Shipboard Observations Research Articles

Examining features of enhanced phytoplankton biomass in the Bay of Bengal using a coupled physical‐biological model

AbstractA coupled bio‐physical ocean model is used to describe areas of enhanced phytoplankton biomass, seen in remotely sensed observations, in the otherwise oligotrophic environment of the Bay of Bengal. The model is based on the Naval Coastal Ocean Model (NCOM), which is one‐way coupled to the 13‐component Carbon, Silicate, and Nitrogen Ecosystem (CoSiNE) model and configured for the Indian Ocean. Model results are compared and evaluated against a set of in situ shipboard observations as well as ocean color data acquired from several remote sensing platforms. The model is shown to successfully simulate the seasonal cycle of phytoplankton, the markedly contrasting scenarios of phytoplankton distribution in the north versus the south Bay of Bengal, and the biological impact from the 1997/1998 Indian Ocean Dipole (IOD) event. The model simulation provides us with vertical cross sections of phytoplankton biomass from summer and winter blooms in the southwest of the bay, information not found in remotely sensed data. It also successfully reproduces the timing of the onset of the blooms and their spatial extent, thereby providing a measure of its potential for augmenting in situ and remotely sensed observations to improve understanding of the dynamics of primary producers and carbon cycling in one of the most poorly sampled regions of the world's oceans.

Dependence of subsurface chlorophyll on seasonal water masses in the Chukchi Sea

AbstractDuring the late summer, phytoplankton in the northeastern Chukchi Sea are typically found in subsurface layers. These layers and their sensitivity to local changes in hydrography and nutrient concentrations are characterized by combining data from a high‐resolution towed sampling platform with traditional shipboard observations. The replacement of surface meltwater and deeper nutrient‐rich Chukchi Winter Water by northward flowing nutrient‐poor Chukchi Summer Water and Remnant Winter Water leads to a net decrease in biomass and smaller phytoplankton. Between 17 and 67% of phytoplankton biomass is contained within the subsurface layers. This estimate is nearly twice as high as previous estimates from sparser shipboard data and suggests subsurface phytoplankton contribute significantly to the net biomass in the Chukchi Sea in late summer.

Nitrogen speciation in various types of aerosols in spring over the northwestern Pacific Ocean

Abstract. The cumulative atmospheric nitrogen deposition has been found to profoundly impact the nutrient stoichiometry of the eastern China seas (ECSs: the Yellow Sea and East China Sea) and the northwestern Pacific Ocean (NWPO). In spite of the potential significance of dry deposition in those regions, shipboard observations of atmospheric aerosols remain insufficient, particularly regarding the compositions of water-soluble nitrogen species (nitrate, ammonium and water-soluble organic nitrogen – WSON). We conducted a cruise covering the ECSs and the NWPO during the spring of 2014 and observed three types of atmospheric aerosols. Aluminum content, air mass backward trajectories, weather conditions, and ion stoichiometry allowed us to discern dust aerosol patches and sea-fog-modified aerosols (widespread over the ECSs) from background aerosols (open ocean). Among the three types, sea-fog-modified aerosols contained the highest concentrations of nitrate (536 ± 300 nmol N m−3), ammonium (442 ± 194 nmol N m−3) and WSON (147 ± 171 nmol N m−3); furthermore, ammonium and nitrate together occupied ∼ 65 % of the molar fraction of total ions. The dust aerosols also contained significant amounts of nitrate (100 ± 23 nmol N m−3) and ammonium (138 ± 24 nmol N m−3) which were obviously larger than those in the background aerosols (26 ± 32 for nitrate and 54 ± 45 nmol N m−3 for ammonium), yet this was not the case for WSON. It appeared that dust aerosols had less of a chance to come in contact with WSON during their transport. In the open ocean, we found that sea salt (e.g., Na+, Cl−, Mg2+), as well as WSON, correlated positively with wind speed. Apparently, marine dissolved organic nitrogen (DON) was emitted from breaking waves. Regardless of the variable wind speeds from 0.8 to as high as 18 m s−1, nitrate and ammonium, by contrast, remained in narrow ranges, implying that some supply and consumption processes of nitrate and ammonium were required to maintain such a quasi-static condition. Mean dry deposition of total dissolved nitrogen (TDN) for sea-fog-modified aerosols (1090 ± 671 µmol N m−2 d−1) was 5 times higher than that for dust aerosols (190 ± 41.6 µmol N m−2 d−1) and around 20 times higher than that for background aerosols (56.8 ± 59.1 µmol N m−2 d−1). Apparently, spring sea fog on the ECSs played an important role in removing atmospheric reactive nitrogen from the Chinese mainland and depositing it into the ECSs, thus effectively preventing its seaward export to the NWPO.

Concentration maxima of volatile organic iodine compounds in the bottom layer water and the cold, dense water over the Chukchi Sea in the western Arctic Ocean: a possibility of production related to the degradation of organic matter

Abstract. We conducted a shipboard observation over the Chukchi Sea and the Canada Basin in the western Arctic Ocean in September and October 2012 to obtain vertical distributions of four volatile organic iodine compounds (VOIs) in seawater. The VOIs observed in this study were iodomethane (CH3I), iodoethane (C2H5I), diiodomethane (CH2I2), and chloroiodomethane (CH2ClI). Maximum concentrations of the four VOIs were found in the bottom layer water over the Chukchi Sea shelf, in which layer the maximum concentration of ammonium (NH4+) also occurred. A significant correlation was observed between C2H5I and NH4+ (correlation coefficient R = 0.93, P < 0.01, n = 64) and between CH3I and NH4+ (R= 0.77, P < 0.01, n = 64), suggesting that the production of these VOIs increased with the degradation of organic matter. Over the northern Chukchi Sea shelf–slope area, concentration maxima of CHI2, CH2ClI, and CH3I were found in the subsurface cold, dense water (CDW). A large nitrogen deficit (N deficit = NH4++ NO3−+ NO2−−16PO43−) occurred simultaneously in this water, suggesting the production of the three VOIs in the sediment or the bottom layer water over the shelf, probably in association with the degradation of organic matter. We conclude that VOI production over the Chukchi Sea shelf can be largely attributed to the degradation of organic matter that is produced in the highly productive shelf water. High concentrations of CH2ClI were also found in the Alaskan Coastal Water (ACW) from the Bering Strait to the surface of the northern Chukchi slope. The VOIs that originated at the Chukchi Sea shelf are expected to be laterally transported to the Arctic Ocean basin through the CDW and the surface ACW.

Air-Sea Interaction in the Bay of Bengal

Recent observations of surface meteorology and exchanges of heat, freshwater, and momentum between the ocean and the atmosphere in the Bay of Bengal are presented. These observations characterize air-sea interaction at 18 degrees N, 89.5 degrees E from December 2014 to January 2016 and also at other locations in the northern Bay of Bengal. Monsoonal variability dominated the records, with winds to the northeast in summer and to the southwest in winter. This variability included a strong annual cycle in the atmospheric forcing of the ocean in the Bay of Bengal, with the winter monsoon marked by sustained ocean heat loss resulting in ocean cooling, and the summer monsoon marked by strong storm events with dark skies and rain that also resulted in ocean cooling. The spring intermonsoon was a period of clear skies and low winds, when strong solar heating and weak wind-driven mixing led to ocean warming. The fall intermonsoon was a transitional period, with some storm events but also with enough clear skies and sunlight that ocean surface temperature rose again. Mooring and shipboard observations are used to examine the ability of model-based surface fluxes to represent air-sea interaction in the Bay of Bengal; the model-based fluxes have significant errors. The surface forcing observed at 18 degrees N is also used together with a one-dimensional ocean model to illustrate the potential for local air-sea interaction to drive upper-ocean variability in the Bay of Bengal.

Decadal Bering Sea seascape change: consequences for Pacific walruses and indigenous hunters.

The most significant factors currently affecting the Pacific walrus (Odobenus rosmarus divergens) population are climate change and consequent changes in sea-ice morphology and dynamics. This paper integrates recent physical sea-ice change in the Bering Sea with biological and ecological conditions of walruses in their winter-spring reproductive habitat. Historically, walrus in winter-spring depended on a critical mass of sea-ice habitat to optimize social networking, reproductive fitness, feeding behavior, migration, and energetic efficiency. During 2003-2013, our cross-disciplinary, multiscale analysis from shipboard observations, satellite imagery, and ice-floe tracking, reinforced by information from indigenous subsistence hunters, documented change of sea-ice structure from a plastic continuum to a "mixing bowl" of ice floes moving more independently. This fragmentation of winter habitat preconditions the walrus population toward dispersal mortality and will also negatively affect the availability of resources for indigenous communities. We urge an expanded research and management agenda that integrates walrus natural history and habitat more completely with changing sea-ice morphology and dynamics at multiple scales, while also meeting the needs of local communities.

Usefulness of deep-ocean water pumping for the seasonal monitoring of mesozooplankton

The Okhotsk Sea is the southernmost seasonally ice-covered ocean in the Northern Hemisphere. Because of the ice coverage during winter, seasonal monitoring of zooplankton is difficult by ordinary ship-board observation. To overcome this issue, zooplankton monitoring of samples collected by deep-ocean water pumping may be useful. In this study, we evaluated seasonal changes in the zooplankton community based on the samples collected by deep-ocean water pumped from a 350-m depth off of Rausu Harbor in the southern Okhotsk Sea at 2.5-day intervals over two and a half years. Zooplankton abundance and biomass ranged from 20–550 inds. m−3 and 6–902 mg WM m−3, respectively. Both parameters showed similar seasonal changes throughout the study period. Copepods were the most dominant taxa throughout the year and accounted for 90% and 76% of the annual mean zooplankton abundance and biomass, respectively. A total of 20 genera and 33 copepod species were observed. Metridia okhotensis and M. pacifica accounted for 61% and 12% of the annual mean copepod abundance, respectively. In general, the abundance and biomass of zooplankton collected by pumped-up deep-ocean water were lower than they were in the epipelagic layer (both characteristics of the former corresponded to approximately 60% of the latter). The combined effects of low zooplankton density at the deep layer (350 m) and the low filtering rate of deep-water pumping are considered to be a possible cause of the low zooplankton abundance and biomass in the pumped-up deep-ocean water. For the zooplankton samples from pumped-up deep-ocean water, there was a high abundance of benthopelagic mysids and copepods (Xanthocalanus spp.). Because the water inlet of the deep-ocean water pumping is located near the sea bottom, the collected samples may be useful for the evaluation of seasonal changes in the population structure of the less-studied benthopelagic species.

Fine-scale structure and mixing across the front between the Tsugaru Warm and Oyashio Currents in summer along the Sanriku Coast, east of Japan

High-resolution shipboard observations were made across the front between the Tsugaru Warm Current (TWC) and the Oyashio Current in July 2013. Fine structure in the frontal zones was successfully captured with an underway conductivity–temperature–depth profiler deployed with a typical horizontal interval of 2–3 nautical miles. The front characterized by marked horizontal gradients in temperature and salinity extended from the subsurface onto the shelf. Along this frontal layer, the minimum frequency for internal waves became substantially lower than the local inertial frequency, mainly due to the strong vertical shear of the geostrophic velocity. Turbulent energy dissipation rates e (vertical diffusivity K ρ ) were frequently elevated along the front and its offshore side up to 3 × 10−8 W kg−1 (10−4 m2 s−1), which may have been caused by an “internal tide chimney”, trapping low-frequency internal waves within the band of strong shear. At the onshore side of the TWC on the shelf, strong mixing with e (K ρ ) exceeding 10−6 W kg−1 (10−3 m2 s−1) was also observed. A large portion of the water columns in the frontal area provided suitable conditions for double diffusion; in some layers with moderate turbulence, temperature microstructures indicative of double diffusion were observed. The vigorous mixing processes around the front are likely to modify the properties of the TWC downstream, which could then produce a latitudinal gradient in environments along the coast.

English

English

Short‐term variability in euphotic zone biogeochemistry and primary productivity at Station ALOHA: A case study of summer 2012

AbstractTime‐series observations are critical to understand the structure, function, and dynamics of marine ecosystems. The Hawaii Ocean Time‐series program has maintained near‐monthly sampling at Station ALOHA (22°45′N, 158°00′W) in the oligotrophic North Pacific Subtropical Gyre (NPSG) since 1988 and has identified ecosystem variability over seasonal to interannual timescales. To further extend the temporal resolution of these near‐monthly time‐series observations, an extensive field campaign was conducted during July–September 2012 at Station ALOHA with near‐daily sampling of upper water‐column biogeochemistry, phytoplankton abundance, and activity. The resulting data set provided biogeochemical measurements at high temporal resolution and documents two important events at Station ALOHA: (1) a prolonged period of low productivity when net community production in the mixed layer shifted to a net heterotrophic state and (2) detection of a distinct sea‐surface salinity minimum feature which was prominent in the upper water column (0–50 m) for a period of approximately 30 days. The shipboard observations during July–September 2012 were supplemented with in situ measurements provided by Seagliders, profiling floats, and remote satellite observations that together revealed the extent of the low productivity and the sea‐surface salinity minimum feature in the NPSG.

The evolution of mode-2 nonlinear internal waves over the northern Heng-Chun Ridge south of Taiwan

Abstract. Two research cruises were conducted from the R/V OCEAN RESEARCHER 3 during 05–16 August 2011 to study the generation and propagation of high-frequency nonlinear internal waves (NLIWs) over the northern Heng-Chun Ridge south of Taiwan. The primary study site was on top of a smaller ridge about 15 km wide by 400 m high atop the primary ridge, with a sill depth of approximately 600 m. A single mooring was used in conjunction with shipboard observations to sample the temperature, salinity and velocity structure over the ridge. All the sensors observed a profusion of mode-2 NLIWs. Some of the waves were solitary, while others had as many as seven evenly spaced waves per packet. The waves all exhibited classic mode-2 velocity structure with a core near 150–200 m and opposing velocities in the layers above and below. At least two and possibly three most common propagation directions emerged from the analysis, suggesting multiple generation sites near the eastern side of the ridge. The turbulent dissipation due to overturns in the wave cores was very high at order 10−4–10−3 W kg−1. The energy budget suggests that the waves cannot persist very far from the ridge and likely do not contribute to the South China Sea transbasin wave phenomenon.

Kara Sea freshwater transport through Vilkitsky Strait: Variability, forcing, and further pathways toward the western Arctic Ocean from a model and observations

Siberian river water is a first-order contribution to the Arctic freshwater budget, with the Ob, Yenisey, and Lena supplying nearly half of the total surface freshwater flux. However, few details are known regarding where, when, and how the freshwater transverses the vast Siberian shelf seas. This paper investigates the mechanism, variability, and pathways of the fresh Kara Sea outflow through Vilkitsky Strait toward the Laptev Sea. We utilize a high-resolution ocean model and recent shipboard observations to characterize the freshwater-laden Vilkitsky Strait Current (VSC), and shed new light on the little-studied region between the Kara and Laptev Seas, characterized by harsh ice conditions, contrasting water masses, straits, and a large submarine canyon. The VSC is 10–20 km wide, surface intensified, and varies seasonally (maximum from August to March) and interannually. Average freshwater (volume) transport is 500 ± 120 km3 a−1 (0.53 ± 0.08 Sv), with a baroclinic flow contribution of 50–90%. Interannual transport variability is explained by a storage-release mechanism, where blocking-favorable summer winds hamper the outflow and cause accumulation of freshwater in the Kara Sea. The year following a blocking event is characterized by enhanced transports driven by a baroclinic flow along the coast that is set up by increased freshwater volumes. Eventually, the VSC merges with a slope current and provides a major pathway for Eurasian river water toward the western Arctic along the Eurasian continental slope. Kara (and Laptev) Sea freshwater transport is not correlated with the Arctic Oscillation, but rather driven by regional summer pressure patterns.

Decadal variability of Subtropical Mode Water subduction and its impact on biogeochemistry

Temperature and salinity data from Argo profiling floats during 2005–2014 were analyzed to examine the decadal variability of the North Pacific Subtropical Mode Water (STMW) in relation to that of the Kuroshio Extension (KE) system. The formation volume of STMW in the southern recirculation gyre of KE in the cooling season was larger during the stable KE period after 2010 than the unstable KE period of 2006–2009 by 50 %. As a result, the volume and spatial extent of STMW increased (decreased) in the formation region during the stable (unstable) KE period, as well as in the southern, downstream region with a time lag of 1–2 years. The decadal expansion and contraction of STMW were also detected by shipboard observations conducted routinely in the most downstream region near the western boundary, in terms of not only physical, but also biogeochemical parameters. After 2010, enhanced subduction of STMW consistently increased dissolved oxygen, pH, and aragonite saturation state and decreased potential vorticity, apparent oxygen utilization, nitrate, and dissolved inorganic carbon, among which changes of dissolved inorganic carbon, pH, and aragonite saturation state were against their long-term trends. These results indicate a new mechanism consisting of westward sea surface height anomaly propagation, the KE state transition, and the STMW formation and subduction, by which the climate variability affects physical and biogeochemical structures in the ocean’s interior and potentially impacts the surface ocean acidification trend and biological production.

Characterization of marine aerosols and precipitation through shipboard observations on the transect between 31° N–32° S in the West Pacific

To characterize the chemical composition, size distributions, and fractional Fe solubility of atmospheric particles over Asian marginal seas, South Indian Ocean and Australian coast, selected water–soluble inorganic and organic species in aerosols and precipitation, trace metals and soluble Fe in aerosols were analyzed by multi–instruments. Results showed that sea salt and non–sea–salt sulfate (nss–SO42–) were the main components in aerosols. Over Asian marginal seas, Cl– and Na+ were the dominant ions in precipitation, accounting for ˜;72% of the total ions. Both SO42– and NO3– accounted for −26% of the total anions, controlling the acidity of the precipitation. Non–sea–salt Ca2+ (nss–Ca2+) accounted for 6.9% of the total cations, dominating the neutralizing component in rainwater. Observed methane sulfonate (MSA) concentrations and MSA/nss–SO42– increased southward. The concentrations of sea salt were affected by wind speeds, which was mainly accumulated in particle size >10μm. Particle size distributions of nss–SO42– and NH4+ mainly peaked in the fine mode, while NO3– was mainly accumulated in the coarse mode. Oxalate presented a bimodal size distribution pattern in both fine and coarse modes. Based on the air mass back trajectories, enrichment factors and Fe/Al, V/Al ratios, aerosol samples collected over Asian marginal seas could be affected by both long–range transported dust and anthropogenic emissions. Good relationship was found between total dissolved iron and nss–SO42–, indicating that acid processing during long–range transport could play an important role in fractional iron solubility in aerosols. The inverse relationship between atmospheric total Fe and fractional Fe solubility fitted in the global–scale trend. This study implicates that dust and acidic air pollutants from continental sources can interact and affect iron solubility in aerosols in the marine atmosphere. However, due to the small size of samples in this study, more investigations need to be conducted in future.

Amazon River water in the northeastern Caribbean Sea and its effect on larval reef fish assemblages during April 2009

AbstractDuring April to June 2009, a large bolus of Amazon River water impacted the northeastern Caribbean Sea. Shipboard observations collected near Saba Bank, the U.S. and British Virgin Islands, and the Anegada Passage showed low surface salinity (35.76 ± 0.05 Practical Salinity Unit (PSU)), elevated surface temperature (26.77 ± 0.14°C), high chlorophyll‐a (1.26 ± 0.21 mg m−3) and high dissolved oxygen (4.90 ± 0.06 mL L−1) in a 20‐ to 30‐m thick surface layer in the riverine plume. The water was ~1°C warmer, 1 PSU fresher, 0.3 mL L−1 higher in oxygen and 1.2 mg m−3 higher in chlorophyll‐a than Atlantic Ocean waters to the north, with Caribbean surface waters showing intermediate values. Plankton net tows obtained in the upper 100 m of the water column revealed larval fish assemblages within the plume that were significantly different from those of the surrounding waters and from those encountered in the area in previous years. The plume waters contained higher concentrations of mesopelagic fish larvae from the families Myctophidae and Nomeidae, which as adults typically inhabit offshore, deep water habitats. Concentrations of larvae from inshore and reef‐associated families such as Scaridae, Serranidae, Labridae and Clupeidae were lower than those found outside the plume in similar shallow areas, particularly in near‐surface waters. An event like the one observed in 2009 had not been documented in at least the past 30 yr, and yet it was followed by another similarly extreme event in 2010. The ecological implications, including any long‐term consequences of such recent extreme events, are important and merit further study.

The origin of the Ulva macroalgal blooms in the Yellow Sea in 2013

Green algal blooms have occurred in the Yellow Sea for seven consecutive years from 2007 to 2013. In this study, satellite image analysis and field shipboard observations indicated that the Ulva blooms in 2013 originated in the Rudong coast. The spatial distribution of Ulva microscopic propagules in the Southern Yellow Sea also supported that the blooms originated in the Rudong coast. In addition, multi-source satellite data were used to evaluate the biomass of green algae on the Pyropia aquaculture rafts. The results showed that approximately 2784 tons of Ulva prolifera were attached to the rafts and possessed the same internal transcribed spacer and 5S rDNA sequence as the dominant species in the 2013 blooms. We conclude that the significant biomass of Ulva species on the Pyropia rafts during the harvesting season in radial tidal sand ridges played an important role in the rapid development of blooms in the Yellow Sea.

Biogeochemical responses associated with the passage of a cyclonic eddy based on shipboard observations in the western North Pacific

A shipboard high-resolution hydrographic survey in the subtropical region of the western North Pacific conducted from October to November 2008 detected part of a cyclonic eddy around 30°N, 145°E. This eddy had propagated westward in the region south of the Kuroshio extension for at least 6 months as a wavelike disturbance. Within this eddy, isopycnals shallowed between a depth of 600 m and just below the surface mixed layer. In addition, maximal dissolved oxygen concentrations were observed in the subsurface layer between depths of 50 and 100 m. Nitrate was depleted within this subsurface maximal oxygen layer. These results suggest that nutrients in the deeper layers were supplied into the euphotic layer as a result of the uplift of isopycnals in the eddy, fueling the photosynthesis of phytoplankton in the subsurface and emitting an excess of oxygen due to new production. Compared with the outside of the eddy, the enhancement of oxygen and the decrease of nitrate in the center of the eddy were estimated to be 2.7 mol O2 m−2 and 0.22 mol N m−2, respectively. The primary productivity calculated using the eddy transition speed of 5.1 km day−1 was 548 mg C m−2 day−1 at the center of the eddy. The enhanced primary productivity due to the passage of the eddy is likely to have an important role in the ecosystem and on material cycling in the subtropical region.

Seasonal variability of primary production and phytoplankton biomass in the western Pacific subarctic gyre: Control by light availability within the mixed layer

AbstractA distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time series station K2 in the western Pacific subarctic gyre (WSG). The mean depth‐integrated primary production was highest (569 ± 162 mg C m−2 d−1) in summer and lowest (101 ± 16 mg C m−2 d−1) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high‐nutrient, low‐chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth‐integrated chlorophyll a was observed in summer (54.6 ± 13.4 mg m−2), but chlorophyll a remained high in winter (45.3 ± 7.7 mg m−2). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.

Heat loss from the Atlantic water layer in the northern Kara Sea: causes and consequences

Abstract. A distinct, subsurface density front along the eastern St. Anna Trough in the northern Kara Sea is inferred from hydrographic observations in 1996 and 2008–2010. Direct velocity measurements show a persistent northward subsurface current (~ 18 cm s−1) along the St. Anna Trough eastern flank. This sheared flow, carrying the outflow from the Barents and Kara seas to the Arctic Ocean, is also evident from shipboard observations as well as from geostrophic velocities and numerical model simulations. Although we cannot substantiate our conclusions by direct observation-based estimates of mixing rates in the area, we hypothesize that the enhanced vertical mixing along the St. Anna Trough eastern flank favors the upward heat loss from the intermediate warm Atlantic water layer. Modeling results support this hypothesis. The upward heat flux inferred from hydrographic data and model simulations is of O(30–100) W m−2. The region of lowered sea ice thickness and concentration seen both in sea ice remote sensing observations and model simulations marks the Atlantic water pathway in the St. Anna Trough and adjacent Nansen Basin continental margin. In fact, the sea ice shows a delayed freeze-up onset during fall and a reduction in the sea ice thickness during winter. This is consistent with our results on the enhanced Atlantic water heat loss along the Atlantic water pathway in the St. Anna Trough.

Global Patterns of Diapycnal Mixing from Measurements of the Turbulent Dissipation Rate

Abstract The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.