Generation Mechanism of Spiciness Anomalies: An OGCM Analysis in the North Atlantic Subtropical Gyre

Alban Lazar,Audine Laurian,Gilles Reverdin

doi:10.1175/2008jpo3896.1

Abstract

Abstract Oceanic teleconnections between the low and midlatitudes are a key mechanism to understanding the climate variability. Spiciness anomalies (density-compensated anomalies) have been shown to transport temperature and salinity signals when propagating along current streamlines in the subtropical gyres of the Atlantic and Pacific Oceans. The generation mechanism of spiciness anomalies in the North Atlantic subtropical gyre is investigated using an analytical model based on the late-winter subduction of salinity and temperature anomalies along isopycnal surfaces. The keystone of this approach is the change of the coordinates frame from isobaric to isopycnic surfaces, suited for subduction problems. The isopycnal nature of spiciness anomalies and the use of a linear density equation allows for the analytical model to depend only upon surface temperature and salinity anomalies, the mean thermocline currents, and the surface density ratio. This model clarifies and above all quantifies the mechanism by which surface temperature and salinity anomalies are modulated by density ratios to produce fully different isopycnal temperature and salinity anomalies. A global run from the ocean GCM (OGCM) Océan Parallélisé (OPA) over the period 1948–2002 provides the reference data in which the North Atlantic subtropical thermocline spiciness variability is analyzed. Two EOF modes are sufficient to explain half of the low-frequency variability in the OGCM: one is maximum over the northeastern subtropics, and the other is in the central basin. The analytical model reproduces well the spatial pattern, amplitude, and sign of these two main modes. It confirms that the two centers of action of the anomalies are conditioned by the surface density ratio, the first corresponding to null salinity gradients and the second to near-density-compensated temperature gradients. Considering that the analytical model has good skills at reproducing the decadal variability of the OGCM spiciness anomalies in the permanent thermocline, it is believed that this is an interesting tool to understand and forecast the ventilation of the North Atlantic subtropical gyre at this time scale.

Highlights

A variable called spiciness was introduced three decades ago in order to characterize water masses and intrusions (Munk 1981; Jackett and McDougall 1985)
To a first-order approximation, spiciness anomalies behave as dynamically passive tracers that are advected by the currents in the thermocline
In the North Atlantic Ocean, spiciness anomalies that are generated in the salinity maximum water (SMW) region propagate at the current speed and emerge close to 308N in the Gulf Stream (LLR)

Summary

Introduction

A variable called spiciness was introduced three decades ago in order to characterize water masses and intrusions (Munk 1981; Jackett and McDougall 1985). Water masses can either be characterized by their temperature and salinity or by their spiciness and density. To a first-order approximation, spiciness anomalies behave as dynamically passive tracers that are advected by the currents in the thermocline In the North Atlantic Ocean, spiciness anomalies that are generated in the salinity maximum water (SMW) region propagate at the current speed and emerge close to 308N in the Gulf Stream (LLR). There, they can interact with the atmosphere and not be density compensated anymore, impacting the sea surface density. Late-winter subduction was suggested to be the mechanism responsible for setting the characteristics of the spiciness anomalies in the thermocline, the details of the generation process of these signals were not the focus of the study

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Conclusion