A Parametric Study Of The Subsurface SST In The Equatorial Pacific In The Zebiak-Cane Model And Its Application

The equatorial Pacific subsurface temperature is an important factor controlling sea surface temperature(SST)and its interannual variability and is also an important link in the dynamic framework of the intermediate coupled model.However,the difficulty of observing and the uncertainty of parameterization schemes of the equatorial Pacific subsurface temperature greatly limit the improvement of ENSO simulation and prediction.In addition,some advanced coupled general circulation models(CGCMs)still have some problems in the simulation of ENSO and even climate mean state,and systematic errors still exist.Based on reanalysis datasets,CMIP5 multi-model simulations as well as Zebiak-Cane(ZC)model,this work investigates the parameterization of the equatorial Pacific subsurface temperature and its application by dynamic diagnosis,theoretical analysis and numerical simulation.Four primary conclusions are summarized as follows:(1)The vertical profile of the equatorial Pacific subsurface temperature can be expressed as a simple function of the thermocline depth and modulated by the warm pool SST and thermocline sharpness.The vertical profile of the subsurface temperature in the equatorial Pacific is highly similar to the hyperbolic tangent function(tanh)in both climate states and inter-annual variability.The ocean isothermal surface rises or decreases as a whole with the change of the thermocline depth for its shape preserving property.Thus,a parameterization of the three dimensional subsurface temperature(Tsub)in the equatorial Pacific centered at the thermocline depth(h)is established,which is bounded by the warm pool SST(Tr)and modulated by the thermocline sharpness(h*).Here h* represents the density of isothermal surface,controlling the rate at which the Tsub decreases with depth.It has a relatively stable distribution of climate mean state,and is about 60 meters in a large range of the upper 300 meters of the equatorial Pacific Ocean.In addition,h* also has moderate interannual variability in the upper subsurface layer near the surface mixied layer,which is mainly related to the SST anomaly(SSTA)and h anomaly related to ENSO variability.This is because the thermocline depth subsides/uplifts during the El Ni(?)o/La Ni(?)a events,and the thermocline sharpness decreases/increases with the stratification structure of the equatorial Pacific subsurface layer more compact/looser.(2)The newly established parameterization of the equatorial Pacific subsurface temperature is fully valid and universal.The parameterized climatological subsurface temperature in the equatorial Pacific is in reasonable agreement with the observation even for the simplest parameterization scheme A1 using a zero-order approximation of h*(60m).It is also a good way to depict the three-Dimentional(3D)subsurface temperature of different types of ENSO events.However,Scheme A1 has some errors in the deep eastern Pacific and upper subsurface layer near the mixed layer.On this basis,Scheme A2 using the 3D distribution of the thermocline sharpness climate mean state can improve the deep layer of the eastern Pacific,but the improvement of the upper subsurface layer is limited.By using multiple linear regression method to establish the first-order approximation of h*,Scheme B can more accurately represent ENSO-related 3D subsurface temperature distributions in the equatorial Pacific,especially in the upper subsurface layer,which is greatly improved compared with Scheme A1.Compared with the parameterization schemes of the subsurface temperature in the original ZC model,the new parameterization is more close to the standard deviation and skewness characteristics in the observation,and the nonlinear relationship between the subsurface temperature and the thermocline depth is also more realistic.(3)Based on the new parameterization,the mean state bias of the equatorial Pacific subsurface temperature from multiple CGCMs in CMIP5 can be attributed to the combined effects of h,h* and Tr.The mean state bias in the equatorial Pacific is an integral phenomenon from sea surface to subsurface.The multi-model Ensemble(MME)mean of the equatorical Pacific SST and subsurface temperature are both colder than observation,and the first inter-model EOF modes both exhibit a pattern of the same sign in the equatorial Pacific,with their first principal components(PC1s)significantly correlated.Based on the new parameterization of the equatorial Pacific subsurface temperature established in observation,it is found that the mean bias between MME mean and the observation is relatively small for h,while Tr is colder and h* is slightly thicker than observation.It's worth noting that the inter-model biases of these three factors all have large standard deviations,reflecting high inter-model variability.The first inter-model EOF modes of h and h* also exhibit a pattern of the same sign in the equatorial Pacific.Furthermore,the inter-model biases of subsurface temperature is linearized by a first-order Taylor expansion approximation,and the contributions of the three factors in different models are quantitatively evaluated by using inter-model EOF projection.The results show that the positive contribution of h is mainly confined to the eastern equatorial Pacific,while the contribution of Tr and h* is mainly in the central and western Pacific with opposite signs.Overall,the three factors all play an important role in the inter-model biases of subsurface temperature,and the contribution of each factor also shows a large inter-model diversity.In addition,the inter-model biases of oceanic and atmospheric variables in the models follow the principle of air-sea interaction which is highly similar to that during CP ENSO.(4)The new parameterization scheme of the equatorial Pacific subsurface temperature can effectively improve the authenticity of ENSO skewness simulation in the ZC model.The simulated SSTA between ENSO cold and warm phases in the original ZC model shows a positive skewness in the eastern Pacific which is similar to the observation.However,based on the heat budget analysis of the mixed layer,it is found that unlike the positive contribution of nonlinear dynamic heating(NDH)in observation,the contribution of NDH to ENSO skewness in the ZC model is almost zero.Further analysis shows that the subsurface temperature anomaly of ENSO events has similar characteristics to that of SSTA,that is,the spatial distribution is limited in the eastern equatorial Pacific and the intensity of La Ni(?)a is suppressed.In the original scheme,two sets of asymmetric parameters were set under the positive and negative phases of the thermocline depth anomaly.After replacing it with the completely symmetric scheme,the positive skewness of SSTA in the equatorial eastern Pacific disappeared.Thus,it can be seen that the untrue skewness distribution in the ZC model is actually due to the artificial asymmetry in the parameterization scheme of the subsurface temperature.The results of replacing the original scheme with the newly established parameterization scheme of equatorial Pacific subsurface temperature(scheme B)can not only improve the spatial distribution of SSTA and subsurface temperature,but also reproduce the positive skewness in the eastern equatorial Pacific and positive effect of NDH similar to the observation.