Variability Of Mixed Layer Depth Research Articles

A comparison of five surface mixed layer models with a year of observations in the North Atlantic

Five upper ocean mixed layer models driven by ERA-Interim surface forcing are compared with a year of hydrographic observations of the upper 1000 m, taken at the Porcupine Abyssal Plain observatory site using profiling gliders. All the models reproduce sea surface temperature (SST) fairly well, with annual mean warm biases of 0.11 °C (PWP model), 0.24 °C (GLS), 0.31 °C (TKE), 0.91 °C (KPP) and 0.36 °C (OSMOSIS). The main exception is that the KPP model has summer SSTs which are higher than the observations by nearly 3°. Mixed layer salinity (MLS) is not reproduced well by the models and the biases are large enough to produce a non-trivial density bias in the Eastern North Atlantic Central Water which forms in this region in winter.All the models develop mixed layers which are too deep in winter, with average winter mixed layer depth (MLD) biases between 160 and 228 m. The high variability in winter MLD is reproduced more successfully by model estimates of the depth of active mixing and/or boundary layer depth than by model MLD based on water column properties. After the spring restratification event, biases in MLD are small and do not appear to be related to the preceding winter biases.There is a very clear relationship between MLD and local wind stress in all models and in the observations during spring and summer, with increased wind speeds leading to deepening mixed layers, but this relationship is not present during autumn and winter. We hypothesize that the deepening of the MLD in autumn is so strongly driven by the annual cycle in surface heat flux that the winds are less significant in the autumn. The surface heat flux drives a diurnal cycle in MLD and SST from March onwards, though this effect is much more significant in the models than in the observations.We are unable to identify one model as definitely better than the others. The only clear differences between the models are KPP’s inability to accurately reproduce summer SSTs, and the OSMOSIS model’s more accurate reproduction of MLS.

Deriving a Proxy for Iron Limitation From Chlorophyll Fluorescence on Buoyancy Gliders

Chlorophyll fluorescence, primarily used to derive phytoplankton biomass, has long been an underutilized source of information on phytoplankton physiology. Diel fluctuations in chlorophyll fluorescence are affected by both photosynthetic efficiency and non-photochemical quenching (NPQ), where NPQ is a decrease in fluorescence through the dissipation of excess energy as heat. NPQ variability is linked to iron and light availability, and has the potential to provide important diagnostic information on phytoplankton physiology. Here we establish a relationship between NPQsv (Stern-Volmer NPQ) and indices of iron limitation from nutrient addition experiments in the sub-Antarctic zone (SAZ) of the Atlantic Southern Ocean, through the derivation of NPQmax (the maximum NPQsv value) and αNPQ (the light limited slope of NPQsv). Significant differences were found for both Fv/Fm and αNPQ for iron versus control treatments, with no significant differences for NPQmax. Similar results from CTDs indicated that changes in NPQ were driven by increasing light availability from late July to December, but by both iron and light from January to February. We propose here that variability in αNPQ, which has removed the effect of light availability, can potentially be used as a proxy for iron limitation (as shown here for the Atlantic SAZ), with higher values being associated with greater iron stress. This approach was transferred to data from a buoyancy glider deployment at the same location by utilising the degree of fluorescence quenching as a proxy for NPQGlider, which was plotted against in situ light to determine αNPQ. Seasonal increases in αNPQ are consistent with increased light availability, shoaling of the mixed layer depth (MLD) and anticipated seasonal iron limitation. The transition from winter to summer, when positive net heat flux dominates stratification, was coincident with a 24% increase in αNPQ variability and a switch in the dominant driver from incident PAR to MLD. The dominant scales of αNPQ variability are consistent with fine scale variability in MLD and a significant positive relationship was observed between these two at a ~10 day window. The results emphasise the important role of fine scale dynamics in driving iron supply, particularly in summer when this micronutrient is limiting.

Variability of the ocean mixed layer depth and the upwelling activity in the Cape Bojador, Morocco

This study examines the seasonality of the patterns of the ocean mixed layer depth (MLD) with the coastal upwelling system, using a model data from marine Copernicus with a resolution of 0.083° × 0.083°. For that, we examined wind forcing, ocean circulation, sea surface temperature, sea surface salinity, sea surface chlorophyll-a, and mixed layer depth seasonality from 2007 to 2017 in the Cape Bojador south of the Moroccan Atlantic coast. We observed that in the fall season the wind speed was low, and the maximum wind speed is in the summer season. Furthermore, we found that coastal areas are related to a generally shallower MLD all the year in the Cape. We also proved that the source of the upwelling is between 25 and 26°N. However, we observed that the sea surface chlorophyll concentration in the same region is minimum, while in the region between 24.5 and 22°N, the CHL-a is maximum. We suggested that the biological richness in Cape Bojador, due to the activity of upwelling, derives to the south because of the northward ocean filament. Despite the different physical and biological parameters used to detect upwelling activity, we found that upwelling detection based on sea surface temperature is the best method to describe the upwelling activity in this region.

On the Emergence of the Atlantic Multidecadal SST Signal: A Key Role of the Mixed Layer Depth Variability Driven by North Atlantic Oscillation

AbstractDespite its wide-ranging potential impacts, the exact cause of the Atlantic multidecadal oscillation/variability (AMO/AMV) is far from settled. While the emergence of the AMO sea surface temperature (SST) pattern has been conventionally attributed to the ocean heat transport, a recent study showed that the atmospheric stochastic forcing is sufficient. In this study, we resolve this conundrum by partitioning the multidecadal SST tendency into a part caused by surface heat fluxes and another by ocean dynamics, using a preindustrial control simulation of a state-of-the-art coupled climate model. In the model, horizontal ocean heat advection primarily acts to warm the subpolar SST as in previous studies; however, when the vertical component is also considered, the ocean dynamics overall acts to cool the region. Alternatively, the heat flux term is primarily responsible for the subpolar North Atlantic SST warming, although the associated surface heat flux anomalies are upward as observed. Further decomposition of the heat flux term reveals that it is the mixed layer depth (MLD) deepening that makes the ocean less susceptible for cooling, thus leading to relative warming by increasing the ocean heat capacity. This role of the MLD variability in the AMO signature had not been addressed in previous studies. The MLD variability is primarily induced by the anomalous salinity transport by the Gulf Stream modulated by the multidecadal North Atlantic Oscillation, with turbulent fluxes playing a secondary role. Thus, depending on how we interpret the MLD variability, our results support the two previously suggested frameworks, yet slightly modifying the previous notions.

Phytoplankton Phenology in the North Atlantic: Insights From Profiling Float Measurements

Phytoplankton division rate (µ), loss rate (l), and specific accumulation rate (r) were calculated using Chlorophyll-a (Chl) and phytoplankton carbon (Cphyto) derived from bio-optical measurements on 12 Argo profiling floats in a north-south section of the western North Atlantic Ocean (40° N to 60° N). The float results were used to quantify the seasonal phytoplankton phenology and bloom dynamics for the region. Latitudinally varying phytoplankton dynamics were observed. In the north, the CPhyto peak was higher, occurred later, and was accompanied by higher total annual CPhyto accumulation. In contrast, in the south, stronger μ-r decoupling occurred despite smaller seasonal variations in mixed layer depth (suggesting the possibility of other ecological forcing), and was accompanied by an increasing portion of winter to total annual production, consistent with relief of nutrient limitation. The float observations of phytoplankton phenology for the mixed layer were compared to ocean color satellite remote sensing observations and found to be similar. A similar comparison to an eddy-resolving ocean simulation found the model only reproduced some of aspects of the observed phytoplankton phenology, indicating possible biases in the simulated physical forcing, turbulent dynamics, and bio-physical interactions. In addition to seasonal patterns in the mixed layer, the float measurements provided information on the vertical distribution of physical and biogeochemical quantities and therefore are complementary to the remote sensing measurements. Seasonal phenology patterns arise from interactions between “bottom-up” (e.g., resources for growth) and “top-down” (e.g., grazing, mortality) factors that involve both biological and physical drivers. The Argo float data are consistent with the disturbance recovery hypothesis over the full, annual seasonal cycle; for the late winter/early spring transition, the float data are also consistent with other bloom hypotheses (e.g. critical photosynthesis, critical division rate, and meso/sub-mesoscale physics) that highlight the importance of brief, episodic boundary layer shoaling for decoupling of division and grazing rates.

An Investigation of the Ocean’s Role in Atlantic Multidecadal Variability

AbstractA mechanistic understanding of the Atlantic multidecadal variability (AMV) is highly desirable since it will considerably aid regional and global climate predictions. Although ocean dynamics have long been invoked to explain the AMV, recent studies have cast doubt on its influence. Here we evaluate the necessity of ocean dynamics for the AMV using an observationally based idealized model that isolates the contribution of atmospheric forcing to the AMV. By demonstrating that this model underestimates the magnitude of the observed sea surface temperature variability in the extratropical North Atlantic, we infer that ocean dynamics contribute significantly to the AMV in this region. This inference holds when we add anthropogenic aerosol forcing and the effects of mixed layer depth variability to the idealized model. Thus, our study suggests that ocean heat transport convergence is needed to explain sea surface temperature variability in the extratropical North Atlantic. Sustained ocean observing systems in the this region will help untangle the physical mechanisms involved.

Dinamika Massa Air di Sekitar Pulau-Pulau Kecil Terluar (PPKT) Perairan Utara Papua

<p class="Papertext"><strong>Water Mass Dynamics Around the Indonesia’s Outer Small Islands at <br /> North Papua Waters. </strong>Coastal area of Indonesia’s Outer Islands (PPKT) at North Papua Waters have a huge potential of marine resources which needs to be managed in sustainable ways. In order to make the processes of protection, conservation, rehabilitation, utilization, and enrichment of coastal resources work well, the study about environmental carrying capacity which includes ocean dynamics analysis around PPKT area is needed. Hereby, the study area were including coastal and off shore area of Jiew, Budd, Fani, Bras and Fanildo Island. The measurement of water mass and ocean current was conducted to analyze the impact of regional ocean dynamics to coastal waters of PPKT. This study found the domination of New Guinea Coastal Current (NGCC) at surface layer, the effect of Indonesian throughflow at subsurface layer (100 – 150m depth), and counter current towards northeast which is believed as a part of New Guinea Coastal Under Current (NGCUC) and Equatorial Under Current (EUC). This ocean current system carried South Pacific Subtropical Water (SPSW) noticed by maximum salinity of 35.4 PSU at 150m depth. There are spatial variability of mixed layer depth and slope of T-S curve at the western and eastern part of study area which might be caused by isopycnal mixing. However, the spatial and temporal variability of Sea Surface Temperature at this area is relatively small due to the presence of Western Paicific Warm Water Pool. With the small annual SST variability, strong ocean current and water mass mixing, this area has a potential for the development of marine ecosystem.</p>

Reconstructed 3-D Ocean Temperature Derived from Remotely Sensed Sea Surface Measurements for Mixed Layer Depth Analysis

The mixed layer depth (MLD) is generally estimated using in situ or model data. However, MLD analyses have limitations due to the sparse resolution of the observed data. Therefore, this study reconstructs three-dimensional (3D) ocean thermal structures using only satellite sea surface measurements for a higher spatial and longer temporal resolution than that of Argo and diagnoses the decadal variation of global MLD variability. To simulate the ocean thermal structures, the relationship between the ocean subsurface temperature and the sea surface fields was computed based on gridded Argo data. Based on this relationship, high spatial resolution and extended periods of satellite-derived altimeter, sea surface temperature (SST), and wind stress data were used to estimate the 3D ocean thermal structures with 0.25° spatial resolution and 26 standard depth levels (5–2000 m) for 24 years (1993–2016). Then, the MLD was calculated using a temperature threshold method (∆T = 0.2 °C) and correlated reasonably well (>0.9) with other MLD datasets. The extended 24-year data enabled us to analyze the decadal variability of the MLD. The global linear trend of the 24-year MLD is −0.110 m yr−1; however, from 1998 to 2012, the linear trend is −0.003 m yr−1 which is an order of magnitude smaller than that of other periods and corresponds to a global warming hiatus period. Via comparisons between the trends of the SST anomalies and the MLD anomalies, we tracked how the MLD trend changes in response to the global warming hiatus.

Long chain 1,14-diols as potential indicators for upper water stratification in the open South China Sea

Abstract Long chain 1,14-diols have been widely studied in typical upwelling areas, but less investigated in open seas. In this study the 1,14-diols in a sediment core, covering a history between 42 ka and 12 ka, were studied to investigate their potential to indicate upper water stratification in the open northern South China Sea (SCS). Downcore records of 1,14-diol fractional abundances, diol index 1 (DI-1) and diol index 2 (DI-2) changed in parallel, with a prominent increase at ~14.5 ka, likely corresponding with the Bolling warming event. This pattern is different from what has been observed in typical upwelling regimes, showing an insignificant relationship between DI-1 and DI-2. Moreover, these records showed a generally inverse variation pattern with records of primary productivity, further suggesting that they are not likely applicable as upwelling indicators in the study area. However, the 1,14-diol records changed positively with sea surface temperature (SST), as well as the difference between SST and subsurface-temperature, hence suggesting their potential to indicate upper water column stratification in the open SCS. This occurrence might be associated with the productivity of 1,14-diol precursor (i.e., Proboscia diatom), which is sensitive to the variation of mixed layer depth in oligotrophic open oceans and lacks of competitiveness relative to the more heavily silicified diatoms under normal Si replete upwelling conditions. Our result also suggested that the East Asian monsoon may have played an important role to modulate riverine terrigenous organic matter input to the study area.

Izmenchivost' tolshchiny peremeshannogo sloya v Chernom more i ee svyaz' s dinamikoy vod i atmosfernym vozdeystviem

Purpose . Spatio-temporal variability of the mixed layer depth (MLD) in different regions of the Black Sea in 1985–2017, its relationship with large- and meso-scale dynamics and wind velocity are investigated. Metods and Results . The study is based on the hydrological data archive for 1985–2017 including the measurements of the ship expeditions, the Argo buoys and the moored buoy “Aqalog”. Seasonal and interannual variability of the mixed layer depth was determined using the density criterion ( dr = = 0,07kg/m 3 ) between the surface layers and the base of the upper mixed layer. Conclusions . In January – March, the large-scale and mesoscale dynamics significantly affects the mixed layer depth variability. Minimum monthly average values of the mixed layer depth in winter are observed in the mesoscale cyclonic eddies and in the center of the sea (20–30 m), the moderate values – on the periphery of the basin (40–45 m) and the maximum ones – in the mesoscale anticyclones (60–70 m). Several times the mixed layer depth values exceeding 150 m were detected in the downwelling areas of the basin. Analysis of the whole period (1985–2017) shows that the mixed layer density was never more than 1015 kg/m 3 . This isopycnal limits the maximum possible depth of the upper mixed layer. The impact of wind velocity on the spatial and temporal variability of the mixed layer thickness is the largest in spring and autumn when the seasonal thermocline is weak. It is less important in summer when solar heating stabilizes the upper layer, and in winter when the mixed layer depth is large. Rise of the mixed layer depth in summer is observed in recent years that is associated with rise of the wind speed in a warm period of a year.

The Black Sea Mixed Layer Depth Variability and Its Relation to the Basin Dynamics and Atmospheric Forcing

The Black Sea Mixed Layer Depth Variability and Its Relation to the Basin Dynamics and Atmospheric Forcing

Production and ecosystem structure in cold‐core vs. warm‐core eddies: Implications for the zooplankton isoscape and rock lobster larvae

AbstractAnticyclonic (warm‐core) mesoscale eddies (WCEs) in the Eastern Indian Ocean carry higher surface chlorophyll signatures than cyclonic (cold‐core) eddies (CCEs). Paradoxically, WCEs host rock lobster larvae (phyllosomas) with lower lipid stores and protein reserves than phyllosomas in CCEs, suggesting a poorer nutritional status. We assess primary productivity and zooplankton isotopic data from eight eddies across four research voyages (2003–2011) to determine how this contradiction might occur. We find that WCEs and CCEs are equally productive per unit chlorophyll a, but depth‐integrated primary production (PP) is greater in eddies with shallower mixed layers (MLs), especially in CCEs. MLs tend to be shallower in CCEs than in WCEs because the pycnocline is closer to the surface. This, in combination with stronger stratification in CCE euphotic zones than those of WCEs, supports greater flagellate and dinoflagellate populations in CCEs. These phytoplankton provide high‐quality nutrition for zooplankton, which feed on average ~ 0.6 trophic level lower in CCEs with the shallowest MLs, accumulating high lipid stores. Conversely, WCEs have, on average, ~ 70 m deeper MLs than CCEs, and host a phytoplankton community with more diatoms. Diatoms provide lower quality food for zooplankton, and zooplankton lipid stores in WCEs decline with trophic level, and possibly, with time after initial (or seasonal) nutrient injection. As a result, phyllosomas in CCEs have higher energy and lipid content than those in warm‐core eddies. The resolution of the paradox, therefore, is that the higher surface chlorophyll signatures of WCEs are not representative of the nutritional value of the prey field of the phyllosoma. We also conclude that interannual variations of mixed layer depth occur at a regional scale, controlling PP.

Upper ocean high resolution regional modeling of the Arabian Sea and Bay of Bengal

In this paper, effort is made to demonstrate the quality of high-resolution regional ocean circulation model in realistically simulating the circulation and variability properties of the northern Indian Ocean (10°S–25°N, 45°–100°E) covering the Arabian Sea (AS) and Bay of Bengal (BoB). The model run using the open boundary conditions is carried out at 10 km horizontal resolution and highest vertical resolution of 2 m in the upper ocean. The surface and sub-surface structure of hydrographic variables (temperature and salinity) and currents is compared against the observations during 1998–2014 (17 years). In particular, the seasonal variability of the sea surface temperature, sea surface salinity, and surface currents over the model domain is studied. The highresolution model’s ability in correct estimation of the spatio-temporal mixed layer depth (MLD) variability of the AS and BoB is also shown. The lowest MLD values are observed during spring (March-April-May) and highest during winter (December-January-February) seasons. The maximum MLD in the AS (BoB) during December to February reaches 150 m (67 m). On the other hand, the minimum MLD in these regions during March-April-May becomes as low as 11–12 m. The influence of wind stress, net heat flux and freshwater flux on the seasonal variability of the MLD is discussed. The physical processes controlling the seasonal cycle of sea surface temperature are investigated by carrying out mixed layer heat budget analysis. It is found that air-sea fluxes play a dominant role in the seasonal evolution of sea surface temperature of the northern Indian Ocean and the contribution of horizontal advection, vertical entrainment and diffusion processes is small. The upper ocean zonal and meridional volume transport across different sections in the AS and BoB is also computed. The seasonal variability of the transports is studied in the context of monsoonal currents.

Climatology of mixed layer depth in the Gulf of Aden derived from in situ temperature profiles

For the first time in the Gulf of Aden, the climatology of mixed layer depth (MLD) is derived from in situ profiles of temperature at a spatial resolution of 0.5° × 0.5°. The climatology has captured all major features of the MLD variability in the Gulf of Aden. Deepening of the mixed layer is noticed during December–February associated with surface heat loss and cooling induced by strong convection. Further, the heat gain during March–May and subsequent warming strengthens the stratification and results in diminished mixing. The westward-moving cyclonic and anticyclonic eddies during winter change the mixed layer, especially along the northern and western Gulf of Aden. The observed permanent anticyclonic eddy during summer in the central part of the Gulf of Aden induces an additional deepening of the MLD by 30–40 m, even in the presence of a relatively strong stratification. The presence of nearly uniform north-easterly winds from October to April makes the MLD of the northern Gulf of Aden greater than the southern side during this period, while the south-westerly winds from June to August reverse the scenario and makes the northern Gulf of Aden shallower than the southern side.

Assessment of the impact of spatial resolution on ROMS simulated upper-ocean biogeochemistry of the Arabian Sea from an operational perspective

ABSTRACTThe resolution of the model emerges to be an important factor in simulating the real oceanic features. In this paper, the performance of two coupled bio-physical models, having spatial resolutions 1/12° (∼9 km) and 1/4° (∼25 km) configured using Regional Ocean Modeling System (ROMS), have been evaluated in simulating upper ocean dynamics of the Arabian Sea. A comparison of the model simulated physical and biogeochemical fields with the observations from remote sensing, in-situ ship-borne, and Biogeochemical-Argo (BGC-Argo) floats showed that the high-resolution model reproduced the ocean physical and biogeochemical dynamics, and their seasonality more efficiently. The upper ocean dynamics associated with the variability of mixed layer depth, persistent occurrence of deep chlorophyll maxima, and seasonal phytoplankton blooms. as well as deep ocean characteristics of oxygen minimum zone were much effectively captured by the high-resolution model than by its counterpart. Similarly, the former model performed very well in reproducing the upwelling dynamics over the eastern continental shelf indicating that the open ocean-coastal coupling has been better established. Our analysis indicates that the realistic representation of the eddy fields by the high-resolution model leads to the better representation of the ocean fields in comparison to the coarse-resolution model.

Seasonal variability of the ocean mixed layer depth depending on the cape Ghir filament and the upwelling in the Moroccan Atlantic coast

Abstract The determination of the monthly variations of the ocean mixed layer depth (MLD), in relationship with upwelling and filament is important in a climate change perspective. This paper presents a monthly change of the mixed layer properties, temperature, salinity, oxygen and fluorescence, to provide a better understanding of the relationship between upwelling, filament and the monthly variability of the MLD in the cape Ghir region latitude 31°N in Moroccan Atlantic coast. Firstly, we compared the MLD using the threshold method and the gradient method of temperature and density from CTD profiles in-situ data. Secondly, we observed the activity of upwelling variability in the region by satellite data. In the other hand, we used monthly mean seawater velocity from Mercator model to identify the filament structure. The general pattern of the in-situ MLD varies between 20m in summer and 150m in winter as the MLD variability derived from Mercator model. The deepest MLD occur in December and February, it coincides with a weak activity of upwelling. During April, June and August, the mixed layer is shallow, accompanied with a high activity of upwelling filament. In October, the mixed layer was deeper following a persistent activity of upwelling.

Processes Controlling Arctic and Antarctic Sea Ice Predictability in the Community Earth System Model

AbstractSea ice predictability is a rapidly growing area of research, with most studies focusing on the Arctic. This study offers new insights by comparing predictability between the Arctic and Antarctic sea ice anomalies, focusing on the effects of regional differences in ice thickness and ocean dynamics. Predictability in simulated regional sea ice area and volume is investigated in long control runs of an Earth system model. Sea ice area predictability in the Arctic agrees with results from other studies, with features of decaying initial persistence and reemergence because of ocean mixed layer processes and memory in thick ice. In pan-Arctic averages, sea ice volume and the area covered by thick ice are the best predictors of September area for lead times greater than 2 months. In the Antarctic, area is generally the best predictor of future area for all times of year. Predictability of area in summer differs between the hemispheres because of unique aspects of the coupling between area and volume. Generally, ice volume only adds to the predictability of summer sea ice area in the Arctic. Predictability patterns vary greatly among different regions of the Arctic but share similar seasonality among regions of the Antarctic. Interactive ocean dynamics influence anomaly reemergence differently in the Antarctic than the Arctic, both for the total and regional area. In the Antarctic, ocean dynamics generally decrease the persistence of area anomalies, reducing predictability. In the Arctic, the presence of ocean dynamics improves ice area predictability, mainly through mixed layer depth variability.

Drivers of Interannual Variability of Summer Mixed Layer Depth in the Southern Ocean Between 2002 and 2011

AbstractClimate change projections indicate that there will be warming and an intensification of westerly winds in the Southern Ocean (SO) in the future. These two forcings potentially have opposing effects on the depth of the surface mixed layer. Here we investigate how interannual to decadal variability of atmospheric surface air temperature (SAT) and zonal wind speed (uwind) impact mixed layer depth (MLD) in the SO (south of 30°S) during summer, the season of main biological activity. We use gridded MLD data from observations and atmospheric reanalysis data of uwind and SAT in the SO to assess summer MLD variability and its potential drivers. With a model‐based sensitivity experiment, we quantify the relative contributions of uwind versus SAT forcing on the summer MLD in the decade 2002–2011. Wind‐induced changes dominate over temperature‐induced changes of the MLD between 2002 and 2011. We find a positive trend of summer MLD in the Antarctic Zone of the Atlantic and Indian Ocean sectors. Our model‐based sensitivity study suggests that the summer MLD shows a zonally asymmetric response to recent atmospheric forcing. In the Pacific and Australian sectors, cooling and intensification of uwind jointly result in a deepening of the mixed layer. In the Atlantic and Indian sectors, the MLD responds differently north and south of the Antarctic Polar Front (APF). A deepening south of the APF is caused by the increase in uwind, whereas the decrease in uwind and warming act in concert to result in a shoaling of the MLD north of the APF.

Decadal variability and trends of oceanic barrier layers in tropical Pacific

Decadal variability and trends of the isothermal layer depth (ILD), mixed layer depth (MLD), and barrier layer thickness (BLT) were analyzed for the tropical Pacific during 1979–2015. The decadal variability of ILD, MLD, and BLT shows a close connection with the Pacific Decadal Oscillation (PDO). At PDO positive phase, the eastward shift of precipitation and weakened trade winds result in thinner BLT in western Pacific and thicker BLT in central and eastern Pacific. The situation is reversed at PDO negative phase. The differences in BLT can be up to 9–15 m. The spatial distributions of decadal trends of ILD and MLD are complex, but a thickening of BLT in the western tropical Pacific is clearly present. The raw trends of ILD, MLD, and BLT averaged in the tropical Pacific (30° N–30° S, 120° E–75° W) from 1979 to 2015 are 1.62, 1.20, and 0.51 m per decade, respectively. PDO can explain about 25% of the increasing trends of BLT, while El Nino-Southern Oscillation (ENSO) only explains about 1.7%. Global warming and/or variability at longer time scales is responsible for the remaining increasing trends. The BLT change is related to the warming and freshening of the western Pacific warm pool in recent decades. The ocean-atmosphere interactions about trade winds, wind-driven ocean circulation, temperature, and precipitation/evaporation are discussed.

Mixed layer depth variability in the Red Sea

Abstract. For the first time, a monthly climatology of mixed layer depth (MLD) in the Red Sea has been derived based on temperature profiles. The general pattern of MLD variability is clearly visible in the Red Sea, with deep MLDs during winter and shallow MLDs during summer. Transitional MLDs have been found during the spring and fall. The northern end of the Red Sea experienced deeper mixing and a higher MLD associated with the winter cooling of the high-saline surface waters. Further, the region north of 19° N experienced deep mixed layers, regardless of the season. Wind stress plays a major role in the MLD variability of the southern Red Sea, while net heat flux and evaporation are the dominating factors in the central and northern Red Sea regions. Ocean eddies and Tokar Gap winds significantly alter the MLD structure in the Red Sea. The dynamics associated with the Tokar Gap winds leads to a difference of more than 20 m in the average MLD between the north and south of the Tokar axis.