Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales

Abstract

This paper evaluates in a realistic context the local contributions of direct atmospheric forcing and intrinsic oceanic processes on interannual sea level anomalies (SLAs). A ¼° global ocean–sea ice general circulation model, driven over 47 yr by the full range of atmospheric time scales, is quantitatively assessed against altimetry and shown to reproduce most observed features of the interannual SLA variability from 1993 to 2004. Comparing this simulation with a second driven only by the climatological annual cycle reveals that the intrinsic part of the total interannual SLA variance exceeds 40% over half of the open-ocean area and exceeds 80% over one-fifth of it. This intrinsic contribution is particularly strong in eddy-active regions (more than 70%–80% in the Southern Ocean and western boundary current extensions) as predicted by idealized studies, as well as within the 20°–35° latitude bands. The atmosphere directly forces most of the interannual SLA variance at low latitudes and in most midlatitude eastern basins, in particular north of about 40°N in the Pacific. The interannual SLA variance is almost entirely due to intrinsic processes south of the Antarctic Circumpolar Current in the Indian Ocean sector, while half of this variance is forced by the atmosphere north of it. The same simulations were performed and analyzed at 2° resolution as well: switching to this laminar regime yields a comparable forced variability (large-scale distribution and magnitude) but almost suppresses the intrinsic variability. This likely explains why laminar ocean models largely underestimate the interannual SLA variance.

Causes of the Regional Variability in Observed Sea Level, Sea Surface Temperature and Ocean Colour Over the Period 1993–2011

“…The remainder variability, which is forced by anomalous surface fluxes of buoyancy along with any nonlinear intrinsic variability (cf. Penduff et al 2011), is in general significantly smaller except in some regions (Fig. 5b, d) such as the region of the Antarctic circumpolar current (ACC), in the Kuroshio extension, in the Arctic, in the North Atlantic, in the North Pacific and in the tropical Pacific, confirming earlier results from Thompson and Ladd (2004), Cabanes et al (2006) and Ponte (2012, 2013).…”

“…5b, 6b) reflects the ocean's adjustment to interannual and decadal changes in buoyancy exchanges as well as any nonlinear intrinsic changes (cf. Penduff et al 2011;Piecuch and Ponte 2012) The perturbation experiments reveal that SSH variability is essentially forced by surface wind stress anomalies (see Fig. 5a, c and also Stammer et al 2013;.…”

“…When this contribution is removed, the remaining variability is explained by buoyancy forcing anomalies (i.e. surface heat and freshwater exchanges) and the intrinsic oceanic variability spontaneously generated by the ocean circulation under the repeated seasonal atmospheric forcing (Penduff et al 2011;Sérazin et al 2015). It is essentially surface heat fluxes anomalies, which explain most of this residual variability (see Sect.…”

Spatial Patterns of Sea Level Variability Associated with Natural Internal Climate Modes

“…(c) Effect of oceanic internal variability Several recent studies examined the effects of oceanic internal variability (instabilities) on sea level variability, by performing experiments using standalone, eddy-permitting (1/3°9 1/3°and 1/4°9 1/4°grids) (e.g., Trenary and Han 2013;Li and Han 2015;Penduff et al 2011) and eddy-resolving (1/12°9 1/ 12°) (Sérazin et al 2015) OGCMs. Over the Indian Ocean, Trenary and Han (2013) and Li and Han (2015) suggested that oceanic internal variability has a significant contribution to decadal sea level variability in the subtropical south Indian Ocean, near the Somali coast and western Bay of Bengal.…”

Oceanic control of multidecadal variability in an idealized coupled GCM

“…Both processes can contribute to increase the role of the atmosphere in the lowfrequency climate variability. Such resolutions remain beyond the scale required to resolve oceanic eddies, but mesoscale turbulence is though to have a minor impact on the existence of multidecadal oceanic variability (Penduff et al 2011;Huck et al 2015). The aim of this study is rather to shed light on the mechanisms of multidecadal variability in a North Atlantic-like ocean at low resolution.…”