OS42B-01
In situ observations of diurnal warming at the ocean surface
Observations of diurnal temperature variability at the ocean surface have been primarily available only from satellite SST retrievals themselves. Since most satellite observations revisit the same location only infrequently, determining how the ocean surface diurnal heating responds to variability in forcing (mainly insolation and wind speed) has been primarily addressed through theoretical modeling or extrapolation of results from in situ (buoy) observations measured 0.5 m to 1.5 m below the skin layer. Diurnal heating in the skin layer may be quite different than heating at 0.5 m as this layer responds very rapidly to changes in heat and momentum. The Explorer of the Seas, a cruise ship, makes weekly cruises on two alternating tracks through the Caribbean Sea. Measurements from the Marine Atmospheric Emitted Radiance Interferometer (M-AERI) carried on the Explorer of the Seas provide one of the few skin SST data sets, along with ancillary measurements necessary for diurnal investigations. Initial analyses show that the surface signature of diurnal warming in the skin layer is chiefly controlled by the wind speed. The daily peak in diurnal warming is directly related to the minimum wind speed during the day, causing the time of the peak to shift depending on when the minimum winds occur. Fluctuations in wind speed can result in multiple peaks in diurnal heating during a single afternoon. Wind speed is negatively lag-correlated with diurnal warming while insolation is positively lag-correlated. The maximum lag-correlation of wind speed (insolation) with diurnal warming is at a time lag of 30 (50) minutes. Several models of diurnal variability exist. A comparison of several models with each other reveals considerable differences in estimates of diurnal warming. Further validation of the models using M-AERI observed diurnal warming again reveals considerable differences in estimates of warming related to model forcing parameterizations.
OS42B-02
Some Processing and Dynamic-Range Issues in Side-Scan Sonar Work
Often side-scan sonar data are collected in such a way that they afford little opportunity to do more than simply display them as images. These images are often limited in dynamic range and stored only in an 8-bit tiff format of numbers representing less than true intensity values. Furthermore, there is little prior knowledge during a survey of the best range in which to set those eight bits. This can result in clipped strong targets and/or the depth of shadows so that the bits that can be recovered from the image are not fully representative of target or bottom backscatter strengths. Several top-of-the-line sonars do have a means of logging high-bit-rate digital data (sometimes only as an option), but only dedicated specialists pay much attention to such data, if they record them at all. Most users of side-scan sonars are interested only in the images. Discussed in this paper are issues related to storing and processing of high-bit-rate digital data to preserve their integrity for future enhanced, after- the-fact use and ability to recover actual backscatter strengths. This papers discusses issues in the use high-bit- rate, digital side-scan sonar data. This work was supported by the Office of Naval Research, Code 321OA, and the Naval Oceanographic Office, Mine Warfare Program.
OS42B-03
Suitability of multichannel seismic systems for imaging the internal structure of the water column
Seismic oceanography is slowly becoming a popular tool to investigate the internal structure of the water column. The principle of this technique is that the energy generated by seismic sources is partially reflected at the boundaries between water masses with contrasting physical properties. The reflected wavefield is recorded and processed to create continuous images of these boundaries. Since the pioneer work of Holbrook was published (Holbrook et al., 2003), numerous papers have appeared showing the potential of seismic oceanography to image the ocean's fine structure with unprecedented lateral resolution (10 m), the spatial coincidence of seismic reflectivity and temperature/salinity contrasts, and the correlation between seismic reflections and internal wave spectra. Despite the relatively large amount of recent work, little has been done concerning the existing issues to adapt seismic systems to oceanographic research. In this work we present a set of basic synthetic tests to illustrate the relative significance of different parameters for imaging the oceanic fine structure using seismic methods. The parameters considered include the frequency content and energy of the source wavelet, the ambient noise level, as well as the shooting rate, signal redundancy and fold. We show that powerful (>200 dB re 1 microPa), low-frequency (20-60 Hz) sources such as those commonly used in deep seismic soundings (DSS) are, purposelessly, well-suited to image also the oceans fine structure at all depth ranges. The reason for this is that, on one hand, the acoustic impedance (i.e., reflection coefficients) associated to intra-oceanic boundaries are two orders of magnitude smaller than those associated to geological boundaries (10-3/10-1), so it is crucial to use energetic sources to overcome ambient noise regardless of the target proximity. On the other hand, the limits between water layers, in contrast to the geological ones, do not show abrupt impedance contrasts but rather smooth gradients within boundary zones of several tens of meters, so the dominant wavelength of common DSS sources (about 25-75 m) is suitable to image them. In addition, we show that for a given system layout one can define the optimal shooting rate that gives the best possible signal-to-noise ratio by taking advantage of system redundancy but at the same time allowing background seismic noise due to repeated shooting to mitigate. Holbrook, W. S., P. Paramo, S. Pearse, and R. W. Schmitt (2003), Thermohaline fine structure in an oceanographic front from seismic reflection profiling, Science, 301, 821- 824.
OS42B-04
Correlations Between Sea-Surface Salinity Tendencies and Freshwater Fluxes in the Pacific Ocean
Temporal changes in sea-surface salinity (SSS) from 21 years of a high resolution model integration of the Pacific Ocean are correlated with the freshwater flux used to force the integration. The correlations are calculated on a 1°x1° grid, and on a monthly scale to assess the possibility of deducing evaporation minus precipitation (E-P) fields from the salinity measurements to be taken by NASA's Aquarius mission. Correlations between the mean seasonal E-P fields and mean seasonal SSS temporal tendencies tend to be zonally- oriented, and are highest where the local precipitation is also relatively high. This result is consistent with Delcroix et al.'s (1996) previous work for large spatial scales in the tropical Pacific. However, the model's fine resolution also shows the result to be true on finer scales and areas not considered by that study. Nonseasonal correlations are highest in the mid-latitudes with relatively smaller correlations in the tropics.