Naval Applications

Antisubmarine Warfare

Oceanographic information is very important for successful Antisubmarine Warfare (ASW). It begins with a climatological and geological data base that includes high-resolution, four-dimensional digital models of temperature and salinity, current and ambient noise information, bathymetry, and magnetic and ambient acoustic characteristics. The Navy, along with civilian agencies, provides and continually updates the necessary information. This information generates the background environmental description for military operations and the basis for global and regional ocean forecasting models.

Remotely-sensed data (especially satellite information) is extremely important for producing an accurate model of the ocean. This type of data, along with modeling, was the basis of the Northwest Atlantic Regional Energetics Experiment (REX). REX developed several fundamental conclusions concerning the dynamics and energetics of the Gulf Stream, specifically:

• Mesoscale variability of the Gulf Stream is at a maximum just west of the New England seamount chain, eastward over the deep Sohm Abyssal Plain, and ending at the Newfoundland Ridge.

• A two-gyre system is exhibited in the recirculation of the annual mean Gulf Stream, due to the seamount chain.

• The Navy's GEOSAT-ERM monitoring of the energy budget of the Gulf Stream verified preexisting energy models based on years of in situ temperature measurements.

• Comparisons of numerical models, satellite imagery, and in situ observations confirm the importance of the North Atlantic's Deep Western Boundary Current in the separation and eastward penetration of the Gulf Stream.

• The vertical structure of ocean density within the Gulf Steam and its rings allows for conversion of surface satellite observations. Such observations include surface topography and inferred vertical profiles of temperature, salinity, and sound speed.

Current day satellite images, data collected from ships and buoys, and climatological data are combined to give the best available three-dimensional view of the ocean. The result is a gridded data field for an area, large or small, that is of interest to the Navy. This data field becomes the input for numerical forecasts of oceanographic and acoustic conditions. The acoustic models form the basis for predicting the performance of any ASW system.

In late 1986,the Gulfcast model was introduced by scientists at Harvard University. Gulfcast is a real time application of the Descriptive-Prediction System to the Gulf Stream Meander and Ring Region. This system is used to produce 1-week dynamical forecasts of the evolving Gulf Stream frontal and ring locations, including ring formation, interaction, and absorption back into the current. In January 1989, the operation was moved to NOAA.

The real-time network used for Gulfcast operations at Harvard was composed of:

• Satellite infrared imagery

• GEOSAT altimetry

• Weekly survey flights using the air-deployed expendable bathythermograph (AXBT).

The combination of the above elements provided a very useful observational tool with very high resolution. The satellite infrared imagery provided synoptic spatial coverage of the Gulf Stream and its rings. The GEOSAT altimeter was used to observe the distortion of the sea-surface topography caused by mesoscale circulation features. The AXBT surveys provided subsurface in situ temperature readings. These readings were then used to fill in the gaps in the remotely sensed data.

New forecasts of the Gulf Stream and ring locations were generated each week by updating the previous week's forecast with the new data set. The Naval Oceaanographic Office also does an analysis of available data from satellites, ships, and aircraft on a daily basis to produce Ocean Feature Analysis charts for the Gulf Stream region and the Gulf of Mexico. The accuracy to which any of the individual data sets can locate the Gulf Stream is about 15 kilometers.

These forecasts are important to the Navy. The 4-dimensional structure of the ocean helps determine how sound will propagate in the water. The Gulf Stream exhibits many properties which may result in such acoustic traits as convergence zone displacement or shadow zones. These phenomenon may prevent the detection of an enemy submarine, or may result in a U.S. submarine being detected.