Do the Atlantic climate modes impact the ventilation of the eastern tropical North Atlantic oxygen minimum zone?

1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany2Christian-Albrechts-Universität zu Kiel, Kiel, Germany

Corresponding author: K. Burmeister (kburmeister@geomar.de)

Key PointsAtlantic Meridional Mode (AMM) associated with meridional shift and anomalous intensity of NEUC

Atlantic Zonal Mode (AZM) associated with anomalous intensity of NEUC

focus on variability of North Equatorial Undercurrent (NEUC)

Kristin Burmeister1 and Joke F. Lübbecke1,2

Fig. 1: Mean zonal velocity (a,b) and dissolved oxygen (c,d) along 23°W for 1972-85 (a,c), 1999-2008 (b,d). Note stronger zonal currents, generally higher oxygen levels; more pronounced oxygen maxima associated with eastward currents during 1972-85. From Brandt et al. (2010).

Variability of the ETNA OMZOxygen levels in the Eastern Tropical North Atlantic (ETNA) Oxygen Minimum Zone (OMZ) vary on daily to multi-decadal time scales. The long-term trend mainly consists of a deoxygenation (Stramma et al., 2008: Brandt et al., 2015).

One important question is how much of the deoxygenation is due to the natural variability of the ETNA zonal current field. Here we focus on the potential impact of the Atlantic climate modes onto the NEUC.

The AMM and AZM dominate tropical Atlantic variability (Fig. 2). Hormann et al. (2012) observed a relationship between NECC variability and the Atlantic climate modes. Our model data confirm their results.

The Atlantic climate modes

Impact of Atlantic Climate Modes on the NEUC

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005−1

−0.5

0

0.5

1

+0.7⋅σ

−0.7⋅σ

Fig. 2: First (b) and second (a) empirical orthogonal function of reanalysis (NOAA) Sea Surface Temperature (SST). March to May mean of AMM index (c; Servain 1991) and June to August mean of AZM index (d; Zebiak 1993) estimated from TRATL01 SST. Black boxes mark region for index calculation.

AMM

AZM1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

−2

−1

0

1

2

+0.7⋅σ

−0.7⋅σ

(a)

(b)

(c)

(d)

Estimation of NEUC central location YCM & intensity INT

Equ. 1 & 2: Central location YCM and intensity INT calculated according to Hsin (2016). Where x is longitude, t is time, z is depth, y is latitude, and u is zonal velocity (only positive values for NEUC).Integration limits for NEUC are the sigma-level of top (bottom) of flow Zt = 24.5 kg m-3 (Zb = 24.5 kg m-3), the southern (northern) limit of flow YS= 3.6°N (YN = 5°N), and the half-mean-width of flow W = 2°.

Model datahigh-resolution (0.1º) tropical Atlantic nest (TRATL01) within global ocean model NEMO-ORCA05 coupled with biogeochemical model

CORE forcing for time period 1958 to 2007

(Duteil et al., 2014)

comparison to observational and reanalysis data

repeat analysis for nNECC and NEC

correlation of variability of the water mass distributions in and the ventilation of the ETNA OMZ

correlation with Sverdrup relation

Outlook

Fig. 3: (a-e) JJA mean of TRATL01 horizontal velocities (arrows) within 100m to 400m depth and NEUC YCM (green line) from 1958 to 2007. Shading marks (a) JJA mean of TRATL01 zonal velocities from 1958 to 2007 and JJA mean of TRATL01 zonal velocity anomalies within 100m to 400m depth during years of (b) positive AMM events, (c) negative AMM events, (d) positive AZM events, (e) negative AZM events. All zonal velocity anomalies are calculated with respect to the JJA mean from 1958 to 2007.

1 m/s

TRATL01 ucur 100−400m JJA neg AMM 1958−2007

60°W 40°W 20°W 0°0°

4°N

8°N

≤ −0.5 −0.3 −0.1 0.1 0.3 ≥ 0.5

velocity in m/s

19−Apr−2017 13:54:04script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/plot_map_NEUC_JJA_TRATL01.mdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur(vcur)/TRATL01_interannual_clim_1958_2007_grid_U(V).nc

TRATL01 ucur 100−400m JJA s AMM 1958−2007

60°W 40°W 20°W 0°0°

4°N

8°N

≤ −0.1 −0.06 −0.02 0.02 0.06 ≥ 0.1

velocity in m/s

12−Apr−2017 16:42:06script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/AMM/plot_map_neg_AMM_NEUC_MAM_TRATL01.mdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur(vcur)/TRATL01_interannual_clim_1958_2007_grid_U(V).ncdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/AMM/TRATL01_neg_AMM_MAM_mean_1958_2007_grid_U.nc

AMM+

TRATL01 ucur 100−400m JJA neg AMM 1958−2007

60°W 40°W 20°W 0°0°

4°N

8°N

≤ −0.1 −0.06 −0.02 0.02 0.06 ≥ 0.1

velocity in m/s

12−Apr−2017 17:04:31script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/AMM/plot_map_neg_AMM_NEUC_MAM_TRATL01.mdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur(vcur)/TRATL01_interannual_clim_1958_2007_grid_U(V).ncdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/AMM/TRATL01_neg_AMM_MAM_mean_1958_2007_grid_U.nc

AMM-

TRATL01 ucur 100−400m JJA pos AZM 1958−2007

60°W 40°W 20°W 0°0°

4°N

8°N

≤ −0.1 −0.06 −0.02 0.02 0.06 ≥ 0.1

velocity in m/s

18−Apr−2017 12:02:36script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/AZM/plot_map_neg_AZM_NEUC_JJA_TRATL01.mdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur(vcur)/TRATL01_interannual_clim_1958_2007_grid_U(V).ncdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/AZM/TRATL01_neg_AZM_jja_mean_1958_2007_grid_U.nc

AZM+

TRATL01 ucur 100−400m JJA neg AZM 1958−2007

60°W 40°W 20°W 0°0°

4°N

8°N

≤ −0.1 −0.06 −0.02 0.02 0.06 ≥ 0.1

velocity in m/s

18−Apr−2017 12:03:40script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/AZM/plot_map_neg_AZM_NEUC_JJA_TRATL01.mdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur(vcur)/TRATL01_interannual_clim_1958_2007_grid_U(V).ncdata: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/AZM/TRATL01_neg_AZM_jja_mean_1958_2007_grid_U.nc

AZM-

JJA

JJA

JJA

JJA

JJA

(a)

(e)

(d)

(c)

(b)

Fig. 4: JJA mean of NEUC YCM (a, e) and INT (c, g) (bold lines) ± corresponding standard deviations (thin lines) for all years from 1958 to 2007 (black), for years of only positive (red) and negative (blue) AMM (a,c) and AZM (e,g) events. JJA anomalies of NEUC YCM (b,f) and INT (d,h) with respect to the JJA mean from 1958 to 2007 for positive (red) and negative (blue) AMM (b,d) and AZM (f,h) events.

AMM

NEUC YCM

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

44.5

55.5

44.5

55.5

19−Apr−2017 11:49:15script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_data 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_N

NEUC YCM anomaly

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

−0.2

0

0.2

0.4

−0.2

0

0.2

0.4

19−Apr−2017 11:49:16script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_Ndata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_N

AZM

degr

ee

latit

ude

NEUC INT

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

2468

10

2468

10

19−Apr−2017 11:49:16script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_Ndata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_N

NEUC YCM

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

19−Apr−2017 11:49:21script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_NEUC_INT_LOC_seasons_amm.mdata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_NEUC_loc_int_mm_24p5_27_3n6p5n_42w15w.mat

NEUC INT

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

40°W 30°W 20°W

2468

10

19−Apr−2017 11:49:22script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_NEUC_INT_LOC_seasons_amm.mdata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_NEUC_loc_int_mm_24p5_27_3n6p5n_42w15w.mat

NEUC INT anomaly

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

19−Apr−2017 12:04:13script: /home/kburmeister/Projekte/S B754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_NEUC_INT_LOC_seasons_amm.mdata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_NEUC_loc_int_mm_24p5_27_3n6p5n_42w15w.mat

NEUC YCM

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

19−Apr−2017 11:49:21script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_NEUC_INT_LOC_seasons_amm.mdata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_NEUC_loc_int_mm_24p5_27_3n6p5n_42w15w.mat

NEUC YCM

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

40°W 30°W 20°W

44.5

55.5

44.5

55.5

44.5

55.5

19−Apr−2017 11:49:21script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_data 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_N

NEUC INT anomaly

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

40°W 30°W 20°W

−1

0

1

−1

0

1

−1

0

1

19−Apr−2017 12:04:00script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_Ndata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_N

Sv

NEUC YCM anomaly

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

40°W 30°W 20°W−0.2

0

0.2

0.4

19−Apr−2017 11:49:22script: /home/kburmeister/Projekte/SFB754/Skripte/Matlab/TRATL01/ucur/current/NEUC/plot_NEUC_INT_LOC_seasons_amm.mdata 1: /home/kburmeister/Projekte/SFB754/Data/TRATL01/ucur/Ycm_INT/NEUC/TRATL01_NEUC_loc_int_mm_24p5_27_3n6p5n_42w15w.mat

(a)

(h)

(g)

(f)

(e)

(d)

(c)

(b)

the Atlantic NEUC is an eastward zonal undercurrent within 100m to 500m depth - during June to August (JJA) it is centered around 4.8°N and has a averaged intensity of 5.6Sv

positive AMM events: NEUC is shifted southward (Fig. 3b and 4 a,b) and weakens up to -1.5Sv (Fig. 4 c,d)

negative AMM events: NEUC is shifted northward (Fig. 3b and 4 a,b) and slightly intensifies (Fig. 4 c,d)

intensity anomalies during negative AMM events are weaker compared to positive AMM events (Fig. 4d)

no anomalous meridional shift of NEUC during AZM events (Fig. 3d,e and 4 e,f)

NEUC intensity strengthens (weakens) up to 1Sv (-1Sv) during positive (negative) AZM events

Brandt, Hormann, Körtzinger, Visbeck, Krahmann, Stramma, Lumpkin, and Schmid (2010), Changes in the Ventilation of the Oxygen Minimum Zone of the Tropical North Atlantic. J. Phys. Oceanogr., 40, 1784-1801. Brandt, Bange, Banyte, Dengler, Didwischus, Fischer, Greatbatch, Hahn, Kanzow, Karstensen, Körtzinger, Krahmann, Schmidtko, Stramma, Tanhua, and Visbeck (2015) On the role of circulation and mixing in the ventilation of oxygen minimum zones with a focus on the eastern tropical North Atlantic. Biogeosciences, 12, 489-512. Duteil, Schwarzkopf, Böning, and Oschlies (2014), Major role of the equatorial current system in setting oxygen levels in the eastern tropical Atlantic Ocean: A high-resolution model study, Geophys. Res. Lett., 41, 2033–2040. Hormann, Lumpkin, & Foltz (2012), Interannual North Equatorial Countercurrent variability and its relation to tropical Atlantic climate modes. J. Geophys. Res., 117(C4). Hsin (2016), Trends of the Pathways and Intensities of Surface Equatorial Current System in the North Pacific Ocean. J. Clim., 29(18), 6693-6710. Servain (1991), Simple climatic indices for the tropical Atlantic Ocean and some applications, J. Geophys. Res., 96(C8), 15137–15146. Stramma, Johnson, Sprintall, and Mohrholz (2008) Expanding oxygen-minimum zones in the tropical oceans. Science, 320, 655–658. Zebiak (1993), Air-sea interaction in the equatorial Atlantic region, J. Clim., 6(8), 1567–1586.

Changes in oxygen levels are associated with changes in the eastward zonal current bands like the NECC, nNECC, or NEUC (Fig. 1; Brandt et al. (2010)).