Study On The Long-term Variability Of The Tropical Pacific Ocean And Its Mechanism

The decadal variablity in the tropical Pacific Ocean is studied. Several data set, including COADS, SODA, HadISST1, ERSST and those from JEDAC are used for the basic analysis. The characteristics of the tropical Pacific dedacal variability are obtained after investigating Sea Surface Temperature(SST), Sea Level Pressure(SLP), sea surface wind and oceanic upper-400m heat content. A fully coupled ocean-atmosphere model (FOAM) is applied to highlight the mechanism of this long-term variability. Basing on FOAM control-run, a series of numerical experiments are conducted, including shutting down ccean-atmosphere coupling in the North Pacific, the tropical Pacific and the South Pacific respectively, and forcing the model with SST anomalies in the mid-latitude of the Pacific Ocean.Rerults from data analysis show that in addition to interannual variability, the tropical Pacific Ocean experiences climate fluctuations on decadal and longer time scales, whose dominant mode is a part of PDO. PDO represents opposite variability between the central North Pacific and the middle and eastern part of the tropical Pacific Ocean, those along North America as well. Its significant period is about 40 to 50 years. Anomalous SLP and wind associated to PDO mainly can be found around the Aleution Low and in the tropical Pacific. The tropical Pacific still has another mode with SST fluctuating in-phase with that north to 30oN, most energetic at period in the 10-15 year band. On the decadal time scale, the upper-400m heat content depictes variability in the western tropics out phase of that in the eastern tropics, which illuminates that decadal variability is not only refered to SST, but also a new distribution of upper-layer heat in the Ocean.The strength of the tropical Pacific decadal variability mainly depends on local ocean-atmosphere coupling. Through atmospheric teleconnetion, SST variability in the tropical Pacific can affect the mid-latitude and induce a simultaneous SST decadal variability. The North Pacific responses more intensely than the South Pacific and results in a spatial pattern with opposite variability between the central and the eastern part. Anomalous SST in the mid-latitude may be transport to the tropics through both the inner oceanic tunnel and ocean-atmosphere interactions. Results from numerical experiments prefer oceanic tunnel in the North Pacific to that in the South Pacifc. However, the former cannot reach to the equator. A positive feedback due to interaction of surface wind, evaporation and SST does spread the mid-latitude variability to the equator. The North Pacific seem to be more influential to the tropics while the South Pacific is modest. Ocean-atmosphere coupling in the mid-latitude can enlarge these extratropical influences very obviously making themselves important to the tropical Pacific decadal variability.Results from experiments support the hypothesis that the (tropical) Pacific decadal variatility originates from interactions between the tropics and the extratropics. The decadal SST anomaly from the extratropics becomes a basin-wide mode by the tropical ocean-atmosphere coupling. Then through atmospheric teleconnection, decadal variability is introduced into the mid-latitude and develops by local atmosphere coupling. The developed variability can affect the tropics strengthening the variability in the west equator. On the other hand, mid-latitude effect can also make tropical SST turn to the opposite variability, and a total cycle forms. During the (tropical) Pacific decadal variability, ocean-atmosphere coupling in the tropical Pacific is most important and those in both South and North Pacific plays less roles.