Deep upwelling occurs throughout the Indian Ocean, although there is strong new evidence of dependence on mid-ocean ridges (Drijfhout and Naveira Garabato, 2008; section 1.2 below). Shallow upwelling is strong in the tropics, and especially in the Arabian Sea. Resolution of the tropical shallow overturning cell (Schott et al., 2002) is important. Because time dependent forcing, circulation, and eddies are intrinsic to this cell, the POP and ECCO-GODAE analyses of the cell and its transports are essential.<\/p>\n
Indian Ocean diapycnal diffusivity is inhomogeneous (section 1.2 below; Kunze et al., 2006; Mackinnon et al., 2008). But diapycnal upwelling also depends on the vertical property structure, so the\u00a0mixing inhomogenity may differ from that of diffusivity which is highest over ridges<\/em><\/strong>\u00a0<\/em>with significant tidal fluxes and around Kerguelen (Kunze et al., 2006).\u00a0 However, it appears that the tropical band has greater diapycnal mixing (Talley et al., 2003). \u00a0This is an important issue, with the first step to be resolved when looking at meridional overturn, but necessarily followed by a regional analysis that defines ridge and boundary regions.<\/p>\n The Indian Ocean gains heat north of about 20 \u00b0S and the ITF transports warm upper ocean water into the Indian Ocean at about 12 \u00b0S. This heat must be exported southwards. At 32\u00b0S, the net southward heat transport is order 1 PW, with a wide range of estimates, as with volume transport (e.g. Ferron and Marotzke, 2003). Talley (2008), with a net of 0.8 PW, found that less than half of the total (~0.3 PW) is carried by the upper Indian Oceans’s subtropical gyre; half (0.4 PW) is associated with the overturning from deep to intermediate\/thermocline waters, and the remainder with the change in temperature of the ITF as it passes through the Indian Ocean (0.1 PW) (e.g. Talley, 2008).\u00a0 The deep upwelling is therefore significant for the global heat budget. For freshwater (FW), the deep upwelling is less significant, while the upper ocean (gyre and ITF) carries much of the northward FW transport required to balance net evaporation in the Indian Ocean (Talley, 2008).\u00a0 While almost all budgets have been constructed with most attention given to the 32\u00b0S section (Fig. 1.1), there are significant meridional variations in freshwater input in the Indian Ocean.\u00a0 What seems most curious about the wide ranges of estimates is that most are based on the same data sets, so sensitivity to choices such as the magnitude of the ITF is important.<\/p>\n In the northern Indian Ocean, net heat gain in the warm pool must be balanced by southward export of heat across the equator and out of the tropics (e.g. Wacongne and Pacanowski, 1996; Loschnigg and<\/p>\n Webster, 2000).\u00a0 Upwelling of deep to thermocline waters remains vigorous even north of the equator, and maybe be responsible for a significant fraction of the heat export. The shallow equatorial ‘roll’ circulation is strongly time dependent, with monsoonal reversals in currents and shallow upwelling; the annual mean is dominated by the SW monsoon conditions (Schott and McCreary, 2002) and exports heat southwards.\u00a0 This same pair of mechanisms (shallow roll and deep upwelling) impacts all of the other properties (carbon, oxygen, freshwater).\u00a0\u00a0\u00a0In the tropics, resolution of the very large time dependence is essential to quantifying the relative importance of the deep and shallow cells for exporting heat, and hence to the sensitivity of the heat budget to interannual and decadal variability in surface forcing (e.g. Indian Ocean Dipole, ENSO, climate change).\u00a0 Even the deep cell is likely to depend on surface forcing if the mixing processes are associated with deep reversing currents and planetary waves that continually adjust the circulation.\u00a0<\/strong>We propose to work intensely with the POP model output on this aspect of the Indian Ocean overturn, with comparisons with the snapshot hydrographic analysis to assess validity of the deeper parts of the modeled circulation.<\/p>\n The Indian Ocean represents a net source of CO2<\/sub>\u00a0to the atmosphere (Bates et al. 2006), while simultaneously storing approximately 20% of the total global inventory of anthropogenic carbon (Sabine et al., 2004) (Figure 1.1b). A number of investigations (Holfort\u00a0et al.,<\/em>\u00a01998; Roson\u00a0et al.<\/em>, 2002; Alvarez\u00a0et al.<\/em>, 2003, Macdonald\u00a0et al<\/em>., 2003) have used observations from long transects to understand how the overturning circulation in the Atlantic affects the transport, divergence and air-sea flux of carbon within that basin.\u00a0 Although similar work has considered the Indian Ocean as part of a global product (Mikaloff-Fletcher et al., 2006; Gruber et al., 2009),\u00a0none has as yet focused on the interaction between the horizontal and vertical circulation and the natural and anthropogenic carbon budgets in this unique basin<\/strong>.<\/p>\n\n 1. Why is the range of overturning transport estimates for section data at 32\u00b0S so large?\u00a0 Some possibilities that will be tested are: (a) Sensitivity to the ITF transport. (b) Sensitivity to details of the\u00a0lateral<\/em>\u00a0circulation within isopycnal layers, especially in the intermediate and deep waters (AAIW layer with northward flow from Southern Ocean and southward flow of upwelled deep water; NADW\/IDW\/CDW layer with northward flow of both NADW and CDW, southward flow of IDW).<\/p>\n 2. Why are GCM and other model-based estimates of Indian Ocean overturning low compared with most estimates based on hydrographic data? Some possibilities to test are: (a) Difficulties with flow through narrow gaps between deep basins, with concomitant high mixing in the gaps. (b) Inaccurate parameterization of deep diffusivities with overly-simplified profiles that are too weak or too uniform.<\/p>\n 3. For meridional overturning, which broadly defined latitude bands are more effective diapycnal mixing locations? Where is the product of regionally dependent diffusivity and regionally dependent vertical property curvature highest?\u00a0 Can deep-reaching reversing currents and eddies in the presence of rough bottom topography, such as in the tropics and north Indian Ocean, create mixing, much like tidal forcing?<\/p>\n 4. Can eddy-driven diffusion (Wolfe et al., 2008) enhance the diffusivity parameterized from ADCP observations (Kunze et al., 2006)?\u00a0 Can lateral stirring by vigorous eddies and reversing currents, especially near fronts where vertical gradients can create hot spots of diapycnal processes?<\/p>\n 5. The POP model shows strong localized upwelling at the equator at all depths. What are the dynamics of this upwelling?\u00a0 What is special about the equator that is not captured in observations of diffusivity (Kunze et al., 2006) itself?<\/p>\n 6. What balance of mechanisms transport heat southwards out of the tropical Indian Ocean? To consider: (a) Deep overturn with downward diffusion of heat in the northern Indian Ocean, (b) Shallow cross-equatorial cell with northward colder flow in the western boundary current and warmer wind-driven southwards flow across the equator, more in mid-ocean (Wacongne and Pacanowski, 1996; Schott et al., 2002), (c) “eddy” flow associate with the large monsoonal variability, especially in the western Indian Ocean, with reversing currents, reversing eddies, and vigorous Rossby\/Kelvin\/Yanai wave activity.<\/p>\n 7. What is the balance of mechanisms for the same questions for carbon transport, given that there is net outgassing in the tropical and northern Indian Ocean?\u00a0 What portions of the circulation transport carbon most vigorously northwards? And the same questions for freshwater transport, given that the Indian Ocean north of 32\u00b0S is net evaporative.<\/p>\n\t\t\t\t\n\t\t\t\t Background Indian Ocean meridional overturn The Indian Ocean converts deep and bottom water to intermediate and thermocline waters that emerge and join either the upper part of the Atlantic’s overturn or the low oxygen upper portion of the Circumpolar Deep Water in the Antarctic, where they eventually upwell to the thermocline (Figs. 1.1).\u00a0 The Pacific…<\/p>\n","protected":false},"author":205,"featured_media":0,"parent":17,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":[],"_links":{"self":[{"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/pages\/244"}],"collection":[{"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/users\/205"}],"replies":[{"embeddable":true,"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/comments?post=244"}],"version-history":[{"count":3,"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/pages\/244\/revisions"}],"predecessor-version":[{"id":279,"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/pages\/244\/revisions\/279"}],"up":[{"embeddable":true,"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/pages\/17"}],"wp:attachment":[{"href":"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-json\/wp\/v2\/media?parent=244"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}Heat and freshwater transports<\/h3>\n
Carbon transports<\/h3>\n
Science questions related to Indian Ocean overturn to be considered by this study<\/h3>\n
![\"Global](\"https:\/\/www2.whoi.edu\/staff\/amacdonald\/wp-content\/uploads\/sites\/77\/2020\/02\/figure3_threeocean_238796.png\" "\\"figure3_threeocean_238796\\"")
\n\t\t\t\t<\/a>\n\t\tFig. 1.1a Global meridional overturn including the Indian Ocean: volume transports. (L. Talley)\n\t\t\t\t\n\t\tFig. 1.1b Carbon uptake and outgassing (arrows at top) superimposed on sections of a carbon anomaly relative to the atmosphere. (Gruber et al., 2009)\n\t\t\t\t\n\t\tFig. 1.2a Strength of Indian Ocean deep-to-shallow overturn from the references listed on the figure. (A. M. Macdonald)\n\t\t\t\t\n\t\tFig. 1.2b Overturning streamfunction for the Atlantic (top), Pacific (plus Indian south of the ITF) (middle), and Indian (north of the ITF), from global 1\/10\u00b0 POP model. (Maltrud and McClean, 2005)\n\n","protected":false},"excerpt":{"rendered":"