OSOcean ScienceOSOcean Sci.1812-0792Copernicus PublicationsGöttingen, Germany10.5194/os-12-861-2016Observed El Niño conditions in the eastern tropical Pacific in October
2015StrammaLotharlstramma@geomar.dehttps://orcid.org/0000-0003-1391-4808FischerTimGrundleDamian S.KrahmannGerdBangeHermann W.https://orcid.org/0000-0003-4053-1394MarandinoChrista A.GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker
Weg 20, 24105 Kiel, Germanynow at: Bermuda Institute of Ocean Sciences (BIOS), 17 Biological
Station, St George's GE 01, BermudaLothar Stramma (lstramma@geomar.de)4July201612486187311March201624March20166June201610June2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://os.copernicus.org/articles/12/861/2016/os-12-861-2016.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/12/861/2016/os-12-861-2016.pdf
A strong El Niño developed in early 2015. Measurements from a research
cruise on the R/V Sonne in October 2015 near the Equator east of the
Galapagos Islands and off the shelf of Peru are used to investigate changes
related to El Niño in the upper ocean in comparison with earlier cruises
in this region. At the Equator at 85∘30′ W, a clear temperature
increase leading to lower densities in the upper 350 m had developed in
October 2015, despite a concurrent salinity increase from 40 to 350 m. Lower
nutrient concentrations were also present in the upper 200 m, and higher
oxygen concentrations were observed between 40 and 130 m. In the equatorial
current field, the Equatorial Undercurrent (EUC) east of the Galapagos
Islands almost disappeared in October 2015, with a transport of only 0.02 Sv
in the equatorial channel between 1∘ S and 1∘ N, and a weak
current band of 0.78 Sv located between 1 and 2∘30′ S. Such
near-disappearances of the EUC in the eastern Pacific seem to occur only
during strong El Niño events. Off the Peruvian shelf at
∼ 9∘ S, characteristics of upwelling were different as warm,
saline, and oxygen-rich water was upwelled. At ∼ 12, ∼ 14, and
∼ 16∘ S, the upwelling of cold, low-salinity, and oxygen-poor
water was still active at the easternmost stations of these three sections,
while further west on these sections a transition to El Niño conditions
appeared. Although from early 2015 the El Niño was strong, the October
measurements in the eastern tropical Pacific only showed developing El
Niño water mass distributions. In particular, the oxygen distribution
indicated the ongoing transition from “typical” to El Niño conditions
progressing southward along the Peruvian shelf.
Introduction
The El Niño–Southern Oscillation (ENSO) cycle of alternating warm El
Niño and cold La Niña events is the dominant year-to-year climate
signal on earth. ENSO originates in the tropical Pacific through interaction
between the ocean and the atmosphere, but its environmental and socioeconomic
impacts are felt worldwide (McPhaden et al., 2006). In the eastern tropical
South Pacific, El Niño events strongly influence the commercial fishery
and weather and impact on the economics and living conditions.
The strongest El Niño events since 1950 were observed in the years
1982/83 and 1997/98, the latter also referred to as “the climate event of
the twentieth century” (Changnon, 2000). Climate models suggest a doubling
in the occurrences of extreme El Niño events in the future in response to
greenhouse warming (Cai et al., 2015). In early 2015, an El Niño with
strength similar to the 1997/98 El Niño developed. Sea surface
temperature anomalies were strongest along the Equator and the tropical North
Pacific, while the development of a temperature anomaly in the eastern
tropical Pacific off Peru was, according to NOAA's “ENSO diagnostic
discussion archive”, strong in April and May, and then weakened and
intensified again from August to October 2015.
El Niño dynamics modulate near-surface temperature, salinity, and
density, as well as the mixed layer depth, oxycline depth, and the vertical
extent of the low oxygen layer (e.g., Fuenzalida et al., 2009). In the
eastern Pacific, ENSO variability is most pronounced along the Equator and
the coasts of Ecuador and Peru (Wang and Fiedler, 2006), but also off Chile
(e.g., Ulloa et al., 2001). Weaker trade winds during El Niño conditions
result in a weaker equatorial circulation with a generally observed weakening
or disappearance of the Equatorial Undercurrent (EUC) (Kessler and McPhaden,
1995; Johnson et al., 2002). During the height of an El Niño event, the
EUC episodically disappears in the western and central Pacific and partially
reverses (Firing et al., 1983; McPhaden et al., 1990; Johnson et al., 2000;
Izumo, 2005), while in the eastern Pacific, episodic disappearance of the EUC
seems rare (Halpern, 1987; McPhaden and Hayes, 1990; Seidel and Giese, 1999;
Johnson et al., 2000; Izumo, 2005). El Niño events lead to a pronounced
eastward extension of the western Pacific warm pool and to a development of
atmospheric convection, and hence a rainfall increase, in the usually cold
and dry eastern Pacific (Cai et al., 2015).
Past El Niño events have been observed to have different local
occurrences and parameter distributions in recent years. There has been
evidence of an increased occurrence of El Niño events in the central
Pacific called Central Pacific (CP) El Niño or “El Niño Modoki”
(e.g., Ashok and Yamagata, 2009; Dewitte et al., 2012), different from the
cold tongue, or Eastern Pacific (EP), El Niño events that develop in the
eastern Pacific. For a typical CP El Niño, the largest sea surface
temperature (SST) increase occurs at the Equator between 130∘ W and
160∘ E, while cooling appears off the shelf of Peru. For the EP El
Niño the SST increases at the Equator east of 180∘ W to South
America and southward along the South American coast to Chile (e.g., Dewitte
et al., 2012).
In the eastern tropical South Pacific (ETSP), a subsurface low oxygen zone
exists with a pronounced minimum in oxygen at ∼ 100 to 500 m depth and
is referred to as an oxygen minimum zone (OMZ) or oxygen deficient zone
(ODZ). This ODZ is suboxic (oxygen concentrations below
∼ 4.5–10.0 µmol kg-1; e.g., Karstensen et al., 2008;
Stramma et al., 2008). In suboxic regions nitrate and nitrite become involved
in respiration processes such as denitrification or anammox (e.g., Kalvelage
et al., 2013). In the eastern equatorial Pacific the oxygen content has been
shown to increase during El Niño events in the upper 300 to 350 m in the
equatorial channel (e.g., Fuenzalida et al., 2009; Czeschel et al., 2012), as
well as off the Peruvian coast (e.g., Helly and Levin, 2004). Coastal winds
during El Niño events are usually upwelling favorable, and thus could not
produce the observed warming (Kessler, 2006). Coastal warming during El
Niño is caused by downwelling Kelvin waves generated by mid-Pacific
westerly wind anomalies that deepen the eastern thermocline, nutricline, and
oxycline and allow warming to occur, independent of the local winds (Kessler,
2006). Consequently, during El Niño events the upwelled water off Peru is
warmer, more oxygen replete and less nutrient rich. El Niño, in general,
results in a depressed thermocline and thus reduced rates of macronutrient
supply and primary production (Pennington et al., 2006) off Peru, which also
contributes to an oxygen increase on the shelf (Gutiérrez et al., 2008).
In the case of strong El Niño events when the oxygen concentration above
the shelf bottom increases from about zero to
> 40 µmol kg-1, the sediments respond with
tremendous changes in ecological state (Gutiérrez et al., 2008). At a
time-series station at ∼ 12∘ S, 77∘30′ W off Lima
from 1996 to 2010 for temperature, salinity, density, oxygen, and nutrients,
the influence of El Niño – especially the strong 1997/98 El Niño –
is clearly visible, with higher temperature, salinity, and oxygen, and lower
density, nitrite, silicate, and phosphate (Graco et al., 2016).
Here we use measurements from an R/V Sonne research cruise in
October 2015 (Fig. 1) from a section across the Equator east of the Galapagos
Islands and from four sections off the Peruvian shelf, to investigate changes
in the upper ocean related to the strong 2015 El Niño in comparison with
earlier cruises in this region. The aim is to unravel the progress of the
transition to El Niño conditions in the eastern Pacific several months
after the start of the El Niño.
Cruise track (blue line) and CTD (conductivity–temperature–depth)
stations (black circles) of cruise R/V Sonne from Guayaquil
5 October to Antofagasta 22 October 2015, as well as equatorial stations
March 1993, February 2009, and November 2012 (yellow “x”), CTD sections off
Peru December 2012 (red “x”), and ADCP sections across the Equator (yellow
line). Topography (color) with depth/height contours in 1000 m intervals
enhanced by the 200 m depth contour.
Data sets and methods
In October 2015 an R/V Sonne transit cruise (So243; 5 to
22 October 2015) from Guayaquil, Ecuador, to Antofagasta, Chile, was carried
out (Fig. 1) (short cruise report available at
https://www.ldf.uni-hamburg.de/sonne/wochenberichte/wochenberichte-sonne/so242-243/so243-scr.pdf),
which allowed us to investigate possible El Niño signals at the Equator
near 85∘30′ W and off the shelf of Peru at sections perpendicular
to the shelf at ∼ 9, ∼ 12, ∼ 14, and
∼ 16∘ S.
A Seabird CTD system with a GO (General Oceanics) rosette with
24 × 10 L water bottles was used for water profiling and discrete
water sampling. The CTD system was used with double sensors for temperature,
conductivity (salinity), and oxygen. The dual CTD temperature sensors
calibrated by the manufacturer are compared during the cruise so that the
deviation is less than 0.002 ∘C, and the accuracy of the temperature
measurements is estimated to be 0.002 ∘C or better. The CTD salinity
calibration with salinometer salinity samples resulted in a rms uncertainty
of 0.0011. The CTD oxygen sensors were calibrated with oxygen measurements
obtained from discrete samples from the rosette applying the classical
Winkler titration method, using a non-electronic titration stand (Winkler,
1888; Hansen, 1999). The rms uncertainty of the CTD oxygen sensor calibration
of cruise So243 was determined to be ±0.8 µmol kg-1.
Oxygen concentrations of less than 3 µmol kg-1 are not
resolved by Winkler titration and values below 3 µmol kg-1
were used as 0 µmol kg-1 for the sensor calibration, as the
H2S smell of the water of related rosette bottles indicated
0 µmol kg-1.
Nutrients were measured on-board with a QuAAtro auto-analyzer (Seal
Analytical). Nitrite (NO2-), nitrate (NO3-), phosphate
(PO43-), and silicid acid (Si(OH)4, referred to as silicate
hereinafter) were measured with an analytical precision of 5.5, 1.3, 0.4, and
0.5 % respectively. The N : P ratio used here was computed as
N : P = (NO3-+ NO2-) : PO43-.
Two vessel-mounted acoustic Doppler current profilers (ADCP) were used to
record ocean velocities in October 2015: an RDI OceanSurveyor 75 kHz ADCP
with 8 m bin spacing provided the velocity distribution to ∼ 650 m
depth, while a 38 kHz ADCP with 32 m bin spacing provided velocity profiles
down to ∼ 1300 m depth. During the entire cruise the navigation data
was of high quality. Due to the interest in the upper ocean, the
higher-resolution 75 kHz ADCP is used here.
Earlier crossings of the Equator (Table 1 and Fig. 1) were accomplished in
March/April 1993 on R/V Knorr (Tsuchiya and Talley, 1998), in
February 2009 on R/V Meteor (Czeschel et al., 2011), and in
November 2012 on R/V Meteor (Stramma et al., 2013) at
85∘50′ W. Sections across the Peruvian shelf between 9 and
16∘ S were made during R/V Meteor cruise M91 in December
2012 (Czeschel et al., 2015; Bange, 2013). Measurement accuracies during
these cruises were similar to October 2015 and the details are described in
the related literature. In contrast to October 2015, the CTD stations in
1993, 2009, and 2012 were not carried out at 2∘30′ S,
85∘30′ W, but at 2∘20′ S and 2∘40′ S at
85∘50′ W, and these two stations were combined for a mean profile
at 2∘30′ S. The sections across the Equator and off the Peruvian
shelf were not at identical geographical coordinates, but we expect that the
offset will be small compared to the differences measured.
Time and geographical location of CTD data used in this study and
the NINO 1+2 and ONI indices for the months of observation or for 2 months
for measurements carried out at the end or beginning of a month listed in the
tables (http://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices and
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml).
TimeLocationNINO 1+2ONI29–31 Mar 19931∘ N–2∘40′ S+0.65 Mar, +0.97 Apr+0.5 March, +0.7 Apr12–13 Feb 20091∘ N–2∘40′ S-0.11-0.71–3 Nov 20121∘ N–2∘40′ S-0.11 Oct, -0.38 Nov+0.4 Oct, +0.2 Nov7–8 Oct 20151∘ N–2∘30′ S+2.52+2.06–23 Dec 2012∼ 9∘ S–∼ 16∘ S-0.68-0.210–19 Oct 2015∼ 9∘ S–∼ 16∘ S+2.52+2.1
Different indices exist to describe the El Niño status and will be used
here to determine the El Niño status at the time of the measurements. The
NINO 1+2 index is the temperature difference compared to the 1982–2005
climatological cycle in the eastern tropical Pacific (0–10∘ S,
80–90∘ W), and is close to the region of the measurements used
here. The Oceanic Niño Index (ONI) has become a standard for identifying
El Niño and La Niña events. It is a running 3-month mean SST anomaly
for the Niño 3.4 region (i.e., 5∘ N–5∘ S,
120–170∘ W) related to the 1981–2010 base period. Events are
defined as five consecutive overlapping 3-month periods at or above the
0.5 ∘C anomaly for warm El Niño events, and at or below the
-0.5 ∘C anomaly for cold La Niña events.
The El Niño in 2015
The SST anomaly for 27 September 2015 to 24 October 2015 was strong along the
Equator to the South American continent and southward off the Peruvian coast
(Fig. 2). The NINO 1+2 index was high at +2.52 in October 2015 (Table 1);
hence, the 2015 El Niño is a clear EP El Niño. The SST distribution
in fall 2015 shows a strong and prominent SST increase along central America
and in the eastern North Pacific at 20–25∘ N that differs from the
typical EP El Niño distribution. This feature, also known as “The
Blob”, is an unrelated positive temperature anomaly that developed in 2013
in the Gulf of Alaska and progressed along the North American continent to
the 20–25∘ N region in mid-2015 (Kintisch, 2015).
Average sea surface temperature anomalies in ∘C for the
period 27 September to 24 October 2015. Image with permission from NOAA
extracted from the
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf
file version from 26 October 2015, redrawn with very minor modifications to
improve the figure quality.
The only El Niño events since 1950 with an October maximum ONI of more
than 1.7, or an overall maximum of 2.0 or larger, are the 1972/73, 1982/83,
and 1997/98 El Niños. In early 2015 the ONI was even larger than the ONI
of these three large El Niño events, while in October 2015 it was at a
similar strength as the three earlier strong El Niños (Fig. 3).
Accordingly, the 2015 El Niño has to be listed as one of the four
strongest El Niños since 1950.
ONI for the strong El Niño years 1972 (dash-dotted), 1982
(dotted), 1997 (dashed), and the years used here: 1993 (green), 2009 (blue),
2012 (red), and 2015 (black line). The months of measurements used here are
marked by colored dots.
The equatorial region east of the Galapagos IslandsThe hydrographic variabilityBackground information
The hydrographic distribution in the eastern equatorial Pacific is influenced
by a seasonal cycle as well as El Niño-related cycles. At 110∘ W
in the eastern Pacific west of the Galapagos Islands, relationships between
zonal velocity, temperature, and salinity in the EUC are all evident in the
seasonal cycle. The EUC peaks in strength around April/May, when it also
surfaces (Johnson et al., 2002). The thermocline is extremely sharp and
shallow. The meridional equatorial spreading of the thermocline associated
with the EUC zonal velocity strength is noticeably stronger during April than
in October, when in April equatorial SST is lowest and the South Equatorial
Current (SEC) is strongest (Johnson et al., 2002). The laterally isolated
salinity maximum within the thermocline just south of the Equator is
strongest when the EUC velocity is at its greatest (Johnson et al., 2002),
and this is also visible in the sea surface salinity (Supplement Fig. S1)
from the MIMOC climatology (Schmidtko et al., 2013). Between austral fall and
winter the minimal oxygen concentration of the ODZ core in the eastern South
Pacific at the Equator changes from 8 to 5 µmol O2 L-1
(Paulmier and Ruiz-Pino, 2009).
Weaker trade winds during El Niño conditions result in a weaker
equatorial circulation, while stronger trade winds during La Niña
conditions lead to a stronger equatorial circulation (Johnson et al., 2002).
During La Niña, the current system at 110∘ W is spun upwards
when compared to El Niño. The cold tongue located in the eastern tropical
Pacific is quite weak during El Niño. Surface salinities are generally
fresher during El Niño than during La Niña, a feature that is at
least partially a product of increased local precipitation associated with
the eastward migration of warm sea surface temperatures and convection, and
partly a result of the reduced trade winds (Johnson et al., 2002). For the
1996–1998 El Niño–La Niña cycle, a fresh mixed layer in the eastern
equatorial Pacific and higher salinity within the pycnocline (defined by the
20 ∘C isotherm) during the El Niño was observed. The higher
salinity was caused by the larger equatorward spreading of the subsurface
salinity maximum of the South Pacific Tropical Water (SPTW) due to anomalous
eastward flow south of the Equator with the relaxation of the South
Equatorial Current and the weaker EUC at the Equator (Johnson et al., 2000).
Hence, during El Niño events higher salinity should be expected near the
pycnocline.
Observations for the 2015 El Niño
SST anomalies for the period 27 September to 24 October 2015 showed an SST
anomaly of 2.0–2.5 ∘C at 85∘30′ W at and south of the
Equator, and of 1.5–2.0 ∘C just north of the Equator (Fig. 2). In
the upper 100 m of the water column, oxygen, temperature, salinity, and
density profiles at the Equator on the ∼ 85∘30′ W meridian
(Fig. 4) reveal differences between March 1993, February 2009, November 2012,
and the El Niño of October 2015. It is important to note that 1993 was
not defined as an official El Niño year as only four, instead of five,
consecutive overlapping 3-month periods were at or above the 0.5 ∘C
anomaly. However, in March and April 1993 the NINO 1+2 index reached
+0.65 and +0.97 (Table 1) and had El Niño-like SST anomalies. To this
end, we will refer to March 1993 as `El Niño-like” hereinafter.
Upper 100 m profiles at the Equator at 85∘50′ W for
30 March 1993 (green), 12 February 2009 (blue), 2 November 2012 (red), and at
the Equator at 85∘30′ W for 7 October 2015 (black)
for (a) oxygen in µmol kg-1, (b) potential
temperature in ∘C, (c) salinity, and (d) potential
density in kg m-3.
In February 2009 the ONI was for the fourth and last month -0.5 or less;
therefore, conditions were similar to a weak La Niña event and we will
refer to it as “La Niña-like” hereinafter. In February 2009, in the
upper 100 m at the Equator at ∼ 85∘30′ W, the oxygen and
temperature were lowest and the density highest compared to the other three
periods (Fig. 4), representing an expected La Niña parameter
distribution. The hydrographic profiles in the neutral ONI period in November
2012 mainly lay between the El Niño profiles for March 1993 and
October 2015, and the La Niña-like profiles in February 2009. The
November 2012 profiles were somewhat closer to the February 2009 profiles.
The El Niño profiles in October 2015 and the El Niño-like profiles in
March 1993 showed slightly higher oxygen concentrations and temperature, and
lower density in the upper 100 m in comparison to November 2012 and
February 2009 (Fig. 4). In October 2015 the salinity compared to the 3 other
years was lowest in a deep thermocline in the upper 40 m, as expected for
the surface layer during an El Niño event because of the increased
precipitation and reduced equatorial upwelling. In contrast, a weak salinity
maximum was located below 40 m, as expected near the pycnocline as saline
warm water progresses from the western Pacific eastward during El Niño.
In October 2015 the higher temperature, higher salinity, and lower density
reached down to ∼ 350 m, while the oxygen profile below 130 m merges
with the profiles from the other measurement periods (Fig. 5).
Same as Fig. 4 but for 100 to 400 m depth.
The strong thermocline/pycnocline of the eastern tropical Pacific is also a
strong nutricline. A consistent general pattern is that nitrate and phosphate
increase with depth to ∼ 500 m with a slight maximum at intermediate
depths, while silicate continues to increase with depth (Fiedler and Talley,
2006). The vertical distribution of nutrients at the Equator at
∼ 85∘30′ W shows lower nitrate, phosphate, and silicate
concentrations in the upper 200 m in October 2015 as well as in the El
Niño-like year 1993 in comparison to the 2009 and 2012 concentrations
(Fig. 6). A primary nitrite maximum (PNM) usually occurs in the lower
euphotic zone, which results from nitrite excretion by phytoplankton and/or a
decoupling of ammonia and nitrite oxidation (i.e., higher rates of ammonia
vs. nitrite oxidation; Fiedler and Talley, 2006; Lomas and Lipschultz, 2006).
At the Equator in the eastern Pacific nitrite is close to undetectable below
100 m depth (Fig. 6). In 1993 and 2015, however, the PNM was located
∼ 25 m deeper and maximum nitrite concentrations were considerably
higher. This reflects the deeper pycnocline in El Niño years. The
enhanced nitrite concentrations seem to be caused by the northward transport
of high nitrite concentration found in the PNM of the SPTW (e.g., Tsuchiya
and Talley, 1998, and as observed during cruise M90) in combination with a
reduced photo inhibition of ammonia oxidation at deeper water depths (see,
e.g., Ward, 2008; Grundle et al., 2013). Furthermore, the deep thermocline
and pycnocline in October 2015 indicate that equatorial upwelling was
reduced. According to the upper ocean hydrographic and nutrient distribution
in October 2015, a clear El Niño situation had adjusted at the Equator at
∼ 85∘30′ W.
Upper 300 m profiles at the Equator at 85∘50′ W for
30 March 1993 (green), 12 February 2009 (blue), 2 November 2012 (red), and at
the Equator at 85∘30′ W for 7 October 2015 (black) for
(a) nitrate in µmol L-1, (b) nitrite in
µmol L-1 (scale change at 0.1µmol L-1),
(c) phosphate in µmol L-1, and (d) silicate
in µmol L-1.
At 1∘ N, ∼ 85∘30∘ W in the 50–300 m layer,
salinity and temperature were higher and the density lower in October 2015
than at the other three times; however, the oxygen was not significantly
higher during this time (Fig. S2). At 2∘30′ S,
∼ 85∘30′ W in the 50 to 250 m layer, the salinity,
temperature, and oxygen were lower and the density higher in the La
Niña-like month of February 2009 than in the other 3 years. The
temperature at 2∘30′ S in the El Niño of October 2015 was
higher in the 50 to 250 m depth range than in the other 3 years and salinity
showed slightly higher values at ∼ 50 to 100 and 150 to 250 m depth.
The oxygen concentration was slightly higher only in the upper ∼ 60 m
for both El Niño events in March 1993 and in October 2015 compared to
2009 and 2012 (Fig. S3). However, earlier selected measurements at
4∘ S, 85∘ W showed a clear oxygen increase to a depth of
∼ 350 m depth for the El Niño years 1982/83 in comparison to
non-El Niño measurements (Czeschel et al., 2012); hence, we conclude that
El Niño influence on the water mass distribution was still weak at
2∘30′ S in October 2015 and only developing in the upper ocean.
Circulation observationsBackground information
The EUC, which carries oxygen-rich water towards the ODZs of the eastern
Pacific, has a seasonal cycle, with a transport peak in the eastern Pacific
at 95∘ W in April/May of ∼ 30 Sv and a minimum in
October/November of a little less than 15 Sv (Johnson et al., 2002; their
Fig. 17). The Galapagos Islands form a barrier to the EUC, which causes it to
bifurcate into a shallow/southern core centered at ∼ 50 m depth (EUCs)
and a deeper/northern core centered at ∼ 150 m depth (EUCd)
(Karnauskas et al., 2010). We are not aware of any El Niño-related EUC
variability observations east of the Galapagos Islands. Results from an ocean
model for 110∘ W show an increase in the surface eastward EUC
current during austral fall, while during other seasons the EUC is at deeper
depth (Cravatte et al., 2007). ROMS (Regional Ocean Model System) model
results (Montes et al., 2011) for El Niño periods east of the Galapagos
Islands show the EUC flowing at a shallower depth associated with lighter
water. The model runs display a weakening and southward shift of the EUC
branches east of the Galapagos Islands with weaker transports, which are
variable, depending on the boundary conditions provided by different ocean
general circulation models (OGCMs). The modeled ROMS EUC transports at
86∘ W between 2∘ N and 2∘ S at 200 m depth for
February/March and October/November depending on the OGCMs are ∼ 10–12
and ∼ 7–8 Sv for OCCAM, ∼ 6–8 and 4–5 Sv for SODA, and
∼ 5.5 and 7–8 SV for ORCA (Echevin et al., 2011).
During the 1997/98 El Niño shipboard current measurements showed that the
EUC virtually disappeared across much of the Pacific basin, associated with
the weakening or even the reversal of the equatorial pressure gradient within
the pycnocline (Johnson et al., 2000). For the 1982–1983 El Niño there
seems to be a strong time delay for the EUC weakening. In September 1982 at
159∘ W, the EUC reversed (Firing et al., 1983); however, at
95∘ W, the EUC was strong in November 1982 before being replaced by
a westward jet in May 1983 (Hayes et al., 1986).
Observations for the 2015 El Niño
The direct velocity observations in October 2015 on the diagonal section from
the Ecuadorian shelf to 1∘ N, 85∘30′ W, show only a weak
signature of the zonal EUC in the upper 100 m located mainly at, and south,
of the Equator (Fig. 7a). The weak transport of the EUC is 0.01 Sv between
1∘ S and 1∘ N, and 0.29 Sv between 2∘30′ S and
1∘ S. The westward flow in the upper 200 m is mainly connected to a
northward flow direction (Fig. 7b) north of 1∘ S. This northwesterly
flow indicates the flow of oxygen-poor water from the ODZ off the South
American continent to the west near the Equator.
ADCP zonal (positive eastward) (a) and meridional (positive
northward) (b) velocity sections (in m s-1) on the diagonal
section from the Ecuadorian shelf at ∼ 2∘30′ S to
1∘ N, 85∘30′ W on 5 and 6 October 2015 (see Fig. 1). The
contour interval is 0.1 m s-1.
The direct velocity observations on the meridional section at
85∘30′ W between 1∘ N and 2∘30′ S and the
diagonal continuation to 5∘ S, 84∘16′ W again show the
weak signature of the EUC in the upper 100 m located mainly at and south of
the Equator north of 2∘30′ S in October 2015 (Fig. 8d). The EUC
transport in the upper 300 m is 0.02 Sv between 1∘ S and
1∘ N and 0.78 Sv between 2∘30′ S and 1∘ S
(Table 2). The stronger eastward flow component in the upper 200 m between
3∘ S and 4∘ S might be a combination of the EUC and the
Southern Subsurface Countercurrent (SSCC; also called the Tsuchiya Jet), as
described for El Niño periods east of the Galapagos Islands in model
results (Montes et al., 2011). The eastward flow near 2∘ S below
200 m is the South Intermediate Countercurrent (SICC).
Zonal ADCP velocity sections (in m s-1; positive eastward;
contour interval 0.1 m s-1) on the meridional path from 1∘ N
to 5∘ S at 85∘50′ W (a) in March 1993,
(b) in February 2009, (c) in November 2012, and
(d) from 1∘ N to 2∘30′ S at 85∘30′ W
and diagonal to 5∘ S, 84∘12′ W in October 2015 (see
Fig. 1).
Summed zonal positive (eastward) and negative (westward) ADCP
transports in Sv (106 m3 s-1) in the equatorial channel at
85∘50′ W in March 1993, February 2009, and November 2012 and at
85∘30′ W in October 2015 as well as the related El Niño
status. The velocity data were slightly smoothed and extrapolated to the
surface.
Time1∘ S–1∘ N0–300 m2∘30′ S–1∘ S0–300 mEl Niño status29–31 Mar 199312.77-0.386.28-0.07Early El Niño-like12–13 Feb 20093.55-1.580.55-1.57Late La Niña-like1–3 Nov 201210.78-0.944.22-0.36Neutral7–8 Oct 20150.02-13.860.78-4.08El Niño
The strongest EUC in our four measurement periods occurred at the end of
March 1993, with 12.77 Sv between 1∘ S and 1∘ N in the
upper 300 m (Table 2; Fig. 8a). This measurement at the end of March was
close to the time of eastern Pacific EUC peak transport in April/May. In
addition, it was at the beginning of an El Niño-like phase, where warmer,
oxygen-rich water is transported from the western to the eastern tropical
Pacific and could enhance the eastward flow component.
In February 2009 the EUC transport between 1∘ S and 1∘ N
was weak (3.55 Sv), although it occurred only 2 months before the time of
the seasonal EUC peak transport. As previously described, February 2009 was
at the end of a short La Niña-like period with an ONI of -0.7, and the
low EUC transport might be related to a generally weak eastward transport of
warm western equatorial Pacific water during La Niña. On a cruise
approximately 1.5 months later in March to April 2009 between the Galapagos
Islands and Ecuador, a region of possible strong cross-hemispheric exchange
was observed immediately to the east of the Galapagos Islands, where a
shallow (200 m) 300 km wide northeastward surface flow transported 7 to
11 Sv (Collins et al., 2013). This northeastward flow might have weakened
the EUC transport at and south of the Equator. The two diagonal sections in
March/April 2009 crossed the 85∘50′ W section at
∼ 1∘50 and 2∘30′ S and, similarly to the
February 2009 measurements, showed a 50 m depth eastward and westward flow
at 1∘50 and 2∘30′ S, respectively, and a westward flow at
both of these latitudes at 200 m depth. In contrast to the velocity
distribution in March 1993, November 2012, and October 2015 (Fig. 8), the
eastward flow component in the upper 200 m south of 2∘30′ S
almost disappeared in February 2009.
The EUC transport in November 2012 at 85∘50′ W between
1∘ S and 1∘ N was 10.78 Sv in the upper 300 m (Table 2).
The months before these measurements had no large ONI values and should
represent the non-El Niño EUC transport in this region for November. The
transport of 10.78 Sv in November at 85∘50′ W is less than the
November minimum at 95∘ W of ∼ 15 Sv (Johnson et al., 2002;
their Fig. 17), and seems to be a reasonable estimate east of the Galapagos
Islands, as the EUC transport decreases in the eastern Pacific. The core of
the EUC below 200 m is quite deep and agrees with the seasonal cycle where
the EUC should be located at deeper depth in austral spring.
The upwelling region off PeruBackground information
Off Peru a highly productive year-round upwelling system is located between 4
and 16∘ S (Chavez and Messié, 2009). Since the 1950s, an SST
decline corresponding to an increase in upwelling has been observed off Peru
(Gutiérrez et al., 2011). The SST off Peru measured at six locations
between 5 and 12∘ S over a period of 6 years shows a seasonal cycle
of 2 to 3 ∘C amplitude with the largest SST near March and the
minimum near October (Montes et al., 2011; their Fig. 4). This seasonal cycle
is also visible in the MIMOC climatology for 9 and 12∘30′ S
(Fig. S1). The time-series station at ∼ 12∘ S,
77∘30′ W shows a seasonal cycle of about 20 m displacement for
the 15 ∘C isotherm, the oxycline depth, and the upper boundary of
the ODZ (Graco et al., 2016). Seasonal eddy fluxes are described along the
coast of Peru, with the largest signal at approximately 15∘ S with a
peak during the austral winter (Vergara et al., 2016). The typical nutrient
distribution along a cross-shelf section at 12∘ S (as seen in
December 2012) shows elevated phosphate concentrations in the surface waters
near the coast, whereas nitrate is depleted in the water column and the
near-surface waters close to the coast (Kock et al., 2016; their Fig. 3).
Conditions that develop along the coast of Ecuador, Peru, and northern Chile
during El Niño events include a strengthening of the poleward flow along
the coast of Peru, persistent deepening of the thermocline, reducing or even
reversing the prevailing upwelling-induced land–sea temperature gradient,
and a southward shift in the position of the ITCZ (Inter-Tropical Convergence
Zone), which brings heavy precipitation to normally arid regions (Strub et
al., 1998). A reduction in coastal cloud cover due to warmer water next to
the coast may enhance insolation and reduce atmospheric pressure over land,
maintaining the pressure difference and winds over the coast. As a result,
upwelling-favorable winds are not greatly reduced when El Niño conditions
are observed in the ocean (Enfield 1981; Huyer et al., 1987; Strub et al.,
1998; Halpern et al., 2002). In general, upwelling-favorable winds and
upwelling continue during El Niño events, and water continues to be drawn
from 50 to 100 m depth to the surface layer, but the thermocline and
nutricline are displaced downward and thickened, so that upwelling during El
Niño brings only warm and nutrient-poor water to the surface (Enfield
1981; Huyer et al., 1987; Strub et al., 1998; Halpern et al., 2002). The
intensity of the upwelling appears to be determined by an interplay between
along-shore, poleward advection and wind intensity, but also by the
cross-shore geostrophic flow and distribution of the water masses on a scale
of 1000 km or more (Colas et al., 2008). In relation to the downward
displacement of the thermocline and nutricline, the oxycline is also
displaced downward. For the 1997/98 El Niño event, Helly and Levin (2004)
described a possible depression of the upper layer of the ODZ (defined by
oxygen concentrations < 0.5 mL L-1;
∼ 22.3µmol L-1) by 100 m, reducing the ODZ area off
Peru and northern Chile (6–20∘ S) by 61 % (from 77 000 to
30 000 km2).
Observations for the 2015 El Niño
The SST anomalies for the period 27 September to 24 October 2015 (Fig. 2)
showed a strong SST anomaly of 1.5–2.0 ∘C between 8 and
14∘ S and a weaker anomaly of 0.5–1.5 ∘C between 14 and
20∘ S. Differing hydrographic distributions were measured off Peru
at ∼ 9∘ S in December 2012 with a neutral ONI status and in
October 2015 with the strong El Niño. In the entire upper 300 m at
∼ 9∘ S, temperature, salinity (Figs. S4 and S5), and oxygen
(Fig. 9) were higher in October 2015 than in December 2012. In contrast to
the typical seasonal cycle that is characterized by lower SST in October than
in December, the SST at 9∘ S was higher in October 2015 than in
December 2012 as a result of the El Niño-related SST increase. Higher
upper water column temperatures in October 2015 also correlated with lower
densities in the upper 300 m (as can be seen from the selected isopycnals in
Fig. 9) despite the concurrent influence on density from the salinity
increase. Accordingly the density changes are temperature dominated. In
December 2012 there was strong upwelling at ∼ 9∘ S with the
< 5 µmol kg-1 O2 layer located below
∼ 30 m depth, while in October 2015 this low oxygen layer was only
found below 240 m depth. The October 2015 nutrient profiles obtained from
shelf stations at ∼ 9∘ S with water depths of little more than
100 m (not shown) highlight the fact that nitrate, phosphate, and silicate
concentrations were lower, and nitrite concentrations were higher in
comparison to profiles from the same location in December 2012, as would be
expected for El Niño periods. Although the isopycnals and parameter
distribution show that upwelling at 9∘ S was occurring in October
2015, it is clear that warmer, saline, and oxygen-replete water was being
upwelled, and that the contribution of oxygen-depleted and nutrient-rich
water was strongly reduced.
Oxygen section (color; in µmol kg-1; same color
scale in both frames) at ∼ 9∘ S off the Peruvian shelf for
December 2012 (a) and October 2015 (b). Three selected
isopycnals are included as white dashed lines. Please note that the section
in October 2015 reaches further west than in December 2012.
At ∼ 12∘ S the measured oxygen distributions for December 2012
and October 2015 are quite similar in the upwelling region at the easternmost
station pair with oxygen concentrations of less than
5 µmol kg-1 (Fig. 10). The oxygen concentration between the
isopycnals σθ= 25.6 and 25.8 kg m-3 was even lower in
October 2015 than in December 2012 in the upwelling region east of
∼ 77∘30′ W (Fig. 10). However, below the oxycline below
50 m depth, temperature, salinity, and oxygen concentrations (Fig. 11f) were
higher in October 2015 than in December 2012 and indicate the transition to
El Niño conditions. The seasonal signal in the time-series station at
∼ 12∘ S, 77∘30′ W shows a shallower 15 ∘C
isotherm and oxycline depth of about 20 m in October than in December (Graco
et al., 2016); hence, the deeper oxycline in October 2015 compared to
December 2012 is not a seasonal signal but an El Niño influence. The
nutrient distribution at the shelf at ∼ 12∘ S (Fig. 11) also
shows El Niño influence with lower phosphate and silicate in October 2015
than in December 2012. This is in agreement with the observed increase in
temperature and salinity, and the lower phosphate and silicate at the
time-series station at ∼ 12∘ S during the strong 1997/1998 El
Niño (Graco et al., 2016). Under El Niño conditions upwelling is
reduced and this prevents nutrients such as phosphate and silicate from
becoming enriched in the mixed layer. Nitrate and nitrite are different,
however, because their distributions are driven more by oxygen availability,
which regulates nitrification and denitrification. Indeed, nitrate was lower
and nitrite was higher in December 2012 than in October 2015, with nitrite
reaching 5.4 µmol L-1 at 75 m in December 2012 (Fig. 11),
consistent with the observations of Kock et al. (2016). At the depths of the
high nitrite concentrations in December 2012, very low oxygen concentrations
of less than 2 µmol kg-1 were measured. Under low oxygen
conditions, incomplete nitrification, incomplete denitrification, or a
combination of both, can result in accumulations of nitrite (e.g., Codispoti
and Christensen, 1985; Gruber, 2008; Brockmann and Morgenroth, 2010), as was
likely the case during 2012. The higher oxygen concentrations in the ODZ at
∼ 12∘ S during October 2015 would have prevented the build-up
of nitrite, as under these conditions denitrification shuts off and
nitrification goes to completion, producing more nitrate.
Oxygen section (color; in µmol kg-1; same color
scale in both frames) at ∼ 12∘ S off the Peruvian shelf for
December 2012 (a) and October 2015 (b). Three selected
isopycnals are included as white dashed lines.
Nutrient profiles at shelf stations with water depth of slightly
more than 100 m in December 2012 at 12∘15′ S, 77∘31′ W
(green) and in October 2015 at 12∘21′ S, 77∘25′ W
(black) for (a) nitrate in µmol L-1,
(b) nitrite in µmol L-1, (scale change at
1 µmol L-1), (c) phosphate in
µmol L-1, (d) silicate in µmol L-1,
(e) N : P ratio, and (f) CTD oxygen in
µmol kg-1.
Another notable difference between December 2012 and October 2015 at
∼ 12∘ S is the lower N : P ratios in the upper 150 m during
2012 vs. 2015 (Fig. S6e). Again, higher oxygen concentrations in the upper
150 m during the 2015 El Niño probably reduced the impact of fixed N
loss processes on the N : P signatures of near-surface waters. These
results imply that El Niño conditions could, at least, partially
alleviate phytoplankton N limitation due to the reduction in the magnitude of
denitrification. While it is beyond the scope of the focus of this study, it
would be interesting to examine whether this increase in N : P ratios
during the 2015 El Niño impacted the phytoplankton communities within
this region. The expectation that they may have impacted the phytoplankton
communities is certainly reasonable (Rousseaux and Gregg, 2012), as Hauss et
al. (2012) observed an increase in diatom biomass when the
NO3- : PO43- ratios of water collected from the Peruvian
upwelling region were increased.
The results from ∼ 12∘ S shelf water indicate that upwelling
of oxygen-poor water was still continuing in October 2015 at 12∘ S
in the near-surface layer, despite the enhanced SST anomaly related to El
Niño. Below the oxycline, however, El Niño conditions were
developing. The observations west of 77∘48′ W in the upper 75 m
show that oxygen as well as temperature (not shown) were lower in
October 2015, maybe related to a stronger poleward flow of the Peru–Chile
Undercurrent (PCUC), which has been shown to be a characteristic of El
Niño events (Strub et al., 1998). The PCUC advects seawater property
anomalies from equatorial to extratropical regions and shoals during El
Niño despite the velocity and transport intensification (Montes et al.
2011; Chaigneau et al., 2013).
At ∼ 14∘ S at the easternmost station near the shelf, the
oxygen distribution is quite similar for December 2012 and October 2015
(Fig. S6), indicating non-El Niño oxygen-poor upwelling near the shelf.
West of 77∘ W, the isopycnals are deeper in October 2015, related to
a deeper thermocline with warmer water in the upper 100 m (not shown) and
higher oxygen in October 2015 compared to December 2012. Similar to
∼ 12∘ S the nutrient distribution shows higher nitrate and
lower phosphate and silicate in October 2015 compared to December 2012 at
∼ 14∘ S. As outlined above, the higher nitrate concentrations
in October 2015 likely result from less denitrification and more complete
nitrication, as a result of the increased oxygen concentrations, and this
again provides evidence of a developing El Niño situation.
At ∼ 16∘ S the oxygen concentrations at the shelf were lower
in October 2015 than in December 2012 (Fig. S7), indicating similarity to
∼ 14∘ S non-El Niño upwelling close to the shelf. The
higher oxygen near the shelf in December 2012 was probably related to an
unusual distribution related to an eddy located near the
∼ 16∘ S section (e.g., Stramma et al., 2013; Czeschel et al.,
2015). The SST at ∼ 14 and ∼ 16∘ S was lower in October
2015 than in December 2012; hence, the slight increase in SST by El Niño
did not compensate for the typical seasonal SST signal. Different to the
sections at ∼ 9∘ S, ∼ 12, and ∼ 14∘ S, at
∼ 16∘ S the density distribution below the thermocline did not
shift to higher densities in October 2015, which shows that the El Niño
influence at 16∘ S was the weakest of the four shelf sections. The
observed transitional feature of normal conditions near-shore and El Niño
conditions offshore is probably a consequence of the cross-shore pattern in
vertical velocity during upwelling. The near-shore vertical velocity is
expected to be substantially larger than the offshore vertical velocity
(Fennel, 1999). A downwelling Kelvin wave could then neutralize the weak
offshore upwelling and bring down the thermocline, while near-shore the
strong upwelling would hardly weaken and for some time still bring up
remnants of cold oxygen-poor water, until supplies feed from the offshore
warmer and oxygen-replete waters. The wind field would not need to change in
order to produce this transition pattern in hydrography.
Conclusions
In this study, hydrographic measurements from a cruise to the eastern
tropical Pacific in October 2015 were used to investigate the signal of the
strong 2015 El Niño in the water mass distribution and in the EUC in
comparison to measurements from the years 1993, 2009, and 2012. An increase
in temperature from the surface to 350 m depth, and salinity in the 40 to
350 m depth layer, appeared at the Equator east of the Galapagos Islands at
85∘30′ W in October 2015. The warmer temperature led to lower
densities despite the concurrent influence of the salinity increase on
density. In October 2015, nitrate, phosphate, and silicate concentrations
were all lower in the upper 200 m when compared with previous non-El
Niño periods; however, higher oxygen concentrations, which are
characteristic of El Niño events, were only located between 40 and 130 m
at the Equator. Except for an oxygen increase in the upper ∼ 60 m at
2∘30′ S, no obvious large vertical oxygen increase appeared at
1∘ N and 2∘30′ S at 85∘30′ W. This weak oxygen
increase at and near the Equator might be related to the weak EUC, which
would otherwise be expected to bring oxygen-richer water eastwards.
Due to the influence of seasonal and El Niño signals, the velocity and
transport observations of the EUC east of the Galapagos Islands were quite
variable in the direct velocity measurements in different years. In addition,
intraseasonal signals with the passage of upwelling and downwelling waves at
intraseasonal timescales (Cravatte et al., 2003; Echevin et al., 2014) might
modify the measurements. As previously observed in the central and western
Pacific, and as predicted from model simulations, the EUC at the Equator
almost disappeared, with a transport of only 0.02 Sv between 1∘ S
and 1∘ N in October 2015 related to the El Niño conditions.
Although weak, the EUC had shifted southward, with a transport of 0.78 Sv
between 2∘30′ and 1∘ S in October 2015. These observations
are in agreement with the predicted weakening and southward shift of the EUC
in model results for El Niño periods (Montes et al., 2011). According to
earlier observations, the disappearance of the EUC in the eastern Pacific
seems to be related mainly to strong El Niño events. For the very strong
1982/83 El Niño, a disappearance of the EUC was described for the eastern
Pacific (Halpern, 1997), whereas for the strong 1997/98 El Niño the EUC
disappeared over all longitudes (Izumo, 2005). In contrast, during the
moderate El Niños of 1986/87 and 1991/92 a disappearance was described in
the western and central Pacific, but only a weakening in the eastern Pacific
(McPhaden et al., 1990; McPhaden and Hayes, 1990; Izumo, 2005; Kessler and
McPhaden, 1995; Seidel and Giese, 1999).
Four hydrographic sections near the Peruvian shelf between ∼ 9 and
∼ 16∘ S had different El Niño related signals in
October 2015. At ∼ 9∘ S there was a large SST increase, and we
observed upwelling of lighter water that was both warmer and more oxygenated,
all of which are characteristic upwelling features of El Niño events.
Between 12 and 16∘ S the SST increase in October 2015 was weaker
than at 9∘ S, and at the easternmost stations near the Peruvian
shelf at ∼ 12, ∼ 14 and ∼ 16∘ S cold and
oxygen-poor water was upwelled as during regular upwelling conditions,
probably some leftover water from the pre-El Niño period. West of the
easternmost stations, El Niño type changes were also observed below the
thermocline and oxycline, a feature that weakened southward and that may be
related to the shoaling and intensification of the PCUC and the influence of
a downwelling Kelvin wave.
The 2015 El Niño started strongly early in the year, and by October 2015
had an ONI similar to earlier major El Niño events. The water
characteristics at 85∘30′ W at the Equator and EUC variability and
upwelling at ∼ 9∘ S also indicated that a strong EP El
Niño had developed. However, at 1∘ N and 2∘30′ S at
85∘30′ W and at the sections near the shelf between 12 and
16∘ S, the El Niño influence was still weak. To this end, the
weak EUC clearly indicated a strong EP El Niño at the Equator, while off
the South American continent the distribution of hydrographic parameters,
oxygen, and nutrients indicated a transition period from regular to El
Niño conditions progressing southward along the Peruvian shelf. Despite
the strong 2015 El Niño, the shift to El Niño distribution in the
eastern Pacific was surprisingly slow. As the ONI increased to the end of
2015, we expect that the El Niño conditions were strengthening in the
eastern Pacific after the cruise in October 2015. Measurements carried out by
CNRS, IRD, and IMARPE with a glider from IFREMER at about 8∘ S off
Peru between 7 November and 17 December 2015 showed an increase in
temperature and oxygen and a decrease in density at ∼ 100 m when
compared to October 2015, thus confirming the expected strengthening of the
El Niño conditions
(https://www.ird.fr/toutel-actualite/actualites/).
In summary, the temperature, salinity, and oxygen measurements all indicate
that during October 2015 the El Niño was strongest along our northern
transects and weakest along our southern transects. This was also apparent in
the nutrient properties between the northern and southern portions of our
study region. As outlined above, at 12∘ S the N : P ratio was
higher and nitrite concentrations were lower during October 2015 when
compared to the non-El Niño period of December 2012, both of which point
to a reduction in the magnitude of denitrification. When comparing the
differences between coastal nutricline N : P ratios and nitrite
concentrations along the coast, we found that the differences between
October 2015 and December 2012 decreased between 12 and 14∘ S, and
again between 14 and 16∘ S (data not shown). This again highlights
the potential for El Niño events to impact N loss processes and upper
water column biogeochemistry.
Data availability
The data from the R/V Knorr cruise in March/April 1993 are available
for ADCP at
ftp://ftp.soest.hawaii.edu/caldwell/adcp/DATABASE/00015.html, for CTD
data as https://doi.pangaea.de/10.1594/PANGAEA.294039, and for
nutrients as https://doi.pangaea.de/10.1594/PANGAEA.837024. The
assembled measurements of the Meteor cruises in February 2009,
November 2012, and December 2012 and the Sonne cruise in
October 2015 used in this paper are available at
https://doi.pangaea.de/10.1594/PANGAEA.861392.
The Supplement related to this article is available online at doi:10.5194/os-12-861-2016-supplement.
Lothar Stramma and Tim Fischer conceived the study,
wrote the manuscript, and carried out the ADCP and hydrographic measurements
on the R/V Sonne cruise in 2015 as well as on some of the R/V
Meteor cruises. Damian S. Grundle was co-chief scientist of the R/V
Sonne cruise in October 2015, organized the nutrient sampling, and
interpreted the nutrient data. Gerd Krahmann calibrated the R/V
Meteor and R/V Sonne CTD data and interpreted the
hydrographic data. Hermann W. Bange was chief scientist on R/V
Meteor in December 2012, he was responsible for the nutrient
measurements on this cruise and interpreted the nutrient data.
Christa A. Marandino was chief scientist on the R/V Sonne cruise in
October 2015 and interpreted the nutrient data. All authors discussed and
modified the manuscript.
Acknowledgements
The Deutsche Forschungsgemeinschaft (DFG) provided support as part of
Sonderforschungsbereich 754: Climate-Biogeochemistry Interactions in the
Tropical Ocean and for the R/V Meteor cruises. The Bundesministerium
für Bildung und Forschung (BMBF) supported this study as part of the
SOPRAN project (03F0611A, 03F0662A) and through funding of the R/V
Sonne cruise in October 2015 (03G0243A). We thank the captains and
crews of the R/V Meteor and R/V Sonne cruises for their
help, R. Czeschel for helpful comments on the graphic software, T. Steinhoff
for co-organizing the R/V Sonne cruise, and M. Lohmann and H. Campen
for the oxygen and nutrient measurements.
Edited by: M. Hoppema
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