ionospheric phase velocity, J. Geophys. Res., 97, pp. 19,373-19,380,1992. Additional Testing Needed to Confirm. Warming of Water off New Jersey Shore.
Eos,Vol. 80, No. 15, April 13,1999 rapid than heretofore appreciated. Web access: A variety of SuperDARN products, including near real-time maps of the global convection pattern, can be viewed at the Johns Hopkins University/Applied Physics Laboratory homepage (http://superdarn. jhuapl.edu/). Acknowledgments This work was supported by the National Science Foundation (NSF) Office of Polar Programs under NSF grant 0PP-9613847 and the Division of Aeronomy under NSF grant ATM-9612834. Support for the SuperDARN radars used in this study was provided by funding agencies in Canada, France, the United Kingdom, and the United States.
Authors J. M. Ruohoniemi and R.A. Greenwald Applied Physics Laboratory, Johns Hopkins University Laurel, Md., USA References Baker, K. B.,and S.Wing, A new magnetic coordinate system for conjugate studies of high latitudes, J. Geophys. Res., 94, pp. 9139-9143,1989. Greenwald, RA., et al., DARN/SuperDARN: A global view of high-latitude convection,Space Sci. Rev., 71, pp. 763-796,1995. Lockwood, M.,A. Pvan Eyken, B. J. I. Bromage, D. M. Willis,and S.WH.Cowley,Eastward propagation of a plasma enhancement following a southward turning of the interplanetary magnetic field, Geo phys. Res. Lett., 13,72-75,1986. Ridley, A. J., G. Lu, C. R. Clauer, and V 0. Papitashvili, Ionospheric convection during nonsteady inter
Additional Testing Needed to Confirm Warming of Water off New Jersey Shore PAGES 173,177 The article by Fisher et al. presents intriguing results. Their analysis of thermal data from boreholes in the sediment suggests large tem perature variability (of order 6-10°C) primarily over the last 60 years with maximum tempera tures approximately 20 years ago at a site (#903) on the continental slope off the south ern Middle Atlantic Bight in approximately 450 m depth. Similar techniques suggest no such temperature change at a site (#902) 4.4 km away in 802 m of water. The waters normally lying along the conti nental slope of the Middle Atlantic Bight and regions to the north are known as slope waters. These waters extend seaward to the northern edge of the Gulf Stream. Temperatures are typically 6-7°C at depths of 450 m or so, increasing almost linearly towards 10-15°C in shallower waters of 200 m [Stommel, 1965]. Below 700 m, temperatures are in the 4-5°C range. Similar temperatures were shown by Sverdrup et al. [1942] at a sta tion along the slope off Chesapeake Bay in 1932. These same authors note the large vari ability in temperatures at around 400 m and attribute it to slope-type waters being inter spersed with short periods of North Atlantic central (Gulf Stream) waters produced by the presence of eddies or shelfward displace ments in the stream itself. As reported by Fisher and his coauthors, in situ observations of near-bottom water tem peratures in the immediate vicinity of the two borehole sites are very limited. A search of a temperature database assembled at the Bedford Institute of Oceanography (BIO) reveals that less than 40 stations are occupied within 10 km of the sites and none prior to 1970. Less than 10 of these had measure ments extending to the depths where the boreholes were drilled. The database con tains all available historical data up to and
including 1998 and is principally derived from holdings at Canada's national oceanographic archive, the Marine Environmental Data Service (MEDS),in Ottawa. It includes the National Oceanic and Atmospheric Administration data, which are periodically downloaded by MEDS. Establishing long-term temperature trends in the waters along the slope is further com plicated by the presence of the intermittent
planetary magnetic field conditions, J. Geophys. Res., 102,pp. 14,563-14,579,1997. Ridley A. J., G. Lu, C. R. Clauer, and V 0. Papitashvili, A statistical study of the ionospheric convection response to changing interplanetary magnetic field conditions using the assimilative mapping of ionospheric electrodynamics technique,./ Geophys. Res., 103, pp. 40234039,1998. Ruohoniemi,J. M.,and R.A.Greenwald,The response of high-latitude convection to a sudden southward IMF turning, Geophys. Res. Lett., 25, pp. 2913-2916,1998. Ruohoniemi, J. M., and K. B. Baker, Large-scale imag ing of high-latitude convection with Super Dual Auroral Radar Network HF radar observations, J. Geophys. Res., 103, pp. 20,797-20,811,1998. Saunders, M. A., M. PFreeman, D. J. Southwood, S.W H. Cowley, M. Lockwood, J. C. Samson, C. J. Farrugia, and T. J. Hughes, Dayside ionospheric convection changes in response to long-period interplanetary magnetic field oscillations: Determination of the ionospheric phase velocity, J. Geophys. Res., 97, pp. 19,373-19,380,1992.
thermal influence of Gulf Stream eddies. In spite of this latter difficulty, low-frequency temperature changes have been observed along the continental slope off the eastern seaboard of the United States and Canada. For example, colder slope water of Labrador Current origin replaced the more typical slope water along the continental slope from the Middle Atlantic Bight to the Scotian Shelf during the 1960s [Worthington, 1964]Petrie and Drinkwater, 1993]. Maximum tempera ture changes were in the 100-300-m depth range with declines of approximately 4-8°C, but similar changes of lower amplitude were also observed at deeper depths.
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Fig. 1. Monthly mean temperatures along the continental shelf of the Middle Atlantic Bight at 450 m (top panel) and at 800 m (bottom panel).
Eos,Vol. 80, No. 15, April 13,1999 By the early 1970s, the Labrador slope water retreated northward and was replaced by warmer waters that remained along the slope from Cape Hatteras to the Scotian Shelf through into the 1990s. Interestingly, Labrador slope water began to penetrate southward again in the autumn of 1997 and by the late spring of 1998 had reached the continental slope off the northern Middle Atlantic Bight [Drinkwater et al, 1998a]. Since large, long-term temperature changes of the type proposed by Fisher et al. typically have rather long horizontal scales in at least the along-shore direction, I examined all avail able temperature data at 450 m and 800 m (±50 m) within an area extending from south of Martha's Vineyard (40° north latitude, 70° west longitude) to east of Chesapeake Bay (38° north latitude, 74° west longitude) from the BIO database. Temperatures at 450 m ranged from 5 to 9°C between 1910 and 1997 (Figure 1). (The quality of the early measure ments was questioned by Fisher and his col leagues, but I have found from an extensive examination of data throughout the north west Atlantic that there is usually consistent agreement between these measurements, gen erally taken with reversing thermometers, and present day observations.) Temperatures in the pre-1970s varied great ly but were generally lower on average than those since 1970 by approximately 1°C. The cool period of the 1960s is clearly evident in the temperature records. No observational evidence exists of a rapid decrease in tem perature in recent years. The observations therefore do not support the large tempera ture changes proposed by Fisher and his colleagues. At 800 m, there are fewer data. They show temperatures in the 4-5°C range with a pattern of long-term variability gener ally similar to that observed at 450 m, but of lower amplitude. Although no evidence exists of a broadscale temperature change of the magnitude suggested by the borehole results at #903, the possibility that the event was indeed local cannot be ruled out. Fisher and his col leagues suggested that the changes might be due to a vertical shift in water masses. Given the long-term mean temperature gradient, a vertical displacement downwards of order 200 m would be required to explain the pro posed temperature change at #903 at 450 m.
This would also imply a 1-2°C temperature increase at 800 m (site #902), assuming the present vertical thermal gradient was con served. No such temperature change at #902 is suggested from the borehole analysis but the small amplitude may be within the uncer tainty of their methods. Another possibility is that the temperature changes could have been caused by a hori zontal shift in water masses such that the slope waters normally occupying the Middle Atlantic Bight slope region were replaced by North Atlantic central (including Gulf Stream) waters. The latter are certainly warm enough to account for the proposed 6-10°C tempera ture change. Because the depth of the Gulf Stream and its associated eddies are typically 1000 m or greater, such a shift would also be expected to lead to a temperature change, but of smaller magnitude, at 800 m on the slope. However, the borehole results indicate no measurable temperature change at the deeper site. If the proposed near-bottom tem perature change was due to a horizontal shift in the Gulf Stream, one would also expect the Stream to have been closer to the shelf in the 1970s, during the time of predicted peak tem peratures, than in the 1980s and 1990s when temperatures were lower. The Gulf Stream location, as measured by the sea surface tem perature, was in fact further offshore in the early 1970s and closer to the Shelf during the 1980s and early 1990s [Taylor, 1995; Drinkwater et al, 1998b]. It therefore seems unlikely that a horizontal shift in the Gulf Stream would explain the borehole results. Examination of the temperature measure ments has, as Fisher and his coauthors duly note, been unable to confirm the near-bottom temperature trends at 450 m suggested by their borehole analysis. The observations do provide some evidence of higher than normal temperatures during the 1970s (the approxi mate time of the proposed peak temperature) but it is of much smaller magnitude than that suggested by the borehole measurements. Also, there is no observational evidence for a recent temperature decline as proposed by Fisher et al. Comparison of the temperatures from the borehole analysis and direct meas urements are tempered by a lack of observa tions, especially in the vicinity of the bore holes and in the pre-1950s period.
Landsat 7 Launch Will Extend Long-Term Earth Remote Sensing Mission PAGE 167 The first pictures of the Earth from deep space that the Apollo 8 spacecraft beamed down in 1968 changed people's perspective
of the planet. So, too, have the images relayed from Landsat satellites. This series of U.S. satellites, the first of which was launched 27 years ago in 1972, has provided the longest continual remote
The possibility of the proposed temperature change being of a local nature cannot be ruled out entirely but it appears unlikely, based on previous observations that such large temperature changes lasting over rela tively long periods usually have extensive spa tial scales. If the temperature change is local, the lack of a temperature response in the borehole analysis at 800 m only a little over 4 km away from the site of the large tempera ture change is puzzling. Temperature changes caused by either a vertical or a horizontal shift in water masses would imply the likeli hood of a small but measurable change at 800 m, a result not evident from the borehole analysis. While further analysis of available temperature measurements may help to demonstrate or refute the hypothesis, addi tional borehole analysis at one or two more sites at different locations along the slope and one site at shallower depths may help to bet ter establish the spatial extent of the thermal event suggested by Fisher and his colleagues. Author K. FDrinkwater Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada References Drinkwater, K. F, D. B. Mountain, and A. Herman, Recent changes in the hydrography of the Scotian Shelf and Gulf of Maine—a return to conditions of the 1960s? 16 pp., North Atlantic Fisheries Organi zation (Dartmouth, Nova Scotia), Scientific Coun cil Research Doc. 98/37,1998a. Drinkwater, K. F, E. Colbourne, and D. Gilbert, Overview of environmental conditions in the North west Atlantic in 1997,8\ pp., North Atlantic Fish eries Organization (Dartmouth, Nova Scotia), Scientific Council Research Doc. 99/38,1998b. Petrie, B. and K. Drinkwater,Temperature and salinity variability on the Scotian Shelf and in the Gulf of Maine 1945-1990,./ Geophys. Res., 98,pp. 20,07920,089,1993. Stommel,H., The Gulf Stream: A physical and dynam ical description, 2nd. edit., 248 pp., Univ. of Calif. Press, Berkeley, 1965. Sverdrup, H. U., M.W Johnson, and R. H. Fleming, The oceans: Their physics, chemistry and general biology, 1087 pp., Prentice-Hall, Englewood Cliffs, N.J., 1942. Taylor, A. H., North-South shifts of the Gulf Stream and their climatic connection with the abundance of zooplankton in the UK and its surrounding seas, ICES J. Mar. Sci, 52, pp. 711-721,1995. Worthington,L.Y, Anomalous conditions in the Slope Water area in 1959,./ Fish. Res. Board Can., 21, pp. 327-333,1964.
sensing record of the Earth's continental surfaces and near coastal regions available for civilian usage. A multispectral scanner on each of the first three Landsats, and a thematic mapper onboard numbers 4 and 5 have, with focussed glances, captured and sent back to Earth more than 4 million scenes of the planet. Bits of data and the resulting imagery have helped researchers to monitor condi-
