Acoustic Thermometry for Arctic Ocean Climate

Peter N. Mikhalevsky Robin Muench

Science Applications International Corporation McLean, VA

Arthur B. Baggeroer

Massachusetts Institute of Technology Cambridge, MA

Abstract

Several climatc models suggest that the Arctic Ocean may be one of the more sensitive indicators of global climate change. In addition to changes in the Arctic Ocean temperature, the ice pack will also respond to these changes; for example, its mean thickness, roughness and the percentage of open water will all be modulated. Low frequency acoustic propagation in the Arctic is strongly influenced hy the ice pack properties since the SOFAR axis is at or near the surface. In addition to the travel time and phase changes that could be observed due to changes in the Arctic Ocean temperature; phase and amplitude coherence, travel times, transmission losses, modal coupling are just a few of the observables which would respond to changes in the pack ice. Current understanding as well as future work and possibilities for acoustic thermometry of Arctic Ocean climate are reviewed.

1. Introduction

The Arctic Ocean can be expected to react to long-term global climate change with changes in temperature, salinity, and ice cover. The Arctic Ocean is an isolated mediterranean basin overlain by a seasonally varying ice cover, whose warm deep water obtains its heat from the Atlantic Ocean via the West Spitsbergen Current in Fram Strait. Global models show increased atmospheric warming at high latitudes; however, Arctic Ocean response is far less pronounced [Manabe et. al., (1990)] and may in fact reflect primarily inflow of warmed Atlantic Water. Other models predict warming [Washington and Meehl (1984), Schlesinger and Mitchell (1987), and Mitchell (1989)l. These model predictions, however, are at odds with observed surface and tropospheric measurements for the 1980's which show less warming at the higher latitudes [Wood (19!90)]. In fact,

some modelled scenarios postulate high latitude cooling, increased precipitation in the form of snow, and increased sea ice thickness and extent, as a precursor to eventual warming [Wood (1990)) Recent coupled ocean and atmospheric models that include a gradual increase of atmospheric C02show that atmospheric warming is still greater, but less so, at the higher latitudes due to greater take up of the heat by the ocean, and the consequent thinning and reduction of sea ice cover [Washington and Meehl (1989)l. A recent evaluation of 15-year records of satellite-derived ice cover information suggests no significant interannual trend in ice cover [Wood (1990)]. In contrast, NASA data [Gloersen arid Carnpbell (1988)l suggest that over recent years sea ice extent is decreasing. This latter data indicated approximately 14% of the area within Arctic ice packs was open water during maximum ice extent. Using the correspondence between surface temperature patterns and ice cover, Gloersen and Campbell suggest that "climate changes in the global average temperature can be detected by observing changes in sea ice extent." Thus acoustic measurements will provide additional observational data on the sea ice that would complement and help to resolve some of these inconsistencies. These conflicting scenarios of higher latitude warming and cooling, as well as fundamental debate concerning the Global Warming issue [Lindzerz (1990)], only highlight the need for reliable data, particularly from this critical part of the world. Among the findings of the Workshop on Arctic Systems Science [Moritz, et al. (1990)] that are particularly relevant to this research are the following: "(1) Despite its relatively small size, the Arctic Ocean and adjacent seas exert a strong influence on the earth's climatic state. Deep water production in this region is a major driver of the global thermohaline circulation, and the ice-cover has an important effect on the planetary albedo. Feedbacks among ice-aient, snow cover, surface albedo and the global heat budget are crucial determinants of the global climate.; and (2) Simulations with global climate models

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portray an arctic marine environment in which global warming would be amplified, due to a combination of effects including sea ice retreat and the stable atmospheric stratification. This potential polar amplification of global change singles out the Arctic as a sensitive and vulnerable region. In view of the importance and inadequate parameterization of key arctic processes and variables in global climate models, it is essential to accelerate research on arctic clouds and radiation, and to improve the formulations of surface pmesses governing sea ice and snow cover. In all of these areas, researrh is hampered by the lack of observations." (Italics added.)

2. Acoustic Monitoring

A summary of amplitude and phase perturbations due to hypothesized changes in Arctic Ocean temperature and ice properties as well as other "signW some of which are the unwanted variability is given in Table 1. These calculations were made for a nominal 2700 km path across the Arctic Ocean. The warming of .007 degrees C/year is based upon model calculations done by Manabe. The estimates of phase variation with ice cover were obtained using the SAFARI code [Schmidt and Jensen (1985)l and additionally on a formulation based on Brekovskikh (1980) which was extended to include compressional and shear wave losses following M c C m m o n and McDaniel (1989, although recent calculations indicate that these losses have a small effect on phase change. Intensity variations were computed using the FFP program [DiNapoli (1971)], calculated using theoretical formulations of Brekovskikh and Lysanov (1982), and compared to empirical results of Marsh (1963). These calculations which are based on the correlation of ice roughness, and hence transmission loss, with ice thickness show a 3 dB per year decrease in transmission loss for a thinning of 10 cm year in average ice thickness [Dinapoli, 19901. Variations in acoustic phase and travel time due to mesoscale effects were computed based upon their characteristics in the Arctic as reported by Newton er al. (1974), Hunkins (1974), Manley and Hunkins (1985), and D ' A s m (1988). Internal waves are not expected to effect the acoustic measurement because of the low energy levels in the Arctic Ocean [Levine et. al. (1985)l. Short-period and annual phase variations due to mesoscale dynamics are estimated to be. 0.1 and 0.01-0.03 cycles, respectively. These estimates show that the signal to noise ratio for these phase observations due to changes in ice thickness is of the order of 10 dB after one year of observations. If one were to use a smaller rate of change for the ice thinning, say 10 cm per U) years, then the S N R would be 7 dB after one year of observations. Table 1 reflects our current understanding. The phase changes shown in Table 1 are within the measurement accuracy

that has already been demonstrated in the Arctic [Mikhalmsky, 19811. Long coherent integrations up to 4OOO secs were accomplished with phase resolved to hundredths of a cycle at 30 Hz.

Planned work will include implementing existing models for the ocean, ice and acoustic propagation, and exercising these models to obtain the estimated acoustic signature in response to climate change. It is natural to model the water volume with a mean sound-speed field, added fluctuations due to mesoscale processes, and further layer-scale perturbations due to warming. The mean field is fairly well determined from knowledge of the mean temperature and water mass structure of the Arctic Basin available from historical data. There is great uncertainty in the expected structure of warming signatures; however, some simple parameterization is useful to estimate the acoustic effects. For example Fig. 1 shows the travel time SNR by mode number for an hypothesized warming of the Atlantic inflow water at 250-300m depth. The noise due to sound speed Perturbations caused by mesoscale fluctuations is assumed to be exponentially decaying from the surface. The climate signal could also include a surface warming effect similar to the temperate seas, as a result of increased atmospheric warming and the presence of open water from leads. This would result in a signal structure that is also an exponential, decaying from the surface. Because of the upward refracting half channel propagation in the Arctic this signal has a stronger effect on travel time changes than in the temperate oceans [Munk, 19931. Fig. 1 shows a modal preference that is important to understand in order to design the appropriate systems for the acoustic monitoring. The analysis techniques developed to compute travel time SNR's will be applied to better estimates of the climate signal and noise processes as they are determined from modeling and experiment. One of the stronger signals affecting the acoustic propagation is that due to the seasonal change in the percentage of open water. This impacts both transmission loss and travel time (see Table 1). These observations would be correlated with satellite data.

In order to characterize the reflection from sea ice we intend to exercise our existing full-wave propagation codes and systematically compare the predicted results for amplitude and phase as a function of frequency, grazing angle (15 degrees and less), and the elastic properties of sea ice. The approaches include building on recent work by Dozier et al. (1991), fiperman and Schmidt (1989), and Fricke (1990). The Kuperman and Schmidt approach can handle full 3D scattering with the boundary characterized in terms of a roughness spectrum and is efficiently implemented within the SAFARI propagation package.

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The Fricke approach solves the full Eulerian elastodynamic equations for wave propagation in complex heterogeneous media; given an input of a realization of the complex under-ice roughness structure, the Fricke code solves for the complete scattered field. Running the code in Monte Carlo fashion for multiple roughness realizations will then yield the variation of the average complex reflection coefficient as a function of ice roughness and thickness statistics. Application of the Fricke code to actual underice roughness measurements from submarine cruises has begun. These complex reflection coefficients will be applied in simpler ray theory models to solve for trans-basin results. The effects of the random distribution of ice draft as reported and modeled by Wadhams (1980), and Hibler et al. (1972) will be included. Because modal dispersion is strongly effected by roughness this too will be investigated as a possible observable that could be correlated with ice property changes [Schmidt, 19931.

3. Conclusions

Preliminary modeling and analysis of both Arctic Ocean climate changes and consequent acoustic modulation are promising and warrant further feasibility study and analysis. Such a program is currently underway as part of the Acoustic Thermometry of Ocean Climate (ATOC) program. The ATOC/Arctic program is international in scope with significant participation by European and Russian researchers. Possible pilot measurements are being proposed in the Arctic in the spring of 1994, as well as measurements in conjunction with the Nansen Drift program 1994-1996.

References

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Brekhovskikh, L.M. and Y. Lysanov, Fundamentals of Ocean Acoustics, Springer-Verlag, New York, 1982, Ch. 9.

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DiNapoli, F.R., private communication. Feb. 1990.

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Moritz, R. E., et al., eds., Arctic Svstem Science: Ocean- m D h e r e Ice Interactions. Report of a Workshop held at the U.C.LA. Lake Arrowhead Conference Center March l2-16,1990 (Joint Oceanographic Institutions Inc., Washington, DC, December 1990).

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Table 1. ThC effects on acoustic phase and amplitude of important Arcti Ocun climatic envimmentd p b e n ~ m e ~ .

.-

0.1

l! 1 z I

0.01 I , 6 10 16 20 0

yo6Mmkr Figure 1. The signal to noise ratio of trml time c h m p due to influx of warming Atlantic water (.U m/s incruse in t n m l time) at a depth of 250 m versus travel time perturbations due to mesacale sound speed changes, plotted as a function of acoustic mode number.

Wood, F.B., Monitoring Global Climate Change: The Case of Greenhouse Warming, Bull. of the Amer. Meteor. SOC., 71(1), p. 42-52, 1990.

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