Multi-scale Variations Of The Surface Circulation In The West Tropical Pacific: Spatial-Tmporal Features And Driving Mechanisms

The western tropical Pacific Ocean is an interesting and important regionfor Ocean and Climate researches, because their dynamics and thermalvariations are closely related with not only climatic modulations in tropicaloceans, but also weather/climate, environment, fishery, and military of China.There are the most complex circulation system of the global in the westernPacific ocean, they are the equatorial current system, western boundary currentsystem, and the Indonesian trhoughflow (ITF) accrossing the ocean basins. Thecrisscrossed currents bring the water mass formed in the different ocean intothe western tropical Pacific Ocean, through the complex mixing, flowing intothe different important oceans, e.g., equatorial Pacific, tropical Indian Ocean,and the East/South China Sea. As a result, the western tropical Pacific Ocean isan crossroads for the water masses (Fine et al.,1994). The variations and pathsof the water masses are affected by the circulation system. It is very necessaryand significative to research the circulation variations and mechanisms, and itwill help us to further understand their influences on the global climatevariations and our country’s environmental changes.This article researches the variations of the different flow branches in thewestern Pacific Ocean using the AVISO data, Quikscat data, ECMWF andOSCAR reanalyze data, combining the linear reduced gravity model.Surrounding the “spatial and temporal variations of the multi-scalecirculation in the tropical western Pacific Ocean”, we expounded somescientific problems and proved some significant/innovative conclusions. Ourresults stressed the importance of the circulations in the Philippine Sea,especially their surface gyre structures.We firstly study the high-frequency variations in the Philippine Sea,especially discussing the mechanisms in controlling the semiannual variability. Pronounced semiannual SSH variations are detected within twozonal bands: one is east of Luzon Strait (19°-22°N) in the northern PS, and theother is southeast of Mindanao coast (4°-7°N) in the southern PS. In the twonear-coast boxes where semiannual harmonic amplitude exceeds4cm, thenorthern box (NB;127°-133°E,19°-22°N) and southern box (SB;127°-133°E,4°-7°N), semiannual changes contribute considerably to the total annual SSHvariance (12%and17%, respectively). Despite prominent SSH variations inthe two boxes, the bifurcation latitude of the North Equatorial Current (NBL)shows weak semiannual fluctuations with a peak-to-peak difference of only0.3°. While the in-phase annual SSH variations between the two boxes worktogether to enhance the annual NBL changes, their out-of-phase semiannualSSH variations work to offset each other in driving displacement ofbifurcation point. Further analysis with a11/2-layer reduced-gravity modelforced by ECMWF wind stress data suggests that, the observed semiannualSSH variations are primarily driven by local wind forcing in the far westernPacific. Rossby waves propagating from eastern/central Pacific are of muchless contribution due to along-path dispersion and canceling. Semiannualsignals of wind forcing field in the northern PS reflect mainly the semiannualchanges of Asian Monsoon system, while those in the southern PS arise fromthe combined effect of Monsoon transitioning and variations of theintertropical convergence zone (ITCZ). The importance of this study in twoaspects, One is that the existence of two pronounced regions of semiannualvariations is newly elucidated. Another is that the driving mechanismspresented using the model is consistent with the satellite observation andreasonable.Another an important research is the role of the MD in the variability ofthe North Equatorial current bifurcation. The Mindanao Dome (MD) featuresprominent oceanic variability and locates geographically close to thebifurcation latitude Ybof the Pacific North Equatorial Current. In this study,the role of the MD in the variability of Ybis examined with20yr of satellite altimetric sea surface height (SSH) data and a1.5-layer linear Rossby wavemodel. It is shown that the seasonal variations of surface Ybare related to notonly the SSH fluctuations near the bifurcation point (bifurcation box;125°-130°E,12°-15°N) but also those outstanding in the MD region (MD box;127°-132°E,6°-9°N). The impact of the MD SSH changes is significant whenbifurcation point stays at southerly latitudes during February-September,which hinders (delays) the southward leap (northward retreat) of YbinApril-May (July-August) and thus leads to the asymmetry of mean Ybseasonal cycle (with a positive skewness of γ=+0.64). Such asymmetry showsalso year-to-year variations depending on yearly mean Ybvalue. A southerlyyearly mean Ybinvolves larger contribution of the MD and thus causes largerasymmetry of Ybseasonal cycle. At interannual and longer timescales, the MDacts to amplify the fluctuations of the bifurcation. It is responsible for about20%of the total low-frequency Ybvariances and plays an important role in the0.12°yr-1southward trend of Ybin the past two decades. The impact of theMD on Ybchanges is becoming more and more significant at varioustimescales as the bifurcation point slowly migrating southward in recentyears.The finally reaches in our article are the surface NECC variations in thewestern Pacific Ocean. Combining the OSCAR and AVISO datas, we firstlydiscuss the seasonal variations. Based on the OSCAR-derived sea surfacecurrents, and AVISO-derived sea surface height anomaly and geostrophiccurrents from the past19yr are used to investigate the annual variability ofthe surface NECC confined in the western Pacific Ocean. Inferred from thesurface current and height data, the headstream (128°-136°E) and downstream(140°-156°E) NECC emerge different seasonal features including the surfaceintensity (INT), position (YCM), and geostrophic/Ekman contribution of theNECC. Aim at the headstream NECC, the slight (solid) semiannual signalsexit in the INT (YCM) annual cycle. The OSCAR-derived INT (INTO) staysaround~1.4×105m2s-1with the weak seasonal amplitude. The southernmost (northernmost) migration of NECC is induced by the expanding of MindanaoDome (Halmahera Eddy) from December (April) through January (July). TheHE SSH increased second time in November causes the semiannual feature ofthe YCM. In the downstream, the minimum (maximum) INT reaches0.6(1.4)×105m2s-1during the first (latter) half of a year. The INTOdisplays twicemaximum respectively in August and November. The seasonal amplitude ofdownstream position is larger (~3°) than headstream. No matter theheadstream or the downstream NECC, the geostrophic transport could notexplain the eastward-flowing one hundred percent. The combined effects ofthe Ekman and geostrophic transports causes the slight semiannual variationsin headstream INTOand double maximum in INTO. Especially to thedownstream, the northern gyre can reflect the NECC intensity through theC-region (4°-6°N;143°-144°E) SSH seasonal variation but not represent thegeostrophic current due to the Ekman transport participation. The seasonalvariability of INT and YCMare both influenced by the signals of wave, thedifferentials are the headstream is modulated by the combined effects of theHE associated with the local westward (eastward)-propagating Rossby(Kelvin) waves from September (March) through November (June) and MDassociated with the westward-propagating Rossby waves from central Pacific.However, the downstream related with the northern gyre is mainlyinfluenced by the westward-propagating Rossby waves from central Pacific inthe northern flank (around5°N).Analysis of the satellite observations during1992-2011revealspronounced interannual-to-decadal variations of the West Pacific NorthEquatorial Countercurrent (NECC) system, including the surface intensity(INT), position (YCM), and path lengthen (LCM) of the NECC jet, together withthe associated recirculation gyres and mesoscale eddies. During the1997-1998and2009-2010El Ni o events, the NECC jet showed increased INT before,and decreased INT, northward shifted position, and lengthened path after themature phase. During the1993-1995and2002-2005central Pacific warming events, it showed also increased INT but no evident changes in YCMand LCM.The varied responses arise from the different natures of individual El Ni oevents. In1998and2010, reflected upwelling Kelvin waves south of theNECC jet, together with the downwelling Rossby waves north of it inducedby the following La Ni a events, weakened the NECC jet through geostrophyand shifted it northward. This process was however absent during the1993-1995and2002-2005events. Through barotropic instability, intensifiedNECC jet during warm condition gives rise to anomalously active mesoscaleeddies which contributed to the unstable state of the system in1998and2010.Hindcast of a linear Rossby wave model reveals a reddening trend of thevariance in the NECC system during the past50years. Variations in the pasttwo decades are dominated by quasi-decadal signals and more contributed bywind forcing in the western Pacific, corresponding to the slow changes of theENSO-related wind forcing pattern.