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Marine heatwaves (MHWs) and cold‐spells (MCSs) are extreme sea surface temperature events with significant impacts on marine ecosystems. However, the connection between these events and mixed layer depth (MLD) variations, as well as how their intensity relates to MLD changes, remains unclear. Integrating OISST V2.1 data with Argo profiles, this analysis finds that during MHWs, MLD decreases by 8.10% globally, while during MCSs, it increases by 8.13%. In 5° × 5° bins, 80.46% of ocean regions show MLD shallowing during MHWs, while 67.69% show deepening during MCSs. A significant global correlation between the intensity of MHWs/MCSs and MLD changes, with coefficients of −0.85 and −0.86, respectively. MHWs are more common in mesoscale anticyclonic eddies (AEs) (19.45%) than in cyclonic eddies (CEs) (10.11%). For MCSs, the pattern reverses, with 8.57% in AEs and 20.82% in CEs. Restratification and mesoscale eddies are two important factors driving MLD changes during these events. Plain Language Summary Marine heatwaves (MHWs) involve prolonged periods of sea surface temperatures (SSTs) above the 90th percentile of the climatological threshold, while marine cold‐spells (MCSs) involve SSTs below the 10th percentile. MHWs and MCSs both significantly impact marine ecosystems, particularly fragile coral reef ecosystems. A substantial amount of literature currently examines the characteristics of MHWs/MCSs, such as their frequency, duration, and cumulative days. However, the relationship between MHWs/MCSs and internal oceanic factors like mixed layer depth (MLD) variation is not fully understood. By combining OISST V2.1 data with Argo profiles, this study finds that MHWs are linked to significant MLD shallowing compared to background values. There is also a strong, statistically significant correlation between MHW intensity and the degree of MLD shallowing, at the 99% confidence level. In contrast, during MCSs, the MLD typically deepens relative to the climatological background. However, the degree of this deepening varies regionally with MCS intensity. Interestingly, MLD shallowing is observed during MCSs when the intensity exceeds −2.4°C. Further analysis indicates that mesoscale eddies and restratification are two mechanisms driving the variation in MLD during MHWs and MCSs. Key Points During marine heatwaves (MHWs), the mixed layer depth (MLD) shallows by 8.10% on average globally, while during MCSs, it deepens by 8.13% A significant correlation between the intensity of MHWs/MCSs and the relative change ratio of MLD Mesoscale eddies occurring alongside MHWs/MCSs can modify the usual trend of MLD shallowing during MHWs and deepening during MCSs

A pronounced tropical cooling (> 4 °C) and high-latitude warming in the annual mean sea surface temperature (SST) climatology is found in a numerical experiment conducted with a coupled model consisting of an atmospheric general circulation model (AGCM) coupled to a slab ocean model (SOM) in which the time-independent SOM thickness is reduced by a factor of two. The results suggest that biases in the ocean mixed layer depth could be contributing to SST biases in coupled atmosphere–ocean general circulation models. These changes in annual mean SST are noteworthy since in simple climate models the SOM thickness controls the amplitude and phase of the SST annual cycle, but it plays no role in determining the annual mean SST. Results from the numerical experiment indicate that halving of the SOM thickness not only changes annual cycle amplitude but results in asymmetrical changes in the annual cycle that rectify onto annual mean SST. The changed SOM thickness is found to primarily affect the surface net solar flux and latent heat flux components. However, due to the SOM equilibrium energy budget constraint, the annual mean net surface heat flux does not change, and the solar and latent heat fluxes changes compensate for each other. There is no such constraint on the annual cycle of heat fluxes, where the solar and latent components act to reinforce each other over much of the ocean, with the notable exception of the tropic Pacific warm pool region. We investigate the influence of the amplitude of the SST annual cycle on the annual mean SST through the net surface heat flux and its components using a two-step approach. First, two prescribed SST AGCM simulations are carried out, both with the annual mean SST of the full-thickness SOM coupled model simulation: one forced with the SST annual cycle of the full-thickness SOM coupled simulation, and the other forced with the SST annual cycle of the half-thickness SOM coupled simulation. Second, the impacts of these changes in the annual cycle on the annual mean SST are assessed in a full-thickness AGCM-SOM coupled simulation forced by the net, annual-mean, surface heat flux changes produced by the SST annual cycle changes in the first step. It is found that these net surface heat flux changes can largely reproduce the annual mean SST sensitivity to SOM thickness found in the coupled SOM thickness halving experiment. The effects on the annual mean heat flux components and annual mean SST from changes in the SOM thickness in an idealized limiting case are also considered.

Severe typhoon Rammasun (2014) was strengthened (super typhoon) in the east of the Leizhou Peninsula before moving into the Beibu Gulf. This study used the empirical orthogonal function (EOF) method to analyze the spatiotemporal variations in the sea surface wind (SSW), surface level anomaly (SLA), sea surface temperature (SST), mixed layer depth (MLD) and precipitation of the SCS. The study found that this typhoon had a significant influence on the SSW, SLA, SST, MLD and precipitation changes. During the typhoon Rammasun, the spatial distribution of precipitation showed obvious 'left-hand-side intensification' relative to the typhoon track. However, the SST decreased rapidly, and cooling showed a clear 'right-hand-side intensification', and two cyclonic eddies in the southwestward were observed from the mode of EOF-1 of the SLA. The spatiotemporal variations of SST, MLD, SLA, SSW and precipitation during the period of super typhoon Rammasun were excellently reflected by the EOF method. The variabilities of SST, MLD, SLA, SSW and precipitation in the SCS during the typhoon period and the relationship between the variabilities were further analyzed.

Planktonic foraminifer Globigerinoides ruber (white) and Trilobatus sacculifer are the most frequently used mixed-layer dwelling species for reconstructing past oceanic environments. Specifically, the Mg/Ca ratios of these two foraminiferal species have been used for reconstructing tropical/subtropical changes in sea surface temperature (SST). However, these two species have different morphotypes, of which the spatial and temporal differences in Mg/Ca ratios and their influencing factors are still unclear. Our objective is to investigate the potential differences between the Mg/Ca ratios of these different morphotypes of G. ruber (white) and T. sacculifer in the western Philippine Sea (WPS) and determine their implications for the reconstruction of SST and upper-ocean structure. Mg/Ca measurements are made on two basic morphotypes of G. ruber (white) [sensu stricto (s.s.) and sensu lato (s.l.)] and T. sacculifer [with (w) and without (w/o) a sac-like final chamber] on samples of Site MD06-3047B from the WPS. Our results reveal that Mg/Ca ratios of different G. ruber morphotypes show consistent differences; and those of T. sacculifer morphotypes show staged variations since MIS 3. It is suggested to select a single morphotype for reconstructing SST changes using the Mg/Ca ratios of G. ruber and T. sacculifer in the WPS. Furthermore, the Mg/Ca ratios between G. ruber s.s. and G. ruber s.l. [Δ(Mg/Ca) G.ruber s.s.−s.l. ] downcore MD06-3047B covaries with indexes of summer monsoon. Combining with the core-top results, showing regional variation of differences in the Δ(Mg/Ca) G.ruber s.s.−s.l. over the western tropical Pacific, we propose that Δ(Mg/Ca) G.ruber s.s.−s.l. may tend to reflect summer mixed layer depth.

To investigate the spatiotemporal variations in the mixed layer depth (MLD) in the Arctic basins, a new criterion to determine the MLD, called the improved maximum angle method (IMAM), was developed. A total of 45 123 potential density profiles collected using Ice-Tethered Profilers (ITPs) in the Arctic basins during 2005–2021 were used to demonstrate the method’s effectiveness. By comparing the results obtained by the fixed threshold method (FTM), percentage threshold method (PTM), and maximum gradient method (MGM) for profiles in the Canada Basin, Makarov Basin, and Eurasian Basin, we determined that the quality index (1.0 for perfect identification of the MLD) of the IMAM regarding the assessment of the MLD determination method reached 0.94, which is much greater than those of other criteria. Moreover, two types of the density profiles were identified based on the mixed layer development stage. The MLDs of the typical profiles determined using the IMAM were found to have better consistency with the original definition. By utilizing the new mixed layer criterion, the seasonal variations and regional differences in the MLD in the Arctic basins were analyzed. Spatially, the summer and winter MLDs in the Canada Basin were the shallowest (13.55 m in summer, 26.76 m in winter) than those in the Makarov (29.51 m in summer, 49.08 m in winter) and Eurasian (20.36 m in summer, 46.81 m in winter) basins due to the stable stratification in the upper ocean and the subsequent small effects of dynamic and thermodynamic processes (wind-driven stirring and brine rejection) in the Canada Basin. Seasonally, in the three Arctic basins, the average MLD was shallowest (22.77 m) in summer; it deepened through autumn and reached a winter maximum (41.12 m).

The present climate simulation and future projection of the mixed layer depth (MLD) and subduction process in the subtropical Southeast Pacific are investigated based on the geophysical fluid dynamics laboratory earth system model (GFDL-ESM2M). The MLD deepens from May and reaches its maximum (>160 m) near (24°S, 104°W) in September in the historical simulation. The MLD spatial pattern in September is non-uniform in the present climate, which shows three characteristics: (1) the deep MLD extends from the Southeast Pacific to the West Pacific and leads to a “deep tongue” until 135°W; (2) the northern boundary of the MLD maximum is smoothly near 18°S, and MLD shallows sharply to the northeast; (3) there is a relatively shallow MLD zone inserted into the MLD maximum eastern boundary near (26°S, 80°W) as a weak “shallow tongue”. The MLD nonuniform spatial pattern generates three strong MLD fronts respectively in the three key regions, promoting the subduction rate. After global warming, the variability of MLD spatial patterns is remarkably diverse, rather than deepening consistently. In all the key regions, the MLD deepens in the south but shoals in the north, strengthing the MLD front. As a result, the subduction rate enhances in these areas. This MLD antisymmetric variability is mainly influenced by various factors, especially the potential-density horizontal advection non-uniform changes. Notice that the freshwater flux change helps to deepen the MLD uniformly in the whole basin, so it hardly works on the regional MLD variability. The study highlights that there are regional differences in the mechanisms of the MLD change, and the MLD front change caused by MLD non-uniform variability is the crucial factor in the subduction response to global warming.

As an economically critical pelagic migratory species, yellowfin tuna ( Thunnus albacores , YFT) is very sensible to physical and environmental conditions, such as sea surface temperature (SST), ocean heat content (OHC), and the mixed layer depth (MLD). We investigated the impact of SST, OHC, and MLD on fluctuations of YFT catch in the western/eastern Indian Ocean using the long time series of 63-year environmental and YFT datasets. We found that the impact of SST on YFT was heavily overestimated in the past, and MLD plays a more critical role in the YFT catch fluctuation. When the MLD deepens (>34.8 m), SST was more influential in predicting the catches of YFT than OHC in the western Indian Ocean, and OHC was more critical to YFT than SST in the eastern Indian Ocean. However, when the MLD shallows (<34.8 m), MLD was more vital to predict the catch per unit effort (CPUE) of YFT than SST/OHC in the western. After 2000, there was an asynchronous pattern of YFT CPUE induced by higher frequency variations and ocean hiatus of SST/OHC signals in the western and eastern Indian Oceans basins. The impact of the subsurface hiatus may induce the decrease of YFT in the eastern Indian Ocean. The above findings clarified a non-stationary relationship between the environmental factors and catches of YFT and provided new insights into variations in YFT abundance.

Ocean chlorophyll (Chl)-induced heating can affect the climate system through the penetration of solar radiation in the upper ocean. Currently, the ocean biology-induced heating (OBH) feedback effects on the climate in the tropical Pacific are still not well understood, and the mechanisms regarding how SST is modulated remain elusive. In this paper, chlorophyll (Chl) data from satellites are combined with physical fields from Argo profiles to estimate OBH-related fields, including the penetration depth (H p ) and the ocean mixed-layer (ML) depth (H m ). In addition, some directly related heating terms with H m and H p are diagnosed, including the absorbed solar radiation component within the ML (denoted as Q abs ), the rate of ML temperature changes that are directly induced by Q abs (denoted as R sr  = Q abs /(ρ 0 C p H m )), and the portion of solar radiation that penetrates through the bottom of the ML (denoted as Q pen ). The structural relationships between these related fields are examined to illustrate how these heating terms are affected by H p and H m . The extent to which R sr and Q pen are modulated by H p is strikingly different during ENSO cycles. In the western-central equatorial Pacific, inter-annual variations in H p tend to be out of phase with those in H m . A decrease (increase) in Q abs from a positive (negative) H p anomaly during El Niño (La Niña) tends to be offset by a negative (positive) H m anomaly. Thus, R sr is not closely related with H p , even though Q abs is highly correlated with H p , indicating that the direct thermal effect through Q abs is not a dominant factor that affects the SST. In contrast, the inter-annual variability of Q pen in the region is significantly enhanced by that of H p , with their high positive correlation. The H p -induced differential heating in the ML and subsurface layers from the Q pen and Q abs terms modifies the thermal contrast, stratification and vertical mixing, which represent a dominant indirect ocean dynamical effect on the SST. The revealed relationships between these related fields provide an observational basis for gaining structural insights into the OBH feedback effects and validating model simulations in the tropical Pacific.

The response of the mixed layer depth (MLD) and subduction rate in the subtropical Northeast Pacific to global warming is investigated based on 9 CMIP5 models. Compared with the present climate in the 9 models, the response of the MLD in the subtropical Northeast Pacific to the increased radiation forcing is spatially nonuniform, with the maximum shoaling about 50 m in the ensemble mean result. The inter-model differences of MLD change are non-negligible, which depend on the various dominated mechanisms. On the north of the MLD front, MLD shallows largely and is influenced by Ekman pumping, heat flux, and upper-ocean cold advection changes. On the south of the MLD front, MLD changes a little in the warmer climate, which is mainly due to the upper-ocean warm advection change. As a result, the MLD front intensity weakens obviously from 0.24 m/km to 0.15 m/km (about 33.9%) in the ensemble mean, not only due to the maximum of MLD shoaling but also dependent on the MLD non-uniform spatial variability. The spatially non-uniform decrease of the subduction rate is primarily dominated by the lateral induction reduction (about 85% in ensemble mean) due to the significant weakening of the MLD front. This research indicates that the ocean advection change impacts the MLD spatially non-uniform change greatly, and then plays an important role in the response of the MLD front and the subduction process to global warming.

The upper mixed layer depth ( h ) has a significant seasonal variation in the real ocean and the low-order statistics of Langmuir turbulence are dramatically influenced by the upper mixed layer depth. To explore the influence of the upper mixed layer depth on Langmuir turbulence under the condition of the wind and wave equilibrium, the changes of Langmuir turbulence characteristics with the idealized variation of the upper mixed layer depth from very shallow ( h =5 m) to deep enough ( h =40 m) are studied using a non-hydrostatic large eddy simulation model. The simulation results show that there is a direct entrainment depth induced by Langmuir turbulence ( h LT ) within the thermocline. The normalized depth-averaged vertical velocity variance is smaller and larger than the downwind velocity variance for the ratio of the upper mixed layer to a direct entrainment depth induced by Langmuir turbulence h/h LT <1 and h/h LT >1, respectively, indicating that turbulence characteristics have the essential change (i.e., depth-averaged vertical velocity variance (DAVV)DADV for Langmuir turbulence) between h/h LT <1 and h/h LT >1. The rate of change of the normalized depth-averaged low-order statistics for h/h LT <1 is much larger than that for h/h LT >1. The reason is that the downward pressure perturbation induced by Langmuir cells is strongly inhibited by the upward reactive force of the strong stratified thermocline for h/h LT <1 and the effect of upward reactive force on the downward pressure perturbation becomes weak for h/h LT >1. Hence, the upper mixed layer depth has significant influences on Langmuir turbulence characteristics.