Fig 1.
Schematic diagram of TCs main development region (MDR) in the Western North Pacific.
The lines represent TC tracks, with color depth indicating TC intensity according to the China Meteorological Administration (CMA) classification standards. Two in situ Argo floats (black asterisks) platform numbers were 5904699 and 5904698, and their corresponding dates were July 16, 2021 and July 21, 2016, respectively.
Table 1.
Weight of UOD index attributes (%).
Table 2.
Top 10 TCs in UOD index.
Fig 2.
The ocean dynamic environment during the Typhoon Sudal and Prapiroon passing through ocean eddy.
(A) and (B) show the ocean relative vorticity fields during and after the passage of typhoon Sudal, respectively. (C) and (D show the ocean relative vorticity fields during and after the passage of Typhoon Prapiroon, respectively. The black box is the study area, and the black dots and lines correspond to the intensity and tracks of TC. The arrows indicate ocean flow fields.
Fig 3.
Accumulative monthly trend change of UOD index.
The red circles represent three mutation periods: A:1993–1997, B:2014–2018, and C:2021. The area between the two red lines represents the 95% significance interval, while the black line represents the critical threshold.
Fig 4.
Relationship between UOD index and Nino3.4 index.
(A) is CWT of UOD index, (B) and (C) show WTC and XWT between UOD index and Nino3.4 index. The white transparent area represents the region within the COI, and the black contour indicates 95% confidence level. The relative phase relationship is shown as black arrows, where the right pointing arrow denotes in-phase, while the left pointing arrow represents anti-phase. The arrow pointing straight down shows the UOD index leading Nino3.4 index changes by 90°.
Table 3.
ENSO events from 1993–2021.
Fig 5.
The temporal changes of UOD index and Nino3.4 index.
Fig 6.
Distribution of environmental factors from 1993 to 2021.
Pink bars, green bars and gray bars show the distribution of environmental factors in El Niño years, La Niña years and neutral years, respectively. The red line represents the median, and the “+” represents outliers. (A)-(C) are typhoon intensity attributes, (D)-(F) are ocean dynamic factors, and (G)-(H) are ocean thermal factors.
Fig 7.
The difference of ocean thermal environment between ENSO years (June–November) and neutral years (June–November).
(A) and (B) respectively show the difference distribution of SST in El Niño years, La Niña years and neutral years; (C) and (D) respectively show the difference distribution of MLD in El Niño years, La Niña years and neutral years. The black box represents the MDR area.
Fig 8.
The trend matrices of the annual mean SST (a), MLD (b) and salinity (c) from 1993–2021 with a time interval of at least ten years used to calculate trends (hypotenuse).
(d)-(f) display changes in temperature, salinity, and buoyancy frequencies of Argo floats. The x-axis and y-axis indicate the start and end year, respectively, and the colors of the rectangles indicate the intensity of the trend (a)-(c).