Observational evidence for propagation of decadal spiciness anomalies in the North Pacific Yoshi N. Sasaki, N. Schneider, N. Maximenko, and K. Lebedev International Pacific Research Center, University of Hawaii

Subduction of water mass In the subtropical region, the mixed layer water in winter is subducted into the main thermocline Subduction plays an important role in the freshwater cycle Qiu and Huang (1995)

Decadal salinity variability Spiciness (density compensated) anomaly –Sa (x, y,  ,t) = S (x, y,  ,t) – Sclm (x, y,   ) –warm/salty or cool/fresh –passive advection by current Subsurface spiciness signals show substantial decadal fluctuations Lukas et al. [2008] from the Hawaii Ocean Time-series (HOT) [PSS-78]

Subduction of spiciness anomaly Several modeling studies showed propagation of spiciness signals from the subtropics to the equator It is difficult to identify basin-wide propagation of spiciness signals from observations due to the sparseness of salinity observations Yeager and Large [2004]   = 25.5 kg m -3

Argo Argo observations provide a lot of T-S profiles with nearly global coverage from the early 2000s  suitable for examining propagation of spiciness signals

Purpose To show observational evidence for basin-wide propagation of spiciness anomalies in the North Pacific –propagation speed? –decay timescale?

Data The Argo profile data are obtained from the US Argo Data Assembly Center The each Argo profile is linearly interpolated to isopycnal surfaces, and the variables are averaged into 3   3  bins for each month We also employ a spatially interpolated dataset using the Variational Interpolation algorithm Number of profiles on 25–25.5  

Montgomery Long-term mean on 25–25.5   Depth [m] outcrop line

Salinity anomaly [  10 PSS-78]

Salinity anomaly [  10 PSS-78]

Salinity anomaly [  10 PSS-78]

Salinity anomaly [  10 PSS-78]

Hovmoller diagram [  10 PSS-78] [  10 3 km]

Trajectory of particles Fifteen parcels are released around the cool/fresh spiciness anomaly positions in 2008, and their backward trajectories are calculated using the mean velocity field

Trajectory of particles

The propagation path and speed show good agreement with advection by the mean geostrophic current

Hovmoller diagram [  10 PSS-78] [  10 3 km]

Hovmoller diagram [  10 PSS-78] [  10 3 km]

Spiciness on streamline coordinate The amplitude of the salinity anomaly of the spiciness signal decreases by about 80% from in 2004 to in 2008 The anomalies are diffused in the course of propagation

Summary Argo observations provide an evidence for propagation of the spiciness anomalies from the eastern subtropics to western tropics The propagation path and speed of the spiciness signals are good agreement with advection by the mean geostrophic current Sasaki, Y. N., N. Schneider, N. Maximenko and K. Lebedev, 2010: Observational evidence for propagation of decadal spiciness anomalies in the North Pacific. GRL, 37, L07708, doi: /2010GL