The Variable Ocean

Bodies of water circulate throughout the oceans both horizontally and vertically as a consequence of physical forces exerted on them according to Newton's Law. The oceanic circulation is not steady in time. Rather motions of water bodies in the ocean are known to vary on a broad range of spatial and temporal scales. The following four examples serve to highlight natural variations of large-scale circulation patterns:

(i) Seasonal variations of the strength of the North Atlantic subtropical gyre at 26°N have the amplitudes of 25 Sv1 (peak to peak). This range is comparable to the time mean strength of the wind-driven, anti-cyclonic basin-scale gyre at this latitude [1].

(ii) The Pacific subtropical cells (STC) a meridional, upper-ocean pattern of circulation that links the subtropical subduction regions north and south of the equator to the equatorial thermocline has seen a decline of 11 Sv or 30% since the 1950s, however, displaying decadal variations of the same order of magnitude [2]. The observed strong decadal and multi-decadal variations in sea surface temperature (SST) in the equatorial Pacific have been shown to be related to changes in STC strength.

(iii) The cyclonic circulation of the North Atlantic subpolar gyre has possibly weakened by 25% and shrunk in size since the mid-1990s [3,4]. This

1 1 Sv 1 x 106 m3 s 1 (unit for volumetric transport, named after Harald Ulrik Sverdrup). For comparison, the Amazon River discharge in the Atlantic is about 0.2 Sv.

has been attributed to the transition of the North-Atlantic Oscillation2 (NAO, [5]) from a comparably strong phase between 1960 and 1995 (manifesting itself in stronger than average westerly winds at mid-latitudes) to a significantly weaker one after 1995. The gyre's weakening and westward retreat has allowed large quantities saline subtropical upper-ocean waters to flow northward past its eastern flank, as a consequence of which a drastic increase in salinities in the Nordic Seas3 has been observed [6]. This is thought to have an impact on the sinking of waters as part of the Atlantic Meridional Overturning Circulation (AMOC), the latter being the primary focus of this chapter.

(iv) Although not having been observed directly, it is commonly thought that temporal changes in the strength of the AMOC a basin wide meridional circulation pattern that links upper-ocean net northward flow of warm, saline waters with cold southward return flow below roughly 1000 m throughout the Atlantic explain large parts of the observed multi-decadal North-Atlantic SST changes [7]. A recent summary and discussion of climate variability and its predictability in the Atlantic sector [8] provides a perspective on the difficulties one has, to distinguish decadal variability from long term, possibly anthropogenic induced trends.

The reason why the different components of the ocean circulation have the potential to change substantially over time is a consequence of the complex forcing at the sea surface (exchange of momentum, heat and freshwater between ocean and atmosphere) on the one hand and internal ocean dynamics on the other. Examples of internal ocean dynamics include advection of water of anomalous density by the mean large-scale ocean circulation, westward energy transfer by off-equatorial planetary waves, the equatorial wave guide, horizontal mixing by meso-scale eddies, deep-water formation due to convection or small-scale vertical mixing, acting to push the cold waters of the oceans' abyss upwards. The large variations that basin-scale circulation patterns may exhibit have the potential to delay the detectability of climate change related shifts in the flow field.

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