Jean-Philippe Boulanger

A Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

In this study, the processes affecting sea surface temperature variability over the 1992-98 period, encompassing the very strong 1997-98 El Nino event, are analyzed. A tropical Pacific Ocean general circulation model, forced by a combination of weekly ERS1-2and TAO wind stresses, and climatological heat and freshwater fluxes, is first validated against observations. The model reproduces the main features of the tropical Pacific mean state, despite a weaker than observed thermal stratification, a 0.1 m s21 too strong (weak) South Equatorial Current (North Equatorial Countercurrent), and a slight underestimate of the Equatorial Undercurrent. Good agreement is found between the model dynamic height and TOPEX/Poseidon sea level variability, with correlation/rms differences of 0.80/4.7 cm on average in the 108N-108S band. The model sea surface temperature variability is a bit weak, but reproduces the main features of interannual variability during the 1992-98 period. The model compares well with the TAO current variability at the equator, with correlation/rms differences of 0.81/0.23 m s 21 for surface currents. The model therefore reproduces well the observed interannual variability, with wind stress as the only interannually varying forcing. This good agreement with observations provides confidence in the comprehensive three-dimensional circulation and thermal structure of the model. A close examination of mixed layer heat balance is thus undertaken, contrasting the mean seasonal cycle of the 1993-96 period and the 1997-98 El Nino. In the eastern Pacific, cooling by exchanges with the subsurface (vertical advection, mixing, and entrainment), the atmospheric forcing, and the eddies (mainly the tropical instability waves) are the three main contributors to the heat budget. In the central-western Pacific, the zonal advection by low-frequency currents becomes the main contributor. Westerly wind bursts (in December 1996 and March and June 1997) were found to play a decisive role in the onset of the 1997-98 El Nino. They contributed to the early warming in the eastern Pacific because the downwelling Kelvin waves that they excited diminished subsurface cooling there. But it is mainly through eastward advection of the warm pool that they generated temperature anomalies in the central Pacific. The end of El Nino can be linked to the large-scale easterly anomalies that developed in the western Pacific and spread eastward, from the end of 1997 onward. In the far-western Pacific, because of the shallower than normal thermocline, these easterlies cooled the SST by vertical processes. In the central Pacific, easterlies pushed the warm pool back to the west. In the east, they led to a shallower thermocline, which ultimately allowed subsurface cooling to resume and to quickly cool the surface layer.

Propagation and reflection of long equatorial waves in the Pacific Ocean during the 1992–1993 El Niño

The TOPEX/POSEIDON satellite, together with the Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) array, provides oceanographic and atmospheric observations which allow a detailed study of the equatorial Pacific variability. During the November 1992 to December 1993 El Nino period, sea level, dynamic height, wind stress, sea surface temperature, and surface zonal current data derived from TOPEX/POSEIDON and TOGA-TAO measurements were used to describe the Pacific ocean-atmosphere system and to understand the role played by long equatorial waves. A potentially important mechanism of the El Nino-Southern Oscillation (ENSO), commonly referred to as the delayed action oscillator, involves Kelvin and long Rossby waves and their reflections at the Pacific western boundary. In order to investigate if this process was at work during the period under study, a method for projecting TOPEX sea level, TOGA-TAO dynamic height, and zonal wind stress onto meridional wave structures was designed both in unbounded and bounded regions. The Kelvin and first three Rossby waves of the first baroclinic mode are propagating at theoretical wave speeds in all data sets. Zonal wind stress projections show that oceanic propagating wave features are strongly linked to wind variability. Reflections are then examined at both boundaries. At the eastern boundary most of the signal reflected from incoming Kelvin waves is either counteracted by unfavorable wind forcing or strongly reinforced and therefore does not seem to play a significant role for generating the major Rossby wave signals during the period under study. In the western Pacific, wind forcing, rather than western boundary reflections, appears to be the main trigger for returning Kelvin waves from the western Pacific to the eastern Pacific. Simultaneously with the weakening of the extended 1991–1993 ENSO event, an upwelling Kelvin wave is observed propagating from the western Pacific in September 1993 to the eastern Pacific in November 1993. This scenario is consistent with some features of the delayed action oscillator mechanism, where an upwelling Kelvin wave is systematically seen returning from the western boundary to the east at the end of warm events. However, here, contrary to the delayed action oscillator, most of this returning Kelvin wave seems to be forced by a strong easterly anomaly located in the western Pacific, rather than by reflection of an upwelling first Rossby wave at the western boundary.

Ocean response to the March 1997 Westerly Wind Event

An Ocean General Circulation Model is used to investigate the oceanic response to the March 1997 Westerly Wind Event that is suggested to have played an important role in the onset of the 1997–1998 El Nino. Our results point out three distinct impacts. First a strong wind-forced downwelling Kelvin wave propagates eastward generating sea surface temperature anomalies up to 1°C and large subsurface temperature and zonal current anomalies, mainly located in the core of the thermocline. Second the northward and westward extension of this wind event is responsible for a surface advection of cold waters from 130°E–5°N to the equator. Third it generates large zonal surface currents at the eastern edge of the warm and fresh pool by a nonlinear interaction between the wind-forced surface jet and the local thermohaline front. Salinity through both its contribution to the local zonal pressure gradient at the front and the barrier layer effect is crucial in the occurrence of this nonlinear mechanism. The fast displacement of the front (2000 km in a month) together with the cooling in the western Pacific is likely to be responsible for the eastward displacement of atmospheric deep convection and eastward winds observed in April–June 1997 and thus to have played a major role in initiating the El Nino of the century.

A Modeling Study of the Impact of Tropical Instability Waves on the Heat Budget of the Eastern Equatorial Pacific

A numerical simulation is used to investigate the mixed layer heat balance of the tropical Pacific Ocean including the equatorial cold tongue and the region of vortices associated with tropical instability waves (TIWs). The study is motivated by a need to quantify the effects that TIWs have on the climatological heat budget of the cold tongue mixed layer; there has been some discrepancy between observations indicating very large equatorward heat transport by TIWs and models that disagree on the full three-dimensional budget. Validation of the model reveals that the TIW-induced circulation patterns are realistic but may have amplitudes about 15% weaker than those in the observations. The SST budget within tropical instabilities is first examined in a frame of reference moving with the associated tropical instability vortices (TIVs). Zonal advection of temperature anomalies and meridional advection of temperature by current anomalies dominate horizontal advection. These effects strongly heat the cold cusps and slightly cool the downwelling areas located at the leading edge of the vortices. Cooling by vertical mixing is structured at the vortex scale and almost compensates for horizontal advective heating in the cold cusps. In contrast to some previous studies, TIW-induced vertical advection is found to be negligible in the SST budget. Cooling by this term is only significant below the mixed layer. The effect of TIWs on the climatological heat budget is then investigated for the region bounded by 2°S–6°N, 160°–90°W, where instabilities are most active. TIWinduced horizontal advection leads to a warming of 0.84°C month 1 , which is of the same order as the 0.77°C month 1 warming effect of atmospheric fluxes, while the mean currents and vertical mixing cool the upper ocean by 0.59°C month 1 and 1.06°C month 1 , respectively. The cooling effect of TIW-induced vertical advection is also negligible in the long-term surface layer heat budget and only becomes significant below the mixed layer. The results above, and in particular the absence of cancellation between horizontal and vertical TIW-induced eddy advection, are robust in three other sensitivity experiments involving different mixing parameterizations and increased vertical resolution.

Influence of the Seasonal Cycle on the Termination of El Niño Events in a Coupled General Circulation Model

In this study, the mechanisms leading to the El Nino peak and demise are explored through a coupled general circulation model ensemble approach evaluated against observations. The results here suggest that the timing of the peak and demise for intense El Nino events is highly predictable as the evolution of the coupled system is strongly driven by a southward shift of the intense equatorial Pacific westerly anomalies during boreal winter. In fact, this systematic late-year shift drives an intense eastern Pacific thermocline shallowing, constraining a rapid El Nino demise in the following months. This wind shift results from a southward displacement in winter of the central Pacific warmest SSTs in response to the seasonal evolution of solar insolation. In contrast, the intensity of this seasonal feedback mechanism and its impact on the coupled system are significantly weaker in moderate El Nino events, resulting in a less pronounced thermocline shallowing. This shallowing transfers the coupled system into an unstable state in spring but is not sufficient to systematically constrain the equatorial Pacific evolution toward a rapid El Nino termination. However, for some moderate events, the occurrence of intense easterly wind anomalies in the eastern Pacific during that period initiate a rapid surge of cold SSTs leading to La Nina conditions. In other cases, weaker trade winds combined with a slightly deeper thermocline allow the coupled system to maintain a broad warm phase evolving through the entire spring and summer and a delayed El Nino demise, an evolution that is similar to the prolonged 1986/87 El Nino event. La Nina events also show a similar tendency to peak in boreal winter, with characteristics and mechanisms mainly symmetric to those described for moderate El Nino cases.

The March 1997 Westerly Wind Event and the Onset of the 1997/98 El Niño: Understanding the Role of the Atmospheric Response

In a previous study, the effect of the March 1997 Westerly Wind Event (WWE) on the evolution of the tropical Pacific Ocean was studied using an ocean general circulation model (GCM). The response was characterized by (i) a cooling of the far western Pacific (;0.88C), (ii) a rapid eastward displacement of the warm pool (2000 km in a month), and (iii) a weak warming of the central eastern Pacific along the path of the oceanic Kelvin wave, excited by the WWE (;0.58C). In this study, the atmospheric response to these aspects of the sea surface temperature (SST) response are investigated using an atmospheric GCM forced with the SST anomalies from the ocean-only experiments. The results have demonstrated that the three aspects of the SST anomaly field, generated by the WWE, .

Role of non-linear oceanic processes in the response to Westerly Wind Events: new implications for the 1997 El Niño onset

In March 1997, a strong westerly wind event (WWE) occurred in the western equatorial Pacific prior to the 1997-1998 El Nino event. It produced downwelling Kelvin waves that interacted non linearly with the surface temperature, salinity and zonal current fronts located at the eastern edge of the warm-fresh pool (EEWP). This non-linear interaction locally increased zonal currents by a factor of three compared to a theoretical linear response, and advected the EEWP at an unexpected rate (? 1m/s) to which the ocean-atmosphere coupled system may have been responding rapidly to trigger El Nino conditions.

CLARIS Project : towards climate downscaling in South America

We explore the bias in monthly and seasonal mean precipitation simulated by ensembles of different regional climate models over South America within the context of the EU-FP6 CLARIS project (A Europe-South America Network for Climate Change Assessment and Impact Studies). We briefly described two series of coordinated simulations: (i) Case studies of anomalous months for south-eastern South America performed with an ensemble of six models, and (ii) A multiyear simulation of the period 1991―2000 performed by four models. The models have been forced with the European Centre for Medium Range Weather Forecasting Reanalysis (ERA-40) and are compared to observational data compiled by the Climatic Research Unit (CRU). The ensemble-mean bias can be large when simulating particular extreme periods in La Plata Basin. Our multi-model analysis suggests that even though the ten-year ensemble mean is able to capture the major regional characteristics of seasonal mean precipitation for South America, models individually display considerable precipitation biases especially in tropical areas. The relatively good performance of the multi-model annual average over La Plata Basin results from the cancelation of offsetting errors in the individual models.

Performance of a multi-RCM ensemble for South Eastern South America

The ability of four regional climate models to reproduce the present-day South American climate is examined with emphasis on La Plata Basin. Models were integrated for the period 1991–2000 with initial and lateral boundary conditions from ERA-40 Reanalysis. The ensemble sea level pressure, maximum and minimum temperatures and precipitation are evaluated in terms of seasonal means and extreme indices based on a percentile approach. Dispersion among the individual models and uncertainties when comparing the ensemble mean with different climatologies are also discussed. The ensemble mean is warmer than the observations in South Eastern South America (SESA), especially for minimum winter temperatures with errors increasing in magnitude towards the tails of the distributions. The ensemble mean reproduces the broad spatial pattern of precipitation, but overestimates the convective precipitation in the tropics and the orographic precipitation along the Andes and over the Brazilian Highlands, and underestimates the precipitation near the monsoon core region. The models overestimate the number of wet days and underestimate the daily intensity of rainfall for both seasons suggesting a premature triggering of convection. The skill of models to simulate the intensity of convective precipitation in summer in SESA and the variability associated with heavy precipitation events (the upper quartile daily precipitation) is far from satisfactory. Owing to the sparseness of the observing network, ensemble and observations uncertainties in seasonal means are comparable for some regions and seasons.

Evaluation of TOPEX and basin-wide Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean sea surface topographies and derived geostrophic currents

Fields of TOPEX-derived sea level anomalies are validated against and compared with fields of Tropical Ocean and Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) dynamic height anomalies for the first 470 days of the TOPEX/POSEIDON mission. At periods longer than 35 days, TOPEX sea level anomalies compare extremely well with TOGA-TAO dynamic height anomalies throughout the whole basin, with a mean correlation of 0.79 and mean rms differences of 2.6 cm. Zonal geostrophic current anomalies are then derived from both data sets and validated against in situ current meter measurements at the equator. TOPEX-derived geostrophic current anomalies at the equator are in good agreement with in situ currents at 0°–165°E, 0°–170°W, 0°–140°W, and 0°–110°W. TOGA-TAO-derived current anomalies compare well with in situ currents at 0°–110°W, 0°–140°W, and 0°–170°W, but poor comparisons at 0°–165°E are thought to be linked to the absence of instantaneous salinity measurements in the dynamic height calculation. Despite localized discrepancies, basin-wide TOGA-TAO geostrophic currents compare reasonably well with TOPEX-derived geostrophic currents. The discrepancies in the comparison between TOPEX and TAO are shown to be mostly associated with gaps in TOGA-TAO space-time sampling and subsequent gridding procedures.