Sensitivity of ENSO Variability to Pacific Freshwater Flux Adjustment in the Community Earth System Model

The effects of freshwater flux （FWF） on modulating ENSO have been of great interest in recent years. Large FWF bias is evident in Coupled General Circulation Models （CGCMs）, especially over the tropical Pacific where large precipitation bias exists due to the so-called ＂double ITCZ＂ problem. By applying an empirical correction to FWF over the tropical Pacific, the sensitivity of ENSO variability is investigated using the new version （version 1.0） of the NCAR＇s Community Earth System Model （CESM1.0）, which tends to overestimate the interannual variability of ENSO accompanied by large FWF into the ocean. In response to a small adjustment of FWF, interannual variability in CESM1.0 is reduced significantly, with the amplitude of FWF being reduced due to the applied adjustment part whose sign is always opposite to that of the original FWF field. Furthermore, it is illustrated that the interannual variability of precipitation weakens as a response to the reduced interannual variability of SST. Process analysis indicates that the interannual variability of SST is damped through a reduced FWF-salt-density-mixing-SST feedback, and also through a reduced SST-wind-thermocline feedback. These results highlight the importance of FWF in modulating ENSO, and thus should be adequately taken into account to improve the simulation of FWF in order to reduce the bias of ENSO simulations by CESM.

Quantitative Analysis of the Feedback Induced by the Freshwater Flux in the Tropical Pacific Using CMIP5

Freshwater flux （FWF） directly affects sea surface salinity （SSS） and hence modulates sea surface temperature （SST） in the tropical Pacific. This paper quantifies a positive correlation between FWF and SST using observations and simulations of the fifth phase of the Coupled Model Intercomparison Project （CMIP5） to analyze the interannual variability in the tropical Pacific. Comparisons among the displacements of FWF, SSS and SST interannual variabilities illustrate that a large FWF variability is located in the west-central equatorial Pacific, covarying with a large SSS variability, whereas a large SST variability is located in the eastern equatorial Pacific. Most CMIP5 models can reproduce the fact that FWF leads to positive feedback to SST through an SSS anomaly as observed. However, the difference in each model＇s performance results from different simulation capabilities of the CMIP5 models in the magnitudes and positions of the interannual variabilities, including the mixed layer depth and the buoyancy flux in the equatorial Pacific. SSS anomalies simulated from the CMIP5 multi-model are sensitive to FWF interannual anomalies, which can lead to differences in feedback to interannual SST variabilities. The relationships among the FWF, SSS and SST interannual variabilities can be derived using linear quantitative measures from observations and the CMIP5 multi-model simulations. A 1 mm d-1 FWF anomaly corresponds to an SSS anomaly of nearly 0.12 psu in the western tropical Pacific and a 0.11℃ SST anomaly in the eastern tropical Pacific.

Modulation of El Nio-Southern Oscillation by Freshwater Flux and Salinity Variability in the Tropical Pacific

The E1 Nifio-Southern Oscillation （ENSO） is emphasized the roles of wind stress and heat flux environmental forcing to the ocean; its effect and modulated by many factors; most previous studies have in the tropical Pacific. Freshwater flux （FWF） is another the related ocean salinity variability in the ENSO region have been of increased interest recently. Currently, accurate quantifications of the FWF roles in the climate remain challenging; the related observations and coupled ocean-atmosphere modeling involve large elements of uncertainty. In this study, we utilized satellite-based data to represent FWF-induced feedback in the tropical Pacific climate system; we then incorporated these data into a hybrid coupled ocean-atmosphere model （HCM） to quantify its effects on ENSO. A new mechanism was revealed by which interannual FWF forcing modulates ENSO in a significant way. As a direct forcing, FWF exerts a significant influence on the ocean through sea surface salinity （SSS） and buoyancy flux （QB） in the western-central tropical Pacific. The SSS perturbations directly induced by ENSO-related interannual FWF variability affect the stability and mixing in the upper ocean. At the same time, the ENSO-induced FWF has a compensating effect on heat flux, acting to reduce interannual Qs variability during ENSO cycles. These FWF-induced processes in the ocean tend to modulate the vertical mixing and entrainment in the upper ocean, enhancing cooling during La Nifia and enhancing warming during E1 Nifio, respectively. The interannual FWF forcing-induced positive feedback acts to enhance ENSO amplitude and lengthen its time scales in the tropical Pacific coupled climate system.

Simulation of Salinity Variability and the Related Freshwater Flux Forcing in the Tropical Pacific: An Evaluation Using the Beijing Normal University Earth System Model(BNU-ESM)

The climatology and interannual variability of sea surface salinity （SSS） and freshwater flux （FWF） in the equatorial Pacific are analyzed and evaluated using simulations from the Beijing Normal University Earth System Model （BNU-ESM）. The simulated annual climatology and interannual variations of SSS, FWF, mixed layer depth （MLD）, and buoyancy flux agree with those observed in the equatorial Pacific. The relationships among the interannual anomaly fields simulated by BNU-ESM are analyzed to illustrate the climate feedbacks induced by FWF in the tropical Pacific. The largest interannual variations of SSS and FWF are located in the western-central equatorial Pacific. A positive FWF feedback effect on sea surface temperature （SST） in the equatorial Pacific is identified. As a response to El Nino-Southern Oscillation （ENSO）, the interannual variation of FWF induces ocean processes which, in turn, enhance ENSO. During El Nino, a positive FWF anomaly in the western-central Pacific （an indication of increased precipitation rates） acts to enhance a negative salinity anomaly and a negative surface ocean density anomaly, leading to stable stratification in the upper ocean. Hence, the vertical mixing and entrainment of subsurface water into the mixed layer are reduced, and the associated E1 Nino is enhanced. Related to this positive feedback, the simulated FWF bias is clearly reflected in SSS and SST simulations, with a positive FWF perturbation into the ocean corresponding to a low SSS and a small surface ocean density in the western-central equatorial Pacific warm pool.

Effects of different freshwater flux representations in an ocean general circulation model of the tropical Pacific

Freshwater flux(FWF) is a major forcing that affects the ocean through several processes. The effects of FWF may be represented in ocean modeling as real freshwater flux(RFF) formulations and virtual salt flux(VSF) methods. RFF formulations have been implemented in the Geophysical Fluid Dynamics Laboratory(GFDL) Modular Ocean Model version 5(MOM5) as a replacement for the non-physical VSF method, which is primarily used in state-of-the-art ocean models. Here, we systematically evaluated the effects of RFF-related processes on the GFDL MOM5-based simulations in the tropical Pacific.When the FWF was treated as the natural boundary condition(NBC), it directly decreased the local temperature and the salinity by changing the volume of the top model layer, and it increased the temperature in the eastern Pacific by triggering an eastward Goldsbrough–Stommel circulation in the subsurface.Moreover, the heat content induced by the FWF tended to counteract the decreasing effects of the NBC on sea surface temperatures(SSTs) in the western-central tropical Pacific. The relationships between SST perturbations and the FWF representation in ocean modeling are also discussed.

Influences of Freshwater from Major Rivers on Global Ocean Circulation and Temperatures in the MIT Ocean General Circulation Model

Responses of global ocean circulation and temperature to freshwater runoff from major rivers were studied by blocking regional runoff in the global ocean general circulation model （OGCM） developed at the Massachusetts Institute of Technology. Runoff into the tropical Atlantic, the western North Pacific, and the Bay of Bengal and northern Arabian Sea were selectively blocked. The blocking of river runoff first resulted in a salinity increase near the river mouths （2 practical salinity units）. The saltier and, therefore, denser water was then transported to higher latitudes in the North Atlantic, North Pacific, and southern Indian Ocean by the mean currents. The subsequent density contrasts between northern and southern hemispheric oceans resulted in changes in major ocean currents. These anomalous ocean currents lead to significant temperature changes （I^C -2~C） by the resulting anomalous heat transports. The current and temperature anomalies created by the blocked river runoff propagated from one ocean basin to others via coastal and equatorial Kelvin waves. This study suggests that river runoff may be playing an important role in oceanic salinity, temperature, and circulations; and that partially or fully blocking major rivers to divert freshwater for societal purposes might significantly change ocean salinity, circulations, temperature, and atmospheric climate. Further studies are necessary to assess the role of river runoff in the coupled atmosphere-ocean system.

Subtropical sea surface salinity maxima in the South Indian Ocean

Subtropical sea surface salinity(SSS)maximum is formed in the subtropical South Indian Ocean(SIO)by excessive evaporation over precipitation and serves as the primary salt source of the SIO.Spaceborne SSS measurements by Aquarius satellite during September 2011-May 2015 detect three disconnected SSS maximum regions(>35.6)in the eastern(105°E-115°E,38°S-28°S),central(60°E-100°E,35°S-25°S),and western(25°E-40°E,38°S-20°S)parts of the subtropical SIO,respectively.Such structure is however not seen in gridded Argo data.Analysis of Argo profile data confirms the existence of the eastern maximum patch and also reveals SSS overestimations of Aquarius near the western and eastern boundaries.Although subjected to large uncertainties,a mixed-layer budget analysis is employed to explain the seasonal cycle of SSS.The eastern and central regions reach the highest salinity in February-March and lowest salinity in August-September,which can be well explained by surface freshwater forcing(SFF)term.SFF is however not controlled by evaporation(E)or precipitation(P).Instead,the large seasonal undulations of mixed layer depth(MLD)is the key factor.The shallow(deep)MLD in austral summer(winter)amplifies(attenuates)the forcing effect of local positive E-P and causes SSS rising(decreasing).Ocean dynamics also play a role.Particularly,activity of mesoscale eddies is a critical factor regulating SSS variability in the eastern and western regions.