5 Discussion
The choke points formed at high latitudes by the separation of the Southern Hemisphere continents with Antarctica, and in the tropics via the Indonesian archipelago, provide natural locations for observing and monitoring interocean exchanges. Indeed, there have been many concerted efforts to sustain long-term monitoring programs in these regions, although in the past the geographical (Southern Ocean) and logistical (Indonesia) isolation of these extreme locations has made this difficult. Nonetheless, in the more recent decades, the many ongoing observational programs along with remotely sensed measurements have successfully provided valuable information on the variability over a range of timescales of interocean exchange through these choke points and their importance to the global climate system.
In the Southern Ocean, the fast-flowing ACC provides an efficient equalizer of interocean properties acting to reduce the contrasts between each of the major ocean basins in the Southern Hemisphere. Nonetheless, energetic eddies in the Agulhas region counteract the strong eastward flow of the ACC and inject salty Indian Ocean water that can be traced across the South Atlantic and potentially influence the MOC. While the strong air–sea interaction, tidal and wind-induced mixing within the Indonesian seas significantly alters the Pacific source water masses that comprise the ITF thermohaline profile that enters the Indian Ocean, its signature appears to be largely overwhelmed possibly by the saltier RSOW by the time the Indonesian waters reach the Indian Ocean western boundary.
As with interocean exchanges, the exchanges between the oceans and their adjacent seas carry different weights with respect to their relevance for variations of the global MOC and climate. The Arctic Sea and the Labrador Sea are the most significant members, with a strong influence on changes in the MOC. The freshwater injection from these subarctic marginal seas will counter against the contribution of saltier waters from the Agulhas system as well as the Mediterranean, and subsequently have a competing influence on the stabilization of the MOC of the North Atlantic. Other marginal seas influence the mean oceanic circulation, but apparently not its variations. However they provide important markers in water mass properties that can be used to identify changes in the transfers between atmosphere and ocean, and in the budgets of heat, salt, carbon, nutrients, and other properties. Notwithstanding the open question of MOW pathways, it is clear that the Mediterranean outflow results in strong property signals in the North Atlantic and in part of the South Atlantic, above or as part of the NADW. Similarly, the outflow from the Red Sea and the Okhotsk Sea has a strong influence on water mass properties at intermediate depths in the Indian and North Pacific Ocean, respectively. However, on decadal timescales, it was shown that source property changes in MOW were too small to have a significant effect on the open Atlantic. This may also prove to be the case for decadal changes in the Red Sea and Okhotsk Sea waters, impacting their adjoining basins. Nevertheless, all these marginal sea inflows can be thought of as indicators of climatic change affecting larger regions.
Deep ocean passages between neighboring oceanic basins permit throughflows of deep and bottom water from one basin to the next. Deep passages are also choke points that, due to their limited extent, potentially provide a relatively easy monitoring site for the amplitude and property variability of the deep branch of the MOC. We focused our discussion on the deep passages that control the spreading of the NADW in the Atlantic Ocean and AABW in the world oceans, and we reviewed the characteristics of these flows. These deep passages are of considerable interest since they are the location of high levels of turbulence, strong water mass modification, and impact the dynamics of the upstream basin (Whitehead, 1998). Mixing is intense (~ 10− 2 m2 s− 1) in deep passages due to the unstable nature of the strongly sheared flows. Downstream of a critical point, the flow becomes supercritical and mixing may be even more intense (values as high as 10− 1 m2 s− 1 were reported by Ferron et al., 1998 for a hydraulic jump region in the Romanche Fracture Zone). This enhanced mixing strongly affects the deep and bottom water properties of the downstream basins. Accurately modeling these regions of intense mixing in general circulation models (GCMs) remains a challenge (Legg et al., 2009).
As documented in other parts of the world oceans, there have been recent significant measurable changes both in the properties and fluxes at interocean and interbasin exchange sites. It is remarkable that all of the examples discussed in this chapter, with the exception of the Red Sea outflow, indicate increasing temperatures over recent decades, thereby strongly suggesting a response of the oceans to global warming. Long-term changes in the Pacific tropical tradewinds have resulted in changes to the ITF transport (Wainwright et al., 2008). While model studies suggest that the poleward shift and intensification of the Southern Ocean westerlies have led to an increased Agulhas leakage (Biastoch et al., 2009), the impact of these wind changes on ACC transport itself remains less clear. Although there is much recent evidence that property changes have occurred in the deep ocean (e.g., Fukasawa et al., 2004; Johnson and Doney, 2006; Kawano et al., 2006; Johnson and Gruber, 2007; Rintoul, 2007; Zenk and Morozov, 2007; McKee et al., 2011), unfortunately no long-term transport measurements in deep passages are presently available. The extreme complexity of abyssal topography along with the technological challenges of making long-term observations of the relatively small signals at low temperatures under immense pressure in remote locations complicates our ability to maintain an optimal sampling array in the deep ocean. Garzoli et al. (2010) recommended the setup of sustainable measurements in the deep passages that are not yet instrumented (e.g., Vema Chanel, Romanche Fracture Zone, Samoan, and Amirante Passages). Indeed, the observed changes highlight the need for long-term monitoring in all interbasin choke points that ultimately connects the MOC system. Such measurements are critical for climate monitoring and GCM validation.
