A Study Of Tropical Pacific Sea-air Variation And Two Types Of El Nin|~o

In this paper, HadISST observational data, and assimilation data of NCEP/NCAR,GODAS et al., have been used to study the sea-air variations of tropical Pacific duringthe two types of El Ni o, by methods of rotated empirical orthogonal function(REOF), rotated singular value decomposition (RSVD), multi taper method-singularvalue decomposition (MTM-SVD), lead/lag regression (correlation) analysis, andcomposite analysis. Main conclusions are listed below:1. REOF method is proven to describe the temporal variation characteristics ofsea surface temperature (SST) anomalies in the central Pacific more objectively thanEOF method does. This paper defines the first and second REOF mode (orstandardized time) as EP and CP leading mode (or index), respectively.2. Lead/lag correlation analysis indicates that the most salient negativecorrelation (up to-0.5above in the inter-annual scale) exists when the EP index leadsthe CP one about20months. This finding is helpful for understanding the formationof the CP El Ni o and the transformation between the two types.3. There are a close relation between the decadal variation (Quasi11-year cycle)of the SST in the tropical Pacific and the CP cycle. More frequent occurrences of theCP El Ni o with an intensifying tendency in the recent decades may be involved withthe strengthening of the tropical Pacific decadal change.4. The coupled variation between the SST and surface zonal wind (SZW)anomalies are studied by RSVD method. Based on the RSVD results, the leadingmodes related with the two types of El Ni o are obtained by REOF method. Further,discrepancy of Sverdrup transport during the two types of El Ni o is examined. Forthe EP El Ni o, the SZW anomalies pattern is favor for poleward Sverdrup transports,and an eastward Sverdrup transport in the equator. While For the CP El Ni o, theSZW anomalies pattern restrains poleward Sverdrup transports, and the zonalSverdrup transports are very weak in the equator. However, transports near the equator in the north and south are opposite to each other.5. The analysis of subsurface ocean temperature (SOT) anomalies variationindicates that the warm signals from the west Pacific warm pool region exist beforethe beginning of both two types of El Ni o, and those for the EP El Ni o migrate intothe eastern basin faster than those for the CP one, which is lifted up significantlywhen they are staying at the central Pacific. Zonal wind force plays an importantcomplementary role for the subsurface warm signals. The location of the downwellinginduced by SZW anomalies on the Equator is about30degrees west for the CP ElNi o than that for the EP one.6. Relation between the propagation SOT anomalies and anomalous equatorialsubsurface current variation is analyzed. For the EP El Ni o, anomalous downwellingin the subsurface of central pacific is not favor for the lifting up the warm signalswhen they pass through the central basin; and the equatorial undercurrents areaccelerated before and around the appearance of El Ni o, which is favor for the warmsignals crashing into the eastern basin fast. While for the CP El Ni o, anomalouswestward currents exist in the west-central Pacific from surface to deep subsurface allalong, leaving a longer time for the warm signals staying the central Pacific andkeeping them from crashing into the eastern Pacific fast; there is a stronger upwellingin the subsurface of the central Pacific. The stronger up-welling is favorable for thewarmer water lifted up onto the mixed layer, and eventually making contributions tothe surface warming in the central basin.