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C. Cassou
and
C. Perigaud

Abstract

Introducing new parameterizations of subsurface temperature and atmospheric convection in the Cane and Zebiak model allows the reproducing of oscillations with a period close to 4 yr; equatorial wind anomalies that are located close to the date line; and realistic amplitudes of SST, wind, and thermocline anomalies in the equatorial Pacific both in warm and cold phases. When the weight given to the atmospheric convection term is increased, the simulated wind maximum along the equator is displaced farther west, and the amplitude and duration of the warm events decrease. Compared to the simulations with the standard parameterization, the ENSO-like oscillations are not lost when the friction is increased with a decay time ranging from 30 to 12 months. Off-equatorial baroclinic ocean and wind anomalies are much less strong, but still necessary for the system to oscillate. Unrealistic easterlies are still present in the eastern Pacific.

Replacing the atmospheric model by a statistical relationship between SST and wind stress anomalies allows further reducing of these deficiencies. The simulated ENSO-like events are in good agreement with observed oceanic and atmospheric fields in terms of amplitude and spatial patterns within 15° of the equator. Sensitivity tests to the statistics prescribed in the atmospheric component show that the model simulates warm events with duration and amplitude that increase with the eastern penetration of the westerlies in the central Pacific, whereas cold events do not last longer and easterlies remain located at the date line whatever their strength. After warm events, the model simulates a gain of heat content on average in the north, and similar features are found in the 1983 and 1998 El Niño events, whereas the Cane and Zebiak model simulates a loss. The gain is due to northerlies along the ITCZ that are associated with an anticyclonic curl and a “recharge” of the oceanic heat content, whereas the loss is explained by the unrealistic easterlies in the eastern Pacific and by the weakness of the meridional winds.

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L. Terray
and
C. Cassou

Abstract

Effects of Atlantic sea surface temperature (SST) anomalies on the North Atlantic low-frequency atmospheric variability are examined by analyzing two ensembles of integrations of the ARPEGE general circulation model (GCM) forced with differently configured observed SSTs and sea ice extents (SIE) over the 1948–98 period. An attempt is made to separate the forced atmospheric response from internal atmospheric variability by using a signal-to-noise maximizing empirical orthogonal function (EOF) analysis. This method yields an estimate of the most detectable common forced response given the knowledge of internal variability provided by the ensemble. Applying the algorithm to North Atlantic atmospheric model data leads to an atmospheric response associated with a tripole pattern in North Atlantic SSTs. The spatial structure of the forced response, which is most consistent in winter, shows a dipole pattern in mean sea level pressure projecting onto the North Atlantic Oscillation. Examination of other atmospheric variables shows a very coherent signal with a quasi-barotropic signature. Additional atmospheric integrations with idealized SST anomaly patterns demonstrate the primary role of the tropical North Atlantic SST anomalies in generating the forced response. The physical mechanism involves related changes in tropical convection, Hadley circulation, and the modulation of the stationary and transient planetary-scale waves by the low-frequency variability in subtropical winds induced by the persistent tropical circulation anomalies.

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C. Perigaud
,
F. Melin
, and
C. Cassou

Abstract

ENSO simulations are investigated in 30-yr integrations of various intermediate coupled models and compared with observed SST, wind, and thermocline depth anomalies over the tropical Pacific. The Cane and Zebiak model simulates warm events with a period close to the observations, but with westerlies that are located 30° east of them and thermocline anomalies in the western Pacific that are much shallower. Between two warm events, the model simulates a series of three weak and short cold SST peaks and hardly ever simulates easterlies. The SST in the eastern equatorial Pacific is not sensitive to thermocline depth anomalies, but to the anomalous downwelling of surface currents induced by Ekman shear. The model simulates a pair of very strong cyclonic wind stress curl anomalies on both sides of the equator in the eastern off-equatorial domain between 7° and 15° lat. These are necessary to maintain the oscillatory regime—so are the ocean meridional Rossby modes higher than 5. The thermocline zonal slopes required to balance the off-equatorial curl anomalies are about three times steeper than the ones required to balance the zonal stress along the equator. Thus the off-equator exerts an excess of zonal pressure, which by continuity affects the equatorial ocean and plays a crucial role in reversing and triggering the growing events. Six months after the warm peaks, the whole ocean between 15°S and 15°N is significantly upwelled. The equatorial oceanic heat content is recharged from the south prior to a warm event.

Contrary to simulations when the model is driven by observed wind anomalies, increasing the friction in the baroclinic ocean does not decrease the off-equatorial variability but significantly alters the low-frequency oscillations that are no longer ENSO-like. Introducing the parameterization of subsurface temperature derived from hydrographic profiles in the ocean component does not allow the coupled model to recover cold events as in a forced context. Introducing the parameterization of convection derived from high-cloud temperature measurements is the most effective improvement, but results still poorly agree with observations and are in contrast with the simulations driven by observed SST, biased toward westerlies in the central Pacific, upwelled thermocline in the west, and warm SST in the east. Thus modifying the ocean component only or the atmosphere only does not have the same impact on simulations as in a forced context. The coupling allows new mechanisms to grow and govern the model behavior. One of them is the slow meridional oceanic mass adjusment in quasi-Sverdrup balance with the winds.

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C. Cassou
,
P. Noyret
,
E. Sevault
,
O. Thual
,
L. Terray
,
D. Beaucourt
, and
M. Imbard

Abstract

The authors present the distribution of a coupled ocean–atmosphere global circulation model. The atmospheric (ARPEGE) and the oceanic (OPA) components run separately at different sites; the coupling is achieved through the exchanges of fluxes via the coupler (OASIS) and the three independent programs communicate together through the 2-Mbit RENATER network. The coupling and distributing procedure is based on the PVM software and is validated by 1-yr simulations. Performances and difficulties raised by the distributed environment are also presented.

An additional study deals with the sensitivity to the precision in exchanged data in coupled mode. This question is addressed by introducing small artificial perturbations into the forcings of each component. The transient growth of these perturbations is first followed over 10 days on specific grid points. A global spatio-temporal analysis is then performed over the planet for 1-yr simulations.

During the first 10 days of the experiments, the “error” dynamics is amplified by the atmosphere with a doubling time of the order of 5 days, while the upper ocean simply relaxes toward equilibrium. For long time ranges of simulation, errors tend to saturate and oscillate around a plateau, following the seasonal cycle. Spatio-temporal studies prove that the most sensitive areas to the precision in exchanged forcings are related to the regions where the variability is the most pronounced. These analyses are integrated into the general studies of predictability in coupled ocean–atmosphere models.

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J. Vialard
,
J. P. Duvel
,
M. J. McPhaden
,
P. Bouruet-Aubertot
,
B. Ward
,
E. Key
,
D. Bourras
,
R. Weller
,
P. Minnett
,
A. Weill
,
C. Cassou
,
L. Eymard
,
T. Fristedt
,
C. Basdevant
,
Y. Dandonneau
,
O. Duteil
,
T. Izumo
,
C. de Boyer Montégut
,
S. Masson
,
F. Marsac
,
C. Menkes
, and
S. Kennan

The Vasco-Cirene program explores how strong air-sea interactions promoted by the shallow thermocline and high sea surface temperature in the Seychelles-Chagos thermocline ridge results in marked variability at synoptic, intraseasonal, and interannual time scales. The Cirene oceanographic cruise collected oceanic, atmospheric, and air-sea flux observations in this region in January–February 2007. The contemporaneous Vasco field experiment complemented these measurements with balloon deployments from the Seychelles. Cirene also contributed to the development of the Indian Ocean observing system via deployment of a mooring and 12 Argo profilers.

Unusual conditions prevailed in the Indian Ocean during January and February 2007, following the Indian Ocean dipole climate anomaly of late 2006. Cirene measurements show that the Seychelles-Chagos thermocline ridge had higher-than-usual heat content with subsurface anomalies up to 7°C. The ocean surface was warmer and fresher than average, and unusual eastward currents prevailed down to 800 m. These anomalous conditions had a major impact on tuna fishing in early 2007. Our dataset also sampled the genesis and maturation of Tropical Cyclone Dora, including high surface temperatures and a strong diurnal cycle before the cyclone, followed by a 1.5°C cooling over 10 days. Balloonborne instruments sampled the surface and boundary layer dynamics of Dora. We observed small-scale structures like dry-air layers in the atmosphere and diurnal warm layers in the near-surface ocean. The Cirene data will quantify the impact of these finescale features on the upper-ocean heat budget and atmospheric deep convection.

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J. Vialard
,
J. P. Duvel
,
M. J. Mcphaden
,
P. Bouruet-Aubertot
,
B. Ward
,
E. Key
,
D. Bourras
,
R. Weller
,
P. Minnett
,
A. Weill
,
C. Cassou
,
L. Eymard
,
T. Fristedt
,
C. Basdevant
,
Y. Dandonneau
,
O. Duteil
,
T. Izumo
,
C. de Boyer Montégut
,
S. Masson
,
F. Marsac
,
C. Menkes
, and
S. Kennan

Abstract

The Vasco—Cirene field experiment, in January—February 2007, targeted the Seychelles—Chagos thermocline ridge (SCTR) region, with the main purpose of investigating Madden—Julian Oscillation (MJO)-related SST events. The Validation of the Aeroclipper System under Convective Occurrences (Vasco) experiment (Duvel et al. 2009) and Cirene cruise were designed to provide complementary views of air—sea interaction in the SCTR region. While meteorological balloons were deployed from the Seychelles as a part of Vasco, the Research Vessel (R/V) Suroît was cruising the SCTR region as a part of Cirene.

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