Eddy and Wind Forced Spiciness Variability in OFES – Formation, Propagation, and Predictability of density compensated anomalies in the Pacific Ocean Thermocline

There is a long history in oceanography of investigation into how the atmosphere drives ocean. The atmosphere’s response to internal ocean dynamics however, is relatively new and has only a few decades of maturity. Part of this emerging relevance is due to increased understanding of low frequency ocean thermocline variation such as the El Nino Southern Oscillation (ENSO) and its effects on global climate. ENSO’s dramatic shifts in the structure of the Pacific’s thermocline/pycnocline leads us to investigate other processes that may explain similar linkages between the ocean and the atmosphere.

Our work uses JAMSTEC Ocean General Circulation Model (OGCM) for the Earth Simulator (OFES), a high resolution (1/10° x 1/10°), eddy resolving model output to look at the Pacific thermocline’s temperature (T) and (S) properties on constant density surfaces (isopycnals) for the period of Jan2005 - Dec2014.

We employ the same OFES parameters as (Furue 2017) with monthly averaged potential T and S data interpolated onto density surfaces and create an ensemble average comprised of five individual model realizations. The intent of this average is to illuminate natural variations of the thermocline and quantify eddy-induced variance of T and S. Our primary focus is on the sigma-theta 25-density surface in the Pacific due to its seasonal exposure to the atmosphere through isopycnal surface outcrops between 60N - 30N and 30S - 40S.

The combination of T and S to describe the composition on an isopycnal is defined as “spiciness”. Spiciness is a thermodynamic variable which follows the isopleths of density contours and is useful in characterizing water masses and intrusions, and behaves as a passive tracer (McDougall and Krzysik 2015, Jacket and McDougall 1985, Lukas 2001). Much of our work looks at anomalies of this value on an isopycnal, where we define anomaly as the difference from the annual cycle (climatology) of a dataset.

Below the mixed layer, isopycnal properties are retained due to little mixing and therefore provide insight into how information about these properties moves in the thermocline and variability in the ocean’s interior. Our findings may illuminate how the ocean’s T and S properties affect the atmosphere when resurfacing to the mixed layer and interaction with the atmosphere occurs.