The role of sub-antarctic mode water in global biological production. Jorge Sarmiento

Transcription

1 The role of sub-antarctic mode water in global biological production Jorge Sarmiento

2 Original motivation Sediment traps suggest that ~one-third of the particulate organic matter flux at 200 m continues past the base of the main thermocline (defined as σ θ = 26.8) If nitrate lost by the above particle sinking were not replaced, the thermocline nitrate would be depleted within ~50 years! QUERY: How do nutrients return from the deep ocean to the thermocline?

3 Hypothesis: The main return pathway for nutrients from the deep ocean is Subantarctic Mode Water (SAMW) (Sarmiento et al., Nature, 2004)

4 Primary evidence: Export production Nutrient depletion south of 30 S (Pg C yr -1 deg -1 ) normal RESULT: ~Three-quarters of biological production N of 30 S is controlled by nutrients fed in from the south. Most of the effect occurs in the density interval corresponding to SAMW and upper AAIW (σ θ < 27.3; LL model; Marinov et al., Nature, 2006)

5 SAMW AAIW Northern Tropical Dye tracer simulations give more detail Dye tracers were used to determine the relative contribution of four water types (black) to the main thermocline (blue) Tracer is set to 1 in black area, set to 0 in white area, conserved in blue area.

6 Fractional contribution of different water types to the main thermocline (σ θ < 27.4) SAMW AAIW Tropical North (LL model)

7 Fractional contribution of different water types to the main thermocline. Average above σ θ = 26.5 (LL model)

8 Three models were used: Typical model Low vertical mixing model High wind model K v = 0.6 cm 2 s -1 A I = 2000 m 2 s -1 HH K v = 0.15 cm 2 s -1 A I = 1000 m 2 s -1 LL-low K v LL with ECMWF winds (higher over Southern Ocean), narrowed Drake Passage, higher surface S in Weddell & Ross Seas, 50 cm 2 s -1 between top two layers, and 1.3 cm 2 s -1 in Southern Ocean. P2A-high wind 2 1 0

9 Fractional contributions of water types to the upper thermocline (σ θ < 26.5) by different models MODEL SAMW TROPICAL NPAC AAIW NATL HH LL-low K v P2A-high wind Annual, global average at Year 400 Simulating a strong SAMW influence requires low vertical mixing and high Southern Ocean winds

10 Phosphate partitioning in nutrient model Total Remin SAMW AAIW Tropical NAtl NPAC South (LL model)

11 Total Phosphate partitioning in LL model - average above 26.5 SAMW NPAC Tropical Remin

12 Phosphate end-members (fractional contribution) Preformed Remineralized HH LL-low K v P2A-high wind Contribution to preformed: SAMW AAIW Tropical NPAC HH LL-low K v P2A-high wind

13 Model new production: Contribution from each end-member

14 Conclusions (1) The processes controlling SAMW formation Low interior vertical mixing shifts NADW return flow from low latitudes to Southern Ocean (and North Pacific) High Southern Ocean winds increase upwelling in Southern Ocean shifting it away from North Pacific and tropics. (2) The mechanisms & pathways by which SAMW enters the upper thermocline Primarily by advection along isopycnals from southeast corner of subtropical gyres followed by upwelling along boundaries Small amount of surface (Ekman) transport to north (3) The the link of these to biological production SAMW accounts directly for about 20% of biological production in the world ocean. Indirectly (including remineralized production) SAMW and AAIW together account for more than two-thirds of biological production north of 30 S - most of this is due to SAMW.

15 Global warming response

16 Salinity decreases Thickness decreases Salinity & isopycnal thickness anomaly with respect to control - Indian Ocean zonal average across 32 S - Scenario A1B, CM2.1

17 Upper ocean (σ θ < 27.4) northward transport decreases Upper figure shows total transport in A1B scenario Control (blue) 2100 (red) 2200 (orange) 2300 (light blue) Lower figure shows anomaly with respect to control. Decreased northward transport reflects reduced meridional overturning.

18 Volume of young water (<50 yrs) doesn t decrease much - and increases in tropics 2 x CO 2 =147.8 x 10 6 km3 Control=156 x 10 6 km 3

19 Changes in ideal age show water becoming younger at 300 m! Gnanadesikan et al. (2006) suggest that this reduced age is due to reduced deep upwelling associated with reduced meridional overturning. - They speculate that oxygen will increase and phosphate drop in O 2 minimum zones!

20 Projected GHG-Induced Warming and Precipitation Signal Looks like IOD More warming Less warming Vecchi et al (2006)

21 Projected Monsoon Rainfall Changes in 21st Century Look Like IOD Rainfall Change During Southwest Monsoon Vecchi et al (2006)

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24 Role of Subantarctic Mode Water in global biological production J.L. Sarmiento, J. Simeon, & A. Gnanadesikan

25 Goals of Study To determine (1) the processes controlling SAMW formation (2) the mechanisms & pathways by which SAMW enters the upper thermocline and (3) the link of these to biological production

26 South Simple Model of Pycnocline Depth Equator North D light water after Stommel and Arons (1960) equatorial upwelling dense water NADW formation Pycnocline depth D sets the NADW formation rate. Our models are configured to all have the same pycnocline depth. Return flow is via equatorial upwelling, and thus a condition is set upon the magnitude of vertical diffusivity Kv.

27 South Include Southern Ocean Processes Equator North D light water Southern Ocean upwelling after Gnanadesikan (1999) equatorial upwelling dense water NADW formation Pycnocline depth D sets the NADW formation rate. Our models are configured to all have the same pycnocline depth. Return flow is a balance of upwelling both at the equator and in the Southern Ocean. This balance is set by along-isopycnal diffusivity Ai and vertical diffusivity Kv.

28 HH NADW Meridional overturning (Sv) LL-low K v NADW P2A-high wind NADW

29 Transport in waters of σ θ < 27.4 P2A-high winds LL-low K v HH Northward flow Southward flow

30 Goals of Study To determine (1) the processes controlling SAMW formation (2) the mechanisms & pathways by which SAMW enters the upper thermocline (3) the link of these to biological production

31 Mass transport vectors at top of SAMW (McCartney, 1977) (LL model)

32 Separation of surface from subsurface SAMW tracer flux HH LL-low K v P2A-high wind

33 Net Phosphate flux through σ θ = 26.5 (mmol m -2 y -1 ) The upward flux of SAMW nutrients (red) occurs primarily at the boundaries on the eastern and western sides of the basins (LL model)

34 Goals of Study To determine (1) the processes controlling SAMW formation (2) the mechanisms & pathways by which SAMW enters the upper thermocline (3) the link of these to biological production

35 Phosphate end-members in upper thermocline above σ θ = 26.5 (fractional contribution) Preformed Remineralized Contribution to preformed: SAMW AAIW Tropical NPAC

36 Phosphate partitioning in LL model - average above 26.5 Total Remin SAMW AAIW Tropical NAtl NPAC South

37 HH LL-low K v Export production from each end member P2A-high wind

38 Northward transport σ θ < 27.4

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40 SAMW Climate response

41 Response of SST, SSS, and Surface Density to Global Warming in 2050

42 How will SAMW respond to global warming? Slide shows response in Hadley model (32 S mean in Indian Ocean). SAMW becomes fresher, less dense, and thicker Banks et al., (2000)

43 Intermediate depth waters in both hemispheres have become fresher in recent decades. Wong et al., 1999

44 IPCC 2000 Fossil fuel emission scenarios A1B B1 (Emissions Scenarios, IPCC 2000)

45 Predicted warming (IPCC) A1B B1

46 Predicted warming (IPCC)

47 Temperature increase between 1990 and 2090 (Scenario A1B) IPCC; Wyman (pers. comm.)

48 % Rainfall increase between 1990 and 2090 (Scenario A1B) IPCC; Wyman (pers. comm.)

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50 Scenario B1 - Anomaly with respect to control - Global zonal average across 32 S

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53 Scenario A1B, CM2.0 - Anomaly with respect to control - Indian Ocean zonal average across 32 S

54 Scenario B1 - Anomaly with respect to control - Global zonal average across 32 S

55 SAMW thickness (m; σ θ )

56 Potential vorticity in 1990

57 Potential vorticity in 2300 (Scenario B1)

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60 Introduction & Background

61 Nutrient depletion south of 30 S normal Evidence: Model simulations of export production (LL model) (Pg C/degree/yr) S 30 S -90% Fractional change in export production north of 30 S following nutrient depletion south of the indicated density outcrop. S 27.1 S 27.3 S 27.4 S % NOTE: Most of the effect occurs in the density interval (σ θ < 27.3) corresponding to SAMW formation Marinov et al., submitted

62 Introduction & Background

63 Model transport

64 Three models were used: Typical model Low vertical mixing model High wind model K v = 0.6 cm 2 s -1 A I = 2000 m 2 s -1 HH K v = 0.15 cm 2 s -1 A I = 1000 m 2 s -1 LL-low K v LL with ECMWF winds (higher over Southern Ocean), narrowed Drake Passage, higher surface S in Weddell & Ross Seas, 50 cm 2 s -1 between top two layers, and 1.3 cm 2 s -1 in Southern Ocean. P2A-high wind gδpd ρε 2 K v A Lxτ S Lx AI D = + S D ρ f L s y Gnanadesikan (1999)

65 LL Meridional overturning (Sv) HH P2A

66 pcfcs on 26.5 surface (patm) TOP: LL model BOTTOM: observations

67 MOM3 Configurations Watermass Transformation Rates by Circulation Model Circulation Model Southern Ocean Upwelling (Sv) Equatorial Upwelling (Sv) NADW Formation (Sv) Pathway of Return Flow Ai low, Kv low LL Ai high, Kv high HH Ai low, Kv high South LHS ECMWF, ndp, 4pt salinity rest, Ai low, Kv med-his (2000m) PSS Ai low, Kv med-his (2000m), 4pt salinity rest RDS Upwelling rates diagnosed from models by analyzing the meridional transport of light (σ 0 < 27.4) waters.

68 Modeled Southern Ocean Anthropogenic CO 2, CFCs, and Radiocarbon No single MOM3 configuration is able to reproduce Southern Ocean CFC, radiocarbon, and anthropogenic CO 2 inventories. Courtesy of Katsumi Matsumoto Southern Ocean 14 C

69 Float trajectories over 100 years Particles released in the SE Pacific SAMW formation region enter the gyres from the SE corner both as SAMW (light red & blue) and as surface waters (dark red). (LL model)

70 LL HH Float trajectories over 100 years P2A

71 Particle trajectories over 59 years Source of particles 30 m = red 60 m = orange 93 m = yellow 125m = green 157m = cyan 189m = violet Colors darken when particle upwells across σ θ = 26.5 surface

72 Vertical flux of SAMW across σ θ = 26.5 surface (m y 1 ) Upwelling Downwelling

73 QuickTime and a TIFF (LZW) decompressor are needed to see this picture.

74 Dye Tracer Simulations

75 But how does SAMW get out of the Southern Ocean and into the upper thermocline (σ θ < 26.5)? A model analysis The deep water types examined are: Subantarctic Mode Water (SAMW) Antarctic Intermediate Water (AAIW) North Atlantic waters (NATL) North Pacific waters (NPAC) Tropical deep waters (TROP)

76 Procedure followed MODELS USED: The MOM3 Princeton/GFDL family of models, with Lo Lo as the "standard" baseline SIMULATIONS: We carry out a series of "One" tracer simulations as follows: Tracer is set to 1 in the region where the particular water type forms Tracer is transported freely in the main thermocline above σ θ = Tracer is set to 0 in the deep water below σ θ = ANALYSIS: We are able to determine: The fractional contribution of each water type to the upper thermocline (σ θ < 26.5) The primary mechanisms of the input. The volumetric input per unit time of each water type

77 Schematic of "One" tracer simulations to determine the influence of different water types on the thermocline SAMW Thermocline SAMW AAIW Thermocline Thermocline SAMW SAMW AAIW AAIW Blue: "One" tracer set to 1 White: "One" tracer set to 0 Tropical Thermocline Green: "One" tracer is conserved AAIW Thermocline SAMW AAIW Thermocline SAMW SAMW AAIW AAIW Isopycnal boundaries are at σ θ = 26.5, 27.1, & 27.4 Northern Note: North Pacific and North Atlantic are separated in Northern.

78 Dye Tracer Simulations SAMW SAMW + (SAMW AAIW) AAIW AAIW + (AAIW SAMW) Northern Northern + (Northern SAMW,AAIW) Tropical Tropical + (Tropical SAMW,AAIW)

79 Fractional contribution of different water types to the main thermocline (σ θ < 27.4)

80 Fractional contribution of different water types to the main thermocline. Average above σ θ = 26.5

81 RESULT 1d: Simulations show SAMW dominates the deep water input to the upper thermocline. What water types contribute to SAMW formation? Total fraction of SAMW From AAIW From "northern" & tropical waters Loss of SAMW to AAIW outcrop

82 Separation of surface from subsurface SAMW tracer flux

83 Fractional contributions of water types to the upper thermocline (σ θ < 26.5). (Annual, global average at Year 400) MODEL SAMW TROPICAL OTHER HH LL (Low vertical mixing) P2A (High Southern Ocean winds) NOTE: Simulating a strong SAMW influence requires low vertical mixing and high Southern Ocean winds

84 Fractional contributions of water types to the upper thermocline (σ θ < 26.5). (Annual, global average at Year 400) MODEL SAMW TROPICAL OTHER HH LL (Low vertical mixing) P2A (High Southern Ocean winds) HH = K v = 0.6 cm 2 s -1, A I = 2000 m 2 s -1. LL = K v = 0.15 cm 2 s -1, A I = 1000 m 2 s -1. P2A = LL with ECMWF winds (higher over Southern Ocean), narrowed Drake Passage, higher surface S in Weddell & Ross Seas, 50 cm 2 s -1 between top two layers, and 1.3 cm 2 s -1 in Southern Ocean. NOTE: Simulating a strong SAMW influence requires low vertical mixing and high Southern Ocean winds

85 Do all models show such a strong influence of SAMW? Fractional contributions of water types to the upper thermocline (σ θ < 26.5). (Annual, global average at Year 400) MODEL SAMW NPAC TROPICAL AAIW NATL LL HH P2A LL = K v = 0.15 cm 2 s -1, A I = 1000 m 2 s -1. HH = K v = 0.6 cm 2 s -1, A I = 2000 m 2 s -1. P2A = LL with ECMWF winds (higher over Southern Ocean), narrowed Drake Passage, higher surface S in Weddell & Ross Seas, 50 cm 2 s -1 between top two layers, and 1.3 cm 2 s -1 in Southern Ocean.

86 Phosphate Simulations

87 Model new production: Contribution from each end-member

88 Export production from each end member

89 Phosphate end-members (fractional contribution) Preformed Remineralized LL HH P2A Contribution to preformed: SAMW AAIW Tropical NPAC LL HH P2A

90 LL HH Net Phosphate flux through 26.5 (mmol m -2 y -1 ) P2A

91 Conclusions

92 Conclusions of model studies SAMW is the primary source of water to the upper thermocline (59%). North Pacific Intermediate Water is second (28%). Main pathway for entry of SAMW is the southeastern corner of the basins as part of the gyre transport Upward transport of SAMW occurs primarily in eastern boundary upwelling regions and in western boundary currents. Most of the oceanic production north of 30 S is driven by remineralized nutrients which originally came from the SAMW

93 Conclusions of model studies SAMW is the primary source of water to the upper thermocline (59%). North Pacific Intermediate Water is second (28%). Main pathway for entry of SAMW is the southeastern corner of the basins as part of the gyre transport Upward transport of SAMW occurs primarily in eastern boundary upwelling regions and in western boundary currents. Most of the oceanic production north of 30 S is driven by remineralized nutrients which originally came from the SAMW

94 Part 2: Conclusions Fractional contribution of each water type to the upper thermocline (σ θ < 26.5) In standard LoLo model, SAMW accounts for 59% and NPAC for 29% of deep water supply to upper thermocline. Tropical supply in LoLo model is 7% but jumps to 25% in the high vertical mixing model. SAMW and NPAC drop by similar amounts: 9% and 11%, respectively. Prince 2A (Swathi) model is 70% SAMW and 19% NPAC, with only 5% Tropical. SAMW is formed predominantly from AAIW (i.e., from south)

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96 Si* Study

97 Consider export across the base of the main thermocline (σ θ = 26.8 surface)

98 We define the base of the upper thermocline as the σ θ = 26.5 surface Figure shows the depth of the σ θ = 26.5 surface Upper: Levitus observations Lower: Lo Lo model

99 We define the base of the upper thermocline as the σ θ = 26.5 surface (figure based on Levitus data)

100 Particle Export (mmol C m -2 d -1 ) Global Export = 12 ± 1 Pg C/yr Dunne and Sarmiento (in prep.) using satellite based primary production

101 Zonal Mean Particulate Organic Carbon Flux in Top 1000 m (g C m- 2 y -1 ) >one-third of the 200 m export flux sinks past the σ θ = 26.8 (SAMW) isopycnal! Thermocline nitrate would be depleted in 53 years. Dunne et al. (in prep.): uses satellite based primary production & the average of Lutz et al. s (2002) sediment trap based remineralization functions.

102 Old paradigm How do nutrients return from abyss? Vertical mixing & upwelling

103 Problems with old vertical mixing and upwelling paradigm: Vertical mixing is at least an order of magnitude smaller than required to explain the vertical distribution of the radiocarbon balance (e.g., Ledwell et al., 1993). Generalized upwelling is inconsistent with surface radiocarbon in Pacific (Toggweiler & Samuels, 1993). So how are nutrients resupplied? The primary supply mechanism must be lateral, presumably along isopycnals (surfaces of constant density), but where does this occur?

104 SAMW forms in deep wintertime mixed layers in the Southern Ocean spanning the Subantarctic Front Density increases from 26.5 to 27.1 in an eastward circuit from W. Atlantic Ocean (McCartney, 1977) Fronts = STF (N&S), SAF, & PF Zones = SAZ and PFZ

105 Southern Ocean Nitrate and Silicic Acid Distributions An unusual characteristic of the waters spanning the Subantarctic Front is their high nitrate and low silicic acid concentrations.

106 We find that Si* = Si(OH) 4 -NO - 3 is an excellent tracer of these low silicic acid high nitrate surface waters Original motivation for Si*: Silicic acid is used by diatoms to construct frustules (shells) Diatoms grown with adequate nutrients and light take up silicic acid in a ratio of Si:N = 1 Negative Si* thus indicates a deficiency relative to diatom Si:N ratio. Positive Si* indicates a surplus relative to diatom ratio.

107 Si* and wintertime mixed layer depth Note that low Si* region (blue on left) matches deep wintertime mixed layer where SAMW forms (red on right).

108 There is nowhere else at the surface of the ocean where Si* is negative But why is Si* so negative in this band?`

109 Schematic of nutrient cycle in Southern Ocean -When diatoms have adequate light and nutrients, they tend to take up Si and nitrate in a ratio close to 1:1 -When stressed (e.g., by iron or light limitation), diatoms tend to build more silicified shells, leading to a Si to NO 3 uptake ratio of 2:1 and higher [Hutchins and Bruland, 1998; Takeda, 1998.] -Hypothesis: iron or light stress in Southern Ocean leads to high Si to NO 3 uptake ratio, which generates negative Si*

110 Si* on σ θ = 26.8 (~SAMW) isopycnal shows global extent of the SAMW influence (NOTE: we are able to demonstrate that Si* is ~conserved at this depth, except in the North Pacific where vertical exchange brings up high Si* water from below.)

111 Si* on σ θ = 27.4 (~AAIW) isopycnal shows that this forms south of the Si depletion zone

112 North Pacific This isopycnal surface is at the depth of the NPIW (North Pacific Intermediate Water), which forms in the Sea of Okhotsk and "mixed water region" between the Kuroshio and Oyashio Currents. Tidal mixing may play a central role. Silicic acid is not removed, perhaps because NPIW formation is due mostly to interior vertical mixing. 70% of Eq. Pac Si(OH) 4 comes from north (Dugdale et al., 2002).

113 Tidally driven vertical mixing at 1000 m Kurile Islands St. Laurent et al. (2003)

114 Problem: How do the nutrients return from the deep ocean into the thermocline? Old paradigm: Vertical mixing & upwelling New Paradigm: Laterally from Southern Ocean (+ North Pacific) (Sarmiento et al., 2004)

115 Implications Southern Ocean (and North Pacific) nutrient dynamics control low latitude biological productivity The SAMW return path for nutrients will likely be very sensitive to climate change. Paleo-implications examined by Brzezinski et al. (2002) & Matsumoto et al. (2002). Global warming simulations show significant impact on SAMW properties (e.g., Banks et al, 2000). The low silicic acid to relative to nitrate of SAMW represents a key factor determining Si limitation of diatoms in low latitudes.

116 Consequences Southern Ocean (and North Pacific) nutrient dynamics control low latitude biological productivity The low silicic acid to relative to nitrate of SAMW represents a key factor determining Si limitation of diatoms in low latitudes. The SAMW return path for nutrients will likely be very sensitive to climate change. Paleo-implications examined by Brzezinski et al. (2002) & Matsumoto et al. (2002).

117 Part 1 Conclusions The primary return path for nutrients into the nutricline is by upwelling in the Southern Ocean and subsequent entrainment into SAMW. The only exception is the North Pacific, where enhanced mixing appears to occur. Without the Southern Ocean return pathway, thermocline nutrients and low latitude biological productivity would plummet. The low silicic acid to relative to nitrate of SAMW represents a key factor determining Si limitation of diatoms in low latitudes. The SAMW return path for nutrients will likely be very sensitive to climate change. Paleo-implications examined by Brzezinski et al. (2002) & Matsumoto et al. (2002).

118 We find in model studies that most low latitude production is controlled by a band between σ θ = 27.3 to 27.4 and 30 S. This is where SAMW (Subantarctic Mode Water) forms.

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120 Thermocline phosphate plummets when Southern Ocean nutrients are depleted:

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122 Silicic acid to nitrate supply ratio across 100 m J opal J organic nitrogen = ( Si(OH) m Si(OH) ) m ( - NO NO - ) m m

123 Two major problems of biogeochemical oceanography 1. How does nitrate return from the deep ocean into the thermocline? 2. Why is silicic acid preferentially trapped in the deep ocean relative to nitrate?

124 WOCE Hydrographic Program Sections

125 Nitrate (μmol/kg) Silicic Acid (μmol/kg)

126 Si* = Si(OH) 4 -NO 3- (μmol/kg) Note that high Si* water gets to surface in the Southern Ocean, but is trapped there!

127 By contrast, nitrate is high in the thermocline because it is NOT trapped in the Southern Ocean Nitrate Preformed nitrate Remineralized nitrate calculated from Apparent Oxygen Utilization