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Swadhin K. Behera
,
Jing Jia Luo
,
Sebastien Masson
,
Suryachandra A. Rao
,
Hirofumi Sakuma
, and
Toshio Yamagata

Abstract

An atmosphere–ocean coupled general circulation model known as the Scale Interaction Experiment Frontier version 1 (SINTEX-F1) model is used to understand the intrinsic variability of the Indian Ocean dipole (IOD). In addition to a globally coupled control experiment, a Pacific decoupled noENSO experiment has been conducted. In the latter, the El Niño–Southern Oscillation (ENSO) variability is suppressed by decoupling the tropical Pacific Ocean from the atmosphere. The ocean–atmosphere conditions related to the IOD are realistically simulated by both experiments including the characteristic east–west dipole in SST anomalies. This demonstrates that the dipole mode in the Indian Ocean is mainly determined by intrinsic processes within the basin. In the EOF analysis of SST anomalies from the noENSO experiment, the IOD takes the dominant seat instead of the basinwide monopole mode. Even the coupled feedback among anomalies of upper-ocean heat content, SST, wind, and Walker circulation over the Indian Ocean is reproduced.

As in the observation, IOD peaks in boreal fall for both model experiments. In the absence of ENSO variability the interannual IOD variability is dominantly biennial. The ENSO variability is found to affect the periodicity, strength, and formation processes of the IOD in years of co-occurrences. The amplitudes of SST anomalies in the western pole of co-occurring IODs are aided by dynamical and thermodynamical modifications related to the ENSO-induced wind variability. Anomalous latent heat flux and vertical heat convergence associated with the modified Walker circulation contribute to the alteration of western anomalies. It is found that 42% of IOD events affected by changes in the Walker circulation are related to the tropical Pacific variabilities including ENSO. The formation is delayed until boreal summer for those IODs, which otherwise form in boreal spring as in the noENSO experiment.

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Swen Jullien
,
Sébastien Masson
,
Véra Oerder
,
Guillaume Samson
,
François Colas
, and
Lionel Renault

Abstract

Ocean mesoscale eddies are characterized by rotating-like and meandering currents that imprint the low-level atmosphere. Such a current feedback (CFB) has been shown to induce a sink of energy from the ocean to the atmosphere, and consequently to damp the eddy kinetic energy (EKE), with an apparent regional disparity. In a context of increasing model resolution, the importance of this feedback and its dependence on oceanic and atmospheric model resolution arise. Using a hierarchy of quasi-global coupled models with spatial resolutions varying from 1/4° to 1/12°, the present study shows that the CFB induces a negative wind work at scales ranging from 100 to 1000 km, and a subsequent damping of the mesoscale activity by ~30% on average, independently of the model resolution. Regional variations of this damping range from ~20% in very rich eddying regions to ~40% in poor eddying regions. This regional modulation is associated with a different balance between the sink of energy by eddy wind work and the source of EKE by ocean intrinsic instabilities. The efficiency of the CFB is also shown to be a function of the surface wind magnitude: the larger the wind, the larger the sink of energy. The CFB impact is thus related to both wind and EKE. Its correct representation requires both an ocean model that resolves the mesoscale field adequately and an atmospheric model resolution that matches the ocean effective resolution and allows a realistic representation of wind patterns. These results are crucial for including adequately mesoscale ocean–atmosphere interactions in coupled general circulation models and have strong implications in climate research.

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Yushi Morioka
,
Tomoki Tozuka
,
Sebastien Masson
,
Pascal Terray
,
Jing-Jia Luo
, and
Toshio Yamagata

Abstract

The growth and decay mechanisms of subtropical dipole modes in the southern Indian and South Atlantic Oceans and their impacts on southern African rainfall are investigated using results from a coupled general circulation model originally developed for predicting tropical climate variations. The second (most) dominant mode of interannual sea surface temperature (SST) variations in the southern Indian (South Atlantic) Ocean represents a northeast–southwest oriented dipole, now called subtropical dipole mode. The positive (negative) SST interannual anomaly pole starts to grow in austral spring and reaches its peak in February. In austral late spring, the suppressed (enhanced) latent heat flux loss associated with the variations in the subtropical high causes a thinner (thicker) than normal mixed layer thickness that, in turn, enhances (reduces) the warming of the mixed layer by the climatological shortwave radiation. The positive (negative) pole gradually decays in austral fall because the mixed layer cooling by the entrainment is enhanced (reduced), mostly owing to the larger (smaller) temperature difference between the mixed layer and the entrained water. The increased (decreased) latent heat loss due to the warmer (colder) SST also contributes to the decay of the positive (negative) pole. Although further verification using longer observational data is required, the present coupled model suggests that the South Atlantic subtropical dipole may play a more important role in rainfall variations over the southern African region than the Indian Ocean subtropical dipole.

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Yushi Morioka
,
Sébastien Masson
,
Pascal Terray
,
Chloé Prodhomme
,
Swadhin K. Behera
, and
Yukio Masumoto

Abstract

Interannual variations of sea surface temperature (SST) in the midlatitudes of the Southern Hemisphere play an important role in the rainfall variability over the surrounding countries by modulating synoptic-scale atmospheric disturbances. These are frequently associated with a northeast–southwest-oriented dipole of positive and negative SST anomalies in each oceanic basin, referred to as a subtropical dipole. This study investigates the role of tropical SST variability on the generation of subtropical dipoles by conducting SST-nudging experiments using a coupled general circulation model. In the experiments where the simulated SST in each tropical basin is nudged to the climatology of the observed SST, the subtropical dipoles tend to occur as frequently as the case in which the simulated SST is allowed to freely interact with the atmosphere. It is found that without the tropical SST variability, the zonally elongated atmospheric mode in the mid- to high latitudes, called the Antarctic Oscillation (AAO), becomes dominant and the stationary Rossby waves related to the AAO induce the sea level pressure (SLP) anomalies in the midlatitudes, which, in turn, generate the subtropical dipoles. These results suggest that the tropical SST variability may not be necessary for generating the subtropical dipoles, and hence provide a useful insight into the important role of the AAO in the midlatitude climate variability.

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Lionel Renault
,
M. Jeroen Molemaker
,
Jonathan Gula
,
Sebastien Masson
, and
James C. McWilliams

Abstract

The Gulf Stream (GS) is known to have a strong influence on climate, for example, by transporting heat from the tropics to higher latitudes. Although the GS transport intensity presents a clear interannual variability, satellite observations reveal its mean path is stable. Numerical models can simulate some characteristics of the mean GS path, but persistent biases keep the GS separation and postseparation unstable and therefore unrealistic. This study investigates how the integration of ocean surface currents into the ocean–atmosphere coupling interface of numerical models impacts the GS. The authors show for the first time that the current feedback, through its eddy killing effect, stabilizes the GS separation and postseparation, resolving long-lasting biases in modeled GS path, at least for the Regional Oceanic Modeling System (ROMS). This key process should therefore be taken into account in oceanic numerical models. Using a set of oceanic and atmospheric coupled and uncoupled simulations, this study shows that the current feedback, by modulating the energy transfer from the atmosphere to the ocean, has two main effects on the ocean. On one hand, by reducing the mean surface stress and thus weakening the mean geostrophic wind work by 30%, the current feedback slows down the whole North Atlantic oceanic gyre, making the GS narrower and its transport weaker. Yet, on the other hand, the current feedback acts as an oceanic eddy killer, reducing the surface eddy kinetic energy by 27%. By inducing a surface stress curl opposite to the current vorticity, it deflects energy from the geostrophic current into the atmosphere and dampens eddies.

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Swadhin K. Behera
,
Jing-Jia Luo
,
Sebastien Masson
,
Pascale Delecluse
,
Silvio Gualdi
,
Antonio Navarra
, and
Toshio Yamagata

Abstract

The variability in the East African short rains is investigated using 41-yr data from the observation and 200-yr data from a coupled general circulation model known as the Scale Interaction Experiment-Frontier Research Center for Global Change, version 1 (SINTEX-F1). The model-simulated data provide a scope to understand the climate variability in the region with a better statistical confidence. Most of the variability in the model short rains is linked to the basinwide large-scale coupled mode, that is, the Indian Ocean dipole (IOD) in the tropical Indian Ocean. The analysis of observed data and model results reveals that the influence of the IOD on short rains is overwhelming as compared to that of the El Niño–Southern Oscillation (ENSO); the correlation between ENSO and short rains is insignificant when the IOD influence is excluded. The IOD–short rains relationship does not change significantly in a model experiment in which the ENSO influence is removed by decoupling the ocean and atmosphere in the tropical Pacific. The partial correlation analysis of the model data demonstrates that a secondary influence comes from a regional mode located near the African coast.

Inconsistent with the observational findings, the model results show a steady evolution of IOD prior to extreme events of short rains. Dynamically consistent evolution of correlations is found in anomalies of the surface winds, currents, sea surface height, and sea surface temperature. Anomalous changes of the Walker circulation provide a necessary driving mechanism for anomalous moisture transport and convection over the coastal East Africa. The model results nicely augment the observational findings and provide us with a physical basis to consider IOD as a predictor for variations of the short rains. This is demonstrated in detail using the statistical analysis method. The prediction skill of the dipole mode SST index in July and August is 92% for the observation, which scales slightly higher for the model index (96%) in August. As observed in data, the model results show decadal weakening in the relationship between IOD and short rains owing to weakening in the IOD activity.

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