Low-frequency eddy modulations in the Hawaiian Lee Countercurrent : observations and connection to the Pacific Decadal Oscillation

Abstract

Interannual-to-decadal time scale eddy variability in the Hawaiian Lee Countercurrent (HLCC) band is investigated using the available sea surface height, sea surface temperature, and surface wind stress data sets. In the HLCC band of 17°N–21.7°N and 170E°–160°W, the prevailing interannual eddy kinetic energy (EKE) signals show enhanced eddy activities in 1993–1998 and 2002–2006, and subpar eddy activities in 1999–2001 and 2007–2009. These interannual EKE signals exhibit little connection to the zonal HLCC velocity changes generated by the dipolar wind stress curl forcing in the immediate lee of the island of Hawaii. Instead, they are highly correlated to the time series of the Pacific Decadal Oscillation (PDO) index. Through a budget analysis for the meridional temperature gradient along the HLCC, we find that during the positive phase of the PDO index, the surface heat flux forcing induces cold (warm) sea surface temperature (SST) anomalies to the north (south) of the HLCC, intensifying the vertical shear between the surface, eastward-flowing HLCC and the subsurface, westward-flowing North Equatorial Current (NEC). This increased vertical shear enhances the baroclinic instability of the HLCC-NEC system and leads to a higher regional EKE level. The opposite processes occur when the PDO switches to a negative phase with the resulting lowered EKE level along the HLCC band. Compared to the surface heat flux forcing, the Ekman flux convergence forcing is found to play a minor role in modifying the meridional SST changes along the HLCC band.