhances the Iranian high. In addition, the anomalous Iranian high was affected by the atmospheric wave trains at middle latitude, which was triggered by the warm anomaly of the Atlantic SST.

Characteristics of Marine Heatwaves in the Indonesian Waters during the PDO, ENSO, and IOD Phases and Their Relationships to Net Surface Heat Flux

We conducted an investigation into the characteristics of marine heatwaves (MHWs) in Indonesian waters, aiming to understand the underlying mechanisms responsible for their formation, particularly the ones generated by net surface heat flux. To accomplish this, we utilized remote sensing data from the National Oceanic and Atmospheric Administration and the European Centre for Medium-Range Weather Forecasts. The dataset covered a 40-year period (1982–2021) encompassing both warm (1982–2007) and cold (2008–2021) phases of the Pacific Decadal Oscillation (PDO). Statistical analysis methods were employed to process the data. Our study reveals significant findings regarding MHWs in Indonesian waters. We observed the highest average frequency and maximum intensity of MHWs, occurring approximately 2–3 times a year and exceeding 1.5 °C, respectively, during the warm phase of PDO (with El Niño events occurring more frequently than La Niña). Conversely, the longest durations of MHWs in Indonesian waters were recorded during the cold phase of PDO (with La Niña events occurring more frequently than El Niño), spanning approximately 7–15 days. We identified local forcing in the form of net surface heat flux as the primary driver of MHW frequency and maximum intensity. During the warm phase of PDO, the net surface heat flux was notably higher, primarily due to increased shortwave radiation (heat gain) and reduced latent heat flux (heat loss) through the evaporation process. These factors collectively contributed to maintaining warmer ocean temperatures. Moreover, our study provides valuable insights into the interannual variability of MHWs through the application of composite calculations. We discovered a strong correlation between the occurrence of El Niño and positive Indian Ocean Dipole (IOD) events during the warm PDO phase and the highest frequency and maximum intensity of MHWs, with approximately 2.52 events and 1.54 °C, respectively. In contrast, we found that MHWs with the longest durations were closely linked to La Niña and negative IOD events during the cold PDO phase, lasting approximately 10.90 days. These findings highlight the complex interplay between climate phenomena and MHW characteristics, further deepening our understanding of their dynamics.

Interpretation of Net Surface Heat Fluxes and Meridional Overturning Circulations in Global Coupled HadGEM3 Climate Simulations

Abstract The annual mean net surface heat fluxes (NSHFs) from the ocean to the atmosphere generated by historical forcing simulations using the HadGEM3-GC3.1 coupled climate model are shown to be relatively independent of resolution, for model horizontal grid spacings between 1° and 1/12°, and to agree well with those based on the DEEP-C (Diagnosing Earth’s Energy Pathways in the Climate System) analyses. Interpretations of the geographical patterns of the NSHFs are suggested that use basic ideas extracted from the theory of the ventilated thermocline and planetary geostrophic layer models. As a step toward investigation of the validity of the assumptions underlying the interpretations, we examine the contributions to the rate of change of the active tracers from the main terms in their prognostic equations as a function of the active tracer and latitude. We find that, consistent with our assumptions, the main contributions from vertical diffusion occur in “near-surface” layers. We also find that, except at high latitudes, the sum of the NSHF and vertical diffusion is mainly balanced by time-mean advection of potential temperature. A corresponding statement holds for potential density but not salinity. We also show that the heat input by latitude bands is dominated by the NSHFs, the time-mean advection, and the equatorial Pacific. It is usually assumed that global integrals of tracer tendencies due to advection as a function of the tracer should be identically zero. We show that nonnegligible contributions to them arise from net freshwater surface fluxes. Significance Statement Our aim is to understand better how the heat and freshwater that are input into the ocean from the atmosphere are then redistributed within the ocean and released back into the atmosphere. We show that the geographical patterns of the heat that is input to or released from the ocean surface in coupled climate models agree well with observations. We outline a dynamically based interpretation of these surface fluxes and provide evidence that supports some of its assumptions. This work might in future help us to understand how the patterns of the surface fluxes will respond to changes in greenhouse gas forcing.

Tracing Oceanic Sources of Heat Content Available for Atlantic Hurricanes

AbstractIn the Main Development Region (MDR) for Atlantic hurricanes, the volume of water warmer than 26.5°C quantifies the potential source of energy for major storms. Taking a Lagrangian perspective, this warm water is backtracked on seasonal timescales in an eddy‐resolving ocean model hindcast spanning 1988–2010. Being confined near the surface and assuming a mixed layer depth of 50 m, net heat fluxes into or out of water parcels advected toward the MDR are inferred from along‐trajectory temperature tendencies. To first order, these heat fluxes match surface net heat fluxes during the months over which water advects into the region. Contributions to this warm water in the preceding 6 months include water resident in the MDR (20%–40%), arriving via the North Brazil Current (NBC, 5%–15%), or via Ekman drift across 10°S. In relative terms, decreased contributions from the NBC and Ekman drift and more in situ warming within the MDR lead to warmer, more active hurricane seasons.

Upper-Oceanic Warming in the Gulf of Mexico between 1950 and 2020

Abstract We estimate ocean heat content (OHC) change in the upper 2000 m in the Gulf of Mexico (GOM) from 1950 to 2020 to improve understanding of regional warming. Our estimates are based on 192 890 temperature profiles from the World Ocean Database. Warming occurs at all depths and in most regions except for a small region at northeastern GOM between 200 and 600 m. GOM OHC in the upper 2000 m increases at a rate of 0.38 ± 0.13 ZJ decade−1 between 1970 and 2020, which is equivalent to 1.21 ± 0.41 terawatts (TW). The GOM sea surface temperature (SST) increased ∼1.0° ± 0.25°C between 1970 and 2020, equivalent to a warming rate of 0.19° ± 0.05°C decade−1. Although SST in the GOM increases at a rate approximately twice that for the global ocean, the full-depth ocean heat storage rate in the GOM (0.86 ± 0.26 W m−2) applied to the entire GOM surface is comparable to that for the global ocean (0.82–1.11 W m−2). The upper-1000-m layer accounts for approximately 80%–90% of the total warming and variations in the upper 2000 m in the GOM. The Loop Current advective net heat flux is estimated to be 40.7 ± 6.3 TW through the GOM. A heat budget analysis shows the difference between the advective heat flux and the ocean heat storage rate (1.76 ± 1.36 TW, 1992–2017) can be roughly balanced with the annual net surface heat flux from ECCO (−37.9 TW).

Ocean Warm Skin Signals Observed by Saildrone at High Latitudes

AbstractThe existence of a cool sea surface skin layer in the global ocean during both day and night is generally recognized. However, a warm skin should be present if the total surface net heat flux () were to be from the atmosphere into ocean. Saildrone, an advanced uncrewed surface vehicle, has been shown to be able to provide sufficiently accurate sea skin temperature (SSTskin) and subsurface temperature (SSTdepth) data at high latitudes. Using those SST data along with meteorological parameters from a Saildrone deployed in the Arctic in the summer of 2019, some warm skin layers were identified due to the gain resulting from the combined effect of positive air‐sea temperature difference, humid surface air and cloudy skies. Furthermore, most warm skins here were found during and shortly after rainfall events. It is essential to incorporate the ability to simulate warm skin layers in the present cool skin models.

The Role of Atmospheric Noise in Decadal SST Variability

Abstract A substantial role for atmospheric noise in simulations of decadal internal variability of SST is demonstrated by a comparison of a multicentury climate model control and a corresponding interactive ensemble (IE) simulation. The IE is designed to reduce atmospheric noise in the heat flux, wind stress, and freshwater flux at the air–sea interface. This comparison suggests that nearly all SST variability on decadal time scales is forced by internal atmospheric variability. The results are examined to determine the relative roles of atmospheric surface heat flux noise and ocean dynamics in the decadal volume-averaged heat budget of the upper ocean. The regional heat budgets in two regions, the South Pacific and the midlatitude North Atlantic, show the net atmospheric surface heat flux to be approximately in equilibrium with the ocean dynamics forcing. The IE and control results are used in the equilibrium heat budget approximation to infer the atmospheric heat flux response to SST, as well as the time series of the control atmospheric noise surface heat flux and ocean dynamics forcings for several regions. The South Pacific region SST is found to be primarily forced by the atmospheric noise surface heat flux and the North Atlantic region SST is forced by the ocean dynamics. Similar strengths for the atmospheric heat flux noise and ocean dynamics forcing, with an interdecadal atmospheric heat flux noise time scale and a centennial ocean dynamics time scale, are found for an Atlantic multidecadal variability region SST.

Impact of the Deep Chlorophyll Maximum in the Equatorial Pacific as Revealed in a Coupled Ocean GCM‐Ecosystem Model

AbstractA coupled ocean general circulation model (OGCM)‐ocean ecosystem model is used to investigate the effects of the deep chlorophyll maximum (DCM) on the ocean state in the equatorial Pacific Ocean. Climatological and interannual three‐dimensional (3‐D) chlorophyll (CHL) fields are captured well in control runs using the coupled ocean physics‐ecosystem model forced by prescribed atmospheric fields. Further sensitivity experiments are performed to assess the effects of 3‐D CHL structure using the OGCM with the prescribed CHL fields taken from the control runs: CHLclim and are runs in which climatological DCM effect is included or only surface CHL effect is included; CHLinter and are runs with interannually varying DCM effects included or not. The differences in the simulated ocean conditions are analyzed to explore DCM effects on sea surface temperature (SST) and amplitude of El Niño‐Southern Oscillation (ENSO). Two competing mechanisms responsible for the DCM effects are revealed: an ocean biology‐induced direct heating (OBH) effect, and an indirect cooling effect due to dynamic processes associated with vertical mixing and shallow meridional overturning circulation. There are three major findings: (a) DCM acts to reduce mean SST by around 0.2°C in the eastern equatorial Pacific, being larger than the surface CHL effects. (b) DCM interannual variability increases the ENSO amplitude to a comparable degree as the surface CHL effects. (c) The total net impact of vertical mixing, currents, and net surface heat flux makes SST drop more under the DCM effect than the surface CHL effect in the eastern Pacific. These findings provide a new insight into the feedback mechanisms for the bioclimate interactions.

Mixed Layer Temperature Budget in the Arabian Sea During Winter 2019 and Spring 2019: The Role of Diapycnal Heat Flux

AbstractSea surface temperatures (SSTs) simulated by almost all models in Coupled Model Inter‐comparison Project phase five are consistently colder (∼−1°C to −4°C) than the observation in the northern Arabian Sea (AS) during winter and spring. These biases significantly weaken the seasonal and extended summer monsoon prediction skills and climate change projection's reliability in this region. To understand the relative contribution of diapycnal heat flux (Jh) compared to other terms in the mixed layer (ML) temperature (MLT) budget during these two seasons, we use time series of vertical profiles of microstructure shear measurements collected in the northeastern AS (NEAS) at 18.4°N and 67.4°E during January 10–17, 2019 (W19) and May 7–21, 2019 (S19). It is found that the vertical processes term and net surface heat flux together determine the bulk of MLT tendency during S19 and W19, and the contribution of the horizontal advection and Jh is relatively smaller. The mean value of Jh (∼−2 W m−2) at the base of the ML shows a comparable magnitude during W19 and S19, and the median values of diapycnal diffusivity (Kρ) at the ML base are not significantly different between W19 (∼3.1 × 10−6 m2 s−1) and S19 (∼5.2 × 10−6 m2 s−1). Besides, Kρ and Jh values were estimated using different Kρ‐Richardson number (Ri) based interior ocean parameterization schemes at the ML base in the NEAS overestimated with respect to observation. The alleged role of overestimation of Kρ and Jh in parameterization schemes on model simulation of cold SST bias in the AS is also discussed.

Heat Flux Estimates From a Synthesis of Satellite Observations and a Hydrodynamic Model (With Application to Long Island Sound)

AbstractEstimating surface heat fluxes via direct covariance measurements or bulk formulae is observation‐intensive and costly. We present a methodology whereby we estimate net surface heat fluxes as the difference between the depth‐integrated heat tendencies and the depth‐integrated horizontal heat exchanges in a hydrodynamic model. We calibrate the model to achieve a good representation of mixing and advection and then assimilate satellite sea surface temperature (SST) observations into the model at an 8‐day scale. The SST data assimilation forces a good representation of observed temperatures and heat tendencies both at the surface and throughout the water column. We estimate the horizontal heat exchange directly from the model output and then infer the surface fluxes required to close the budget. When we apply this methodology to a model with prescribed surface heat fluxes and without data assimilation, we can recover the prescribed fluxes with an RMS error of ±10 W m−2 and an r2 of 0.998. When we compare our results to those estimated using Coupled Ocean‐Atmosphere Response Experiment bulk formulae with observations in western Long Island Sound, we find similarly good agreement.

Seasonal cycle of sea surface temperature in the tropical Angolan Upwelling System

Abstract. The Angolan shelf system represents a highly productive ecosystem. Throughout the year the sea surface is cooler near the coast than further offshore. The lowest sea surface temperature (SST), strongest cross-shore temperature gradient, and maximum productivity occur in austral winter when seasonally prevailing upwelling-favourable winds are weakest. Here, we investigate the seasonal mixed layer heat budget to identify atmospheric and oceanic causes for heat content variability. By using different satellite and in situ data, we derive monthly estimates of surface heat fluxes, mean horizontal advection, and local heat content change. We calculate the heat budgets for the near-coastal and offshore regions separately to explore processes that lead to the observed SST differences. The results show that the net surface heat flux warms the coastal ocean stronger than further offshore, thus acting to damp spatial SST differences. Mean horizontal heat advection is dominated by meridional advection of warm water along the Angolan coast. However, its contribution to the heat budget is small. Ocean turbulence data suggest that the heat flux, due to turbulent mixing across the base of the mixed layer, is an important cooling term. This turbulent cooling, being strongest in shallow shelf regions, is capable of explaining the observed negative cross-shore temperature gradient. The residuum of the mixed layer heat budget and uncertainties of budget terms are discussed.

Indian Ocean mixed layer depth changes under global warming

The surface ocean mixed layer (OML) is critical for climate and biological systems. Changes in ocean mixed layer depth (MLD) of the Indian Ocean under global warming are examined utilizing outputs from 24 climate models in the Coupled Model Intercomparison Project phase 6 (CMIP6) models and the Community Earth System Model 1.0 with Community Atmosphere Model version 5 (CESM1–CAM5). The results show that the MLD generally decreases in low- and high-emissions Shared Socioeconomic Pathway (SSP) scenarios (ssp126 and ssp585). In ssp126 and ssp585, the multi-model ensemble-mean OML, respectively shoals about 5 and 10% over both the northern tropics and southern subtropics, with high model consistency. This robust OML shoaling appears in the 1980s and is closely associated with increased surface buoyancy forcing and weakened winds. In contrast, the OML in the south equatorial Indian Ocean slightly deepens and displays large intermodel differences in the sign and magnitude of the changes. The effects of direct CO2 increase and wind changes on OML changes are further quantified by CESM1–CAM5 partially coupled experiments. The results show that the increased surface net heat flux from direct CO2 increase dominates OML shoaling in the northern tropics. In the southern subtropics, the increased surface heat flux, reduced wind speed, and wind-driven divergence all facilitate the OML shoaling. In the south equatorial Indian Ocean, wind changes generally deepen the OML, consistent with the CMIP6 results. Moreover, the OML shoaling-related upper ocean stratification changes are contributed by both temperature and salinity changes in the northern tropics but dominated by temperature changes south of 10°S. These results highlight the regional differences in MLD changes and their forcing, which is important for understanding regional climate changes and corresponding changes in extreme events and biological systems under global warming.

Characteristics of Marine Heatwaves (2008–2021) in the Savu Sea, East Nusa Tenggara

In this paper, we investigate the characteristics and the mechanism underlying the formation of marine heatwaves (MHWs) in the Savu Sea, known as the richest marine biodiversity area in the world. The focus is on identifying the frequency, intensity, and duration of occurrence MHWs generated by local forcing in the form of net surface heat flux. The study utilizes the remote sensing data from NOAA Optimum Interpolation (OI) Sea Surface Temperature (SST) for SST and The European Centre for Medium-Range Weather Forecasts Reanalysis v5 (ERA-5) for net surface heat flux components (shortwave radiation, longwave radiation, sensible heat flux, and latent heat flux). All data covers the period for the last 14 years (2008–2021) during the cold phase of the Pacific Decadal Oscillation (PDO) and then processed by statistical analysis methods. The results showed that the mean frequency, intensity, and duration of occurrence MHWs in the Savu Sea were around two times a year, 1.4–1.6 °C, and 10–12 days, respectively. The highest frequency and intensity MHWs occurred at around 122–124 °E and 8.30–9.30 °S (northern Savu Sea) generated by the most significant net surface heat flux in these areas. This study reveals the most dominant contribution of net surface heat flux in the northern Savu Sea comes from high shortwave radiation (heat gain) and low latent heat flux through the evaporation process (heat loss), thus keeping the ocean warm.

Submesoscale processes observed by high-frequency float in the western South China sea

Using in situ high-frequency data obtained from a new type of float (Navis-SL1) with a thermal engine, this study documents that steep isopycnals result in submesoscale processes during a strong wind event in the western South China Sea. The mixing induced by strong wind promoted the appearance of vertical isopycnals and weakened stratification in the mixed layer. The background isopycnals did not have a traditional frontal structure with a single inclined direction (i.e., a cold dense filament configuration). Submesoscale characteristics induced by the mixed-layer instability (MLI) were found in the mixed layer. By using the parameterization of streamfunction of MLI, two ageostrophic overturning circulation cells formed. In the center of the two cells, the vertical velocity was downward, while it was upwelling on both sides. The horizontal velocity diagnosis showed convergence/divergence at the top/bottom of the mixed layer. The vertical equivalent heat flux induced by MLI was compared with the daily averaged surface net heat flux and was found to have an important impact on the buoyancy budget in the mixed layer.

ENSO Asymmetry in CMIP6 Models

Abstract An interesting aspect of the El Niño–Southern Oscillation (ENSO) phenomenon is the asymmetry between its two phases. This paper evaluates the simulations of this property of ENSO by the Coupled Model Intercomparison Project phase 6 (CMIP6) models. Both the surface and subsurface signals of ENSO are examined for this purpose. The results show that the models still underestimate ENSO asymmetry as shown in the SST field, but do a better job in the subsurface. A much weaker negative feedback from the net surface heat flux during La Niña in the models is identified as a factor causing the degradation of the ENSO asymmetry at the surface. The simulated asymmetry in the subsurface is still weaker than the observations owing to a weaker dynamic coupling between the atmosphere and ocean. Consistent with the finding of a weaker dynamic coupling strength, the precipitation response to the SST changes is also found to be weaker in the models. The results underscore that a more objective assessment of the simulation of ENSO by climate models may have to involve the examination of the subsurface signals. Future improvements in simulating ENSO will likely require a better simulation of the surface heat flux feedback from the atmosphere as well as the dynamical coupling strength between the atmosphere and ocean. Significance Statement The ENSO phenomenon affects weather and climate worldwide. An interesting aspect of this phenomenon is the asymmetry between its two phases. Previous studies have reported a weaker asymmetry in the simulations by climate models. But these studies have focused on the ENSO asymmetry at the surface. Here by examining the ENSO asymmetry at the surface and the subsurface, we have found that ENSO asymmetry is better simulated in the subsurface than at the surface. We have also identified factors that are responsible for the degradation of the ENSO asymmetry at the surface as well as the remaining weakness in the subsurface, pointing out specific pathways to take to further improve ENSO simulations by coupled climate models.

Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Abstract. Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these is that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighbouring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. We identify the onset of the surface phytoplankton growth as concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic carbon derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favours carbon uptake by phytoplankton lasting a few days. Simultaneously, the air–sea net heat flux switches from negative to positive, linked to changes in the latent air–sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the surface phytoplankton bloom. We conclude that decreases in the mixing- and mixed-layer depths lead to the onset of the phytoplankton growth due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 d between 2016 and 2019 as between 38 and 191 mmol C m−2. These results have important implications, as biological processes play a major role in the seasonal evolution of surface pCO2 and thereby the rate of reduction in atmospheric CO2 by exchange at the air–sea interface.

Surface net heat flux estimated from drifter observations

The ocean mixed layer temperature equation is used to estimate the surface net heat flux from drifter measurements. The net heat flux is determined for both the climatologic and tropical cyclone (TC) conditions. The spatial distributions of the drifter-derived heat fluxes under both the two conditions are similar to those derived from satellite observations. However, the drifter-derived climatologic heat flux appears to be weaker in magnitude than that derived from satellites, and performs better in closing the energy budget with a global mean value of 3.9 W m−2. The drifter-derived heat flux also performs better than the satellite-derived heat flux under TCs, using the buoy observations as a reference considering metrics such as the meen error, mean absolute error, root mean-square error and percent bias. The spatially averaged mean net heat flux derived from drifters under TCs is −124 W m−2 at 10° N, and decreases to −85 W m−2 at 30° N, however, these values are much larger than those obtained from satellites (−63 W m−2 and −21 W m−2, respectively). As additional components for the mixed layer temperature equation, both the entrainment velocity and eddy diffusivity in climatology show large amplitudes in regions with strong currents such as the Western Boundary Current and Antarctic Circumpolar Current. However, under TC conditions large values of the entrainment velocity and eddy diffusivity mostly appear in regions with strong winds.

Thermal-Moisture Dynamics at Different Underlying Surfaces in Permafrost Regions of the Central Tibetan Plateau by considering the Effect of Rainfall

The Qinghai-Tibetan Plateau (QTP) has undergone an increase in rainfall and a drastic alteration in the moisture-heat regime in active layers and engineering. To investigate the water and heat responses of natural ground and engineering to rainfall, the differences in energy on the ground surface and the thermal-moisture dynamics of different permafrost underlying surfaces were discussed. Based on the meteorological data in 2013 observed at the Beiluhe observation station, three types of underlying surfaces (i.e., natural ground, asphalt pavement, and gravel pavement) were selected to compare the differences in energy balance at the ground surface, water-energy transport process, and coupling mechanism in active layers under rainfall conditions by a coupled water vapor-heat model of the unsaturated frozen soil. The results show that the asphalt pavement greatly increases the surface net radiation and soil surface heat flux, decreases the surface evaporation latent heat, and cuts off the moisture migration between the atmosphere and the active layer. The gravel pavement significantly increases the surface evaporation latent heat to lower the soil surface heat flux, and the amount of moisture in shallow soil is strongly influenced by rainfall and evaporation. Therefore, the moisture migration and accumulation under the asphalt pavement are dominated by the water vapor flux under thermal gradients, whereas the liquid water under the water potential gradients is the major source of moisture migration under the gravel pavement. The heat transfer in the shallow active layer is dominated by heat conduction. The effect of heat conduction, water vapor migration, and phase transition on the soil temperature is evident for the asphalt pavement, while the impact of liquid water migration on the shallow soil temperature for the gravel pavement is significant in the thawing period. As a result, the soil temperature relationship between different underlying surfaces is asphalt pavement>gravel pavement>natural ground. The thickness of the active layer gradually decreases. Although rainfall infiltration promotes the liquid water convection of the gravel pavement, the decrease in heat flux is less than the increase in thermal conductivity. In general, the construction of asphalt pavement and gravel pavement accelerates the degradation of permafrost. The results can provide theoretical and simulated guidance for the stability prediction and analysis of various underlying surfaces in the central QTP where rainfall is increasing.