Strong North Pacific Subtropical Mode Water Volume and Density Decrease in Year 1999

Abstract

A strong decrease of volume and density of North Pacific Subtropical Mode Water (NPSTMW) in 1999 was analyzed in a regional high-resolution (0.1°) numerical ocean circulation model simulation. Both shoaling of the bottom and deepening of the top of the NPSTMW layer contributed equally to volume decrease. They were locally governed by different physical processes, but both seem to be associated with basin-wide changes in wind. A westward propagating negative thermocline depth anomaly that developed in the Central Pacific when the Pacific Decadal Oscillation index changed from a positive to a negative phase in 1998 caused shoaling of the bottom of the NPSTMW layer. Deepening of the top of the NPSTMW layer was due to an increase in the near surface stratification, caused by an increase in wind-driven lateral heat transport convergence by the Kuroshio Extension jet starting in 1997. Both processes increased the potential vorticity in the NPSTMW region, decreasing the volume of water in the NPSTMW density range that satisfied the low potential vorticity constraint that is part of the definition of “mode water.” The strong near-surface density decrease provided preconditioning for preferential surface formation of a lighter variety of NPSTMW, further decreasing its density. It also resulted in decrease of the outcrop window in the NPSTMW density range, strongly reducing its formation rate in 1998 and 1999 despite strong surface heat loss.