The Seasonal Zonal Equatorial Undercurrent Strength variation with ENSO

Abstract

The Equatorial Undercurrent (EUC) is an important current both physically and biologically. It is not only key to equatorial circulation but also closely related to primary production at the Equator. Seasonal variation of EUC is caused by the seasonality of the equatorial thermocline. This study retrieves in situ EUC velocity data using an ADCP from the TN427 cruise, traveling from 5°N to 5°S at 170°W in December and January, from 2023 - 2024. During the first crossing through equator, the maximum EUC velocity was measured to be 0.42 ! " and mean EUC velocity was 0.02 ! " . Calculated transport equals to 10.77 Sverdrup. During the second crossing, the velocity is measured to be 0.59 m/s with 0.04 ! " mean velocity. The transport for the second crossing was 14.78 Sverdrup. The maximum EUC velocity at the beginning of 2024 is much lower than the value in 2023, which was caused by the increase in upwelling during El Niño. According to the most recent ENSO SSTA indices of Niño 3.4 from NOAA State of Ocean Climate, the peak of Niño 3.4 index was in December 2023 and January 2024, followed by a decrease in Niño 3.4 index. Future prediction states that an increase of maximum EUC velocity will be observed within the next few months.