200
because the large internal variability in the climate system
might conceal small forced changes. Furthermore, the internal variability itself might also change under global warming. In our study, we analyze changes in the rainfall variability
in the tropical Atlantic region. We use a 100-member
ensemble of historical (1850-2005) model simulations with
the Max Planck Institute for Meteorology Earth System
Model (MPI-ESM) to separate the directly forced response
and internal variability. To investigate the effects of global
warming, we employ an additional ensemble of model simulations with stronger external forcing (1% CO 2 -increase per
year, same integration length as the historical simulations)
with 68 ensemble members. We find changes in the internal
variability of tropical rainfall in all ocean basins. However,
the magnitude and sign of the change is regionally different.
In the tropical Atlantic, the strongest internal variability is
simulated on the southern flank of the Intertropical
Convergence Zone (ITCZ). Under global warming the variability on the southern ITCZ flank increases and is located
further north than under preindustrial conditions, coinciding
with a narrowing and intensification of the mean rainfall in
the Atlantic ITCZ.
1.2.3 Linking the Tropical and Subtropical
Atlantic: A Benguela Upwelling Perspective
Lydia Siegfried
1*
, Martin Schmidt
1
, Volker Mohrholz
1
1
Leibniz Institute for Baltic Sea Research, Warnemünde,
Seestraße 15, 18119 Rostock-Warnemünde, Germany
*corresponding author: lydia.siegfried@io- warnemuende.
de
Keywords: SACW, Benguela, Regional model, Upwelling
Upwelling in the Benguela region is mainly fed by two
different Central Water types. The interplay between the
tropical South Atlantic Central Water (SACW) and the subtropical ESACW (Eastern SACW) determines the physical
and chemical properties of the Benguela ecosystem. We use
a regional circulation model (Modular Ocean Model) to
analyze the source region of the upwelled water. Passive
tracers are employed to study propagation pathways of
Central Water Masses and their temporal and spatial variation. We show that water originating from the equatorial
current system contributes substantially to the upwelling in
the northern Benguela system. The Equatorial UnderCurrent
(EUC) is one of the strongest currents in the South-east
Atlantic and its water is transported to the Benguela system
by two mechanisms: the trapping of water inside the equatorial and coastal wave guides and by a poleward bending of
the EUC between 8°W and 0°E feeding the South Equatorial
UnderCurrent (SEUC). Water loss from the EUC into the
SEUC dominates in austral summer whereas the strength of
transport in the wave guides is relatively constant throughout the year. The SE-transport in the open ocean might be
established by a Sverdrup balance. A strong interannual
transport anomaly was found for the Benguela Niño
2010/11. Transport from the EUC to the Benguela upwelling system was enhanced, maximal SEUC transport peaked
earlier in the winter season and coastal upwelling was
reduced. Once transported to the Benguela upwelling system, part of the surface and subsurface water contributes to
coastal upwelling in the Kunene Cell and is advected westward due to Ekman transport. The remaining part of the
water feeds the poleward undercurrent which is the main
pathway for tropical SACW transport inside the Benguela
system. Strength and location of the poleward undercurrent
are determined, among other things, by wind forcing and
shelf topography.
1.2.4 Bottom Currents in the Tropical
Fractures of the Northern Mid-Atlantic Ridge
D. I. Frey
1*
, E. H. Morozov
1
, N. I. Makarenko
2
1
Shirshov Institute of Oceanology, Russian Academy of
Sciences, Moscow, Russia
2
Lavrentiev Institute of Hydrodynamics, Russian
Academy of Sciences, Novosibirsk, Russia
*corresponding author: dima.frey@gmail.com
Keywords: Abyssal currents, LADCP measurements,
Numerical modeling, Ocean circulation
The properties of bottom flows in the tropical part of the
Atlantic were studied on the basis of hydrographic measurements and numerical modeling of the oceanic circulation.
Antarctic Bottom Water (AABW, potential temperature
θ<2 °C) occupies the deepest layer of the major part of the
Atlantic Ocean. This water propagates from the Weddell Sea
to the north filling the lower part of the ocean basins.
Interbasin exchange of bottom waters occurs through the
depressions in the topographic obstacles. Mean velocities of
the northward propagation of AABW are less than 1 cm/s;
however, when AABW flows through narrow abyssal channels, the current strongly accelerates. Thermohaline properties and velocities of the currents in the region of the
Mid-Atlantic Ridge were measured onboard the R/V
“Akademik Sergey Vavilov” by the scientists of the Shirshov
Institute of Oceanology in 2014–2016. A part of these hydrographic observations at several fracture zones was carried
out for the first time. The numerical simulation was performed using the Institute of Numerical Mathematics Ocean
Model (INMOM). The observations of velocities were used
for verification of the numerical model. The simulated threedimensional velocity fields with high spatial resolution in the
lower layer allow us to study the bottom currents over their
entire length. This research was supported by the Russian
Science Foundation (project 16-17-10149).
Appendices
because the large internal variability in the climate system
might conceal small forced changes. Furthermore, the internal variability itself might also change under global warming. In our study, we analyze changes in the rainfall variability
in the tropical Atlantic region. We use a 100-member
ensemble of historical (1850-2005) model simulations with
the Max Planck Institute for Meteorology Earth System
Model (MPI-ESM) to separate the directly forced response
and internal variability. To investigate the effects of global
warming, we employ an additional ensemble of model simulations with stronger external forcing (1% CO 2 -increase per
year, same integration length as the historical simulations)
with 68 ensemble members. We find changes in the internal
variability of tropical rainfall in all ocean basins. However,
the magnitude and sign of the change is regionally different.
In the tropical Atlantic, the strongest internal variability is
simulated on the southern flank of the Intertropical
Convergence Zone (ITCZ). Under global warming the variability on the southern ITCZ flank increases and is located
further north than under preindustrial conditions, coinciding
with a narrowing and intensification of the mean rainfall in
the Atlantic ITCZ.
1.2.3 Linking the Tropical and Subtropical
Atlantic: A Benguela Upwelling Perspective
Lydia Siegfried
1*
, Martin Schmidt
1
, Volker Mohrholz
1
1
Leibniz Institute for Baltic Sea Research, Warnemünde,
Seestraße 15, 18119 Rostock-Warnemünde, Germany
*corresponding author: lydia.siegfried@io- warnemuende.
de
Keywords: SACW, Benguela, Regional model, Upwelling
Upwelling in the Benguela region is mainly fed by two
different Central Water types. The interplay between the
tropical South Atlantic Central Water (SACW) and the subtropical ESACW (Eastern SACW) determines the physical
and chemical properties of the Benguela ecosystem. We use
a regional circulation model (Modular Ocean Model) to
analyze the source region of the upwelled water. Passive
tracers are employed to study propagation pathways of
Central Water Masses and their temporal and spatial variation. We show that water originating from the equatorial
current system contributes substantially to the upwelling in
the northern Benguela system. The Equatorial UnderCurrent
(EUC) is one of the strongest currents in the South-east
Atlantic and its water is transported to the Benguela system
by two mechanisms: the trapping of water inside the equatorial and coastal wave guides and by a poleward bending of
the EUC between 8°W and 0°E feeding the South Equatorial
UnderCurrent (SEUC). Water loss from the EUC into the
SEUC dominates in austral summer whereas the strength of
transport in the wave guides is relatively constant throughout the year. The SE-transport in the open ocean might be
established by a Sverdrup balance. A strong interannual
transport anomaly was found for the Benguela Niño
2010/11. Transport from the EUC to the Benguela upwelling system was enhanced, maximal SEUC transport peaked
earlier in the winter season and coastal upwelling was
reduced. Once transported to the Benguela upwelling system, part of the surface and subsurface water contributes to
coastal upwelling in the Kunene Cell and is advected westward due to Ekman transport. The remaining part of the
water feeds the poleward undercurrent which is the main
pathway for tropical SACW transport inside the Benguela
system. Strength and location of the poleward undercurrent
are determined, among other things, by wind forcing and
shelf topography.
1.2.4 Bottom Currents in the Tropical
Fractures of the Northern Mid-Atlantic Ridge
D. I. Frey
1*
, E. H. Morozov
1
, N. I. Makarenko
2
1
Shirshov Institute of Oceanology, Russian Academy of
Sciences, Moscow, Russia
2
Lavrentiev Institute of Hydrodynamics, Russian
Academy of Sciences, Novosibirsk, Russia
*corresponding author: dima.frey@gmail.com
Keywords: Abyssal currents, LADCP measurements,
Numerical modeling, Ocean circulation
The properties of bottom flows in the tropical part of the
Atlantic were studied on the basis of hydrographic measurements and numerical modeling of the oceanic circulation.
Antarctic Bottom Water (AABW, potential temperature
θ<2 °C) occupies the deepest layer of the major part of the
Atlantic Ocean. This water propagates from the Weddell Sea
to the north filling the lower part of the ocean basins.
Interbasin exchange of bottom waters occurs through the
depressions in the topographic obstacles. Mean velocities of
the northward propagation of AABW are less than 1 cm/s;
however, when AABW flows through narrow abyssal channels, the current strongly accelerates. Thermohaline properties and velocities of the currents in the region of the
Mid-Atlantic Ridge were measured onboard the R/V
“Akademik Sergey Vavilov” by the scientists of the Shirshov
Institute of Oceanology in 2014–2016. A part of these hydrographic observations at several fracture zones was carried
out for the first time. The numerical simulation was performed using the Institute of Numerical Mathematics Ocean
Model (INMOM). The observations of velocities were used
for verification of the numerical model. The simulated threedimensional velocity fields with high spatial resolution in the
lower layer allow us to study the bottom currents over their
entire length. This research was supported by the Russian
Science Foundation (project 16-17-10149).
Appendices