Interannual Variability Of The North Equatorial Current Bifurcation And Relative Ocean-Atmosphere Coupled Responses In The Western Pacific

The seasonal and interannual variability of the bifurcation latitude of the North Equatorial Current (NEC), Walker Circulation variation during 20th century and relative ocean-atmosphere coupled responses are investigated with a control simulation in the Kiel Climate Model (KCM) and an atmospheric general circulation model (AGCM) forced by observed SSTs. There are two oceanic circulation modes about the interannual variability of the NEC bifurcation latitude (NBL), which is NMK Mode related to the variations of the NEC-Mindanao Current (MC)-Kuroshio Current (KC) (NMK) system and MK Mode related to the variations of MC and KC. We analyze the mechanism and the connection with El Ni?o-Southern Oscillation (ENSO) through ocean-atmosphere coupled responses within the two modes. We re-investigate the behavior of the Walker Circulation using different observational datasets and ensemble integrations. The Walker Circulation may have strengthened forced by the spatial structure of the long-term trend in tropical SST.As it shows in KCM, the seasonal cycle of the NBL is dominated by East Asian Monsoon through the local Ekman Pumping with in southern part during spring and summer, while in northern part during autumn and winter. The NEC bifurcated in higher (lower) latitude during El Ni?o (La Ni?a) events, with a period time as 2-4 years and a longer time in the interannual and interdecadal variability of NBL.The interannual variability of NBL is highly related to the first two Empirical Orthogonal Function (EOF) modes for the meridional velocity averaged within a 4°-longitude band off the Philippine coast. The first mode named NMK Mode with a variance of 51% is mainly related to variations of the Subtropical Gyre (STG) and Tropical Gyre (TG) as a 4-year period time. It shows an enhancement of the NMK system with low latitude of NEC bifurcation. The linear regressions of ocean and atmosphere onto NMK Mode show strengthens of the STG and TG with a dipole for the atmospheric and oceanic circulations with warming and more precipitation in Warm Pool, equatorial Pacific and KC region while cooling and less precipitation in MC region. It is highly influenced by ENSO cycle with 2-season lag for the formation, evolvement and decay of the NMK Mode. During the developing and mature of ENSO warm (cold) event, there are both atmospheric and oceanic anti-cyclone (cyclone) circulations in northern part of the equatorial Western North Pacific due to the teleconnection, then cyclone (anti-cyclone) circulations in southern part due to the ENSO positive feedback and Hadley Circulation change to compose the dipoles for atmospheric and oceanic circulations. Thus, both subtropical gyre and tropical gyre are intensified (weakened) in turn, corresponding with the NMK Mode and southern (northern) part of NBL. The climate changes are high related to the oceanic-atmospheric coupled responses with a warming (cooling) and more (less) precipitation in East China Sea, but cooling (warming) and less (more) precipitation in Philippine coast. The anomalous atmospheric circulation decreases during the decay of ENSO warm (cold) events. Then the NMK Mode starts to decay due to weaken of the STG and TG with a northward shift of NBL. The NMK Mode is highly influenced by the ENSO cycle with high connection to the ocean-atmosphere coupled responses. However, it shows the independence upon ENSO to some extent.The second mode named MK Mode with a variance of 26% is mainly related to variations of the western currents of NMK system as a quasi-biennial oscillation (QBO). It shows an increase of MC but decrease of KC with high latitude of NBL. The linear regressions of ocean and atmosphere onto MK Mode show a cold band from KC to the Philippine coast, an El Ni?o-like SST pattern in the equator and both atmospheric and oceanic cyclone circulation in the Tropical Western Pacific with more precipitation. The MK Mode is highly correlated to the QBO of East Asian Monsoon with 1-2 seasons leading ENSO cycle. The atmospheric cyclone (anti-cyclone) circulation in the Philippine Sea corresponding with the strong (weak) East Asian winter monsoon anomaly resulted in the NBL at high (low) latitude with decrease (increase) of KC while increase (decrease) of MC, cooling (warming) and less (more) precipitation, meanwhile, there is a warming (cooling) and more (less) precipitation in east of Philippine coast. Then, it forms local air-sea dynamics and thermal positive feedbacks which make the MK Mode persistence and warming (cooling) in Equatorial Central and Eastern Pacific via oceanic process. This may excite the El Ni?o (La Ni?a) event and an anti-cyclone (cyclone) in the Philippine Sea through teleconnection, which will lead East Asian winter monsoon to be weak (strong) and then into the other phase of MK Mode. MK Mode is favor of ENSO through oceanic process via changes of MC due to the NBL shifts by wind variations. However, it is not a substantial condition. Recent studies indicate a weakening of the Walker Circulation during the 20th century. Here, we present evidence from sea surface temperature (SST) observations and an atmospheric general circulation model that the Walker Circulation may have intensified rather than weakened. Observed Equatorial Indo-Pacific Sector sea surface temperature since 1870 exhibited a zonally asymmetric evolution: While the Equatorial Pacific shows only a weak warming, the Equatorial Indian Ocean exhibited a rather strong warming. This has resulted in a strong increase of the SST gradient between the two ocean basins along the Equator. The change in the SST gradient drove an anomalous atmospheric circulation, with an enhancement of both Walker and Hadley Circulation, as inferred from ensemble experiments with the atmosphere model forced by the observed SSTs. Anomalously strong precipitation is simulated over the Indian Ocean and anomalously weak precipitation over the western Pacific, with corresponding changes in the surface wind pattern. Sensitivity to the forcing SST, however, is noticed. The results may be important in the context of Global Warming, as regional climate changes in the Equatorial Sector may critically depend on inter-basin interactions.