A Studv Of ENSO Diversity And Its Mechanisms

ENSO diversity refers to the emergence in recent decades of different El Nino types in the tropical Pacific,each with significant but different global impacts on weather and climate.This thesis mainly discusses the warm phase of ENSO:El Nino.Different from canonical El Ni(?)o,the newly emergent type,called Central Pacific El Nino(CP El Nino)or ENSO Modoki,has its center of warming shifted more to the central equatorial Pacific.Ongoing debates concern whether ENSO should be described as a diverse continuum or two distinct modes of variability.A large number of ad hoc indices of ENSO diversity have previously been studied.Various mechanisms have been proposed but so far there is no consensus,likely hampered by two factors.First,the original definition of ENSO Modoki was based on the limited domain Empirical Orthogonal Function(EOF)analysis,which is not robust and highly dependent on the artificial boundaries.Second,the diversity indices involve other non-distinguishing factors in addition to the essential factors in various proportions.Besides that,in conventional EOF analysis,there exists mode mixing in frequency domain of modes,which make it difficult to describe the physical mechanism of each mode.The Rotated Empirical Orthogonal Function has been used in this thesis to eliminate the mode mixing problem.The modes obtained can explain most of pan-Pacific SST variability.Therefore,the four fundamental modes in pan-Pacific SST are:ENSO-cycle mode,pan-Pacific Decadal Oscillation mode(PDO mode),Central Pacific Variability mode(CPV mode)and North Pacific Gyre Variability mode(NPGV mode).To describe the ENSO variability,only three of the four modes are found to be necessary.Of the three,the recent reported emergence of the ENSO diversity can be accounted for by changes in two modes,the PDO and CPV.The formation of the CP El Nino is somewhat dependent on the CPV mode at a significant time.The newly introduced fundamental mode,CPV,has a horseshoe shaped warm SST pattern in the central Pacific flanked by cold SST to the east.Positive CPV is associated with a shallower thermocline in the east and a deeper one in the central Pacific,favoring the formation of CP El Ni(?)o.The newly emergent CPV mode is likely the result of the intensification of the tropical Pacific trade winds since late 1970s.Intensified trade wind tends to keep some El Ni(?)o in the central Pacific,forming a CP El Ni(?)o,while unimpeded El Ni(?)o will eventually form an EP El Ni(?)o.The mean difference of the two types of El Ni(?)o in their thermocline depth variation in the equatorial Pacific can be entirely explained by(CPV+PDO)minus ENSO.Paleoclimate records appear to indicate that ENSO is "modulated" from decade to decade.The modulation could be through interactions between the tropical and extratropical Pacific,or arising through variability intrinsic to the tropical Pacific.However,the nature of such an influence is not yet understood.There is no doubt that subtropical decadal variability like PDO can affect the tropical sea-surface temperature(SST),mostly in the equatorial Central Pacific.We found that this influence is not through the nonlinear mechanism of amplitude or frequency modulation,but through linear superposition of the two climate modes on tropical SST.To fully understand the relationship between PDO and ENSO,we need to know whether the tropical SST contributes to the subtropical SST.PDO is reconstructed using Hasselmann models with different forcing such as ENSO or North Pacific Index(NPI).The result shows that PDO exists independently in the interannual variability of tropical ENSO.Because of the impact of the ENSO phenomenon on regional and global weather and climate,its potential long-term predictability is an important area of study.Since each phase of the PDO has decadal persistence,it was hoped that its influence on the ENSO phenomenon might have decadal predictability.We suggest that the problem of decadal predictability should be recast into predicting not the ENSO itself,but the tropical Pacific SST.