The Regulation of Sea Ice Thickness by Double‐Diffusive Processes in the Ross Gyre

RESEARCH ARTICLE The Regulation of Sea Ice Thickness by Double‐Diffusive 10.1029/2019JC015247 Processes in the Ross Gyre Key Points: Yana Bebieva1 and Kevin Speer1 • Double‐diffusive staircases are observed within the main 1Geophysical Fluid Dynamics Institute, Florida State University, Tallahassee, FL, USA pycnocline underlying the surface mixed layer in the central Ross Gyre • The presence of a double‐diffusive staircase structure enhances sea ice Abstract New fine‐scale observations from the central Ross Gyre reveal the presence of double‐ formation and UCDW entrainment diffusive staircase structures underlying the surface mixed layer. These structures are persistent over fl by suppression of upward heat uxes seasons, with more developed mixed layers within the double‐diffusive staircase in winter months. The • Double‐diffusive heat flux suppression forms a feedback that sensitivity of the ice formation rate with respect to mixing processes within the main pycnocline maintains staircase structure (double‐diffusive versus purely turbulent mixing) is investigated with the 1‐D model. A scenario with purely turbulent mixing results in significant underestimates of sea ice thickness. However, a scenario when double‐diffusive mixing operates in the presence of weak shear yields plausible ranges for sea ice Correspondence to: thickness that agrees well with the observations. The model results and observations suggest a peculiar Y. Bebieva, feedback mechanism that promotes the self‐maintenance of double‐diffusive staircases. Suppression of [email protected] the vertical heat fluxes due to the presence of a double‐diffusive staircase, compared to purely turbulent case, allows Upper Circumpolar Deep Water to be more exposed to surface buoyancy fluxes. Our Citation: results shed light on the process—double diffusion—that might account for estimated rates of winter Bebieva, Y., & Speer, K. (2019). The regulation of sea ice thickness by water mass transformation in the central Ross Gyre. double‐diffusive processes in the Ross Gyre. Journal of Geophysical Research: Oceans, 124. https://doi.org/10.1029/ 2019JC015247 1. Introduction Received 26 APR 2019 The general stratification in the Southern Ocean south of the Antarctic Circumpolar Current is such that Accepted 2 SEP 2019 Accepted article online 8 SEP 2019 relatively warm and salty deep water (Upper Circumpolar Deep Water, UCDW) underlies a relatively cold and fresh surface mixed layer. Such vertical stratification, when both temperature (θ) and salinity (S) increase with depth, is amenable to diffusive convection, leading to the formation of a double‐diffusive staircase. These structures are widespread in a range of polar settings when background turbulent mixing is suf- ficiently weak to allow the formation of the layers (Bebieva & Timmermans, 2016; Shaw & Stanton, 2014; Shibley et al., 2017). In the Southern Ocean, robust staircase structures are observed directly below the surface mixed layer within the main pycnocline in winter. In other seasons, the mixed layers are less pro- nounced and intermittently disrupted by turbulent mixing events. Sea ice thickness and distribution are directly related to the physical processes in the surface mixed layer. The growth rate of sea ice is mainly controlled by the exchange processes between the atmosphere and the deeper ocean at the top and bottom boundaries of the surface mixed layer (for an overview, see, e.g., Hobbs et al., 2016). These processes include heat and salt fluxes, and both temperature and salinity play key roles in the evolution of sea ice (Gordon, 1981). Heat flux from the relatively warm UCDW below the mixed layer can counteract heat loss to the atmosphere and melt ice (this is referred to as the “thermal barrier” mechanism, Martinson, 1990). Sea ice formation generates dense plumes of higher salinity water that penetrate and deepen the mixed layer base, eroding the strength of the main pycnocline and entraining UCDW into the mixed layer. Thus, double‐diffusive processes within the main pycnocline have the potential to determine the strength of the thermal barrier effect. The purpose of this study is to demonstrate how double‐diffusive mixing influences the background stratification, surface mixed layer properties, and sea ice thickness in the Ross Gyre (Figures 1a and 1b). We also address the effect of double‐diffusive mixing on the wintertime water mass transformation, as it has been suggested that the UCDW transforms into the surface mixed layer water in winter and then forms the Intermediate Water (IW) the following season in the upper cell of the meridional overturning circulation (e.g., Abernathey et al., 2016; Evans et al., 2018; Pellichero et al., 2018). The fl ©2019. American Geophysical Union. interior vertical uxes responsible for the distribution and strength of this transformation are not All Rights Reserved. well known. BEBIEVA AND SPEER 1 Journal of Geophysical Research: Oceans 10.1029/2019JC015247 Figure 1. (a) Map showing a region of the Ross Gyre where Argo floats (blue dots) were operating from 2012 to 2018 and the general locations of MRV‐11036, MRV‐11038, and MRV‐11039 (red dots) in 2018; (b) UCDW maximum potential temperature θUCDW (black and red dots show Argo and MRV drift tracks, respectively); (c) the thermocline erosion index Jθ; (d) the halocline erosion index JS. The white contour in (b) and black contours in (c) and (d) show the 1.65 °C isotherm. Red contour in (b) shows time mean 10% sea ice concentration for winter months (June–October) averaged between 2012 and 2018. UCDW = Upper Circumpolar Deep Water; MRV = Marine Robotic Vehicles. Only the central region of the Ross Gyre is considered here, where mean Ekman pumping is weaker and spa- tial gradients and seasonal changes in the UCDW core properties are minimal (Figures 1c and 1d; minimal erosion indices for temperature and salinity Jθ and JS are shown and, along with Ekman pumping, will be discussed further below). We aim to explore these questions by analyzing recent fine‐scale observations from the central Ross Gyre. We consider the gyre in a highly simplified situation where lateral fluxes are neglected and the halocline deepens in the central basin only due to vertical diffusion and employ a 1‐D surface mixed layer model (modified after Martinson, 1990). The paper is organized as follows. The next section describes the data used for the analysis followed by section 3 where we characterize seasonal variability of the surface mixed layer and the underlying pycnocline. In section 4, we formulate the governing equations of the model and use observations to contextualize model results in section 5. Section 6 summarizes the findings and outlines the implication of the results. 2. Data Data analyzed here are derived mainly from a subset of Argo floats (Argo, 2000) to obtain background characteristics of the Ross Gyre. In addition, we deployed several floats with higher temporal and vertical resolution to sample the onset of the seasonal cycle in the upper water column. These latter floats (manufactured by MRV, or Marine Robotic Vehicles) help to characterize fine‐scale double‐diffusive staircases. Recent Argo program deployments are also occurring with floats that sample at higher vertical resolution. 2.1. Argo Floats We use data from Argo floats that drifted through a region bounded by 55°S and 75°S, 160°E and 130°W from 2012 to 2018 (Figure 1a) with some of the profiles measured under ice by the Argo floats equipped with the ice avoidance mechanism (see, e.g., Wong & Riser, 2011). The Argo data include profiles with different vertical resolution ranging from 0.25 to 10 m. To derive the background characteristics of the Ross Gyre all the profiles were first resampled using a rolling average (where applicable) or interpolated onto a regular vertical 1‐m grid from 1 to 500 m. In order to explore the effects of vertical mixing on ice formation rates, the gyre boundaries, where strong lateral exchange occurs, were excluded. To identify the central Ross Gyre we consider the UCDW temperature maximum (θUCDW; Gouretski, 1999) and use only the profiles where the θUCDW<1.65 °C (Figure 1b). An area BEBIEVA AND SPEER 2 Journal of Geophysical Research: Oceans 10.1029/2019JC015247 Figure 2. Sequence of (a) potential temperature θ and (b) salinity S profiles from 15 June 2018 to 21 July 2018 (only upper 250 m is shown to highlight staircase structures) measured by MRV‐11039. Each potential temperature and salinity profile is offset by 1.1 °C and 0.18 from the leftmost profile, respectively. Insets show the closeup structures between ∼100‐ and ∼160‐m depth for the profiles measured on 29 June 2018. (c) Potential temperature and salinity values (within 0‐ to 500‐m depth interval) measured by Argo and MRV from the beginning of June to the end of October; the solid black line shows the mean profile for these winter months. Thin gray contours indicate potential density anomaly (kg/m3) referenced to the surface. MRV = Marine Robotic Vehicles. selected using this criterion shows effectively uniform potential density within the temperature maximum layer (Figure 2c, only winter profiles are shown), indicating that mixing associated with the lateral fluxes due to baroclinic instability is restricted. The data set thus constructed contains approximately 4,500 profiles from 66 Argo floats with the varying number of profiles from 300 to 400 per month. We also characterize the lateral structure of the gyre by comparing pycnocline erosion rates over the winter season. The thermocline erosion index (Jθ) is computed as an absolute value of an integral of potential temperature difference between averaged profiles for June and October from the base of the mixed layer hML 500m (defined below) to 500 m, that is, Jθ ¼ ∫ ðθ −θ Þdz . The halocline erosion index is computed simi- hML Oct Jun 500m larly but using the salinity profilesJS ¼ ∫ ðθ −θ Þdz .

Load more