7
© The Author(s) 2018
S. Jungblut et al. (eds.), YOUMARES 8 – Oceans Across Boundaries: Learning from each other,
https://doi.org/10.1007/978-3-319-93284-2_2
Can Climate Models Simulate
the Observed Strong Summer Surface
Cooling in the Equatorial Atlantic?
Tina Dippe, Martin Krebs, Jan Harlaß,
and Joke F. Lübbecke
Abstract
Variability in the tropical Atlantic Ocean is dominated by
the seasonal cycle. A defining feature is the migration of
the inter-tropical convergence zone into the northern
hemisphere and the formation of a so-called cold tongue
in sea surface temperatures (SSTs) in late boreal spring.
Between April and August, cooling leads to a drop in
SSTs of approximately 5°. The pronounced seasonal
cycle in the equatorial Atlantic affects surrounding continents, and even minor deviations from it can have striking
consequences for local agricultures.
Here, we report how state-of-the-art coupled global
climate models (CGCMs) still struggle to simulate the
observed seasonal cycle in the equatorial Atlantic, focusing on the formation of the cold tongue. We review the
basic processes that establish the observed seasonal cycle
in the tropical Atlantic, highlight common biases and
their potential origins, and discuss how they relate to the
dynamics of the real world. We also briefly discuss the
implications of the equatorial Atlantic warm bias for
CGCM-based reliable, socio-economically relevant seasonal predictions in the region.
The Equatorial Atlantic: A Climate Hot Spot
The tropical oceans are a crucial element of the global climate system. Defined here as the ocean area between 15°N
and 15°S, they occupy only about 13% of the earth’s surface,
but receive approximately 30% of the global net surface
insolation.
1
Processes both in the ocean and the atmosphere
redistribute surplus heat from low to higher latitudes. Without
these mechanisms, the tropics would get steadily warmer,
while the polar regions would radiate away more heat than
they receive and hence continue to cool. The oceans help to
establish the overall radiative equilibrium that is responsible
for our relatively stable climate (Trenberth and Caron 2001).
Apart from the energy surplus, another defining feature of
an equatorial ocean is that the effect of the earth’s rotation
vanishes at the equator, giving rise to a physical framework
that is subtly different from its higher-latitude counterpart.
The effect of the earth’s rotation manifests in a pseudo-force
that is called the Coriolis force. It deflects large-scale motion
towards the right of the movement on the northern hemisphere and towards the left on the southern hemisphere. It
provides rotation to large weather systems and explains why
large-scale movement curves or even becomes circular. An
exception is the equator, where the Coriolis force vanishes
and movement can be straightforward. Additionally, the nonexistent Coriolis force at the equator acts as a barrier for the
transmission of information within the ocean, for example
1 Based on data by Trenberth et al. (2009).
T. Dippe (*) · M. Krebs · J. Harlaß
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
e-mail: tdippe@geomar.de; jharlass@geomar.de
J. F. Lübbecke
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
Faculty of Mathematics and Natural Sciences, Christian Albrechts
University, Kiel, Germany
e-mail: jluebbecke@geomar.de
© The Author(s) 2018
S. Jungblut et al. (eds.), YOUMARES 8 – Oceans Across Boundaries: Learning from each other,
https://doi.org/10.1007/978-3-319-93284-2_2
Can Climate Models Simulate
the Observed Strong Summer Surface
Cooling in the Equatorial Atlantic?
Tina Dippe, Martin Krebs, Jan Harlaß,
and Joke F. Lübbecke
Abstract
Variability in the tropical Atlantic Ocean is dominated by
the seasonal cycle. A defining feature is the migration of
the inter-tropical convergence zone into the northern
hemisphere and the formation of a so-called cold tongue
in sea surface temperatures (SSTs) in late boreal spring.
Between April and August, cooling leads to a drop in
SSTs of approximately 5°. The pronounced seasonal
cycle in the equatorial Atlantic affects surrounding continents, and even minor deviations from it can have striking
consequences for local agricultures.
Here, we report how state-of-the-art coupled global
climate models (CGCMs) still struggle to simulate the
observed seasonal cycle in the equatorial Atlantic, focusing on the formation of the cold tongue. We review the
basic processes that establish the observed seasonal cycle
in the tropical Atlantic, highlight common biases and
their potential origins, and discuss how they relate to the
dynamics of the real world. We also briefly discuss the
implications of the equatorial Atlantic warm bias for
CGCM-based reliable, socio-economically relevant seasonal predictions in the region.
The Equatorial Atlantic: A Climate Hot Spot
The tropical oceans are a crucial element of the global climate system. Defined here as the ocean area between 15°N
and 15°S, they occupy only about 13% of the earth’s surface,
but receive approximately 30% of the global net surface
insolation.
1
Processes both in the ocean and the atmosphere
redistribute surplus heat from low to higher latitudes. Without
these mechanisms, the tropics would get steadily warmer,
while the polar regions would radiate away more heat than
they receive and hence continue to cool. The oceans help to
establish the overall radiative equilibrium that is responsible
for our relatively stable climate (Trenberth and Caron 2001).
Apart from the energy surplus, another defining feature of
an equatorial ocean is that the effect of the earth’s rotation
vanishes at the equator, giving rise to a physical framework
that is subtly different from its higher-latitude counterpart.
The effect of the earth’s rotation manifests in a pseudo-force
that is called the Coriolis force. It deflects large-scale motion
towards the right of the movement on the northern hemisphere and towards the left on the southern hemisphere. It
provides rotation to large weather systems and explains why
large-scale movement curves or even becomes circular. An
exception is the equator, where the Coriolis force vanishes
and movement can be straightforward. Additionally, the nonexistent Coriolis force at the equator acts as a barrier for the
transmission of information within the ocean, for example
1 Based on data by Trenberth et al. (2009).
T. Dippe (*) · M. Krebs · J. Harlaß
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
e-mail: tdippe@geomar.de; jharlass@geomar.de
J. F. Lübbecke
GEOMAR Helmholtz Centre for Ocean Research Kiel,
Kiel, Germany
Faculty of Mathematics and Natural Sciences, Christian Albrechts
University, Kiel, Germany
e-mail: jluebbecke@geomar.de