A deep Argo pilot experiment in the Australian Antarctic Basin. Steve Rintoul

Transcription

1 A deep Argo pilot experiment in the Australian Antarctic Basin Steve Rintoul

2 Outline 1. Why the Australian Antarctic Basin? 2. Evidence for change and variability in the deep layers of the basin Multi-decadal trends Interannual variability Seasonal variability 3. Strategy and status

3 Why the Australian Antarctic Basin (AAB)? Close to Antarctic Bottom Water (AABW) sources, so signal (seasonal, interannual, multi-decadal trend) is large Best-ventilated deep basin in the Southern Ocean, supplied by Ross Sea and Adelie Land sources (40% of total AABW) Relevant to ocean-ice shelf interaction / future of Antarctic Ice Sheet / sea-level rise Signal poorly-sampled by sparse repeat hydrography Basin is accessible, with multiple deployment opportunities

4 CFC concentration in AABW layer Australian Antarctic Basin has the highest CFC concentrations in the abyssal ocean. Reflects inflow from Ross Sea and Adelie Land sources of AABW. Orsi et al., 1999

5 Strongest freshening signal in the AABW Purkey & Johnson, 2013

6 AABW change in Australian Antarctic Basin Freshening of AABW is largest near the sources (blue bars). Colored bars indicate decadal trends in temperature (red) and salinity (blue) of the deepest 300 m of the water column. Warming signal strengthens downstream in the boundary current (red bars). van Wijk & Rintoul, 2014

7 AABW - S at SR3 (140E) 2015

8 : ± dec : ± dec -1 Changes in AABW at 140 E : -86±29 dbar dec : -122±60 dbar dec -1 Aoki et al, JGR, 2013

9 Katsumata et al., 2015 Change on S4 ( 63S), 1995 to 2012

10 Interannual variability of AABW -S Mean,S of bottom 200 dbar in core of AABW layer at southern end of SR3, using only summer data

11 Seasonality of AABW export to deep ocean Mooring at 2632 m depth shows strong seasonal cycle in bottom water transport and properties over the continental slope. Fukamachi et al 2000

12 Seasonal change in near-bottom SR3 Repeat hydrography shows strong seasonal cycle in deep stratification on lower slope. Aoki et al, in prep

13 Sea ice cover in AAB Red line is northern limit of region covered by sea ice for more than 40 days/yr Massom et al., PLOS ONE, 2013

14 van Wijk & Rintoul 2014 Change in thickness of AABW layer P11S 150E SR3 140E I9S 115E Latitude Latitude Latitude Red (black) line indicates summer (winter) ice edge. Most of the basin is ice-free in summer, and a strong thinning signal extends into latitudes that are ice-free year-round

15 Summary of variability & change in deep AAB Strong seasonal, interannual and multi-decadal signals Seasonal and interannual variability of T & S is strongest near the BW sources Freshening and contraction of volume is largest near BW sources and damped downstream Decadal temperature trend on isobars strengthens downstream due to deflation of cold layers Trend of temperature ( /decade) and contraction (>100 m/decade) detectable in a 5 year deployment Salinity trend (0.01 / decade) too small to detect?

16 Strategy and status Use a mix of deep float types: deep Ninja/Arvor over the slope, APEX with ice-avoidance in seasonal sea ice zone, SOLO in ice-free waters in northern basin Interest in deep Argo pilot from Japan (2 deep Ninja), France (3+ deep Arvor), Australia (5-8?) and possibly others Deep hydrography planned: SR3 (140E) in Jan 2018 I9S (115E) in Jan 2020 CTDs of opportunity may be possible on resupply voyages

17 Summary The Australian-Antarctic Basin is a promising target for a deep Argo pilot experiment: Strong signal Accessible Reasonable historical coverage Deep hydrography provides deployment & calibration opportunities Main drawback is presence of sea ice in part of the basin, for part of the year; requires ice avoidance Primary target is seasonal and interannual variability, both to understand dynamics and to help interpret trends from sparse deep hydrography

18

19 Large contraction of AABW layer in the Australian Antarctic Basin Purkey & Johnson, 2013

20 Mean duration of sea ice cover Massom et al., PLOS ONE, 2013

21 Trend estimates Theta ( C/dec) SAB avg 0.21±0.19 W/m2 AAB avg 0.24±0.14 W/m2 Salinity (/dec) Thickness (m/dec) Period Reference Not sig PJ10, PJ12 (Temp trend is implied heat flux across 4000 m) PJ12, PJ13 (S trend is for = -0.5C; trend is for = 0C) AABW 150E to 2011 vw&r14 (for 300 m thick bottom layer; thickness change is for AABW layer below gamma = kg/m3) AABW 140E to 2011 As above AABW 115E- Ant to 2011 As above AABW 84E AABW 115E- MOR