Search Results

You are looking at 1 - 4 of 4 items for :

  • Author or Editor: Karen J. Heywood x
  • Journal of Atmospheric and Oceanic Technology x
  • Refine by Access: Content accessible to me x
Clear All Modify Search
Michael P. Meredith, John M. Vassie, Robert Spencer, and Karen J. Heywood

Abstract

Bottom pressure recorders (BPRs) have been deployed at Drake Passage (DP) to monitor changes in the volume transport of the Antarctic Circumpolar Current (ACC) through the passage. The use of inverted echo sounders (IESs) in assisting the interpretation of the BPR data is presented. The initial data processing of the IES data is outlined, and the accuracy of the data described. The most significant limitation on the application of the data is the presence of a sea-state bias with a root-mean-square value of around 0.4 ms. IES data are shown to perform well at determining whether individual changes in bottom pressure are due to changes in cross-passage-averaged barotropic transport or due to the effect of meanders, eddies, and/or lateral shifts of ACC fronts.

The conversion of acoustic travel time to more useful oceanographic parameters (dynamic height, baroclinic pressure, inverse-barometer-corrected sea level) is described. A method for improving the performance of very deep bottom pressure in monitoring ACC barotropic transport changes is described for the case where the BPR–IES instrumentation is deployed near an ACC front. Reasons for the inability of this method to improve the ability of shallower pressure records in monitoring the ACC are discussed, and suggestions for future refinements are outlined.

Full access
Paul A. Dodd, Martin R. Price, Karen J. Heywood, and Miles Pebody

Abstract

A compact water sampler rated to full ocean depth has been deployed from an autonomous underwater vehicle (AUV) to enable oceanographic tracer measurements. Techniques developed to allow the instrument to collect up to 49 samples of sufficient purity for tracer measurement without the need for extensive flushing have increased its sampling frequency, allowing a 200-mL seawater sample to be collected in 10 min. This is achieved by flushing the instrument and sample containers before deployment with a fluid of known properties that can be detected after recovery using salinity analysis. A deployment in which water samples were collected for oxygen isotope ratio analysis is presented as an example. Factors limiting the reliability of the instrument when deployed from an AUV are identified and procedures are developed to address critical problems.

Full access
Pierre Cauchy, Karen J. Heywood, Nathan D. Merchant, Bastien Y. Queste, and Pierre Testor

Abstract

Wind speed measurements are needed to understand ocean–atmosphere coupling processes and their effects on climate. Satellite observations provide sufficient spatial and temporal coverage but are lacking adequate calibration, while ship- and mooring-based observations are spatially limited and have technical shortcomings. However, wind-generated underwater noise can be used to measure wind speed, a method known as Weather Observations Through Ambient Noise (WOTAN). Here, we adapt the WOTAN technique for application to ocean gliders, enabling calibrated wind speed measurements to be combined with contemporaneous oceanographic profiles over extended spatial and temporal scales. We demonstrate the methodology in three glider surveys in the Mediterranean Sea during winter 2012/13. Wind speeds ranged from 2 to 21.5 m s−1, and the relationship to underwater ambient noise measured from the glider was quantified. A two-regime linear model is proposed, which validates a previous linear model for light winds (below 12 m s−1) and identifies a regime change in the noise generation mechanism at higher wind speeds. This proposed model improves on previous work by extending the validated model range to strong winds of up to 21.5 m s−1. The acquisition, data processing, and calibration steps are described. Future applications for glider-based wind speed observations and the development of a global wind speed estimation model are discussed.

Open access
Marina Frants, Gillian M. Damerell, Sarah T. Gille, Karen J. Heywood, Jennifer MacKinnon, and Janet Sprintall

Abstract

Finescale estimates of diapycnal diffusivity κ 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 κ 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.

Full access