Abstract
The importance of deep ocean observations has been recognized with regard to changes in the deep ocean such as global bottom-water warming. Therefore, sustainable deep ocean monitoring networks that use autonomous profiling floats have been widely proposed, and a number of deep-float deployment initiatives have begun around the world. Deployed floats promise to provide unprecedented deep ocean information. However, present deep-float data are known to have biases. In particular, a depth-dependent bias in salinity data is a major issue that prevents us from constructing global deep ocean monitoring networks. This paper proposes a new approach to utilize ongoing deep-float salinity data to reduce the bias in estimates of the global full-depth ocean state. It reports results from comparative experiments with and without deep-float data by using the proposed approach to examine the impact of data from currently operating deep floats on ocean state estimates. The results demonstrate that available float data possibly contribute local corrections to the modeled climate ocean state. Furthermore, we clarify how interannual basin-scale estimations are controlled by available deep-float salinity data in two specific regions of the Southern and Indian Oceans.
Highlights
The deep ocean has gained attention from climate researchers since Fukasawa et al (2004) documented bottom-water warming in the abyssal North Pacific Ocean
The problem is that the bias is different for each float; the values range from − 0.005 to − 0.02 at depths of 3500–4000 m based on analysis of float data for which there are comparable ship-based Conductivity Temperature Depth profiler (CTD) data obtained at the same time as the float deployments (Fig. 2; Kobayashi 2016)
Local impact of deep-float salinity data To examine the temporal development of the estimated subsurface ocean state, we focus on two areas of 50° longitude by 10° latitude (Fig. 1): the Southern Ocean area off Antarctica’s Wilkes Land (100–150° E, 60–70° S) and the South Indian Ocean area (40–90° E, 40–50° S)
Summary
The deep ocean has gained attention from climate researchers since Fukasawa et al (2004) documented bottom-water warming in the abyssal North Pacific Ocean. The warming was detected by repeated high-accuracy ship-based observations by the World Ocean Circulation Experiment (WOCE) Hydrographic Program on its P1 survey line across the subarctic Pacific along 47° N, and Johnson et al (2008) reported similar bottom-water warming in the Indian basin, from revisit cruises. Subsequent researchers have detected global bottom-water warming (e.g., Purkey and Johnson 2010, Kouketsu et al 2011). Recent observational studies have indicated that Antarctic Bottom Water (AABW) has been freshening in some observational sections (e.g., Aoki et al 2005, Rintoul 2007, Jacobs and Giulivi 2010). Deep ocean observations remain too sparse to resolve the temporal evolution of the deep ocean state
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