The Impact Of Global Warming On The Tropical Pacific Ocean And The Possible Mechanisms

The global climate has gradually been warmer since the end of the19thcenturybased on observational, reanalysis and model simulation data. Global warming is ahot issue as it causes significant impacts on weather and climate worldwide. On theother hand, the tropical Pacific is the largest one among different tropical oceans andshows remarkable ocean-atmosphere interactions as well. In this paper, a globalcoupled model CAM3.1-1.5RGO is used to study the response of the tropical Pacificunder global warming, which may not only help to deepen the understanding oftropical coupled ocean-atmosphere physical processes but also to estimate the impactsof global warming. Three aspects including mean state (SST), variability (ENSO) andteleconnection (the impact of tropical north Atlantic) are selected to be focused on inthis study.Firstly, the debate of the tropical Pacific response pattern under global warmingin model simulations is preliminarily solved. Regarding the mean state, a robust ElNi o-like warming pattern is found in the equatorial Pacific under CO2doubling inthe CAM3.1-1.5RGO model. The surface heat budget analyses suggest that the ElNi o-like pattern is resulted from a weakening of the Walker circulation. In thewestern equatorial Pacific, all the heat flux components are important to warm theocean, with the vast majority cancelled by entraiment cooling related to increasedstratification. In the central-eastern Pacific, the oceanic horizontal advections alongwith longwave radiation and latent heat flux act to warm the ocean, with entrainment,shortwave radiation and horizontal diffusion acting as damping terms. Although theocean model used here is a simple reduce-gravity model, the El Ni o-like responsesupports the results of some full ocean-atmosphere general circulation models (GCMs)performed for the World Climate Research Programme (WCRP) Coupled ModelIntercomparison Project (CMIP) phase-5. However, this result forms a sharp contrast to that in the Zebiak-Cane coupled model (La Ni a-like) where a simplifiedatmosphere model is used. After comparisons between ZC and CAM3.1-1.5RGOmodel, the atmospheric component, especially the radiation and cloud feedbackprocesses, is found to play a dominant role in determining the equatorial Pacific SSTresponse to doubled-CO2forcing in model simulations.Secondly, changes in the propagations of the SSTA during El Ni o events as wellas extreme El Ni o under global warming are further explained. Based on three setsof numerical experiments with different CO2concentrations (CTRL,2CO2and4CO2),the characteristics of ENSO are found to be changed associated with the tropicalPacific mean state variations. More specifically, in the higher CO2concentrationscenarios, the period of ENSO increases and the amplitude decreases. According tothe Bjerknes instability (BJ) index analysis, these phenomena are resulted from largerincrease in the damping effects (mean advection damping, thermodynamic dampingand ocean adjustment damping) than in the positive effects (zonal advective feedback,thermocline feedback and Ekman feedback) of the tropical Pacific ocean-atmosphereinteractions. In addition, as the CO2concentration increased, more El Ni o eventsfeature prominent eastward propagation, i.e., SST anomalies in the El Ni o eventstend to propagate eastwards. The surface heat budget and BJ index analysis indicatethat the zonal advective feedback supports westward propagation and the thermoclinefeedback supports eastward propagation. In a warmer climate, the thermoclinefeedback enhances and the zonal advective feedback weakens due to weakenedbackground mean currents and winds, which promotes more eastward propagationevents. At last, because of weakened background zonal and meridional SST gradient,southward immigration of ITCZ as well as increased sensitivity of atmosphere toocean heating, more extreme El Ni o events occur when the CO2concentration ishigher. Therefore, a weaker change in SST may cause stronger El Ni o events andgreater climate impacts under global warming.Thirdly, changes in the impact of the tropical north Atlantic on the tropicalPacific under global warming are revealed. By performing ensemble experiments with different CO2concentrations (CTRL,2CO2and4CO2), the north tropical Atlantic(NTA) warm SST anomalies in January are found to be able to induce a La Ni a-likecooling in the following autumn (SON) and winter (DJF) over the tropical Pacific.Meanwhile, this cooling pattern weakens as the climate warms. The mechanismsaccording to the heat budget analysis are listed below. In spring (MAM), both thestrength and shape of the meridional SST dipole (warm in the north and cold in thesouth) in the tropical eastern Pacific remain the same in different CO2concentrationscenarios. However, as the CO2concentration increased, the equatorialwarddisplacement of ITCZ suppresses the northward extension of the southeast trade windas well as the ocean dynamic processes, which weaken the cold SST anomaliesformation over the tropical eastern Pacific in the following summer (JJA). Thisweakened response leads to weakened zonal SST gradient changes and thecorresponding Bjerknes positive feedback, resulted in weakened growth of local coldSST anomalies in autumn. In winter, the vertical advection of mean temperature byanomalous upwelling is reduced due to weakened east wind anomalies at the westside of the SST cooling. At the same time, the weakened background mean currentstogether with less changed meridional SST gradient causes reduced meridionaladvection of anomalous temperature by mean medional currents. Both of them areimportant in weakening the intensity of the cold SST anomalies over the tropicalPacific. Moreover, the zonal advection effects of mean temperature by anomalouszonal currents decrease due to weakened background mean zonal SST gradient, whichis the major factor in limiting the west extension of the La Ni a-like cooling.