Sea surface temperature associations with the late Indian summer monsoon

International audienceThis paper uses recent gridded and historical data in order to assess the relationships betweeninterannual variability of the Indian Summer Monsoon (ISM) and Sea Surface Temperature (SST)anomaly patterns over the Indian and Pacific oceans.Interannual variability of ISM rainfall and dynamical indices for the traditional summer monsoonseason (June-September) are strongly influenced by rainfall and circulation anomalies observedduring August and September, or the Late Indian Summer Monsoon (LISM). Anomalous monsoonsare linked to well-defined LISM rainfall and large-scale circulation anomalies. The east-westWalker and local Hadley circulations fluctuate during the LISM of anomalous ISM years. LISMcirculation is weakened and shifted eastward during weak ISM years. Therefore, we focus on thepredictability of the LISM in this study.Strong (weak) (L)ISMs are preceded by significant positive (negative) SST anomalies in thesoutheastern subtropical Indian Ocean, off Australia, during boreal winter. These SST anomaliesare mainly linked to south Indian Ocean dipole events, recently studied by Behera and Yamagata(2001), and to the El Niño-Southern Oscillation (ENSO) phenomenon. These SST anomalies arehighly persistent and affect the northwestward translation of the Mascarene high from austral toboreal summer. The southeastward (northwestward) shift of this subtropical high associated withcold (warm) SST anomalies off Australia causes a weakening (strengthening) of the wholemonsoon circulation through a modulation of the local Hadley cell during the LISM. Furthermore, itis suggested that the Mascarene high interacts with the underlying SST anomalies through apositive dynamical feedback mechanism, maintaining its anomalous position during the LISM.Our results also explain why a strong ISM is preceded by a transition in boreal spring from an ElNiño to a La Niña state in the Pacific and vice versa. An El Niño event and the associated warmSST anomalies over the southeastern Indian Ocean during boreal winter may play a key role in thedevelopment of a strong ISM by strengthening the local Hadley circulation during the LISM. On theother hand, a developing La Niña event in boreal spring and summer may also enhance the eastwestWalker circulation and the monsoon as demonstrated in many previous studies

Monthly mean wind stress and Sverdrup transport in the North Atlantic: A comparison of the Hellerman-Rosenstein and Isemer-Hasse climatologies

The monthly mean wind stress climatology of Hellerman and Rosenstein (HR) is compared with the climatology of Isemer and Hasse (IH), which represents a version of the Bunker atlas (BU) for the North Atlantic based on revised parameterizations. The drag coefficients adopted by IH are 21% smaller than the values of BU and HR, and the calculation of wind speed from marine estimates of Beaufort force (Bft) is based on a revised Beaufort equivalent scale similar to the scientific scale recommended by WMO. The latter choice significantly increases wind speed below Bft 8, and effectively counteracts the reduction of the drag coefficients. Comparing the IH stresses with HR reveals substantially enhanced magnitudes in the trade wind region throughout the year. At 15°N the mean easterly stress increases from about 0.9 (HR) to about 1.2 dyn cm−1 (IH). Annual mean differences are smaller in the region of the westerlies. In winter, the effect due to the reduced drag coefficient dominates and leads to smaller stress values in IH; during summer season the revision of the Beaufort equivalents is more effective and leads to increased stresses. Implications of the different wind stress climatologies for forcing the large-scale ocean circulation are discussed by means of the Sverdrup transport streamfunction (ψs): Throughout the subtropical gyre a significant intensification of ψs takes place with IH. At 27°N, differences of more than 10 Sv (1 Sv ≡ 106 m3 s−1) are found near the western boundary. Differences in the seasonality of ψs are more pronounced in near-equatorial regions where IH increase the amplitude of the annual cycle by about 50%. An eddy-resolving model of the North Atlantic circulation is used to examine the effect of the different wind stresses on the seasonal cycle of the Florida Current. The transport predicted by the numerical model is in much better agreement with observations when the circulation is forced by IH than by HR, regarding both the annual mean (29.1 Sv vs 23.2 Sv) and the seasonal range (6.3 Sv vs 3.4 Sv)

Role of the southern Indian Ocean in the transitions of the monsoon-ENSO system during recent decades

International audienceThe focus of this study is to document the possible role of the southern subtropical IndianOcean in the transitions of the monsoon-ENSO system during recent decades.Composite analyses of Sea Surface Temperature (SST) fields prior to El Niño-SouthernOscillation (ENSO), Indian Summer Monsoon (ISM), AUstralian Summer Monsoon(AUSM), Tropical Indian Ocean Dipole (TIOD) and Maritime Continent Rainfall (MCR)indices reveal the South East Indian Ocean (SEIO) SSTs during late boreal winter as theunique common SST precursor of these various phenomena after the 1976-1977 regime shift.Weak (strong) ISMs and AUSMs, El Niños (La Niñas) and positive (negative) TIOD eventsare preceded by significant negative (positive) SST anomalies in the SEIO, off Australiaduring boreal winter. These SST anomalies are mainly linked to subtropical Indian Oceandipole events, recently studied by Behera and Yamagata (2001). A wavelet analysis of aFebruary-March SEIO SST time series shows significant spectral peaks at 2 and 4-8 yearstime scales as for ENSO, ISM or AUSM indices. A composite analysis with respect toFebruary-March SEIO SSTs shows that cold (warm) SEIO SST anomalies are highlypersistent and affect the westward translation of the Mascarene high from austral to borealsummer, inducing a weakening (strengthening) of the whole ISM circulation through amodulation of the local Hadley cell during late boreal summer. At the same time, thesesubtropical SST anomalies and the associated SEIO anomalous anticyclone may be a triggerfor both the wind-evaporation-SST and wind-thermocline-SST positive feedbacks betweenAustralia and Sumatra during boreal spring and early summer. These positive feedbacksexplain the extraordinary persistence of the SEIO anomalous anticyclone from boreal springto fall. Meanwhile, the SEIO anomalous anticyclone favors persistent southeasterly windanomalies along the west coast of Sumatra and westerly wind anomalies over the westernPacific, which are well-known key-factors for the evolution of positive TIOD and El Niñoevents, respectively. A correlation analysis supports these results and shows that SEIO SSTsin February-March has higher predictive skill than other well-established ENSO predictors forforecasting Niño3.4 SST at the end of the year. This suggests again that SEIO SST anomaliesexert a fundamental influence on the transitions of the whole monsoon-ENSO system duringrecent decades

Impact of westerly wind bursts on the warm pool of the TOGA-COARE domain in an OGCM

International audienceA primitive equation model is used to investigate the warm pool equilibrium of the tropical Pacific ocean. Attention is focused on the upper ocean. The oceanic response is described using an isothermal approach applied to warm waters contained in the TOGA-COARE domain. The heat balance shows that all the terms, atmospheric surface fluxes, advection and diffusion, operate in the heat budget with different time scales. Over long periods, diffusive heat fluxes transfer heat received from the atmosphere out of the warm pool trough the top of the main thermocline. Over short periods, the impact of westerly wind bursts modifies this balance: atmospheric heating is reversed, diffusion is enhanced and advective heat transports out of the warm pool operate through zonal and vertical contributions. We were able to relate the two latter processes to zonal jets and Ekman pumping, respectively. Conversely, the meridional contribution always represents a source of heat, mainly due to the tropical wind convergence. The modelling results clearly show that except during strong wind events, entrainment cooling is not an important component of the budget. The inability to remove heat is due to the salt stratification which needs to be first reduced or even destroyed by westerly wind bursts to activate heat entrainment into deeper layers. Finally, we suggest that the near zero estimate for the surface heat flux entering the warm pool may be extended to longer periods including seasonal to interannual time scale

Quantifying the AMOC feedbacks during a 2×CO₂ stabilization experiment with land-ice melting

International audienceThe response of the Atlantic Meridional Overturning Circulation (AMOC) to an increase in atmospheric CO2 concentration is analyzed using the IPSL-CM4 coupled ocean-atmosphere model. Two simulations are integrated for 70 years with 1%/year increase in CO2 concentration until 2×CO2, and are then stabilized for further 430 years. The first simulation takes land-ice melting into account, via a simple parameterization, which results in a strong freshwater input of about 0.13 Sv at high latitudes in a warmer climate. During this scenario, the AMOC shuts down. A second simulation does not include this land-ice melting and herein, the AMOC recovers after 200 years. This behavior shows that this model is close to an AMOC shutdown threshold under global warming conditions, due to continuous input of land-ice melting. The analysis of the origin of density changes in the Northern Hemisphere convection sites allows an identification as to the origin of the changes in the AMOC. The processes that decrease the AMOC are the reduction of surface cooling due to the reduction in the air-sea temperature gradient as the atmosphere warms and the local freshening of convection sites that results from the increase in local freshwater forcing. Two processes also control the recovery of the AMOC: the northward advection of positive salinity anomalies from the tropics and the decrease in sea-ice transport through the Fram Strait toward the convection sites. The quantification of the AMOC related feedbacks shows that the salinity related processes contribute to a strong positive feedback, while feedback related to temperature processes is negative but remains small as there is a compensation between heat transport and surface heat flux in ocean-atmosphere coupled model. We conclude that in our model, AMOC feedbacks amplify land-ice melting perturbation by 2.5

: chapitre 10

International audienc

Ocean-atmosphere exchanges and water mass formation: chapitre 11

International audienc

Dynamics of the Indian monsoon and ENSO relationships in the SINTEX global coupled model

International audienceThis paper uses recent gridded climatological data and a coupled General CirculationModel (GCM) simulation in order to assess the relationships between the interannualvariability of the Indian Summer Monsoon (ISM) and the El Niño-Southern Oscillation(ENSO). The focus is on the dynamics of the ISM-ENSO relationships and the abilityof a state-of-the-art coupled GCM to reproduce the complex lead-lag relationshipsbetween the ISM and ENSO.The coupled GCM is successful in reproducing the ISM circulation and rainfallclimatology in the Indian areas even though the entire ISM circulation is weakerrelative to that observed. In both observations and in the simulation, ISM rainfallanomalies are significantly associated with fluctuations of the Hadley circulation and200 hPa zonal wind anomalies over the Indian Ocean. A quasi-biennial time-scale isfound to structure ISM dynamical and rainfall indices in both cases. Moreover, ISMindices have a similar interannual variability in the simulation and observations.The coupled model is less successful in simulating the annual cycle in the tropicalPacific. A major model bias is the eastward displacement of the western North PacificInter Tropical Convergence Zone (ITCZ), near the dateline, during northern summer.This introduces a strong semi-annual component in Pacific Walker circulation indicesand central equatorial Pacific sea surface temperatures. Another weakness of thecoupled model is a less-than-adequate simulation of the Southern Oscillation due to anerroneous eastward extension of the Southern Pacific Convergence Zone (SPCZ) yearround.Despite these problems, the coupled model captures some aspects of theinterannual variability in the tropical Pacific. ENSO events are phase-locked with theannual cycle as observed, but are of reduced amplitude relative to the observations.Wavelet analysis of the model Niño34 time series shows enhanced power in the 2-4year band, as compared to the 2-8 year range for observations during the 1950-2000period.The ISM circulation is weakened during ENSO years in both the simulation and theobservations. However, the model fails to reproduce the lead-lag relationship betweenthe ISM and Niño34 SSTs. Furthermore, lag correlations show that the delayedresponse of the wind stress over the central Pacific to ISM variability is insignificant inthe simulation. These features are mainly due to the unrealistic interannual variabilitysimulated by the model in the western North Pacific. The amplitude and even the signof the simulated surface and upper-level wind anomalies in these areas are notconsistent with observed patterns during weak/strong ISM years. The ISM and westernnorth Pacific ITCZ fluctuate independently in the observations, while they arenegatively and significantly correlated in the simulation. This isolates the PacificWalker circulation from the ISM forcing. These systematic errors may also contributeto the reduced amplitude of ENSO variability in the coupled simulation. Most of theunrealistic features in simulating the Indo-Pacific interannual variability may be tracedback to systematic errors in the base state of the coupled model