Observing the ice-covered oceans around Antarctica by profiling floats

Transcription

1 Observing the ice-covered oceans around Antarctica by profiling floats Annie Wong, Stephen Riser School of Oceanography University of Washington, USA

2 Aug Since 2007, UW has deployed 83 profiling floats in the seasonal ice zone around Antarctica.

3 RSV Aurora Australis R/V Nathaniel B. Palmer R/V Polarstern

4 Photo courtesy of Paul Mauricio, chief engineer on the R/V Roger Revelle I8S cruise 2007 Spirit of Enderby (Professor Khromov) S.A. Agulhas

5 3 dbar TEMPERATURE-BASED CRITERIA Mixed layer sampling Decision 20 dbar p = 2.5 dbar Test 1: Between 50 and 20 dbar compute the median temperature T ml. If T ml is greater than a threshold temperature T th for 2 successive profiles, continue ascent to the surface. If not 2 successive profiles or if T ml < T th, then store the profile and descend. Test 2 (continue ascent): (a) No ice; transmit the profile. (b) The float hits the bottom of the ice; if no contact with a satellite in 2 hours, store the profile and descend. 50 dbar Test 3 (on or off test): In designated summer months, turn off Tests 1-2. Schematic of the ice-avoidance algorithm

6 Yearly distribution of number of CTD profiles collected by UW ice floats 56% sea ice free 44% under sea ice first UW ice float deployed in Feb 2007 data to April 2012 only The most long-lasting UW ice float has been operating since September 2007 and is still doing well (6 winters under ice). Average lifespan of the UW ice floats is 3 years so far.

7 11389 good quality CTD profiles as of April % sea ice free 44% under sea ice 0 60 E 60 W 120 E Dispersion of floats from initial positions led to good data coverage in the seasonal ice zone seaward of the 2000 m isobath. 120 W 60 S 50 S August 2009 sea ice edge m bathymetry 4000 m bathymetry

8 T/S time series seaward of the 2000-m isobath under sea ice under sea ice under sea ice

9 MARCH sea ice free WMO ID Cycles 127 to 130 JULY under sea ice WMO ID Cycles 91 to 95 stratified and stable only marginally stable In winter, the mixed layer under sea ice is very weakly stratified because of convective mixing as a result of brine rejection and entrainment of water from the permanent pycnocline.

10 onto the Antarctic continental slope Signature of the westward-flowing Antarctic Slope Current Continental slope < 2000 m C Circumpolar Deep Water is found at shallow depths near the Antarctic continental slope, due to upwelling associated with the Antarctic Divergence. Away from the influence of Circumpolar Deep Water, mixed layer depth increases (not just because of winter).

11 Martinson (1990), JGR, Vol. 95, No. C7, A 1-D model of the winter mixed layer under a sea ice cover Heat loss from the winter mixed layer to the atmosphere through ice and leads Snow Ice Lead Snow Ice Sea ice growth introduces a salt flux; convection drives entrainment of deep water Turbulence from relative ice motion and wind maintains the well-mixed surface layer Heat and salt flux across the permanent pycnocline by entrainment and mixing Deep water (CDW or WDW)

12 The Martinson (1990) model found solutions for: 1. Winter mixed layer salinity change 2. Entrainment depth, h e 3. Thermodynamic sea ice growth, h i As a function of: ( = Argo under ice data) a. Initial winter mixed layer depth b. T and S gradients across the permanent pycnocline c. Winter ocean-atm heat loss through leads and ice, F atm Under ice T/S data can be used in conjunction with a quantitative winter mixed layer model to estimate variables such as h e, h i, F atm.

13 Combining winter under ice T/S observations with Martinson (1990) Winter entrainment ~ 49 ± 11 m over 5 months Winter entrainment heat flux to the base of the ML ~ 34 ± 8 Wm 2 Freshwater needed to balance the salt flux ~ 73 ± 16 cm per year Thermodynamic sea ice growth ~ 26 ± 14 cm over 5 months Winter ocean-atm heat loss through leads and ice ~ Wm 2 Wong and Riser (2011), JPO, 41(6),

14 Conclusions 1. Technology Argo floats that use the Iridium satellite transmission system can be used successfully in the seasonal ice zone around Antarctica. The key to success is a workable ice-avoidance algorithm that can be programmed into the float controller. Many ice floats from UW carry biogeochemical sensors, such as dissolved oxygen and nitrate.

15 2. Scientific values Estimate heat flux supplied to the base of the winter mixed layer as a result of deep water entrainment due to ice growth-induced convection. Estimate winter ocean-atmosphere heat loss through leads and ice in the seasonal ice zone. Study how thermodynamic sea ice growth rates are affected by the strength of the permanent pycnocline Earth s radiative balance. Monitor potential spots for formation of sensible heat polynyas, and therefore deep water ventilation. Estimate annual surface freshwater balance in the seasonal ice zone. Improve representation of ocean-ice-atmosphere interactions in global climate models. Observe time-sensitive oceanographic events in active areas.