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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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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. Perigaud
and
J. F. Minster

Abstract

Three methods of extraction of mesoscale variability from altimetry are compared in the Somali current area: crossover residuals after minimization to reduce orbit errors, along-track variability using quasi-repetitive tracks, and difference relative to the altimetric mean sea surface of Marsh et al. The signals are filtered using an optimal estimation technique with a zero-crossing of the spatial correlation function of 250 km and an e-folding decay time of 30 days. The comparison is made by spatial correlation between the variation maps produced by the three techniques and with XBT data along the tanker sea lane. Probably because of insufficient data defining the reference mean sea surface in the area, the third method leaves large topographic features related to the geoid. Similarly, the horizontal distance between the quasi-repetitive tracks is too large and their difference is contaminated by the geoid signal. The comparison between the topographic heights deduced from crossover residuals and those estimated from the XBT sections is excellent down to 7 cm rms. These maps reveal an eastward extension of the “Great Whirl” in August. The variation of topography is so swift that it is impossible to sample the system correctly with a single oceanographic vessel.

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C. Perigaud
,
P. Delecluse
, and
J. F. Minster

Abstract

Seasat scatterometer data over the Arabian Sea are used to build wind stress fields during July and August 1978. They are first compared with 3-day wind analyses from ship data along the Somali coast. Seasat scatterometer specifications of 2 m s−1and 20° accuracy are fulfilled in almost all cases. The exceptions are for winds stronger than 14 m s−1, which are underestimated by the scatterometer by 15%.

Wind stress is derived from these wind data using bulk formula with a drag coefficient depending on the wind intensity. A successive correction objective analysis is used to build the wind stress field over the Arabian Sea with a 2°×2°and 6 day resolution. The final wind stress fields are not significantly dependent on the objective analysis because of the dense coverage of the scatterometer. The combination of scatterometer and coastal ship data gives the best coverage to resolve monsoon wind structures even close to the coast.

The final wind stress fields show wind features consistent with other monthly mean wind stress fields. However, a high variability is observed on the 6-day time scale. In particular, a monsoon break is present on 16 July with a minimum wind stress intensity in the southern part of the Arabian Sea and and a drastic decrease over the whole region. Compared to climatology, the southwest monsoon is stronger and lasts longer in 1978: no clear decline is observed, but alternating strong increases and decreases in wind intensity takes place until the end of August. Scatterometer data are powerful to monitor such rapid and extensive wind variations.

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