Variability Of Mixed Layer Depth Research Articles

Evaluation of Interannual Simulations and Indian Ocean Dipole Events During 2000–2014 from a Basin Scale General Circulation Model

Interannual simulations performed using an eddy-permitting ocean general circulation model (OGCM) for years 2000–2014 were validated against buoy measurements over the Indian Ocean (IO). Model-simulated fields were evaluated extensively using multiple statistical metrics to quantify the quality of model in reproducing variability in oceanic surface and subsurface features. The model-simulated sea surface temperature (SST) at different moored buoy locations exhibits high (close to + 0.9) correlation coefficient (R) with the ranges of standard deviation (SD) of simulated SST consistent with the corresponding buoy observations. The root mean square difference (RMSD) estimated between the buoy and simulated SST was found to be less than 0.6 °C at most of the buoy locations. The model-simulated subsurface temperature profile, including the thermocline, resembled good agreement with the buoy profiles. Intraseasonal and interannual variability of 20 °C isotherm (D20) was simulated reasonably well as observed at the respective buoy locations. Mean error in surface currents was low; however, the meridional component from model simulations showed a better agreement (RMSD < 0.25 m/s) with the observations as compared to zonal components (< 0.4 m/s) for the periods of buoy data availability. The Dipole Mode Index (DMI) derived from simulated SST reproduces the positive/negative Indian Ocean Dipole (IOD) events that occurred during the simulation period. Interannual variability in temperature, currents and oceanic mixed layer depth was analyzed in response to IOD events. The anomalies in equatorial currents were found to affect the strength of coastal currents along the Indian coastlines. Model simulations showed the enhanced (suppressed) coastal upwelling process along the Sumatra coast that leads to anomalous cooling (warming) off the Sumatra coast during the positive (negative) IOD events.

Seasonal Variation of Submesoscale Flow Features in a Mesoscale Eddy-dominant Region in the East Sea

Seasonal changes in the distribution of submesoscale (SM) flow features were examined using a fine-resolution numerical simulation. The SM flows are expected to be strong where mesoscale (MS) eddies actively develop and also when the mixed layer depth (MLD) is deep due to enhanced baroclinic instability. In the East Sea (ES), MS eddies more actively develop in summer while the MLD is deeper in winter, which provided the motivation to conduct this study to test the effects of MLD and MS eddies on the SM activity in this region. Finite-scale Liapunov exponents and the vertical velocity components were employed to analyze the SM activities. It was found that the SM intensity was marked by seasonality: it is stronger in winter when the mixed layer is deep but weaker in summer - despite the greater eddy kinetic energy. This is because in summer the mixed layer is so thin that there is not enough available potential energy. When the SM activity was quantified based on parameterization, (MLD × density gradient), it was determined that the seasonal variation of MLD plays a more important role than the lateral density gradient variation on SM flow motion in the ES.

Generation and Decay Mechanisms of Ningaloo Niño/Niña

AbstractUsing an ocean model, generation and decay mechanisms of warm/cool sea surface temperature anomalies (SSTAs) off Western Australia, or Ningaloo Niño/Niña, are investigated through the calculation of a mixed‐layer temperature (MLT) balance taking the mixed‐layer depth (MLD) variation into account. Since Ningaloo Niño/Niña develops owing to local air‐sea interaction and/or remote forcing, events are classified into two cases based on alongshore wind anomalies and analyzed separately. It is revealed that the anomalous meridional advection associated with the stronger Leeuwin Current and the enhanced warming by the climatological shortwave radiation because of the shallower MLD generate warm SSTAs in the coastal region for both cases of Ningaloo Niño. On the other hand, the latent heat flux damps SSTAs only in a case without northerly alongshore wind anomalies. In the decay, larger sensible heat loss is important. Because of the reduced meridional temperature gradient, the meridional advection eventually damps SSTAs. The sensitivity change to the climatological shortwave radiation owing to MLD anomalies explains offshore MLT tendency anomalies for both cases throughout the events. The mechanisms for Ningaloo Niña are close to a mirror image of Ningaloo Niño but differ in that the latent heat flux damps offshore SSTAs. The seasonal phase‐locking nature of Ningaloo Niño/Niña is related to the seasonal variations of MLD and surface heat fluxes, which regulate the amplitude and sign of the sensitivity change to surface heat fluxes. It is also related to the seasonal variations of the Leeuwin Current and meridional temperature gradient through advection anomalies.

Investigating the role of air-sea forcing on the variability of hydrography, circulation, and mixed layer depth in the Arabian Sea and Bay of Bengal

Summary An effort is made to understand and quantify the influence of near surface zonal and meridional winds, incoming shortwave radiation, and freshwater flux air-sea forcings on the seasonal variability of the hydrography, circulation, and mixed layer depth of the Arabian Sea (AS) and Bay of Bengal (BoB). Sensitivity experiments using an ocean general circulation model are carried out for this purpose in the Indian ocean around 65°–95°E, 5°–22°N during 1998–2014 (17 years). In the absence of near surface wind forcing, the sea surface temperature of the region greatly increases in all the seasons, whereas, in the absence of incoming shortwave radiation forcing, exactly opposite is the case. The sea surface salinity of the AS and BoB decreases in the absence of wind and shortwave radiation forcings, whereas, in the northern BoB it increases in the absence of freshwater flux forcing. The sub-surface changes in the stratification of temperature and salinity are also investigated. The influence of the air-sea forcings on the mixed layer depth of the region is found to be highly seasonally dependent. The effect of air-sea forcings on the seasonal variability of the upper ocean vertical stability is studied using the vertical shear of the horizontal velocity, buoyancy frequency, and energy required for mixing as quantifiers. The near surface wind forcing has highest contribution in changing the surface circulation of the region.

Bay of Bengal salinity stratification and Indian summer monsoon intraseasonal oscillation: 1. Intraseasonal variability and causes

AbstractThe huge freshwater flux of the Indian summer monsoon (ISM; May–October) gives rise to strong salinity stratification in the Bay of Bengal (BoB), causing a shallow mixed layer and a thick barrier layer, which potentially affects intraseasonal oscillations of the monsoon (MISOs). In this study, intraseasonal variability of the mixed‐layer depth (MLD) and barrier layer thickness (BLT) is investigated using in situ observations from Argo floats and moored buoys and an ocean general circulation model (OGCM). The average MLD in the BoB is typically 20–30 m during the ISM, while the BLT increases from ∼10 m in May–June to 20–40 m in September–October. MISOs induce in‐phase variations in MLD and isothermal layer depth (ILD), both of which are deepened by 8–15 m during MISO active phase, while the change of BLT is small and within the error range of Argo data sampling. In the northern (southern) bay, BLT increases by ∼5 m (2 m) during MISOs owing to a larger deepening of ILD than MLD. OGCM experiments are performed to understand the underlying mechanism. In the BoB intraseasonal variations of MLD, ILD and BLT arise largely from ocean internal instability, whereas those induced by MISOs are weaker. The in‐phase variations of MLD and ILD during MISOs are induced by different processes. The MLD deepening is primarily caused by wind stress forcing, while the ILD deepening is driven by surface heat fluxes via surface cooling. The limited variability of BLT is due to the offsetting of different forcing processes.

Seasonal variation of the global mixed layer depth: comparison between Argo data and FIO-ESM

The present study evaluates a simulation of the global ocean mixed layer depth (MLD) using the First Institute of Oceanography-Earth System Model (FIOESM). The seasonal variation of the global MLD from the FIO-ESM simulation is compared to Argo observational data. The Argo data show that the global ocean MLD has a strong seasonal variation with a deep MLD in winter and a shallow MLD in summer, while the spring and fall seasons act as transitional periods. Overall, the FIO-ESM simulation accurately captures the seasonal variation in MLD in most areas. It exhibits a better performance during summer and fall than during winter and spring. The simulated MLD in the Southern Hemisphere is much closer to observations than that in the Northern Hemisphere. In general, the simulated MLD over the South Atlantic Ocean matches the observation best among the six areas. Additionally, the model slightly underestimates the MLD in parts of the North Atlantic Ocean, and slightly overestimates the MLD over the other ocean basins.

Observed mesoscale eddy signatures in Southern Ocean surface mixed‐layer depth

AbstractCombining satellite altimetry with Argo profile data a systematic observational estimate of mesoscale eddy signatures in surface mixed‐layer depth (MLD) is provided across the Southern Ocean (SO). Eddy composite MLD anomalies are shallow in cyclones, deep in anticyclones, and increase in magnitude with eddy amplitude. Their magnitudes show a pronounced seasonal modulation roughly following the depth of the climatological mixed layer. Weak eddies of the relatively quiescent SO subtropics feature peak late‐winter perturbations of ±10 m. Much larger MLD perturbations occur over the vigorous eddies originating along the Antarctic Circumpolar Current (ACC) and SO western boundary current systems, with late‐winter peaks of −30 m and +60 m in the average over cyclonic and anticyclonic eddy cores (a difference of ≈ 100 m). The asymmetry between modest shallow cyclonic and pronounced deep anticyclonic anomalies is systematic and not accompanied by corresponding asymmetries in eddy amplitude. Nonetheless, the net deepening of the climatological SO mixed layer by this asymmetry in eddy MLD perturbations is estimated to be small (few meters). Eddies are shown to enhance SO MLD variability with peaks in late winter and eddy‐intense regions. Anomalously deep late‐winter mixed layers occur disproportionately within the cores of anticyclonic eddies, suggesting the mesoscale heightens the frequency of deep winter surface‐mixing events along the eddy‐intense regions of the SO. The eddy modulation in MLD reported here provides a pathway via which the oceanic mesoscale can impact air‐sea fluxes of heat and carbon, the ventilation of water masses, and biological productivity across the SO.

Lagrangian pathways of upwelling in the Southern Ocean

AbstractThe spatial and temporal variability of upwelling into the mixed layer in the Southern Ocean is studied using a 1/10 ocean general circulation model. Virtual drifters are released in a regularly spaced pattern across the Southern Ocean at depths of 250, 500, and 1000 m during both summer and winter months. The drifters are advected along isopycnals for a period of 4 years, unless they outcrop into the mixed layer, where lateral advection and a parameterization of vertical mixing are applied. The focus of this study is on the discrete exchange between the model mixed layer and the interior. Localization of interior‐mixed layer exchange occurs downstream of major topographic features across the Indian and Pacific basins, creating “hotspots” of outcropping. Minimal outcropping occurs in the Atlantic basin, while 59% of drifters outcrop in the Pacific sector and in Drake Passage (the region from 140 W to 40 W), a disproportionately large amount even when considering the relative basin sizes. Due to spatial and temporal variations in mixed layer depth, the Lagrangian trajectories provide a statistical measure of mixed layer residence times. For each exchange into the mixed layer, the residence time has a Rayleigh distribution with a mean of 30 days; the cumulative residence time of the drifters is 261 ± 194 days, over a period of 4 years. These results suggest that certain oceanic gas concentrations, such as CO2 and 14C, will likely not reach equilibrium with the atmosphere before being resubducted.

North Atlantic Subtropical Underwater and Its Year-to-Year Variability in Annual Subduction Rate during the Argo Period

AbstractSubtropical underwater (STUW) and its year-to-year variability in annual subduction rate are investigated using recently available Argo data in the North Atlantic. For the period of observation (2002–14), the mean annual subduction rate of the STUW is 7.3 ± 1.2 Sv (1 Sv = 106 m3 s−1) within the density range between 25.0 and 26.0 kg m−3. Once subducted, the STUW spreads in the subtropical gyre as a vertical salinity maximum. In the mean, the spatial changes in temperature and salinity of the STUW tend to compensate each other, and the density of the water mass remains rather stable near 25.5 kg m−3 in the southwestern part of the subtropical gyre. The annual subduction rate of the STUW varies from year to year, and most of this variability is due to lateral induction, which in turn is directly linked to the variability of the winter mixed layer depth. Through modulation of surface buoyancy, wind anomalies associated with the North Atlantic Oscillation are primarily responsible for this variability. Sea surface salinity anomalies in the formation region of the STUW are conveyed into the thermocline, but their westward propagation cannot be detected by the present data.

Ekman Pumping and Mixed Layer Depth Variability over the Indo-Pacific Oceans during the El Nino and IOD Events

The following study addresses the variability of Mixed Layer Depth (MLD) and Ekman pumping (W EK ) during the extreme El Nino and Indian Ocean Dipole (IOD) over the Indo-Pacific regions. Monthly anomalies considering the climatology of the period of 1980 through 2011 show that during the El Nino years, Ekman suction (positive Ekman pumping) is replaced with Ekman pumping (negative Ekman pumping) in the tropical eastern Pacific Ocean (88˚W-90˚W and 13˚S-15˚S) resulting in positive MLD anomalies, and the strong Ekman pumping may be the source for the deepened thermocline during El Nino. In the La Nina events shallow MLD exits in the tropical eastern Pacific Ocean, due to positive Ekman pumping. During the positive IOD (PIOD) events in the south eastern Indian Ocean (97˚E-100˚E and 2˚S-5˚S) MLD becomes shallow and positive Ekman pumping anomalies occur. During the negative IOD (NIOD) years opposite signs take place. Composite events of El Nino are compared with those of IOD, showing more Ekman pumping anomalies during IOD events as against less deviation in SST.

Adrift Upon a Salinity-Stratified Sea: A View of Upper-Ocean Processes in the Bay of Bengal During the Southwest Monsoon

The structure and variability of upper-ocean properties in the Bay of Bengal (BoB) modulate air-sea interactions, which profoundly influence the pattern and intensity of monsoonal precipitation across the Indian subcontinent. In turn, the bay receives a massive amount of freshwater through river input at its boundaries and from heavy local rainfall, leading to a salinity-stratified surface ocean and shallow mixed layers. Small-scale oceanographic processes that drive variability in near-surface BoB waters complicate the tight coupling between ocean and atmosphere implicit in this seasonal feedback. Unraveling these ocean dynamics and their impact on air-sea interactions is critical to improving the forecasting of intraseasonal variability in the southwest monsoon. To that end, we deployed a wave-powered, rapidly profiling system capable of measuring the structure and variability of the upper 100 m of the BoB. The evolution of upper-ocean structure along the trajectory of the instrument's roughly two-week drift, along with direct estimates of vertical fluxes of salt and heat, permit assessment of the contributions of various phenomena to temporal and spatial variability in the surface mixed layer depth. Further, these observations suggest that the particular ``barrier-layer'' stratification found in the BoB may decrease the influence of the wind on mixing processes in the interior, thus isolating the upper ocean from the interior below, and tightening its coupling to the atmosphere above.

Weakening of subduction in the Subtropical Mode Water formation region observed during 2003–2013

AbstractSubduction plays an important role in the formation of the Subtropical Mode Water (STMW). Variation of subduction in the western North Pacific was studied using gridded monthly data from Argo float profiles. The results revealed that there exists a weakening trend of subduction in the STMW formation region (28°N–35°N, 142°E–175°W) due to decreasing winter mixed layer depth (MLD) during 2003–2013. In the STMW formation region, the mean subduction rate was about 64 m yr−1 and showed a decreasing trend at −3.44 ± 2.47 m yr−2 during 2003–2013. Meanwhile, the late winter (March) MLD showed a decreasing trend at −4.02 ± 2.41 m yr−1. Associated with the weakening subduction, the STMW volume had a similar decreasing trend in late winter. Diagnostic calculation indicated that change of the mixed layer temperature (MLT) is the key factor in determining the MLD variations in the STMW formation region. It is demonstrated that the increasing MLT tends to decrease oceanic density and stabilize the upper ocean. This oceanic processes act to weaken the vertical mixing and decrease the MLD, resulting in the weakening of subduction.

Seasonal variability in mixed layer depth in Nansha area in 2013

Seasonal variability in mixed layer depth in Nansha area in 2013

Intraseasonal variability of mixed layer depth in the tropical Indian Ocean

In this paper, we use an observational dataset built from Argo in situ profiles to describe the main large-scale patterns of intraseasonal mixed layer depth (MLD) variations in the Indian Ocean. An eddy permitting (0.25°) regional ocean model that generally agrees well with those observed estimates is then used to investigate the mechanisms that drive MLD intraseasonal variations and to assess their potential impact on the related SST response. During summer, intraseasonal MLD variations in the Bay of Bengal and eastern equatorial Indian Ocean primarily respond to active/break convective phases of the summer monsoon. In the southern Arabian Sea, summer MLD variations are largely driven by seemingly-independent intraseasonal fluctuations of the Findlater jet intensity. During winter, the Madden–Julian Oscillation drives most of the intraseasonal MLD variability in the eastern equatorial Indian Ocean. Large winter MLD signals in northern Arabian Sea can, on the other hand, be related to advection of continental temperature anomalies from the northern end of the basin. In all the aforementioned regions, peak-to-peak MLD variations usually reach 10 m, but can exceed 20 m for the largest events. Buoyancy flux and wind stirring contribute to intraseasonal MLD fluctuations in roughly equal proportions, except for the Northern Arabian Sea in winter, where buoyancy fluxes dominate. A simple slab ocean analysis finally suggests that the impact of these MLD fluctuations on intraseasonal sea surface temperature variability is probably rather weak, because of the compensating effects of thermal capacity and sunlight penetration: a thin mixed-layer is more efficiently warmed at the surface by heat fluxes but loses more solar flux through its lower base.

Dynamics of ocean surface mixed layer variability in the Indian Ocean

AbstractWe present a new methodology that allows quantifying the impact of individual terms of the temperature and salinity conservation within the mixed layer on mixed layer depth (MLD). The method is applied to output from an ocean general circulation model in the Indian Ocean to investigate variability and changes in MLD. On seasonal timescales and for most areas of the Indian Ocean variability of MLD is tightly linked to all thermohaline budget terms. In the Indian Ocean at approximately 20°S the MLD covaries with surface heat and freshwater fluxes on intraseasonal and interannual timescales. The geography of the region includes the Leeuwin Current, plus the tropical eastern Indian Ocean for interannual surface freshwater fluxes. The range of seasonal amplitudes of MLD variability varies with individual budget terms but is typically within 1 m/month to 100 m/month. The ocean footprints of an intraseasonal tropical cyclone, tropical and midlatitude seasonal temperature and salinity budgets and interannual variability associated with the Indian Ocean Dipole Mode are analyzed. The results reveal close relationships of the thermohaline budgets within the mixed layer with the variability of the MLD. The associated tendencies of changes in MLD are consistent with Argo and satellite‐based observations of tendencies within the mixed layer and sea‐surface temperature and salinity.

Low‐salinity water off West Luzon Island in summer

AbstractLow‐salinity water with two cores is found off West Luzon Island in the South China Sea (SCS) during summer. A series of salinity observations and model results show that the low‐salinity water begins to appear in June, reaches its lowest salinity in September, and disappears after October. Rainfall associated with the summer monsoon impinging on the Philippine mountain ranges plays an important role in the formation of the low‐salinity water, while upward Ekman pumping of high‐salinity subsurface water caused by the strong winter monsoon is important for its disappearance. Variation in mixed layer depth is responsible for the formation of the two cores of the low‐salinity water, while advection also contributes. The study further demonstrates that the low‐salinity water has considerable interannual variability associated with El Niño‐Southern Oscillation (ENSO): during the summer of the decaying year of an El Niño, an anticyclonic wind anomaly occurs in the SCS. The anticyclonic wind anomaly is associated with a northeasterly anomaly south of 18°N, reducing precipitation and causing salting of the low‐salinity water off West Luzon Island. The situation is reversed during the summer of the decaying year of a La Niña.

Sverdrup critical depth and the role of water clarity in Norwegian Coastal Water

Abstract The critical depth concept was first recognized by Gran and Braarud (1935). During summer, in the Bay of Fundy, they observed an unexpected no bloom situation. Their interpretation was that high amounts of detritus of terrestrial origin caused too murky water and insufficient light for the tidally mixed phytoplankton. Almost 20 years later, this was elaborated by Sverdrup (1953) into a hypothesis for the initiation of the spring bloom in the North Atlantic Water (NAW) masses. Since then, variations in mixed layer depth have been a key in phytoplankton modelling. As illustrated by the study of Gran and Braarud, variation in the non-phytoplankton light attenuation coefficient is also a key to understand phytoplankton bloom conditions. Due to lack of accurate parameterizations, however, non-phytoplankton light attenuation is often assumed invariant in phytoplankton modelling. Here, I report spatial variation in a proxy for the pre-bloom light attenuation in Norwegian Coastal Water (NCW). It is shown that this variation can be partially accounted for by variations in salinity and dissolved oxygen. The light attenuation coefficient at 440 nm increased by 0.041 and 0.032 m−1 with drops in salinity and dissolved oxygen of 1PSU and 1 ml O2 l−1, respectively. Consequences for the euphotic depth, Sverdrup critical depth, and the nutricline depth are discussed. I conclude that phytoplankton modelling, particularly across coastal and oceanic waters, such as NCW and NAW, needs to account for variations in the non-phytoplankton light attenuation and that salinity might be a useful proxy for regional parameterizations.

Chlorophyll variability in the oligotrophic gyres: mechanisms, seasonality and trends

A 16-year (1998-2013) analysis of trends and seasonal patterns was conducted for the five subtropical ocean gyres using satellite data: chlorophyll-a (Chl-a) retrievals from ocean color, sea surface temperature (SST), and sea-level anomaly (SLA). Trend analysis was also performed on mixed-layer data derived from ocean model gridded temperature and salinity profiles (1998-2010). The Chl-a monthly composites were constructed from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate-resolution Imaging Spectroradiometer (MODIS) on Aqua using two different algorithms: the standard algorithm (STD) that has been in use since the start of the SeaWiFS mission in 1997, and a more recently developed Ocean Color Index (OCI) algorithm with improved accuracy in low Chl-a waters. Trends were obtained for all gyres using both STD and OCI algorithms, which demonstrated generally consistent results. The North Pacific, Indian Ocean, North Atlantic and South Atlantic gyres showed significant downward trends in Chl-a, while the South Pacific gyre has a much weaker upward trend with no statistical significance. Time series of satellite-derived net primary production (NPP) showed downward trends for all the gyres, while all five gyres exhibited positive trends in SST and SLA. The seasonal variability of Chl-a in each gyre is tightly coupled to the variability in mixed layer depth (MLD) with peak values in winter in both hemispheres when vertical mixing is more vigorous, reaching depths approaching the nutricline. On a seasonal basis, Chl-a concentrations increase when the MLD approaches or is deeper than the nutricline depth, in agreement with the concept that vertical mixing is the major driving mechanism for phytoplankton photosynthesis in the interior of the gyres. The combination of surface warming trends and biomass reduction over the 16-year period has the potential to reduce atmospheric CO2 uptake by the gyres and therefore influence the global carbon cycle.

Seasonal and interannual variability of pan-Arctic surface mixed layer properties from 1979 to 2012 from hydrographic data, and the dominance of stratification for multiyear mixed layer depth shoaling

Using 21,406 hydrographic profiles from 1979 to 2012, we present the first observational, pan-Arctic assessment of Mixed Layer (ML) properties, including quantification of seasonal and interannual variability, and identification of multiyear ML depth shoaling.Arctic Mixed Layer Depths (MLDs) vary strongly seasonally, being deeper (∼25 to >50m) in winter than summer (∼5–30m). Eastern Arctic MLDs (regional mean ∼20m in summer, ∼70 to 100+m in winter) are deeper than western Arctic MLDs (∼8m in summer, 30m in winter). Patchiness, likely related to small-scale sea ice cover variability, is large – standard deviations ∼40% of the regional mean.By binning data into 6 regions (i.e., Chukchi Sea, Southern Beaufort Sea, Canada Basin, Makarov Basin, Eurasian Basin and Barents Sea), we quantify regional seasonal climatologies and interannual variability of ML depth, temperature and salinity. In most regions, ML changes are consistent with seasonal ice melt (∼1–3m) with a ∼1.5 times greater sea ice change required in the western Arctic than in the eastern Arctic. In the Southern Beaufort Sea and the Canada Basin, however, other freshwater sources contribute to observed seasonality.MLDs are significantly correlated with wind only during ice-free times, and even then the relationship only explains 1–20% of the MLD variance. The same wind is 2–3 times more effective at deepening the ML in the eastern Arctic than in the (more stratified) western Arctic. Changes in underlying stratification (Δρ) explain ∼60% of the MLD variance, with MLDs proportional to Δρ−0.45. Weak eastern Arctic stratification permits a wind–MLD coupling comparable to an Ekman model, while the stronger western Arctic stratifications reduce the wind’s effectiveness by a factor of 6.Remarkably, record-length (up to 30-year) trends indicate almost ubiquitous ML shoaling, order 0.5–1m/yr, both in winter and summer over all the high Arctic (Canada, Makarov and Eurasian basins) and in winter in the peripheral seas (Chukchi, Southern Beaufort and Barents seas), coincident with ML freshening and increased stratification, while wind speed trends are either not significant or decreasing. The freshwater change related to this shoaling is small – order 100km3/yr. In contrast, the Southern Beaufort Sea shows ML deepening, coincident with decreasing stratification, possibly related to river water being driven away from the coast. Changes in T–S space suggest decreased convection in the Eurasian Basin in the 2000s.Although in these results it is the absence of sea ice that allows wind-driven ML deepening, the dominance of stratification over wind in determining MLD suggests that even small changes in the Arctic freshwater budget may control MLD variability, with implications for mixing nutrients and heat up into the surface layer and photic zone.

Interannual Variability of the Mixed Layer Winter Convection and Spice Injection in the Eastern Subtropical North Atlantic

AbstractThe Argo dataset is used to study the winter upper-ocean conditions in the northeastern subtropical (NEA) Atlantic during 2006–12. During late winter 2010, the mixed layer depth is abnormally shallow and a negative anomaly of density-compensated salinity, the so-called spiciness, is generated in the permanent pycnocline. This is primarily explained by unusual weak air–sea buoyancy flux during the late winter 2010, in contrast with the five other studied winters. Particularly deep mixed layers and strong spiciness anomalies are observed during late winter 2012. The 2010 winter conditions appear to be related to historically low North Atlantic Oscillation (NAO) and high tropical North Atlantic index (TNA). Interannual variability of the eastern subtropical mixed layer is further investigated using a simple 1D bulk model of mean temperature and salinity linear profiles, based on turbulent kinetic energy conservation in the upper-ocean layer, and forced only with seasonal air–sea buoyancy forcing corresponding to fall–winter 2006–12. It suggests that year-to-year variability of the winter convective mixing driven by atmospheric buoyancy flux is able to generate interannual variability of both late winter mixed layer depth and spiciness in a strongly compensated layer at the base of the mixed layer and in the permanent pycnocline.