| The North Pacific Victoria Mode(VM)is the second or third mode of EOF analysis of sea surface temperature(SST)in the North Pacific north of 20°N.Over the Maritime Continent,autumn is a dry season with frequent fires,and the large amount of greenhouse gases and aerosols produced by wildfires has a bad impact on local air quality and the global climate.In-depth understanding of the variations of VM is conducive to deepen the understanding of VM climate effects,and establishing the connection between VM and autumn precipitation over the Maritime Continent is conducive to the prediction of wildfires.This article uses atmospheric reanalysis data,SST and EN4 subsurface ocean temperature data to analyze the characteristics of the interdecadal shift of the seasonal variations,spatial structure and magnitude of VM,and explore interdecadal shift of interannual relationship between VM and autumn precipitation over the Maritime Continent.The main conclusions are as follows:(1)The characteristics of interdecadal shifts in North Pacific SST mode in the late 1980s are mainly reflected on the EOF2(SST trend)and EOF3(VM).Only after 1990,consistent warming can be seen in the North Pacific.The spatial structure of the VM was a meridional dipole from 1950 to 1989,while the structure transformed into a meridional tripole from 1990 to 2022.This is because SST at the Gulf of Alaska(56°N～61°N,173°E～150°W,namely the northern SST key area)is only significantly related to the northern part of the North Pacific(38°N～47°N,172°W～140°W,namely the central SST key area)and the central part of the North Pacific(26°N～35°N,157°E～170°W,namely the southern SST key area)after 1990.The VM has seasonal phase-locking characteristics.From 1950 to 1989,it was the strongest in spring,followed by winter,but from 1990 to 2022,it was the strongest in winter,followed by spring.With regard to the internal causes of the ocean,the reason for the relative increase of wintertime VM from 1990 to 2022 is that the amplitudes of the three key areas of SST all increase,resulting in the enhancement of the meridional SST gradient in the North Pacific.The reason for the relative decrease of springtime VM is that the amplitude of southern SST key area weakens,resulting in the weakening of the meridional SST gradient.The North Pacific Oscillation(NPO)is the external factor of the interdecadal shifts of wintertime and springtime VM intensity.The relationship between NPO and wintertime VM strengthened after 1990,whereas the relationship with springtime VM weakened after 1990.(2)In the North Pacific SST mode,only the springtime VM has an impact on the autumn precipitation over the Maritime Continent at the interannual scale.When springtime VM is in positive(negative)phase,it is usually accompanied by the positive(negative)phase of the Pacific meridional mode and the cold(warm)SST in the Western North Pacific.The SST gradient would lead to the westerly(easterly)anomaly in the equatorial Pacific,impacting eastward propagation of equatorial Kelvin wave and leading to warm(cold)SST anomaly in the central and eastern Pacific.SST gradient and westerly(easterly)anomaly persist into autumn through Bjerknes feedback,so the mature El Nino(La Nina)event forms in autumn.Then,through the Walker circulation,the abnormal sinking(rising)movement,low-level divergence and high-level convergence(low-level convergence and high-level divergence)over the Maritime Continent occur,resulting in precipitation decrease(increase).This interannual relationship between the variation of springtime VM and the autumn precipitation over the Maritime Continent has changed in 2003,which was a significant negative correlation from 1979 to 2002,but was insignificant from 2003 to 2022.The reason for this interdecadal shift is that the magnitude and position of the North Pacific Oscillation altered from 2003 to 2022,which led to the weakening of the influence of the North Pacific Oscillation on the southern SST key area of the VM,thus weakening the amplitude,weakening the meridional gradient of the SST key areas between the southern and central part of VM,weakening the VM amplitude,and finally reducing the correlation coefficient. |
