Glider estimates and model diagnoses of low-latitude western boundary current temperature advection into the Pacific equatorial strip

Abstract:

Estimates of mass transport into the Pacific equatorial strip have shown its importance in the evolution of ENSO and lower-frequency variability. The low-latitude western boundary currents (LLWBCs) tend to be anti-correlated with flows across the long zonal sections to the east, consistent with ENSO oscillator ideas. Thirteen years of continuous glider sections across the LLWBC in the Solomon Sea show the mass and temperature anomalies carried from the South Pacific to the equator.

The LLWBCs are fast and narrow; reference-level geostrophy is inadequate, requiring the dense sampling including absolute velocity provided by glider sections. The co-located glider measurements of T,S and cross-track velocity adds the new ability to estimate the influence of temperature advection on the tropical strip (spanning the basin within 9S-9N).

A novel method to combine the dense glider LLWBC sections with more traditional (Ekman plus geostrophic) estimates across the long sections east of the western boundary layer defines the advective contribution to heat content of the entire equatorial strip. Time series of equatorward temperature advection can be estimated entirely from glider and Argo in situ observations, and scatterometer winds, showing the variation and differing roles of the individual elements of advection through the boundaries of the strip. Despite its small size, advection through the Solomon Sea is a major contribution to the rate of change of temperature of the equatorial strip. The largest source of uncertainty probably comes from sparse sampling of the northern LLWBC, the Mindanao Current.

A calculation in a 1/10th-degree model is able to reproduce the glider LLWBC observations, and estimates the missing Mindanao term and the role of the Indonesian Throughflow. It describes the whole picture, including the surface flux terms, of the time-dependent heat balance of the equatorial strip. The model adds the important detail of the higher frequencies - unresolved by the glider - especially intraseasonal variations, and illuminates the external and internal controls on heat transport to the equator.