General AW Circulation Schemes

Transcription

1 General AW Circulation Schemes Aagaard, topographically steered boundary current along slopes and ridges - interior flow weak, dominated by eddies (based on current meters) Rudels et al, mixing off St Anna - cyclonic (anti-clockwise) circulation (based on T-S and tracers)

2 Tracers of Atlantic Water TEMPERATURE-SALINITY - presence of Tmax - form in TS space - following a warming (see later) CHEMICAL TRACERS - usually atmospheric source - mixed into surface water, and then isolated from the atmosphere CFCs (ChloroFluoroCarbons) - solvents from 1960s onwards - CCl4 ( carbon-tet ) is oldest then CFC11, CFC12 and newest CFC113 - atmospheric concentrations KNOWN - use presence or ratios to give age Cs and I from Nuclear Reprocessing - on-going, - concentrations KNOWN - use presence or ratios to give age - also dissolved oxygen?? - Delta O18?? (not really) - N:P ratios (perhaps) NON-CONSERVATIVE (weakness and strength) Bomb Tritium - atomic bomb tests 1950s - surface layer of tritium (isotope of H) - decays to Helium-3 (half-life ~ 12.4 yrs) CONSERVATIVE - but mixing important

3 CFCs JGR, 2000 High Low High

4 JGR, 1999 Cs and I High Low High High Low High Smith et al., 1999

5 Pathways and Ages (transit time?) High Low High High High Low Low Smethie et al., 2000 Smith et al., JGR, ages much higher than from advection speeds, possibly due to CFC mixing model

6 Routes in the Arctic AW circulation very different to PW circulation - AW follows topography (with exceptions?) - PW follows ice?? Jones et al., Arctic 2001, Change Polar 2009 Research - Woodgate

7 JGR, 2005 How AW varies in the Arctic EWG data year mean of Temperature in each box Boxes numbered in order of our best guess at time since Fram Strait Does AW cool as it progresses?? - if so, why? What else might be happening?

8 Quadfasel et al., Nature, 1991 Changing AW DSR, Rossiya tourist cruise, XBT section Significant Warming of AW

9 Atlantic Layer warming in 1990s (collation/model by Karcher et al., 2003) Modelled (full fields) and observed (circles) AW core temperatures in various years Warmer since 1990s,... but slight cooling following the warming... USE THIS AS TRACER!!

10 Changing Atlantic Inflow 2005

11 JGR, 2005 How unique is this warming? EWG data Temperature at AW depths in Box 3 Have been previous warm periods

12 JClim, 2004 (Real data, especially sparse real data can be very very messy) Normalised anomalies of AW Core Temperature

13 Do these warmings agree?? Swift et al, 2005 AO, courtesy I.Rigor Polyakov et al., 2004

14 GRL, in 2000s,hydrography at NP returning to pre-1990s state - may relate to AO (magenta ~ red marks)

15 Along-Lomo Ridge transit speed of cm/s (v close to 2 cm/s of Woodgate et al, 2001, moorings) IS M1 in the boundary current?

16 Warming in the Chukchi Borderland warming progressing through the Chukchi Borderland Woodgate et al, 2007

17 The Warming of the 1990s 1993 Larsen - 1 deg warmer on the Mendeleev Ridge - inversions in temperature and salinity Carmack et al, 95, and McLaughlin et al, 96

18 Atlantic Water zigzags AOS94 Line up/ Nest all through the Arctic - ~ 5,000km Angles of the Zigzags match double diffusive theory Potential Temperature (deg C) Salinity (psu) Carmack et al, 1997

19 Structure in the Atlantic Water core Pacific Nutrient Max - very different T-S structures - very sharp fronts 1) The temperature maximum is very dependent on location 2) Why are the structures so different? 3) Can we follow pathways using the T-S structure? Woodgate et al, in press

20 Mixing and Double Diffusion in T-S Space TEMPERATURE TEMPERATURE SALINITY SALINITY Mechanical mixing in TS space creates straight lines between water masses In double diffusive processes, heat diffuses faster than salt. So, in TS space resultant waters are not on a straight line between the parent water masses

21 Theories for formation, and for growing to a large amplitude steady state (Turner, Ruddick, Toole, Georgi, McDougall, Walsh, Carmack, Rudels, May ) Diffusive Convection Regime Cold Fresh / Warm Salty unstable in temperature Salt fingering Regime Warm Salty / Cold Fresh unstable in salt Temperature unstable Salinity unstable

22 T-S Zigzags in the Arctic Interaction of two water columns - therefore can learn something about origins Diffusive Convection Regime Cold Fresh / Warm Salty unstable in temperature Salt fingering Regime Warm Salty / Cold Fresh unstable in salt Line up throughout Arctic (~ 5000km) - therefore LOW ENERGY environment Spread by?? - self propagating? (spread at 90deg to front) - fossil intrusions? (carried advectively) Can be used as a tracer of the boundary current??? Many refs, - most Carmack, Walsh or McDougal for overview, see Woodgate et al, 2007

23 Chukchi Borderland Atlantic Water Circulation Only shown Fram Strait Branch Water, but Barents Branch very similar Woodgate et al, 2007, JGR

24 Seasonal Atlantic Water Variability ~260 m, i.e. Fram Strait Branch Dmitrenko et al, in press - amplitude ~ 0.5 deg C!! - aliasing problems with CTD sections - different timing at depth ~ 840 m, i.e. Barents Sea Branch

25 Details still unclear From Woodgate and Steele, 2006, NSF proposal

26 The Arctic Ocean Boundary Current - mean flow weak, but with strong eddies - topography following - equivalent barotropic (i.e. velocity well correlated at all levels) - NOT SEASONALLY VARYING - current ~ 50 km wide found over ~ m isobaths - centered over ~ 1700m isobath

27 ~ 2 cm/s ~ 2 cm/s The Arctic Ocean Boundary Current BUT STILL DON T KNOW Woodgate et al., 2001, DSR ~ 5 cm/s = what type of current it is - equivalent barotropic jet? (Killworth and Hughes, 2002) Transports large uncertainties!!, but consistent with inputs Advection speeds a few cm/s, consistent with Tmax translation, faster than CFC ages. (? Mixing?) Flow split by Ridge one branch N along Lomonosov Ridge, one branch into Canadian Basin Some waters cross ridge S of 88N Small exchange of Deep Waters (CBDW/EBDW Canadian/Eurasian Basin Deep Water) = what drives it - Neptune interaction of eddies and topography (Holloway, 1987) - Potential Vorticity forcing (Karcher et al., 2007) - Windstress from Greenland Sea (Nost and Isachsen, 2003)

28 What drives the Boundary Current? PROPERTIES TO EXPLAIN: - equivalent barotropic structure - steered by topography - weak - variability NOT local - width (~ 100 km)??? - pathways - variability -direction Greg Holloway Neptune effect (Holloway, 1987; Nazarenko et al, 1998) Eddy-topography interactions drive a mean along isobath flow - makes model look like our bias,.. BUT IS TUNED - can we quantify this effect given number of eddies? - topostrophy = v x grad D = tendency of flow to follow topog (Holloway et al, 2007)

29 PV of inflows? JPO, 2005

30 Eurasian Basin = cyclonic due to Barents Sea inflow Canadian Basin = anticyclonic due to surface wind stress

31 f/h contours JMR, m/s Ekman pumping > Hydrographic Forcing == wind forcing greater than hydrographic == forcing into f/h band is OUTSIDE the Arctic, i.e. AO boundary driven by wind-stress in Nordic Seas

32 Canadian Eurasian JMR, 2003 RED = EKMAN BLUE = Hydrographic Triangles = Canadian Basin; Circles = Eurasian Basin Both basins Ekman bigger Basins have different signs Modelled circulation again matches our bias