New developments in mode-water research: an introduction

Abstract

Two landmark papers appeared in 1969, laying the foundation for this special section in Journal of Oceanography. Masuzawa (1969) coined the term ‘‘subtropical mode water’’ (STMW) in the North Pacific, characterizing it as a thick, vertically uniform layer of rh 25.2–25.6 that forms a distinct mode in water volume censuses as a function of temperature, salinity or density. By carefully studying historical hydrographic observations, Uda and Hasunuma (1969) discovered an eastward subtropical countercurrent (STCC) in the southern subtropical gyre where the Sverdrup theory predicts a broad westward current. This countercurrent is peculiar in its direction against both the prevailing northeast trade winds and the westward current underneath. Uda and Hasunuma noted in passing that the STMW resides north of the STCC, splitting the thermocline into a northward-shoaling upper branch and a downward-sloping lower thermocline. Since then, studies of mode water and STCC went their separate ways. The STMW is a favorite subject for water-mass analysts, with other mode waters found in the 1990s in the central and eastern North Pacific. The strong deviation of STCC from the barotropic Sverdrup streamfunction, on the other hand, captured the imagination of dynamists, who proposed a flurry of theories that attempt to explain this peculiar current. A self-consistent theory of STCC had to wait for 30 years, identifying none other than mode waters as the culprit. As often happens in history, seemingly distinct phenomena—mode water and STCC in this case—turn out to be closely related from a fresh perspective under the light of new theories. Based on a ventilated thermocline model, Kubokawa (1999) showed that mode waters of different densities, formed in the deep winter mixed layer around the Kuroshio Extension east of Japan, will cross their paths on their way south in the subtropical gyre, stack up in the vertical, and shoal the upper thermocline, creating a surface-intensified STCC on the south flank of the modewater pool. Subsequent observational and modeling studies lend support for this theory of STCC. Thus, mode waters are far more than just thick water masses moving around passively in the subtropical gyre but assume the role of a dynamical agent that causes surface currents to deviate substantially from the Sverdrup dynamics. In retrospect, the dynamical effect of mode water may not be too surprising given that mode waters are characterized by their low potential vorticity, and that potential vorticity is a dynamical tracer conservative following a parcel’s trajectory on an isopycnal. S.-P. Xie (&) International Pacific Research Center and Department of Meteorology, SOEST, University of Hawaii at Manoa, Honolulu, HI 96822, USA e-mail: xie@hawaii.edu