Miles McPhee and the MaudNESS Science Group*

The Maud Rise Nonlinear Equation of State Study (MaudNESS), in the eastern Weddell Sea, Antarctica

Acknowledgements

The MaudNESS program is supported by the National Science Foundation, Office of Polar Programs, Antarctic Ocean and Climate Systems. We are indebted to the Raytheon Polar Services group and the officers and crew of the Nathaniel B. Palmer for exemplary support throughout.

Synopsis A study of upper ocean mixing in a weakly stratified environment under thin sea ice was executed in late austral winter, 2005, from the research icebreaker, Nathaniel B Palmer, in the vicinity of the Maud Rise (MR)seamount. Our goal was to gauge the impact of nonlinear equation-of-state characteristics on the surface energy balance and possible midwinter polynya formation. The experiment comprised primarily three elements: •Phase 1: A rapid, shallow (mostly ~700 m) CTD survey (105 stations), emphasizing the margins of MR. •Phase 2: Two ice drift stations with instrument systems deployed on the drifting ice and from the moored ship. •Phase 3: A series of short, repeated drifts ranging in duration from 10 to 39 hours, with instrumentation deployed exclusively from the Palmer.

Field Program Summary

• Phase 1 CTD survey: 105 stations, mostly to 700 m, some deeper• Phase 2 Ice station drifts: Drift P2D1: 35 hours, Drift P2D2: 103 h• Phase 3 Passive ship drifts: 11 total ranging from 10 to 39 h. • Ice Buoy Deployment: 4 ea• Atmospheric lead experiment using “kite-sondes.”• Ad hoc “MaudBERG” experiment to measure upper ocean response in the wake of a small drifting iceberg• Float deployment: 13 AWI rafos floats, 6 UW lagrangian floats

• We did NOT encounter widespread deep convection during August, 2005 in the Weddell Sea• BUT it was close: 5-10 cm of additional ice growth (in the absence of other factors) would have made the mixed layer in the SW margin of Maud Rise as dense as the underlying warm, deep water.

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MaudNESS Sta 91 was similar to one of the least stable ANZFLUX 94 stations (#46). Phase 3 was thus concentrated in this region. The density contrast at the base of the mixed layer was < 0.02 km m-3.

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Locations of Phase 1 survey stations plotted on Maud Rise bathymetry. Black curves emanating from squares indicate Phase 2 and Phase 3 drift trajectories.

Phase 1: Rapid, shallow CTD survey over Maud Rise

Phase 1 was initially designed to cross MR, then concentrate on the ENE sectors, where previous large polynyas had first appeared. We found that region to be relatively stable compared with previous winter data, and ended up concentrating on the SW sector, where the water column was less stable as indicated in the lower panel.

Phase 2: Ship-supported ice drift station

Currents measured relative to drifting ice at about 2.85 m below the interface. 1-h averages from Sontek ADV

Elevation of mixed layer temperature above freezing.

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Phase 2 was designed to characterize air-sea-ice interaction in the absence ofstrong convection near the center of the MR Taylor cap. During P2, several instrument systems (2 turbulence masts, VMP, met tower, ice studies) were deployed on the ice adjacent to the NBP. The first station drifted rapidly to the NE and broke up as we approached the NE flank. The 2nd station was deployed near the western margin, in a region of strong tidal activity, which eventually led to its demise as well.Turbulence studies from Phase 2, Drift 1 (shown below) indicate that the ice underside was very smooth, and that heat exchange between the ice and ocean (characterized by cH) was similar to previous studies in both the Antarctic and Arctic.

Phase 3: Short drift stations deployed from the NBP

Phase 3 comprised a series of 11 short drift stations with instrumentation deployed exclusively from the NBP. Juggling the watch circles of 4 wires hanging from the ship (2 profilers and 2 turbulence masts) along with occasional encroachment of drifting ice, presented a significant challenge, met by highly skilled ship handling and constant vigilance. During several drifts in the SW corner of the region, conditions came tantalizingly close to being thermobarically unstable (as illustrated for Phase 3, Drift 4 below), but a lack of very cold conditions conducive to ice growth, and relatively weak wind stirring, allowed the upper ocean to remain marginally stable throughout the exercise.

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A. Phase 3 Drift 5 salinity contours.

B. Difference in temperature between the mixed layer and level 10 m lower, indicating substantial heat just below the mixed layer.

C. Same except salinity. Red curve described below.

D. Difference in density at pressure corresponding to mixed layer depth. The red curve in C shows the change in salinity that would make density neutral (no contrast) across the thermocline when thermobaricity is considered. It reaches a minimum of 0.01 psu, which for a 150-m thick mixed layer, could be provided by as little as 7 cm of mean ice growth.

Deep “pycnocline-following” mast

Turbulence clusters (separated by 6 m)

1.2 MHz ADP (upward)

0.6 MHz ADP (downward)

8-m thermistor string

Sea-Bird 9+ CTD

Photo: Behrens

Moonpool cycling CTD/microstructure pkg.

MaudNESS Instrumentation

Instrumentation included:

•NBP/Raytheon shipboard complement: (ocean/met/navigation/bathymetry)•UWAPL/NPS Moonpool automated cycling CTD/microstructure•NPS/MRC deep “pycnocline-following” turbulence/thermistor/ADP mast•MRC midlevel turbulence mast •U. Bergen shallow turbulence/ADP mast (ice station only)•ESR loose-tethered vertical microstructure profiler (VMP)•NPS shipboard/ice station met tower/radiosondes•NPS ice drift buoys•UWAPL lagrangian vertical ly•AWI deep ocean floats•Ice coring/temperature/sampling•AsPect observations•XBT survey during ice egress

First routine use of the moonpool during ice operations. Cleared by plunger.

Mid-level turbulence cluster mast

SBE T/C/C

Sontek ADVOcean

Phase 3 Baltic room deployment

Photo: Behrens

*Poster figures and photographs prepared by the MaudNESS Science Group: G. Behrens, L. de Steur, D. Goldberg, P. Guest, R. Harcourt, R. Lindsay, M. McPhee, D. Morison, J. Morison, R. Muench, D. Notz, M. Ohmart, L. Padman, B. Powell, K. Richter, W. Shaw, A. Sirevaag, T. Stanton, and J. Stockel.

A strong onboard modeling effort was maintained throughout the project, driven by observed conditions and daily NCAR AMPS MM5 regional weather forecasts. We found these to be generally quite accurate, and they were invaluable in predicting ice drift and optimal ship positioning during the Phase 3 operations.

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