Oxygen variability in the eastern tropical North Atlantic oxygen minimum zone

(1)

Oxygen variability in the eastern tropical North Atlantic oxygen minimum zone

Peter Brandt

with contributions from

Johannes Hahn, Florian Schütte, Tim Fischer, Johannes Karstensen,

Richard J. Greatbatch, Marcus Dengler, Arne Körtzinger, Gerd Krahmann,

Sunke Schmidtko, Lothar Stramma, Toste Tanhua, and Martin Visbeck

(2)

T,S,v N,P,Fe

O₂

SFB754

t

(3)

Oxygen Depletion in the North Atlantic OMZ

Oxygen data show a reduction of dissolved oxygen in the North Atlantic OMZ over the last 40 years.

Stramma et al. 2008

mmol/kg

Motivation

(4)

Habitat Reduction for Pelagic Fishes

Stramma et al. 2011

Motivation

(5)

Global Model Simulations

Oschlies, pers. comm.

Annual mean oxygen [μmol/kg] at 300m in observations (WOA) and different state-of-the-art global models

Motivation

(6)

Stramma et al. 2013

Mismatch between Observed and Modeled Trends

Pattern correlation between simulated (upper right) and observed (bottom) patterns of past oxygen change over the last 50 yr is negative

Oxygen (left,

μmol/kg) and oxygen trend (right, μmol/kg/yr) at 300m.

Motivation

(7)

Outline



Structure of the

Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ)

• Mean structure

• Shallow OMZ

• Equatorial oxygen maximum



Oxygen Budget

• Consumption

• Diapycnal mixing

• Lateral mixing

• Advection



Long-term Oxygen Changes



Summary

(8)

Ventilated Thermocline

Transport processes at the boundary between ventilated and

unventilated thermocline:

advection (solid arrow) and diffusive flux (open

arrow) Luyten et al. 1983

FLAME simulation, C. Eden

Simulation of OMZs involve physical

processes from large to small scales: circulation, jets, eddies, filaments, turbulent mixing.

Structure of the ETNA OMZ: Mean Structure

(9)

Oxygen Distribution at 600m [ccm/l]

Left: From METEOR expedition 1925/27

(Wattenberg 1939)

Right: From WOA’09 (same style, courtesy Florian Schütte)

Note, oxygen maximum at the equator

Wattenberg 1939 WOA‘09

(10)

Mean Circulation and Oxygen Distribution

Complex zonal current system connects high-oxygen western boundary regime with sluggish flow in the eastern basin.

Brandt et al. 2015

(11)

Measurement Programme

Repeat ship section along 23°W; moored observations;

microstructure measurements; 2 tracer release experiments

Brandt et al. 2015

(12)

Mean 23°W Section

Equatorial oxygen maximum Deep

oxycline at about 300m or s_q=26.8 kg/m³

OMZ is ventilated from the west by zonal currents

(13)

Oxygen at Deep Oxygen Minimum

Deep OMZ (below 200m) located in the interior with slightly enhanced oxygen

concentration

toward the eastern boundary

From shipboard CTD/O2 profiles taken during more than 30 research cruises

(14)

Oxygen at Shallow Oxygen Minimum

Shallow OMZ

(above 200m) close to the eastern

boundary upwelling region

Single low oxygen events also in the region of the deep OMZ

(15)

Mean 18°N Section

More More SACW

NACW

Shallow OMZ

Structure of the ETNA OMZ: Shallow OMZc

(16)

Oxygen at CVOO Mooring

Oxygen at 40-60m (black), 140m (grey) and

oxygen saturation (red) Karstensen et al., 2015

CVOO

(17)

Passage of a Mode-Water Eddy at the CVOO Mooring

Low oxygen zones present just below the mixed-layer of a high productive anticyclonic modewater eddies (oxygen at 42m and 170m, salinity, meridional velocity [m/s])

Karstensen et al., 2015

(18)

Low Oxygen Eddies Identified in Available Hydrographic Data

Schütte et al., 2016 Anticyclonic

modewater eddies

(ACME) Cyclonic eddies

Eddy tracking using SLA data

(19)

Mean Surface Signatures

(20)

Mean Vertical Structure

Obtained from oxygen anomaly profiles, i.e. difference between profiles inside and outside of an eddy.

(21)

Summary: Shallow OMZ

Inside of cyclones and ACMEs very low oxygen environment are found just below the mixed layer

Eddy cores are well-isolated, i.e., hydrographically constant, while respiration is elevated and

reduces oxygen during westward eddy migration

Locally increased consumption (aOUR) within these eddies represents an

essential part of the total consumption and is partly responsible for the

formation of the shallow oxygen minimum zone

Schütte et al., 2016; and Biogeosciences special issue on Hydrography, biogeochemistry, and biology of

"dead-zone eddies" in the eastern tropical North Atlantic aOUR (µmol kg^-1 y^-1)

-15 -10 -5 0

Depth (m)

0 50 100 150 200 250 300 350 400 450 500

ACME Cyclone Total

Consumption after Kartsensen et al. 2008

(22)

Equatorial oxygen and velocity distribution



Why there is on oxygen maximum at the equator?



Why it is largely missing in global Earth System Models?

Oschlies, pers. comm.

Structure of the ETNA OMZ: Equatorial oxygen maximum

(23)

Zonal Velocity in the Equatorial Atlantic at 23°W

Equatorial Deep Jets are a dominant flow feature below the Equatorial Undercurrent and oscillate with a period of about 4.5 years

Downward phase and

upward energy propagation

Shipboard mean and

moored time series

(24)

Energy of Zonal Flow at 23°W:

Basin Mode Oscillations

Most of the energy is concentrated on only three frequencies

All peaks are associated with resonant linear equatorial basin modes

Black line: basin mode characteristic

from PhD thesis, M. Claus, 2016

25 Cane and Moore (1981)

described low-

frequency standing equatorial modes

composed of equatorial Kelvin and long Rossby waves

T = 4 L

c

_gw

(25)

Equatorial Basin Mode

Greatbatch et al.

(2012) used a reduced- gravity model to

simulate regular high- baroclinic-mode

oscillations with a period of 4.5 years

Width of the EDJs could be correctly

simulated by including lateral eddy viscosity of about 200-300m²/s

Greatbatch et al. 2012

(26)

Advection-Diffusion Model

Model is forced by the velocity field of basin mode of EDJs

It includes a restoring to western boundary oxygen concentrations within a boundary layer and oxygen consumption (van Geen et al.

2006)

Simulation are performed until a constantly oscillating state is reached (about 160 yr)

Mean relative oxygen shows ventilation of the equatorial band due to basin mode oscillations

Equator

Brandt et al. 2012

(27)

Reduced-Gravity Model with EDJ and Mean Advection

a) Mean zonal flow field, i.e.

latitudinally

alternating zonal jets

b) Mean oxygen distribution

c) Oxygen anomaly along 23°W

d) Mean Oxygen along the equator

(28)

Summary: Equatorial Atlantic Ventilation



Equatorial Deep Jets and the mean eastward flow at 2°N/S with westward flow in between results in

strong mixing between western boundary regime and eastern equatorial Atlantic producing the equatorial oxygen maximum



High-resolution models with better representation of equatorial circulation allow simulation of more

realistic oxygen distribution (e.g. Duteil et al. 2014)

(29)

Oxygen Budget of the ETNA OMZ



Oxygen tendency



Oxygen sink

• Heterotrophic respiration



Oxygen source or sink:

• Diapycnal mixing

• Meridional eddy fluxes

• Advection by latitudinally alternating zonal jets

Oxygen Budget

¶O

₂

¶t = -C z ( ) ⁺ ^K

r

¶

²

O

₂

¶z

²

+ K

_y

¶

²

O

₂

¶y

²

- u ¶O

₂

¶x + ...

(30)

Respiration Estimates

AOUR: apparent oxygen utilization rate

Derived as the ratio of AOU and CFC11 ages (data from the subtropics)

Exponential decay of AOUR downward is assumed

Karstensen et al. 2008

Oxygen Budget: Consumption

(31)

OUR from Different Tracer-Based Age Concepts

Mean age from the transit time distribution (TTD) is calculated by D/G=1, with D the width and G the mean age of the TTD

„classical“ tracer age is with D/G=0

Problems: very old water masses, mixing of different water masses

Large uncertainty

Mean age (TTD)

“classical”

tracer age

“classical”

tracer age

Oxygen Budget: Consumption

(32)

Diapycnal Mixing

Microstructure measurements yield a diapycnal diffusivity, K, that is relatively

constant with

depth in the depth range of the OMZ

Fischer et al. 2013

Oxygen Budget: Diapycnal Mixing

(33)

Tracer Release Experiment

Diapycnal and lateral mixing estimated from tracer spreading:

K_r = (1.19±0.18) x 10^-5 m² s^-1

K_x = 1200±600 m² s^-1, K_y= 500±200 m² s^-1

Banyte et al. 2012, 2013

Oxygen Budget: Diapycnal Mixing

(34)

Hahn et al. 2014

Meridional Eddy Fluxes

Two Methods

Eddy correlation method applied to moored observations of oxygen and meridional velocity (here at 5°N,

23°W)

Flux gradient parameterization based on repeat ship sections

Oxygen Budget: Lateral Mixing

F_O

2 = v'O₂ '

dy K dO

F = - _e ²

(35)

Mean Eddy Diffusivity Profile K

_e

Brandt et al. (2010)

NATRE: Ferrari and Polzin (2005)

GUTRE: Banyte et al. (2013) TNEA: Hahn et al. (2014)

(36)

Eddy Flux Divergence

Oxygen supply due to

meridional eddy flux

Hahn et al. 2014

Meridional eddy diffusivity

(37)

Oxygen Budget of the ETNA OMZ

Hahn et al. 2014

Oxygen Budget

(38)

Mean 23°W Section

Equatorial oxygen maximum Deep

oxycline at about 300m or s_q=26.8 kg/m³

OMZ is ventilated from the west by zonal currents

(39)

Oxygen Depletion in the North

Atlantic OMZ = Climate Change?

Oxygen data show a reduction of dissolved oxygen in the North Atlantic OMZ over the last 40 years.

Stramma et al. 2008

mmol/kg

Long-term Oxygen Changes

(40)

Ocean Deoxygenation

Increased stratification and a corresponding reduction of ventilation, or solubility changes associated with a warming of subducted water masses (Bopp et al. 2002; Matear and Hirst 2003)

Increase in heterotrophic respiration along the pathways of ventilating water masses due to excess organic carbon

formed at higher CO₂ levels (Oschlies et al. 2008)

Simulated global O₂ changes in response to external forcing (90% confidence), but Atlantic O₂ changes undistinguishable from internal variability (Andrews et al. 2013)

Observations indicate circulation changes:

e.g. weakening of zonal jets (Brandt et al. 2010)

(41)

150-300m, 9-15°N, 20-26°W

350-700m, 9-15°N, 20-26°W

(42)

Oxygen Budget with Trends

Hahn et al. 2016, in preparation

(43)

Decadal Water Mass Changes

Salinity increase (more high-saline NAW)

associated with oxygen decrease and vice versa

Oxygen Trend 2006-2014

Salinity Trend 2006-2014

(44)

Summary



Advection dominates ventilation in the upper 300m



Deoxygenation associated with anthropogenic

climate change might not be the dominant signal on regional scale



Strong decadal

oxygen changes likely associated with

circulation variability



Exact mechanisms are still unknown

Trend 2006-2014

(45)

Acknowledgements



This study was supported by the German Science Foundation (DFG) as part of the

Sonderforschungsbereich 754 “Climate-

Biogeochemistry Interactions in the Tropical Ocean” , by the German Federal Ministry of

Education and Research as part of the co-operative projects “NORDATLANTIK”, “RACE”, “SOPRAN”, and “AWA” as well as by the Cluster of Excellence

“The Future Ocean”



Oxygen variability in the eastern tropical North Atlantic oxygen minimum zone