Journal of Atmospheric and Oceanic Technology, Nov 1, 2013
ABSTRACT Finescale estimates of diapycnal diffusivity are computed from CTD and expendable CTD (X... more ABSTRACT Finescale estimates of diapycnal diffusivity are computed from CTD and expendable CTD (XCTD) data sampled in Drake Passage and in the eastern Pacific sector of the Southern Ocean and are compared against microstructure measurements from the same times and locations. The microstructure data show vertical diffusivities that are one-third to one-fifth as large over the smooth abyssal plain in the southeastern Pacific as they are in Drake Passage, where diffusivities are thought to be enhanced by the flow of the Antarctic Circumpolar Current over rough topography. Finescale methods based on vertical strain estimates are successful at capturing the spatial variability between the low-mixing regime in the southeastern Pacific and the high-mixing regime of Drake Passage. Thorpe-scale estimates for the same dataset fail to capture the differences between Drake Passage and eastern Pacific estimates. XCTD profiles have lower vertical resolution and higher noise levels after filtering than CTD profiles, resulting in XCTD estimates that are, on average, an order of magnitude higher than CTD estimates. Overall, microstructure diffusivity estimates are better matched by strain-based estimates than by estimates based on Thorpe scales, and CTD data appear to perform better than XCTD data. However, even the CTD-based strain diffusivity estimates can differ from microstructure diffusivities by nearly an order of magnitude, suggesting that density-based fine-structure methods of estimating mixing from CTD or XCTD data have real limitations in low-stratification regimes such as the Southern Ocean.
The mixed-layer heat balance in the Southern Ocean is examined by combining remotely sensed measu... more The mixed-layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from June 1, 2002 to May 31, 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer (AMSR-E) sea surface temperature measurements are available. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion and air-sea flux are largely consistent with the evolution of the mixed-layer temperature. The mixed-layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air-sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributors to the surface heat budget error are thought to be the air-sea heat fluxes, as only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. Sensitivity tests suggest that a proper representation of the mixed-layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed-layer depth.
Egu General Assembly Conference Abstracts, Apr 1, 2009
Mixing generated by mesoscale eddies is believed to play an important role in the transfer of wat... more Mixing generated by mesoscale eddies is believed to play an important role in the transfer of water masses and tracers across the Antarctic Circumpolar Current (ACC). While eddy activity is strong in regions with strong currents, such as the ACC, strong currents can also act as mixing barriers, leading to small effective eddy diffusivities. Here, we discuss the horizontal and vertical distributions of isopycnal eddy diffusivities (ΰ) estimated from the dispersion of numerical floats released in the core regions of the ACC in the Parallel Ocean Program (POP), and we test the relationship between these diffusivities, eddy kinetic energy (EKE) and mean flow. Estimated Lagrangian ΰ are horizontally highly variable and can be high in the core of the ACC and to the north of the ACC in regions where eddy kinetic energy is high. Elevated values are found near topographic features and close to the Brazil-Malvinas Confluence Zone and Agulhas Retroflection. At each depth interval, ΰ is correlated with the eddy velocity, and there is little evidence for any correlation with the strength of the mean flow. Vertically, cross-stream eddy length scales increase with depth, suggesting that eddy mixing may be enhanced at depth where PV gradients are weaker. This effect is masked by the strong decrease with depth of eddy velocities, leading to depth invariant cross-stream ΰ averaging 1300 ± 150 m2 s-1 around the Polar Frontal Zone. The results imply that in the core of the ACC, subsurface values of ΰ cannot simply be mapped from surface eddy kinetic energy in the core of the ACC and that the meridional variations of ΰ change with longitude.
Southern Ocean winds are distinctive both because they are strong, and because their standard dev... more Southern Ocean winds are distinctive both because they are strong, and because their standard deviation is comparable in magnitude to their mean. A number of schemes have been developed to map SeaWinds scatterometry, either from the wind fields alone or by blending scatterometry with numerical forecast winds. Results of this analysis show that regional means are not strongly dependent on the mapping methodology. In contrast variances of wind stress can vary by a factor of 3 depending on the spatial and temporal resolution used to represent the wind fields. In addition, the kurtosis, which indicates whether the winds have a Gaussian distribution, varies with mapping scheme. Spectral analysis is used to evaluate the time and length scales of dominant wind variability.
In southern Drake Passage, the Shackleton Fracture Zone (SFZ) marks a transition, with consistent... more In southern Drake Passage, the Shackleton Fracture Zone (SFZ) marks a transition, with consistently low chlorophyll-a (Chl-a) values to the west and high values in the Ona Basin to the east. Iron incubation experiments in the region suggest that the elevated Chl-a levels are the result of iron that is entrained into shelf waters along the Antarctic Peninsula and then advected offshore. However, the year-round persistence of the Chl-a gradient cannot be explained by a simple horizontal advection model, implying that additional processes, such as small-scale vertical mixing, may contribute to the process. We examine the spatial pattern of vertical mixing in the SFZ using shipboard observations of temperature and salinity profiles. Profiles were sampled with expendable CTD probes in winter 2006, and with rosette-mounted CTDs in winter 2006 and summer 2004. Diapycnal eddy diffusivities are computed from Thorpe-scale density overturns in the profiles, and vertical velocities are computed from the diffusivities to estimate the rate at which iron can be entrained into the mixed layer by small-scale mixing.
Bulletin of the American Meteorological Society, 2012
... Research Mark Bourassa 1 , Sarah Gille 2 , Cecilia Bitz 3 , David Carlson 4 , Ivana Cerovecki... more ... Research Mark Bourassa 1 , Sarah Gille 2 , Cecilia Bitz 3 , David Carlson 4 , Ivana Cerovecki 2 , Meghan Cronin 5 , Will ... 1999). The parameterization of König-Langlo and Augstein (1994) reproduced the observations with root-mean-square (RMS) deviations of less than 16 W m ...
Probability density functions (pdfs) of surface velocity and surface velocity gradients in the oc... more Probability density functions (pdfs) of surface velocity and surface velocity gradients in the ocean are calculated using altimetric data from the Topex/Poseidon satellite. These provide information about turbulence in a high-Reynolds-number geophysical flow. Both velocity pdfs and velocity gradient pdfs calculated over small regions are Gaussian but have more exponential shapes as the size of the region increases. We develop a simple explanation for the non-Gaussianity of velocity pdfs based on the inhomogeneity of eddy kinetic energy in the ocean.
Journal of Atmospheric and Oceanic Technology, Nov 1, 2013
ABSTRACT Finescale estimates of diapycnal diffusivity are computed from CTD and expendable CTD (X... more ABSTRACT Finescale estimates of diapycnal diffusivity are computed from CTD and expendable CTD (XCTD) data sampled in Drake Passage and in the eastern Pacific sector of the Southern Ocean and are compared against microstructure measurements from the same times and locations. The microstructure data show vertical diffusivities that are one-third to one-fifth as large over the smooth abyssal plain in the southeastern Pacific as they are in Drake Passage, where diffusivities are thought to be enhanced by the flow of the Antarctic Circumpolar Current over rough topography. Finescale methods based on vertical strain estimates are successful at capturing the spatial variability between the low-mixing regime in the southeastern Pacific and the high-mixing regime of Drake Passage. Thorpe-scale estimates for the same dataset fail to capture the differences between Drake Passage and eastern Pacific estimates. XCTD profiles have lower vertical resolution and higher noise levels after filtering than CTD profiles, resulting in XCTD estimates that are, on average, an order of magnitude higher than CTD estimates. Overall, microstructure diffusivity estimates are better matched by strain-based estimates than by estimates based on Thorpe scales, and CTD data appear to perform better than XCTD data. However, even the CTD-based strain diffusivity estimates can differ from microstructure diffusivities by nearly an order of magnitude, suggesting that density-based fine-structure methods of estimating mixing from CTD or XCTD data have real limitations in low-stratification regimes such as the Southern Ocean.
The mixed-layer heat balance in the Southern Ocean is examined by combining remotely sensed measu... more The mixed-layer heat balance in the Southern Ocean is examined by combining remotely sensed measurements and in situ observations from June 1, 2002 to May 31, 2006, coinciding with the period during which Advanced Microwave Scanning Radiometer (AMSR-E) sea surface temperature measurements are available. All terms in the heat budget are estimated directly from available data. The domain-averaged terms of oceanic heat advection, entrainment, diffusion and air-sea flux are largely consistent with the evolution of the mixed-layer temperature. The mixed-layer temperature undergoes a strong seasonal cycle, which is largely attributed to the air-sea heat fluxes. Entrainment plays a secondary role. Oceanic advection also experiences a seasonal cycle, although it is relatively weak. Most of the variations in the advection term come from the Ekman advection, in contrast with western boundary current regions where geostrophic advection controls the total advection. Substantial imbalances exist in the regional heat budgets, especially near the northern boundary of the Antarctic Circumpolar Current. The biggest contributors to the surface heat budget error are thought to be the air-sea heat fluxes, as only limited Southern Hemisphere data are available for the reanalysis products, and hence these fluxes have large uncertainties. Sensitivity tests suggest that a proper representation of the mixed-layer depth is important to close the budget. Salinity influences the stratification in the Southern Ocean; temperature alone provides an imperfect estimate of mixed-layer depth.
Egu General Assembly Conference Abstracts, Apr 1, 2009
Mixing generated by mesoscale eddies is believed to play an important role in the transfer of wat... more Mixing generated by mesoscale eddies is believed to play an important role in the transfer of water masses and tracers across the Antarctic Circumpolar Current (ACC). While eddy activity is strong in regions with strong currents, such as the ACC, strong currents can also act as mixing barriers, leading to small effective eddy diffusivities. Here, we discuss the horizontal and vertical distributions of isopycnal eddy diffusivities (ΰ) estimated from the dispersion of numerical floats released in the core regions of the ACC in the Parallel Ocean Program (POP), and we test the relationship between these diffusivities, eddy kinetic energy (EKE) and mean flow. Estimated Lagrangian ΰ are horizontally highly variable and can be high in the core of the ACC and to the north of the ACC in regions where eddy kinetic energy is high. Elevated values are found near topographic features and close to the Brazil-Malvinas Confluence Zone and Agulhas Retroflection. At each depth interval, ΰ is correlated with the eddy velocity, and there is little evidence for any correlation with the strength of the mean flow. Vertically, cross-stream eddy length scales increase with depth, suggesting that eddy mixing may be enhanced at depth where PV gradients are weaker. This effect is masked by the strong decrease with depth of eddy velocities, leading to depth invariant cross-stream ΰ averaging 1300 ± 150 m2 s-1 around the Polar Frontal Zone. The results imply that in the core of the ACC, subsurface values of ΰ cannot simply be mapped from surface eddy kinetic energy in the core of the ACC and that the meridional variations of ΰ change with longitude.
Southern Ocean winds are distinctive both because they are strong, and because their standard dev... more Southern Ocean winds are distinctive both because they are strong, and because their standard deviation is comparable in magnitude to their mean. A number of schemes have been developed to map SeaWinds scatterometry, either from the wind fields alone or by blending scatterometry with numerical forecast winds. Results of this analysis show that regional means are not strongly dependent on the mapping methodology. In contrast variances of wind stress can vary by a factor of 3 depending on the spatial and temporal resolution used to represent the wind fields. In addition, the kurtosis, which indicates whether the winds have a Gaussian distribution, varies with mapping scheme. Spectral analysis is used to evaluate the time and length scales of dominant wind variability.
In southern Drake Passage, the Shackleton Fracture Zone (SFZ) marks a transition, with consistent... more In southern Drake Passage, the Shackleton Fracture Zone (SFZ) marks a transition, with consistently low chlorophyll-a (Chl-a) values to the west and high values in the Ona Basin to the east. Iron incubation experiments in the region suggest that the elevated Chl-a levels are the result of iron that is entrained into shelf waters along the Antarctic Peninsula and then advected offshore. However, the year-round persistence of the Chl-a gradient cannot be explained by a simple horizontal advection model, implying that additional processes, such as small-scale vertical mixing, may contribute to the process. We examine the spatial pattern of vertical mixing in the SFZ using shipboard observations of temperature and salinity profiles. Profiles were sampled with expendable CTD probes in winter 2006, and with rosette-mounted CTDs in winter 2006 and summer 2004. Diapycnal eddy diffusivities are computed from Thorpe-scale density overturns in the profiles, and vertical velocities are computed from the diffusivities to estimate the rate at which iron can be entrained into the mixed layer by small-scale mixing.
Bulletin of the American Meteorological Society, 2012
... Research Mark Bourassa 1 , Sarah Gille 2 , Cecilia Bitz 3 , David Carlson 4 , Ivana Cerovecki... more ... Research Mark Bourassa 1 , Sarah Gille 2 , Cecilia Bitz 3 , David Carlson 4 , Ivana Cerovecki 2 , Meghan Cronin 5 , Will ... 1999). The parameterization of König-Langlo and Augstein (1994) reproduced the observations with root-mean-square (RMS) deviations of less than 16 W m ...
Probability density functions (pdfs) of surface velocity and surface velocity gradients in the oc... more Probability density functions (pdfs) of surface velocity and surface velocity gradients in the ocean are calculated using altimetric data from the Topex/Poseidon satellite. These provide information about turbulence in a high-Reynolds-number geophysical flow. Both velocity pdfs and velocity gradient pdfs calculated over small regions are Gaussian but have more exponential shapes as the size of the region increases. We develop a simple explanation for the non-Gaussianity of velocity pdfs based on the inhomogeneity of eddy kinetic energy in the ocean.
Uploads
Papers by Sarah Gille