Changes In Mixed Layer Depth Research Articles

Marine Heatwaves/Cold‐Spells Associated With Mixed Layer Depth Variation Globally

AbstractMarine 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.

Temporal variability in the relationship between line height absorption and chlorophyll concentration: a case study from the Northern Gulf of Alaska

The Line Height Absorption (LHA) method uses absorption of light to estimate chlorophyll-a. While most users consider regional variability and apply corrections, the effect of temporal variability is typically not explored. The Northern Gulf of Alaska (NGA) was selected for this study because there was no published regional value and its large swings in temporal productivity would make it a good candidate to evaluate the effect of temporal variability on the relationship. The mean NGA value of 0.0114 obtained here should be treated with caution, as variation in the slope of the relationship (aLH*), and thus chlorophyll-a estimates, in the NGA region varied by ∼25% between spring (aLH* = 0.0109) and summer (aLH* = 0.0137). Results suggest that this change is driven by a shift in pigment packaging and cell size associated with changes in mixed layer depth and stratification. Consideration of how temporal variability may affect the accuracy of the LHA method in other regions is thus recommended.

Weakened Seasonality of the Ocean Surface Mixed Layer Depth in the Southern Indian Ocean During 1980–2019

AbstractTemporal and spatial variations in the ocean surface mixed layer are important for the climate and ecological systems. During 1980–2019, the Southern Indian Ocean (SIO) mixed layer depth (MLD) displays a basin‐wide shoaling trend that is absent in the other basins within 40°S–40°N. The SIO MLD shoaling is mostly prominent in austral winter with deep climatology MLD, substantially weakening the MLD seasonality. Moreover, the SIO MLD changes are primarily caused by a southward shift of the subtropical anticyclonic winds and hence ocean gyre, associated with a strengthening of the Southern Annular Mode, in recent decades for both winter and summer. However, the poleward‐shifted subtropical ocean circulation preferentially shoals the SIO MLD in winter when the meridional MLD gradient is sharp but not in summer when the gradient is flat. This highlights the distinct subtropical MLD response to meridional mitigation in winds due to different background oceanic conditions across seasons.

Evaluation of the mechanisms acting on the Atlantic Meridional Overturning Circulation in CESM2 for the 1pctCO2 experiment

The Atlantic Meridional Overturning Circulation (AMOC) is a crucial component of the Earth's climate system due to its fundamental role in heat distribution, carbon and oxygen transport, and the weather. Other climate components, such as the atmosphere and sea ice, influence the AMOC. Evaluating the physical mechanisms of those interactions is paramount to increasing knowledge about AMOC's functioning. In this study, the authors used outputs from the Community Earth System Model version 2 and observational data to investigate changes in the AMOC and the associated physical processes. Two DECK experiments were evaluated: piControl and 1pctCO2 , with an annual increase of 1% of atmospheric CO2 . The analysis revealed a significant decrease in the AMOC, associated with changes in mixed layer depth and buoyancy in high latitudes of the North Atlantic, resulting in the shutdown of deep convection and potentially affecting the formation of North Atlantic Deep Water and Antarctic Bottom Water. A vital aspect observed in this study is the association between increased runoff and reduced water evaporation, giving rise to a positive feedback process. Consequently, the rates of freshwater spreading have intensified during this period, which could lead to an accelerated disruption of the AMOC beyond the projections of existing models.

Observed amplification of the seasonal CO2 cycle at the Southern Ocean Time Series

The Subantarctic Zone, the circumpolar region of the Southern Ocean between the Subtropical and Subantarctic fronts, plays an important role in air-sea CO2 exchange, the storage of anthropogenic CO2, and the ventilation of the lower thermocline. Here we use a time series from moored platforms deployed between 2011 and 2021 as part of the Southern Ocean Time Series (SOTS) observatory to investigate the seasonality and interannual variability of upper ocean hydrography and seawater CO2 partial pressure (pCO2). The region is a net sink for atmospheric CO2 over the nearly 10-year record, with trends revealing that the ocean pCO2 may be increasing slightly faster than the atmosphere, suggesting that oceanic as well as anthropogenic atmospheric forcing contributes to the decadal change, which includes a decline in pH on the order of 0.003 yr−1. The observations also show an amplification of the seasonal cycle in pCO2, potentially linked to changes in mixed layer depth and biological productivity.

Spatial and seasonal variations of near-inertial kinetic energy in the upper Ross Sea and the controlling factors

The spatial distribution and seasonal variation of near-inertial kinetic energy (NIKE) in the upper Ross Sea (RS) are examined using the 1/4° NEMO3.6-LIM3 sea ice physical–biological coupled model. The annual-mean surface and mixed layer-integrated NIKE have large values at the shelf break around Iselin Bank and the northeast area outside the continental shelf. The spatial distribution of the surface and mixed layer-integrated NIKE in the austral summer RS is mainly controlled by the near-inertial wind stress magnitude (NIWSM), which acts as the energy source. The northeast high NIKE agrees with the large NIWSM there. Semidiurnal tides only contribute to the NIKE peak around the Iselin Bank and shelf break. The change of mixed layer depth (MLD) can induce spatial discrepancies between surface and mixed layer-integrated NIKE. The shallower MLD in the western RS limits the near-inertial energy in a thin layer, which has induced the large surface NIKE there. The NIKE in the upper RS has a pronounced seasonal variation with the largest surface and mixed layer-integrated NIKE both observed during austral summer. The seasonal variation of NIWSM and sea ice concentration play an important role in inducing the seasonal variation of NIKE. While the NIWSM acts as the main energy source of NIKE, the presence of sea ice can prevent the wind from directly acting on the surface ocean, inducing the low NIKE during austral winter.

Marine phytoplankton downregulate core photosynthesis and carbon storage genes upon rapid mixed layer shallowing

Marine phytoplankton are a diverse group of photoautotrophic organisms and key mediators in the global carbon cycle. Phytoplankton physiology and biomass accumulation are closely tied to mixed layer depth, but the intracellular metabolic pathways activated in response to changes in mixed layer depth remain less explored. Here, metatranscriptomics was used to characterize the phytoplankton community response to a mixed layer shallowing (from 233 to 5 m) over the course of two days during the late spring in the Northwest Atlantic. Most phytoplankton genera downregulated core photosynthesis, carbon storage, and carbon fixation genes as the system transitioned from a deep to a shallow mixed layer and shifted towards catabolism of stored carbon supportive of rapid cell growth. In contrast, phytoplankton genera exhibited divergent transcriptional patterns for photosystem light harvesting complex genes during this transition. Active virus infection, taken as the ratio of virus to host transcripts, increased in the Bacillariophyta (diatom) phylum and decreased in the Chlorophyta (green algae) phylum upon mixed layer shallowing. A conceptual model is proposed to provide ecophysiological context for our findings, in which integrated light limitation and lower division rates during transient deep mixing are hypothesized to disrupt resource-driven, oscillating transcript levels related to photosynthesis, carbon fixation, and carbon storage. Our findings highlight shared and unique transcriptional response strategies within phytoplankton communities acclimating to the dynamic light environment associated with transient deep mixing and shallowing events during the annual North Atlantic bloom.

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.

Oceanographic controls on Southern Ocean dimethyl sulfide distributions revealed by machine learning algorithms

AbstractWe developed two machine learning models to map the Southern Ocean distribution of the climate‐active gas dimethyl sulfide (DMS) at 20 km resolution. Results obtained from ensembles of random forest regressions and artificial neural networks reproduce observed DMS distributions with significantly higher accuracy than traditional statistical techniques, and are less prone to biases and spatial distortions than existing interpolation‐based climatologies. Both models predict persistently low offshore DMS concentrations associated with the Antarctic circumpolar current, suggesting that wind‐driven overturning mixing is the dominant regional control on DMS distributions. In addition, 60% of the variance in DMS seasonality is explained by changes in mixed layer depth and sea surface temperature, with a significant correlation between DMS concentrations and sea‐ice cover in coastal waters. We further identify the tracer Si* (defined as ) as a potentially important predictor for regional DMS distributions in Southern Ocean waters. At finer scales, our models capture various oceanographic features, including eddies, hydrographic fronts and jets that appear to play a role in driving DMS variability. Our results yield an estimated Southern Ocean sea‐air DMS flux of 8.7 ± 2.1 Tg S integrated across the phytoplankton growing season (October to April), representing 30.8% of total global oceanic S emissions, and highlighting the region's importance to the marine sulfur cycle. Our work provides new insights into the drivers of spatial variability in Southern Ocean DMS concentrations and sea‐air fluxes, and their potential responses to future climate‐dependent changes.

The effect of normal and abnormal eddies on the mixed layer depth in the global ocean

Mesoscale eddies are broadly distributed over the global ocean and play a significant role in modulating the spatiotemporal evolution of mixed layer depth (MLD). The presence of abnormal eddies in the ocean has been shown; however, the precise quantification of the effect of eddies on MLD, given the case of abnormal eddies, has not been carried out thus far. Differently from the previous approach to identify abnormal eddies through sea surface temperature, we therefore, proposed a method to identify abnormal eddies, using potential density based on Argo data (Array for Real-time Geostrophic Oceanography) for 15 years from 2003 to 2017. Results showed that abnormal anticyclonic eddies (AAE) and abnormal cyclonic eddies (ACE) accounted for 21.67% and 20.17% of total matching anticyclonic eddies (TAE) and the total matching cyclonic eddies (TCE), respectively, in the global ocean. The proportions of abnormal eddies were relatively higher in tropical regions but lower in regions with the boundary current and strong eddy kinetic energy. The MLD changes caused by normal and abnormal eddies were estimated combining satellite altimetry data. The normal eddies were the total matching eddies with the removal of the abnormal eddies, separately called normal anticyclonic eddies (NAE) and normal cyclonic eddies (NCE). The overall influence of NAE (NCE) was more significant on MLD deepening (uplifting) than that of TAE (TCE). Globally, NAE (NCE) changed MLD deepening (uplifting) from ~66 m (~54 m) to ~67 m (~53 m) and exhibited a more pronounced change in the Indian Ocean sector of the Southern Ocean region, from ~111 m (~94 m) to ~115 m (~92 m) in the winter. AAE (ACE), exerted a relatively weak but opposite effect on MLD deepening (uplifting). In other words, the global average MLD caused by them shoaled (deepened) from ~66 m (~54 m) to ~59 m (~59 m), and the North Pacific Ocean shoaled (deepened) from ~61 m (~47 m) to ~49 m (~57 m) in winter. Given the above results, abnormal eddies should be accounted for when the impact of ocean eddies is evaluated on the global climate system.

Seasonal variation of surface seawater radiocarbon in the Andaman Sea as recorded in coral

Corals provide high-resolution radiocarbon record of the surface ocean. These high-resolution records can provide understanding of the surface ocean conditions and processes regulating these conditions. A Porites coral from the Andaman Sea was investigated for its high-resolution radiocarbon record between 2007 and 2014. The radiocarbon measurement of the coral shows a post-bomb period decline trend (2.7‰ yr−1) along with seasonal variations. A positive correlation is observed between the seasonal radiocarbon changes and the stable oxygen isotope values of the coral. The coral registers the seasonal changes in mixed layer depth and sea surface temperature between the monsoon and non-monsoon periods. Recent radiocarbon values of the Andaman Sea surface water have been found to be higher compared to the contemporary atmospheric radiocarbon values.

Chasing the offshore wind farm wind-wake-induced upwelling/downwelling dipole

The operational principle of offshore wind farms (OWF) is to extract kinetic energy from the atmosphere and convert it into electricity. Consequently, a region of reduced wind speed in the shadow zone of an OWF, the so-called wind-wake, is generated. As there is a horizontal wind speed deficit between the wind-wake and the undisturbed neighboring regions, the locally reduced surface stress results in an adjusted Ekman transport. Subsequently, the creation of a dipole pattern in sea surface elevation induces corresponding anomalies in the vertical water velocities. The dynamics of these OWF wind-wake induced upwelling/downwelling dipoles have been analyzed in earlier model studies, and strong impacts on stratified pelagic ecosystems have been predicted. Here we provide for the first time empirical evidence of the existence of such upwelling/downwelling dipoles. The data were obtained by towing a remotely operated vehicle (TRIAXUS ROTV) through leeward regions of operational OWFs in the summer stratified North Sea. The undulating TRIAXUS transects provided high-resolution CTD data which enabled the characterization of three different phases of the ephemeral life cycle of a wind-wake-induced upwelling/downwelling dipole: development, operation, and erosion. We identified two characteristic hydrographic signatures of OWF-induced dipoles: distinct changes in mixed layer depth and potential energy anomaly over a distance < 5 km and a diagonal excursion of the thermocline of ~10–14 m over a dipole dimension of ~10–12 km. Whether these anthropogenically induced abrupt changes are significantly different from the corridor of natural variability awaits further investigations.

Impact of symmetric instability parametrization scheme on the upper ocean layer in a high-resolution global ocean model

The primary effect of symmetric instability (SI) is the shoaling of the mixed layer. This has been shown to improve the vertical mixing of a passive tracer between the surface mixed layer and the ocean interior in several ideal models. In this study, we apply an SI parameterization scheme on a resolved front in a realistic, high-resolution global ocean model, and evaluate global-scale SI effects. We analyze the model outputs for two typical front regions: the Kuroshio Extension (KE) shows conditions favorable for SI generation whereas in the Subantarctic Front (SAF), east of Australia, SI is weak. We demonstrate that the SI parameterization has a detectable effect on the global-scale sea surface temperature (SST), mixed layer depth (MLD), especially in the strong front and current-topography interactions regions, in winter in the northern and southern hemispheres. The variation of MLD changes by 30% in a local region when SI parameterization scheme is considered. These spatial and temporal distribution characteristics are consistent with those expected for the SI mechanism. However, the SI parameterization only results in submesoscale positive and negative staggered patch structures in the MLD and SST, analogous to turbulence structures. Therefore, its global-scale direct impacts on the MLD and SST are not systematic. The submesoscale disturbance induced by the SI parameterization is of the same order of magnitude as the errors in the current SST observation data; as such, it is assumed to be statistically insignificant in the scope of the current study in a global model. The submesoscale MLD and SST disturbance induced by SI is probably important for local air-sea exchange, but this requires further investigation with the use of long-time-integral coupled models in the future.

Mixed Layer Depth Promotes Trophic Amplification on a Seasonal Scale

AbstractThe Humboldt Upwelling System is of global interest due to its importance to fisheries, though the origin of its high productivity remains elusive. In regional physical‐biogeochemical model simulations, the seasonal amplitude of mesozooplankton net production exceeds that of phytoplankton, indicating “seasonal trophic amplification.” An analytical approach identifies amplification to be driven by a seasonally varying trophic transfer efficiency due to mixed layer variations. The latter alters the vertical distribution of phytoplankton and thus the zooplankton and phytoplankton encounters, with lower encounters occurring in a deeper mixed layer where phytoplankton are diluted. In global model simulations, mixed layer depth appears to affect trophic transfer similarly in other productive regions. Our results highlight the importance of mixed layer depth for trophodynamics on a seasonal scale with potential significant implications, given mixed layer depth changes projected under climate change.

Effects of Mixed Layer Depth on Phytoplankton Biomass in a Tropical Marginal Ocean: A Multiple Timescale Analysis

AbstractIn open oceans, changes in mixed layer depth (MLD) may affect phytoplankton growth and biomass variations via the regulation of nutrient supply from deep waters. Estimates of relationships between variability in phytoplankton dynamics and the MLD remain limited, especially at different time scales. We compiled and analyzed averaged euphotic‐depth‐integrated chlorophyll‐a (IChl‐a) and surface chlorophyll‐a (SChl‐a) concentrations collected from 27 cruises during the period of 1999–2019 in the tropical northern South China Sea (SCS). Seasonal differences existed in both averaged IChl‐a and SChl‐a concentrations, with significantly high concentrations in the cold season. Inconsistent relationships between the averaged IChl‐a and SChl‐a concentrations between seasons implied that the use of SChl‐a concentration as a common indicator of phytoplankton biomass dynamics should be performed with caution. Over the past decades in the northern SCS, the averaged IChl‐a, SChl‐a, and MLD decreased to a greater extent in the cold season than in the warm season, while sea surface temperature (SST) rose rapidly and dramatically in both seasons. The MLD was observed to have better correlations with the averaged IChl‐a and SChl‐a concentrations than the SST in the time‐series data. Our results highlight the importance of IChl‐a concentration, which is an overall measure of phytoplankton responses to euphotic zone conditions, and the MLD could be used as a good index for changes in phytoplankton biomass under climate change.

Seasonal mixed layer depth shapes phytoplankton physiology, viral production, and accumulation in the North Atlantic

Seasonal shifts in phytoplankton accumulation and loss largely follow changes in mixed layer depth, but the impact of mixed layer depth on cell physiology remains unexplored. Here, we investigate the physiological state of phytoplankton populations associated with distinct bloom phases and mixing regimes in the North Atlantic. Stratification and deep mixing alter community physiology and viral production, effectively shaping accumulation rates. Communities in relatively deep, early-spring mixed layers are characterized by low levels of stress and high accumulation rates, while those in the recently shallowed mixed layers in late-spring have high levels of oxidative stress. Prolonged stratification into early autumn manifests in negative accumulation rates, along with pronounced signatures of compromised membranes, death-related protease activity, virus production, nutrient drawdown, and lipid markers indicative of nutrient stress. Positive accumulation renews during mixed layer deepening with transition into winter, concomitant with enhanced nutrient supply and lessened viral pressure.

Role of ocean dynamics in the evolution of mixed-layer temperature in the Bay of Bengal during the summer monsoon

Earlier studies suggest that the mixed-layer (ML) temperature or sea surface temperature (SST) of the Bay of Bengal during the summer monsoon is determined primarily by air–sea fluxes. We use an oceanic general circulation model (OGCM) to show that oceanic processes also play a significant role. Model heat-budget computations show that horizontal and vertical advection, constituting the direct role of dynamics, contribute significantly to the SST tendency in the western bay. The eastern limit of this direct-role regime is determined by downwelling Rossby waves, which are forced largely from the equatorial Indian Ocean, but are modified by alongshore winds at the eastern boundary and Ekman pumping along their westward path across the bay. The current and, as a result, advection weaken behind the Rossby waves. To the east of the Rossby wavefront, the thermocline is deep, permitting a deeper ML, but the actual ML depth (MLD) depends on the wind speed, with salinity also playing a role in the northern bay. Yet, there is negligible change in the SST even when MLD changes significantly because the deep thermocline decouples the changes in MLD and SST. In contrast, the shallower thermocline in the western bay limits the potential MLD, leading to larger changes in SST. The upwelling (downwelling) Rossby wave essentially conditions the upper ocean by decreasing (increasing) the potential depth of the mixed layer. SST variation weakens only when the thermocline deepens during downwelling events, which occur later in the western bay because Rossby waves propagate westward. The significant, but subtle, role of the Rossby waves in decoupling the changes in MLD and SST via downwelling is indirect, unlike the direct role of advection estimated in a standard heat-budget computation, and has been largely ignored in earlier studies.

Response of the mixed layer depth and subduction rate in the subtropical Northeast Pacific to global warming

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.

Salt Sinking in the Upper South Pacific Subtropical Gyre From 2004 to 2016

AbstractThe salinity in the 50–300‐m water column of the South Pacific subtropical gyre (10°S–30°S) increased from 2004 to 2016. The observed changes are primarily associated with changes in the South Pacific Tropical Water (SPTW) volume, which increased at a rate of (3.17 ± 0.25) 1014 m3 per decade in the region of 10–30°S and 150°E–90°W. The increases in the SPTW volume are caused by increased SPTW production, which increased at a rate of 3.45 ± 2.65 Sv per decade. The temporal variability in the mixed layer accounts for 75 ± 32% of the observed increase in SPTW production. The increases in temporal changes in the mixed layer are due to a deeper mixed layer depth (MLD) in austral winter and a shallower MLD in austral spring. The possible drivers of MLD changes at the air‐sea surface are examined. Among the wind stress curl, buoyancy fluxes, and the turbulence induced by wind stress, changes in the surface buoyancy fluxes play the most important role in the deepening of the MLD during austral winter, and changes in the wind stress play a leading role in the shoaling of the MLD during austral spring. Changes in the buoyancy fluxes during winter and changes in the wind stress in spring can be attributed to increases in the Southern Annular Mode from 2004 to 2016. This study highlights the strong modulation of SPTW formation by decadal climate variability over the subtropical South Pacific.

Mixed Layer Depth and Sea Surface Warming under Diurnally Cycling Surface Heat Flux in the Heating Season

AbstractIn the present study, large-eddy simulations (LESs) were performed to investigate mixed layer depth (MLD) and sea surface warming (SSW) under diurnally cycling surface heat flux in the heating season, in which a mixed layer (ML) is shoaling on intraseasonal time scales. The LES results showed that the diurnal cycle makes the MLD greater (smaller) at lower (higher) latitudes than the MLD without the cycle. Time scales of the wind-induced shear and the surface heat are a key to understand this latitudinal dependence of the diurnal cycle effects. The wind-induced shear-driven turbulence developed from early morning and became strongest at half the inertial period (Ti/2), while nighttime cooling weakened the ML stratification until the end of the nighttime (T24 = 24 h). At lower latitudes where Ti/2 > T24 (lower than 15°), the shear-driven turbulence continued to grow after T24 and determined the time of the greatest MLD. Thus, the shear-driven turbulence shaped the latitudinal dependence of the MLD, though convective turbulence helped further deepening of the ML. At higher latitudes (Ti/2 < T24), on the other hand, the shear-driven turbulence ceased growing before the nighttime cooling ended. However, reduced stratification due to the nighttime cooling supported the shear-driven turbulence to continue deepening the ML. Thus, the nighttime cooling shaped the latitudinal dependence of the MLD at higher latitudes. The MLD change induced by the diurnal cycle altered the SSW rate. At higher latitudes, the diurnal cycle is expected to reduce the MLD and increase the SSW by 10% in the heating season.