Willis
Joshua K.
Willis
Joshua K.
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ArticleQuantifying spread in spatiotemporal changes of upper-ocean heat content estimates: an internationally coordinated comparison(American Meteorological Society, 2022-01-15) Savita, Abhishek ; Domingues, Catia M. ; Boyer, Tim ; Gouretski, Viktor ; Ishii, Masayoshi ; Johnson, Gregory C. ; Lyman, John ; Willis, Joshua K. ; Marsland, Simon ; Hobbs, William ; Church, John A. ; Monselesan, Didier Paolo ; Dobrohotoff, Peter ; Cowley, Rebecca ; Wijffels, Susan E.The Earth system is accumulating energy due to human-induced activities. More than 90% of this energy has been stored in the ocean as heat since 1970, with ∼60% of that in the upper 700 m. Differences in upper-ocean heat content anomaly (OHCA) estimates, however, exist. Here, we use a dataset protocol for 1970–2008—with six instrumental bias adjustments applied to expendable bathythermograph (XBT) data, and mapped by six research groups—to evaluate the spatiotemporal spread in upper OHCA estimates arising from two choices: 1) those arising from instrumental bias adjustments and 2) those arising from mathematical (i.e., mapping) techniques to interpolate and extrapolate data in space and time. We also examined the effect of a common ocean mask, which reveals that exclusion of shallow seas can reduce global OHCA estimates up to 13%. Spread due to mapping method is largest in the Indian Ocean and in the eddy-rich and frontal regions of all basins. Spread due to XBT bias adjustment is largest in the Pacific Ocean within 30°N–30°S. In both mapping and XBT cases, spread is higher for 1990–2004. Statistically different trends among mapping methods are found not only in the poorly observed Southern Ocean but also in the well-observed northwest Atlantic. Our results cannot determine the best mapping or bias adjustment schemes, but they identify where important sensitivities exist, and thus where further understanding will help to refine OHCA estimates. These results highlight the need for further coordinated OHCA studies to evaluate the performance of existing mapping methods along with comprehensive assessment of uncertainty estimates.
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ArticleCharacteristic depths, fluxes and timescales for Greenland’s tidewater glacier fjords from subglacial discharge‐driven upwelling during summer(American Geophysical Union, 2022-03-02) Slater, Donald A. ; Carroll, Dustin ; Oliver, Hilde ; Hopwood, Mark J. ; Straneo, Fiamma ; Wood, Michael ; Willis, Joshua K. ; Morlighem, MathieuGreenland's glacial fjords are a key bottleneck in the earth system, regulating exchange of heat, freshwater and nutrients between the ice sheet and ocean and hosting societally important fisheries. We combine recent bathymetric, atmospheric, and oceanographic data with a buoyant plume model to show that summer subglacial discharge from 136 tidewater glaciers, amounting to 0.02 Sv of freshwater, drives 0.6–1.6 Sv of upwelling. Bathymetric analysis suggests that this is sufficient to renew most major fjords within a single summer, and that these fjords provide a path to the continental shelf that is deeper than 200 m for two-thirds of the glaciers. Our study provides a first pan-Greenland inventory of tidewater glacier fjords and quantifies regional and ice sheet-wide upwelling fluxes. This analysis provides important context for site-specific studies and is a step toward implementing fjord-scale heat, freshwater and nutrient fluxes in large-scale ice sheet and climate models.
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ArticleThe Atlantic Meridional Overturning Circulation at 35°N from deep moorings, floats, and satellite altimeter(American Geophysical Union, 2023-05-13) Le Bras, Isabela Alexander‐Astiz ; Willis, Josh ; Fenty, IanFrom 2004 to 2014, the Line W moorings measured a 0.7 Sv yr−1 slowing of the deep western boundary current (DWBC) offshore of Cape Cod. Here, we combine these deep mooring observations with float and satellite altimeter data and find that this DWBC change corresponded to a slowing of the cross‐basin Atlantic Meridional Overturning Circulation (AMOC) of about 0.3 Sv yr−1. Our AMOC transport time series corresponds well with the Estimating the Circulation and Climate of the Ocean state estimate, particularly when the Line W mooring data influences our volume closure. We compare our 35°N time series with a similar time series at 41°N as well as the 26°N RAPID AMOC, and find AMOC declines across datasets from 2004 to 2014. However, when we extend our analysis to 2004–2019, there are no significant trends at any latitude. These observations suggest that AMOC decadal variability is meridionally coherent from 26°N to 41°N and that the DWBC may reflect this variability.Plain Language SummaryThe Atlantic ocean hosts an overturning circulation that is thought to be an important piece of our climate system. This circulation pattern spans the width of the basin, making it difficult and costly to measure, so direct observations of the overturning circulation are scarce. In this study we combine existing mooring, float, and satellite altimeter observations to estimate the overturning circulation at a new latitude (35°N), and compare it to existing estimates at 26°N and 41°N as well as the ECCO ocean state estimate. We find that the long term (about 10 year) AMOC variability is consistent across latitudes and data products. While we cannot rule out a decreasing AMOC trend during the 20th century, we find that natural variability is too large to detect a net AMOC decrease in direct observations since 2004.Key PointsWe compile an Atlantic Meridional Overturning Circulation (AMOC) time series at 35°N from deep moorings, floats, and altimeter that agrees with the Estimating the Circulation and Climate of the Ocean state estimateThe 2004 to 2014 slowing of the deep western boundary current corresponded to an AMOC decline at 35°NWe find no evidence of long‐term AMOC decline, but consistent decadal variability across 26°N, 35°N, and 41°N