Anomalous warming occurred in the equatorial central-eastern Pacific in early May 2014,attracting much attention to the possible occurrence of an extreme El Ni?o event that year because of its similarity to the situation in early 1997. However,the subsequent variation in sea surface temperature anomalies(SSTAs) during summer 2014 in the tropical Pacific was evidently different to that in 1997,but somewhat similar to the situation of the 1990 aborted El Ni?o event. Based on NCEP(National Centers for Environmental Prediction) oceanic and atmospheric reanalysis data,the physical processes responsible for the strength of El Ni?o events are examined by comparing the dominant factors in 2014 in terms of the preceding instability of the coupled ocean–atmosphere system and westerly wind bursts(WWBs) with those in 1997 and 1990,separately. Although the unstable ocean–atmosphere system formed over the tropical Pacific in the preceding winter of 2014,the strength of the preceding instability was relatively weak. Weak oceanic eastward-propagating downwelling Kelvin waves were forced by the weak WWBs over the equatorial western Pacific in March 2014,as in February 1990. The consequent positive upper-oceanic heat content anomalies in the spring of 2014 induced only weak positive SSTAs in the central-eastern Pacific–unfavorable for the subsequent generation of summertime WWB sequences. Moreover,the equatorial western Pacific was not cooled,indicating the absence of positive Bjerknes feedback in early summer 2014. Therefore,the development of El Ni?o was suspended in summer 2014.
Using a new global ocean reanalysis of the second generation Global Ocean Data Assimilation System of the Beijing Climate Center(BCC_GODAS2.0) spanning the period 1990-2009,we firstly quantify the accuracy of BCC_GODAS2.0 in representing the temperature and salinity by comparing with OISST and SODA data.The results show that the assimilation system may effectively improve the estimations of temperature and salinity by assimilating all kinds of observations,especially in the equatorial eastern Pacific.Moreover,the root mean square errors of monthly temperature and salinity are respectively reduced by 0.53℃ and 0.28 psu,compared with the model control simulation results.Then,the applicability of this ocean reanalysis for sea surface temperature(SST) anomaly variability in the tropical Pacific is evaluated with the observational HadISST data.The NINO3 index of the new reanalysis shows a good agreement with that of HadISST,with a correlation of 93.6%.Variations in SST from BCC_GODAS2.0 are similar to those obtained from HadISST data along the equator,showing the major large zonal-scale features such as the strong magnitude of seasonal cycle.The amplitude of SST anomaly standard deviation in the equatorial eastern Pacific is also closer to observations(HadISST) than NCEP GODAS does.Besides,the first two leading empirical orthogonal function(EOF) modes of the monthly SST anomalies over the tropical Pacific region are explored.The EOF1 pattern of BCC_GODAS2.0 captures a traditional El Ni o pattern,which improves magnitudes of the positive SST anomaly in the cold tongue of the eastern Pacific.The EOF2 pattern exhibits a El Ni o Modoki pattern.Comparatively,the EOF2 pattern of BCC_GODAS2.0 extends more strongly toward the subtropics.It also overcomes the problem that negative loadings are confined in the narrow equatorial eastern Pacific.Consequently,the magnitude and spatial distribution of the leading EOF patterns of BCC_GODAS2.0 are well consistent with those of HadISST.
WANG DongXiaoQIN YingHaoXIAO XianJunZHANG ZuQiangWU FengMin
Simulated outgoing longwave radiation(OLR) outputs by two versions of the grid-point atmospheric general circulation model(GAMIL) were analyzed to assess the influences of improvements in cloud microphysics and convective parameterization schemes on the simulation of the Madden-Julian oscillation(MJO) and other tropical waves.The wavenumber-frequency spectral analysis was applied to isolate dominant modes of convectively coupled equatorial waves,including the MJO,Kelvin,equatorial Rossby(ER),mixed Rossby-gravity(MRG),and inertio-gravity(IG) waves.The performances of different versions of the GAMIL model(version 1.0(GAMIL1.0) and version 2.0(GAMIL2.0)) were evaluated by comparing the power spectrum distributions of these waves among GAMIL1.0,GAMIL2.0,and observational data.GAMIL1.0 shows a weak MJO signal,with the maximum variability occurring separately at wavenumbers 1 and 4 rather than being concentrated on wavenumbers 1-3,suggesting that GAMIL1.0 could not effectively capture the intraseasonal variability.However,GAMIL2.0 is able to effectively reproduce both the symmetric and anti-symmetric waves,and the significant spectra of the MJO,Kelvin,and MRG waves are in agreement with observational data,indicating that the ability of GAMIL2.0 to simulate the MJO and other tropical waves is enhanced by improving the cloud microphysics and convective parameterization schemes and implying that such improvements are crucial to further improving this model's performance.
