Turbulent mixing in the ocean is key to regulate the transport of heat, freshwater and biogeochemical tracers, with strong implications for Earth's climate. In the deep ocean, tides supply much of ...the mechanical energy required to sustain mixing via the generation of internal waves, known as internal tides, whose fate-the relative importance of their local versus remote breaking into turbulence-remains uncertain. Here, we combine a semi-analytical model of internal tide generation with satellite and in situ measurements to show that from an energetic viewpoint, small-scale internal tides, hitherto overlooked, account for the bulk (>50%) of global internal tide generation, breaking and mixing. Furthermore, we unveil the pronounced geographical variations of their energy proportion, ignored by current parameterisations of mixing in climate-scale models. Based on these results, we propose a physically consistent, observationally supported approach to accurately represent the dissipation of small-scale internal tides and their induced mixing in climate-scale models.
A global database of infrared (IR) land surface emissivity is introduced to support more accurate retrievals of atmospheric properties such as temperature and moisture profiles from multispectral ...satellite radiance measurements. Emissivity is derived using input from the Moderate Resolution Imaging Spectroradiometer (MODIS) operational land surface emissivity product (MOD11). The baseline fit method, based on a conceptual model developed from laboratory measurements of surface emissivity, is applied to fill in the spectral gaps between the six emissivity wavelengths available in MOD11. The six available MOD11 wavelengths span only three spectral regions (3.8–4, 8.6, and 11–12μm), while the retrievals of atmospheric temperature and moisture from satellite IR sounder radiances require surface emissivity at higher spectral resolution. Emissivity in the database presented here is available globally at 10 wavelengths (3.6, 4.3, 5.0, 5.8, 7.6, 8.3, 9.3, 10.8, 12.1, and 14.3μm) with 0.05° spatial resolution. The wavelengths in the database were chosen as hinge points to capture as much of the shape of the higher-resolution emissivity spectra as possible between 3.6 and 14.3μm. The surface emissivity from this database is applied to the IR regression retrieval of atmospheric moisture profiles using radiances from MODIS, and improvement is shown over retrievals made with the typical assumption of constant emissivity.
Mixed‐layer depth (MLD) is often used in a mixed‐layer heat budget to relate air‐sea exchange to changes in the near‐surface ocean temperature. In this study, reanalysis heat flux products and ...profiles from a 15 year time series of high‐resolution, near‐repeat expendable bathythermograph/expendable conductivity‐temperature‐depth (XBT/XCTD) sampling in Drake Passage are used to examine the nature of MLD variations and their impact on a first‐order, one‐dimensional heat budget for the upper ocean in the regions north and south of the Polar Front. Results show that temperature and density criteria yield different MLD estimates, and that these estimates can be sensitive to the choice of threshold. The difficulty of defining MLD in low‐stratification regions, the large amplitude of wintertime MLD (up to 700 m in Drake Passage), and the natural small‐scale variability of the upper ocean result in considerable cast‐to‐cast variability in MLD, with changes of up to 200 m over 10 km horizontal distance. In contrast, the heat content over a fixed‐depth interval of the upper ocean shows greater cast‐to‐cast stability and clearly measures the ocean response to surface heat fluxes. In particular, an annual cycle in upper ocean heat content is in good agreement with the annual cycle in heat flux forcing, which explains ∼24% of the variance in heat content above 400 m depth north of the Polar Front and ∼63% of the variance in heat content south of the Polar Front.
Key Points
Mixed‐layer depth is definition‐dependent and varies over small spatial scales
Heat content is a more robust measure of ocean variability in Drake Passage
Seasonal cycles in heat content agree well with surface heat flux forcing
Iceberg C-18a (35×7×0.184
km) was studied repeatedly by five circumnavigational surveys in March-April 2009. During the period of the surveys, C-18a travelled 109 nautical miles in 23 days covering ...an area of 8.1×10
3
km
2. This iceberg was formed from iceberg C-18 (76×7
km) that originated from the Ross Ice Shelf in May, 2002. Ship-based measurements show that this iceberg produced fresh meltwater above the seasonal pycnocline that diluted and chilled the water it passed through from the surface to a depth of approximately 50
m (summer mixed layer). The surface meltwater effects were detectable as far away as 19
km and persisted for at least 10 days. We also found evidence that this iceberg was disrupting the Weddell Deep Water to depths up to 1500
m. If we include these deep effects through the water column, the estimate of ocean water altered by this single iceberg reaches 3×10
12
m
3 over 23 days. Chemical and biological effects were detected at the same space and time scales as the physical properties, with decreasing partial pressure of carbon dioxide (pCO
2) close to the iceberg and lower particle and chlorophyll concentration. Ten days after the passage of C-18a, chlorophyll-
a had increased by 15%. These results are consistent with alternative hypotheses regarding the role of icebergs as mediators of a localized geophysical disturbance (H
1) as well as promoters of chlorophyll-
a production (H
2).
Temperature and current measurements from two moorings onshore of the Celtic Sea shelf break, a well‐known hot spot for tidal energy conversion, show the impact of passing summer storms on the ...baroclinic wavefield. Wind‐driven vertical mixing changed stratification to permit an increased on‐shelf energy transport, and baroclinic energy in the semidiurnal band appeared at the moorings 1–4 days after the storm mixed the upper 50 m of the water column. The timing of the maximum in the baroclinic energy flux is consistent with the propagation of the semidiurnal internal tide from generation sites at the shelf break to the moorings 40 km away. Also, the ∼3 day duration of the peak in M2 baroclinic energy flux at the moorings corresponds to the restratification time scale following the first storm.
Key Points
Depth‐averaged N governs criticality for first mode internal waves on the shelf
Wind‐mixing causes 3 day subcritical window for IT to propagate on‐shelf
Peaks in M2 energy flux at shelf moorings are delayed relative to wind‐mixing
Abstract
A simple model of an internal wave advected by an oscillating barotropic flow suggests flaws in standard approaches to estimating properties of the internal tide. When the M
2
barotropic ...tidal current amplitude is of similar size to the phase speed of the M
2
baroclinic tide, spectral and harmonic analysis techniques lead to erroneous estimates of the amplitude, phase, and energy in the M
2
internal tide. In general, harmonic fits and bandpass or low-pass filters that attempt to isolate the lowest M
2
harmonic significantly underestimate the strength of M
2
baroclinic energy fluxes in shelf seas. Baroclinic energy flux estimates may show artificial spatial variability, giving the illusion of sources and sinks of energy where none are actually present. Analysis of previously published estimates of baroclinic energy fluxes in the Celtic Sea suggests this mechanism may lead to values being 25%–60% too low.
Subsurface melting of a free-floating Antarctic iceberg Stephenson, Gordon R.; Sprintall, Janet; Gille, Sarah T. ...
Deep-sea research. Part II, Topical studies in oceanography,
06/2011, Volume:
58, Issue:
11
Journal Article
Peer reviewed
Observations near a large tabular iceberg in the Weddell Sea in March and April 2009 show evidence that water from ice melting below the surface is dispersed in two distinct ways. Warm, salty ...anomalies in
T–
S diagrams suggest that water from the permanent thermocline is transported vertically as a result of turbulent entrainment of meltwater at the iceberg's base. Stepped profiles of temperature, salinity, and density in the seasonal thermocline are more characteristic of double-diffusive processes that transfer meltwater horizontally away from the vertical ice face. These processes contribute comparable amounts of meltwater–O(0.1
m
3) to the upper 200
m of a 1
m
2 water column–but only basal melting results in significant upwelling of water from below the Winter Water layer into the seasonal thermocline, suggesting that these two processes may have different effects on vertical nutrient transport near an iceberg.
Observations made near the Celtic Sea shelf edge are used to investigate the interaction between wind-generated near-inertial oscillations and the semidiurnal internal tide. Linear, baroclinic energy ...fluxes within the near-inertial (f) and semidiurnal (M sub(2)) wave bands are calculated from measurements of velocity and density structure at two moorings located 40 km from the internal tidal generation zone. Over the 2 week deployment period, the semidiurnal tide drove 28-48 W m super(-1) of energy directly on-shelf. Little spring-neap variability could be detected. Horizontal near-inertial energy fluxes were an order of magnitude weaker, but nonlinear interaction between the vertical shear of inertial oscillations and the vertical velocity associated with the semidiurnal internal tide led to a 25-43% increase in positive on-shelf energy flux. The phase relationship between f and M sub(2) determines whether this nonlinear interaction enhances or dampens the linear tidal component of the flux, and introduces a 2 day counter-clockwise beating to the energy transport. Two very clear contrasting regimes of (a) tidally and (b) inertially driven shear and energy flux are captured in the observations. Key Points * Internal wavefield energy fluxes in a stormy, stratified shelf sea * Inertial-tidal interaction enhances on-shelf internal wave energy transport * Two distinct shear and flux regimes: tidal and inertial
We describe the results from a spatial cyberinfrastructure developed to characterize the meltwater field around individual icebergs and integrate the results with regional- and global-scale data. ...During the course of the cyberinfrastructure development, it became clear that we were also building an integrated sampling planning capability across multidisciplinary teams that provided greater agility in allocating expedition resources resulting in new scientific insights. The cyberinfrastructure-enabled method is a complement to the conventional methods of hydrographic sampling in which the ship provides a static platform on a station-by-station basis. We adapted a sea-floor mapping method to more rapidly characterize the sea surface geophysically and biologically. By jointly analyzing the multisource, continuously sampled biological, chemical, and physical parameters, using Global Positioning System time as the data fusion key, this surface-mapping method enables us to examine the relationship between the meltwater field of the iceberg to the larger-scale marine ecosystem of the Southern Ocean. Through geospatial data fusion, we are able to combine very fine-scale maps of dynamic processes with more synoptic but lower-resolution data from satellite systems. Our results illustrate the importance of spatial cyberinfrastructure in the overall scientific enterprise and identify key interfaces and sources of error that require improved controls for the development of future Earth observing systems as we move into an era of peta- and exascale, data-intensive computing.
A 16 year record of expendable bathythermograph transects across Drake Passage is used to examine variability in upper‐ocean heat content that is not associated with the annual cycle. Links between ...upper‐ocean heat content and anomalous heat fluxes, winds, two large‐scale climate indices, and mesoscale eddies and meanders are examined. Results suggest that interannual variations in surface heat fluxes explain ∼5 to 10% of the variance in upper‐ocean heat content. Anomalous surface heat fluxes are linked to meridional wind anomalies upstream of Drake Passage, which in turn are linked to forcing by El Niño/Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). ENSO and SAM are correlated with upper‐ocean heat content at near‐zero lags, and statistically significant correlations occur at longer time lags as well. The impact of mesoscale eddies and meanders on upper‐ocean heat content is explored with the use of a tracked eddy database. An empirical relationship is constructed relating upper‐ocean heat content anomalies to eddy length scales and amplitudes. Eddies and meanders are estimated to account for more than one third of the nonannual cycle variance in Drake Passage upper‐ocean heat content.
Key Points
Upper‐ocean heat content in Drake Passage examined using 16‐year XBT timeseries.
Interannual variability of heat content linked to surface heat flux anomalies.
Eddies account for one third of variance in heat content anomalies above 400 m.
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