EFFECT OF REVISING WARM WATER EXEMPTION LATITUDES In Congressional hearings and in comments contained in the docket leading to the February 1984 final rule, there was much discussion about the water temperature at which exposure suits should be required, and where, geographically, that temperature was likely to occur. There was general consensus that temperatures of 60°F or below should be the criterion for requiring exposure suits, but experts disagreed as to where temperatures this low would occur for a significant portion of the year. This is not surprising, since the science of predictive oceanography is based on probability analysis of very sketchy data culled from a variety of sources, some of questionable accuracy. The 35 N and 35° S boundary lines correlate remarkably well with the 60° F isotherms (lines of equal water temperature) in the open ocean, using the best sources available. It is only near shore that the isotherms begin to behave erratically. In the North Atlantic, for example, the influence of the Gulf Stream pushes a finger of the 68° F isotherm as far north as 37° N in January, but a countercurrent inshore of the Gulf Stream causes cold water to flow south parallel to the coast, pulling the 60° F isotherm all the way down to 33 N at its intersection with the coast. However, the closer to shore a casualty occurs, the sooner rescue forces can reach the scene, and the less time a victim will spend immersed in the cold water. The isotherms along the coasts of North and South Carolina and Georgia are virtually parallel to the shoreline, and so close together that the water temperature increases 1° F for each additional 7-8 miles distance from the coast, out to about 80 miles. Furthermore, January is the only month in which any portion of the 60° F isotherm dips below 35° N latitude in the North Atlantic. In the North Pacific, cold water reaches somewhat further south in winter than in the Atlantic. The 60° F isotherm swings as low as 33° N in mid-ocean in January, the only time of year it moves that far south. Near Japan, Korea, and the China Sea, there is a similar, though not as pronounced, irregularity to that on the East Coast of the U. S. The near shore isotherms close to the Asian continent dip below the 35th parallel during five months of the year. More significant is a larger, "U" shaped dip in the isotherm off the coast of California. Because the isotherms there are not closely spaced lines parallel to the shore as they are in the Atlantic, it is actually more dangerous to operate without exposure suits 100 miles off the coast of southern California in January than at an equal distance off the coast of North Carolina. In the southern hemisphere the seasons are reversed. July and August are the most dangerous winter months, yet even during these months the 60° F isotherm does not move north of the 35° S parallel of latitude in the open ocean between South America and Australia. As in the northern hemisphere, there are convolutions of the isotherms near shore, but in August, the 60° F isotherm intersects the west coast of South America as far north as 13° South. Yet the isotherm runs almost due south, parallel to the coast, all the way down to 35° S latitude before swinging sharply out to sea. West of Australia, the 60° F isotherm swings north of 35° S latitude from June through October, but only by about 5°, and in an area not traversed by many U. S. vessels in normal trade. Then the isotherm returns to the 35th parallel and follows it to the East Coast of Africa. Near the coast of Africa the isotherm swings northward. In the South Atlantic, the 35° S latitude line is again an accurate approximation of the 60° F isotherm in the open ocean in winter. The isotherm turns sharply north before intersecting the west coast of Africa, behaving much like it does in the Pacific with respect to the West Coast of South America. It also swings slightly north as it intersects the East Coast of South America. In summary, the 35th parallel of latitude is an accurate approximation of the 60° F. isotherm in winter in both the northern and southern hemispheres in the open ocean. The final regulatory analysis prepared for the Coast Guard regulations is in Appendix III. It discusses several alternatives including a fixed geographic boundary, a constantly changing boundary based on actual water temperature at the time, and a seasonal boundary similar to that used in the load line regulations. In the end, it appeared that a single year-round geographic boundary was the most practical. That boundary was set at 35° N and 35° S latitudes, based on the best information available to us on water temperatures during the coldest months of the year in each hemisphere. Appendix IV contains maps of the East and West Coasts of the U. S., showing the location of the 31, 32°, and 35° N parallels of latitide to place the existing and proposed boundaries into perspective. Also included is a reduced copy of a U. S. Navy Hydrographic Office Sea Surface Temperature Chart for the Eastern Pacific Ocean in February. This chart was selected for inclusion because it shows the isotherms in the Atlantic and Pacific Oceans during the coldest month of the year in the northern hemisphere. The atlas containing this chart is large and does not reproduce well when reduced to 8 1/2 x 11 inch size, but is available in the DOT Library for reference. Tables 1 and 2 summarize the water temperatures and estimated survival times for an unprotected person in the coastal waters of the United States in the months of coldest water temperature. The coastal areas are where the greatest variation from the 60° F criterion occurs. For both the Atlantic coast and the Pacific coast, the figures are for 35° N, which is the "world-wide" warm water exemption line used in the Coast Guard regulations published in February 1984, 32 N which is the latitude in the Atlantic Ocean mandated in the 1984 Authorization Act, and 31° N which is the latitude required to be analyzed in this report. The reliability of the water temperature data used in tables 1 and 2 needs to be considered. The sources of the data do not include any information on how the temperatures may vary. In order to get some idea as to what kind of variations may occur, the water temperature reported in six of the casualties listed in Appendix II was compared to the average temperature from the atlases. (The seventh casualty in the appendix occurred on a sound which was not covered in the atlas.) Four of the actual temperatures were 1° F below the average temperature in the atlas. One was 5° F below the atlas temperature, and the sixth was 8° F below the atlas temperature. All of these casualties were in coastal waters where the greatest variability would be expected. information is not sufficient to make a general statement as to the reliability of the data, but it is clear that significant differences can occur. This |