Piecuch
Christopher G.
Piecuch
Christopher G.
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ArticleHow is New England coastal sea level related to the Atlantic meridional overturning circulation at 26 degrees N?(American Geophysical Union, 2019-05-01) Piecuch, Christopher G. ; Dangendorf, Sönke ; Gawarkiewicz, Glen G. ; Little, Christopher M. ; Ponte, Rui M. ; Yang, JiayanMonthly observations are used to study the relationship between the Atlantic meridional overturning circulation (AMOC) at 26° N and sea level (ζ) on the New England coast (northeastern United States) over nonseasonal timescales during 2004–2017. Variability in ζ is anticorrelated with AMOC on intraseasonal and interannual timescales. This anticorrelation reflects the stronger underlying antiphase relationship between ageostrophic Ekman‐related AMOC transports due to local zonal winds across 26° N and ζ changes arising from local wind and pressure forcing along the coast. These distinct local atmospheric variations across 26° N and along coastal New England are temporally correlated with one another on account of large‐scale atmospheric teleconnection patterns. Geostrophic AMOC contributions from the Gulf Stream through the Florida Straits and upper‐mid‐ocean transport across the basin are together uncorrelated with ζ. This interpretation contrasts with past studies that understood ζ and AMOC as being in geostrophic balance with one another.
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ArticleMeridional asymmetry in recent decadal sea-level trends in the subtropical Pacific Ocean(American Geophysical Union, 2021-03-02) Schloesser, Fabian ; Thompson, Philip R. ; Piecuch, Christopher G.Recent sea surface height (SSH) trends in the South Pacific are substantially greater than trends in the North Pacific. Here, we use the Estimating the Climate and Circulation of the Ocean Version 4 Release 4 ocean state estimate and the Ocean Reanalysis System 5 to identify the forcing and mechanisms underlying that meridional asymmetry during 2005–2015. Thermosteric contributions dominate the spatial structure in Pacific SSH trends, but contributions from local surface heat fluxes are small. Wind stress trends drive a spin-up of the South Pacific subtropical gyre and a northward shift of the North Pacific subtropical gyre. A reduced gravity model forced with reanalysis winds qualitatively reproduces the meridional seesaw in sea level, suggesting that asymmetric trends in subtropical wind stress drive a cross-equatorial heat transport. A reversal in forcing associated with this process could impact near-term rates of coastal sea-level change, particularly in Pacific Island communities.
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ArticleIntraseasonal sea level variability in the Persian Gulf(American Meteorological Society, 2021-05-01) Piecuch, Christopher G. ; Fukumori, Ichiro ; Ponte, Rui M.Satellite observations are used to establish the dominant magnitudes, scales, and mechanisms of intraseasonal variability in ocean dynamic sea level (ζ) in the Persian Gulf over 2002–15. Empirical orthogonal function (EOF) analysis applied to altimetry data reveals a basinwide, single-signed intraseasonal fluctuation that contributes importantly to ζ variance in the Persian Gulf at monthly to decadal time scales. An EOF analysis of Gravity Recovery and Climate Experiment (GRACE) observations over the same period returns a similar large-scale mode of intraseasonal variability, suggesting that the basinwide intraseasonal ζ variation has a predominantly barotropic nature. A linear barotropic theory is developed to interpret the data. The theory represents Persian Gulf average ζ (¯ζ) in terms of local freshwater flux, barometric pressure, and wind stress forcing, as well as ζ at the boundary in the Gulf of Oman. The theory is tested using a multiple linear regression with these freshwater flux, barometric pressure, wind stress, and boundary ζ quantities as input and ¯ζ as output. The regression explains 70% ± 9% (95% confidence interval) of the intraseasonal ¯ζ variance. Numerical values of regression coefficients computed empirically from the data are consistent with theoretical expectations from first principles. Results point to a substantial nonisostatic response to surface loading. The Gulf of Oman ζ boundary condition shows lagged correlation with ζ upstream along the Indian subcontinent, Maritime Continent, and equatorial Indian Ocean, suggesting a large-scale Indian Ocean influence on intraseasonal ¯ζ variation mediated by coastal and equatorial waves and hinting at potential predictability. This study highlights the value of GRACE for understanding sea level in an understudied marginal sea.
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ArticleRiver-discharge effects on United States Atlantic and Gulf coast sea-level changes(National Academy of Sciences, 2018-07-09) Piecuch, Christopher G. ; Bittermann, Klaus ; Kemp, Andrew C. ; Ponte, Rui Vasques de Melo ; Little, Christopher M. ; Engelhart, Simon E. ; Lentz, Steven J.Identifying physical processes responsible for historical coastal sea-level changes is important for anticipating future impacts. Recent studies sought to understand the drivers of interannual to multidecadal sea-level changes on the United States Atlantic and Gulf coasts. Ocean dynamics, terrestrial water storage, vertical land motion, and melting of land ice were highlighted as important mechanisms of sea-level change along this densely populated coast on these time scales. While known to exert an important control on coastal ocean circulation, variable river discharge has been absent from recent discussions of drivers of sea-level change. We update calculations from the 1970s, comparing annual river-discharge and coastal sea-level data along the Gulf of Maine, Mid-Atlantic Bight, South Atlantic Bight, and Gulf of Mexico during 1910–2017. We show that river-discharge and sea-level changes are significantly correlated (p<0.01), such that sea level rises between 0.01 and 0.08 cm for a 1 km3 annual river-discharge increase, depending on region. We formulate a theory that describes the relation between river-discharge and halosteric sea-level changes (i.e., changes in sea level related to salinity) as a function of river discharge, Earth’s rotation, and density stratification. This theory correctly predicts the order of observed increment sea-level change per unit river-discharge anomaly, suggesting a causal relation. Our results have implications for remote sensing, climate modeling, interpreting Common Era proxy sea-level reconstructions, and projecting coastal flood risk.
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ArticleLocal and remote forcing of interannual sea‐level variability at Nantucket Island(American Geophysical Union, 2022-06-07) Wang, Ou ; Lee, Tong ; Piecuch, Christopher G. ; Fukumori, Ichiro ; Fenty, Ian ; Frederikse, Thomas ; Menemenlis, Dimitris ; Ponte, Rui M. ; Zhang, HongThe relative contributions of local and remote wind stress and air-sea buoyancy forcing to sea-level variations along the East Coast of the United States are not well quantified, hindering the understanding of sea-level predictability there. Here, we use an adjoint sensitivity analysis together with an Estimating the Circulation and Climate of the Ocean (ECCO) ocean state estimate to establish the causality of interannual variations in Nantucket dynamic sea level. Wind forcing explains 67% of the Nantucket interannual sea-level variance, while wind and buoyancy forcing together explain 97% of the variance. Wind stress contribution is near-local, primarily from the New England shelf northeast of Nantucket. We disprove a previous hypothesis about Labrador Sea wind stress being an important driver of Nantucket sea-level variations. Buoyancy forcing, as important as wind stress in some years, includes local contributions as well as remote contributions from the subpolar North Atlantic that influence Nantucket sea level a few years later. Our rigorous adjoint-based analysis corroborates previous correlation-based studies indicating that sea-level variations in the subpolar gyre and along the United States northeast coast can both be influenced by subpolar buoyancy forcing. Forward perturbation experiments further indicate remote buoyancy forcing affects Nantucket sea level mostly through slow advective processes, although coastally trapped waves can cause rapid Nantucket sea level response within a few weeks.
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PreprintOrigin of spatial variation in US East Coast sea-level trends during 1900-2017(Nature Research, 2018-12-18) Piecuch, Christopher G. ; Huybers, Peter ; Hay, Carling C. ; Kemp, Andrew C. ; Little, Christopher M. ; Mitrovica, Jerry X. ; Ponte, Rui M. ; Tingley, Martin P.Identifying the causes of historical trends in relative sea level—the height of the sea surface relative to Earth’s crust—is a prerequisite for predicting future changes. Rates of change along the U.S. East Coast during the last century were spatially variable, and relative sea level rose faster along the Mid-Atlantic Bight than the South Atlantic Bight and Gulf of Maine. Past studies suggest that Earth’s ongoing response to the last deglaciation1–5, surface redistribution of ice and water 5–9, and changes in ocean circulation9–13 contributed importantly to this large-scale spatial pattern. Here we analyze instrumental data14, 15 and proxy reconstructions4, 12 using probabilistic methods16–18 to show that vertical motions of Earth’s crust exerted the dominant control on regional spatial differences in relative sea level trends along the U.S. East Coast during 1900–2017, explaining a majority of the large-scale spatial variance. Rates of coastal subsidence caused by ongoing relaxation of the peripheral forebulge associated with the last deglaciation are strongest near North Carolina,Maryland, and Virginia. Such structure indicates that Earth’s elastic lithosphere is thicker than has been assumed in other models19–22. We also find a significant coastal gradient in relative sea level trends over this period that is unrelated to deglaciation, and suggests contributions from twentieth-century redistribution of ice and water. Our results indicate that the majority of large-scale spatial variation in longterm rates of relative sea level rise on the U.S. East Coast was due to geological processes that will persist at similar rates for centuries into the future.
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ArticlePutting it all together: Adding value to the global ocean and climate observing systems with complete self-consistent ocean state and parameter estimates.(Frontiers Media, 2019-03-04) Heimbach, Patrick ; Fukumori, Ichiro ; Hill, Christopher N. ; Ponte, Rui M. ; Stammer, Detlef ; Wunsch, Carl ; Campin, Jean-Michel ; Cornuelle, Bruce D. ; Fenty, Ian ; Forget, Gael ; Kohl, Armin ; Mazloff, Matthew R. ; Menemenlis, Dimitris ; Nguyen, An T. ; Piecuch, Christopher G. ; Trossman, David S. ; Verdy, Ariane ; Wang, Ou ; Zhang, HongIn 1999, the consortium on Estimating the Circulation and Climate of the Ocean (ECCO) set out to synthesize the hydrographic data collected by the World Ocean Circulation Experiment (WOCE) and the satellite sea surface height measurements into a complete and coherent description of the ocean, afforded by an ocean general circulation model. Twenty years later, the versatility of ECCO's estimation framework enables the production of global and regional ocean and sea-ice state estimates, that incorporate not only the initial suite of data and its successors, but nearly all data streams available today. New observations include measurements from Argo floats, marine mammal-based hydrography, satellite retrievals of ocean bottom pressure and sea surface salinity, as well as ice-tethered profiled data in polar regions. The framework also produces improved estimates of uncertain inputs, including initial conditions, surface atmospheric state variables, and mixing parameters. The freely available state estimates and related efforts are property-conserving, allowing closed budget calculations that are a requisite to detect, quantify, and understand the evolution of climate-relevant signals, as mandated by the Coupled Model Intercomparison Project Phase 6 (CMIP6) protocol. The solutions can be reproduced by users through provision of the underlying modeling and assimilation machinery. Regional efforts have spun off that offer increased spatial resolution to better resolve relevant processes. Emerging foci of ECCO are on a global sea level changes, in particular contributions from polar ice sheets, and the increased use of biogeochemical and ecosystem data to constrain global cycles of carbon, nitrogen and oxygen. Challenges in the coming decade include provision of uncertainties, informing observing system design, globally increased resolution, and moving toward a coupled Earth system estimation with consistent momentum, heat and freshwater fluxes between the ocean, atmosphere, cryosphere and land.
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ArticleRiver effects on sea-level rise in the Rio de la Plata estuary during the past century(European Geosciences Union, 2023-01-18) Piecuch, Christopher G.Identifying the causes for historical sea-level changes in coastal tide-gauge records is important for constraining oceanographic, geologic, and climatic processes. The Río de la Plata estuary in South America features the longest tide-gauge records in the South Atlantic. Despite the relevance of these data for large-scale circulation and climate studies, the mechanisms underlying relative sea-level changes in this region during the past century have not been firmly established. I study annual data from tide gauges in the Río de la Plata and stream gauges along the Río Paraná and Río Uruguay to establish relationships between river streamflow and sea level over 1931–2014. Regression analysis suggests that streamflow explains 59 %±17 % of the total sea-level variance at Buenos Aires, Argentina, and 28 %±21 % at Montevideo, Uruguay (95 % confidence intervals). A long-term streamflow increase effected sea-level trends of 0.71±0.35 mm yr−1 at Buenos Aires and 0.48±0.38 mm yr−1 at Montevideo. More generally, sea level at Buenos Aires and Montevideo respectively rises by (7.3±1.8)×10-6 m and (4.7±2.6)×10-6 m per 1 m3 s−1 streamflow increase. These observational results are consistent with simple theories for the coastal sea-level response to streamflow forcing, suggesting a causal relationship between streamflow and sea level mediated by ocean dynamics. Findings advance understanding of local, regional, and global sea-level changes; clarify sea-level physics; inform future projections of coastal sea level and the interpretation of satellite data and proxy reconstructions; and highlight future research directions. Specifically, local and regional river effects should be accounted for in basin-scale and global mean sea-level budgets as well as reconstructions based on sparse tide-gauge records.
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ArticleOcean mass, sterodynamic effects, and vertical land motion largely explain US coast relative sea level rise(Nature Research, 2021-11-09) Harvey, Thomas C. ; Hamlington, Benjamin D. ; Frederikse, Thomas ; Nerem, R. Steven ; Piecuch, Christopher G. ; Hammond, William C. ; Blewitt, Geoffrey ; Thompson, Philip R. ; Bekaert, David P. S. ; Landerer, Felix ; Reager, John T. ; Kopp, Robert E. ; Chandanpurkar, Hrishikesh A. ; Fenty, Ian ; Trossman, David S. ; Walker, Jennifer S. ; Boening, CarmenRegional sea-level changes are caused by several physical processes that vary both in space and time. As a result of these processes, large regional departures from the long-term rate of global mean sea-level rise can occur. Identifying and understanding these processes at particular locations is the first step toward generating reliable projections and assisting in improved decision making. Here we quantify to what degree contemporary ocean mass change, sterodynamic effects, and vertical land motion influence sea-level rise observed by tide-gauge locations around the contiguous U.S. from 1993 to 2018. We are able to explain tide gauge-observed relative sea-level trends at 47 of 55 sampled locations. Locations where we cannot explain observed trends are potentially indicative of shortcomings in our coastal sea-level observational network or estimates of uncertainty.
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ArticleOrigin of interannual variability in global mean sea level(National Academy of Sciences, 2020-06-08) Hamlington, Benjamin D. ; Piecuch, Christopher G. ; Reager, John T. ; Chandanpurkar, Hrishikesh A. ; Frederikse, Thomas ; Nerem, R. Steven ; Fasullo, John T. ; Cheon, Se-HyeonThe two dominant drivers of the global mean sea level (GMSL) variability at interannual timescales are steric changes due to changes in ocean heat content and barystatic changes due to the exchange of water mass between land and ocean. With Gravity Recovery and Climate Experiment (GRACE) satellites and Argo profiling floats, it has been possible to measure the relative steric and barystatic contributions to GMSL since 2004. While efforts to “close the GMSL budget” with satellite altimetry and other observing systems have been largely successful with regards to trends, the short time period covered by these records prohibits a full understanding of the drivers of interannual to decadal variability in GMSL. One particular area of focus is the link between variations in the El Niño−Southern Oscillation (ENSO) and GMSL. Recent literature disagrees on the relative importance of steric and barystatic contributions to interannual to decadal variability in GMSL. Here, we use a multivariate data analysis technique to estimate variability in barystatic and steric contributions to GMSL back to 1982. These independent estimates explain most of the observed interannual variability in satellite altimeter-measured GMSL. Both processes, which are highly correlated with ENSO variations, contribute about equally to observed interannual GMSL variability. A theoretical scaling analysis corroborates the observational results. The improved understanding of the origins of interannual variability in GMSL has important implications for our understanding of long-term trends in sea level, the hydrological cycle, and the planet’s radiation imbalance.
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ArticleLow-frequency dynamic ocean response to barometric-pressure loading(American Meteorological Society, 2022-10-17) Piecuch, Christopher G. ; Fukumori, Ichiro ; Ponte, Rui M. ; Schindelegger, Michael ; Wang, Ou ; Zhao, MengnanChanges in dynamic manometric sea level ζm represent mass-related sea level changes associated with ocean circulation and climate. We use twin model experiments to quantify magnitudes and spatiotemporal scales of ζm variability caused by barometric pressure pa loading at long periods (≳1 month) and large scales (≳300km) relevant to Gravity Recovery and Climate Experiment (GRACE) ocean data. Loading by pa drives basin-scale monthly ζm variability with magnitudes as large as a few centimeters. Largest ζm signals occur over abyssal plains, on the shelf, and in marginal seas. Correlation patterns of modeled ζm are determined by continental coasts and H/f contours (H is ocean depth and f is Coriolis parameter). On average, ζm signals forced by pa represent departures of ≲10% and ≲1% from the inverted-barometer effect ζib on monthly and annual periods, respectively. Basic magnitudes, spatial patterns, and spectral behaviors of ζm from the model are consistent with scaling arguments from barotropic potential vorticity conservation. We also compare ζm from the model driven by pa to ζm from GRACE observations. Modeled and observed ζm are significantly correlated across parts of the tropical and extratropical oceans, on shelf and slope regions, and in marginal seas. Ratios of modeled to observed ζm magnitudes are as large as ∼0.2 (largest in the Arctic Ocean) and qualitatively agree with analytical theory for the gain of the transfer function between ζm forced by pa and wind stress. Results demonstrate that pa loading is a secondary but nevertheless important contributor to monthly mass variability from GRACE over the ocean.
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ArticleUnderstanding of contemporary regional sea-level change and the implications for the future(American Geophysical Union, 2020-04-17) Hamlington, Benjamin D. ; Gardner, Alex S. ; Ivins, Erik ; Lenaerts, Jan T. M. ; Reager, John T. ; Trossman, David S. ; Zaron, Edward D. ; Adhikari, Surendra ; Arendt, Anthony ; Aschwanden, Andy ; Beckley, Brian D. ; Bekaert, David P. S. ; Blewitt, Geoffrey ; Caron, Lambert ; Chambers, Don P. ; Chandanpurkar, Hrishikesh A. ; Christianson, Knut ; Csatho, Beata ; Cullather, Richard I. ; DeConto, Robert M. ; Fasullo, John T. ; Frederikse, Thomas ; Freymueller, Jeffrey T. ; Gilford, Daniel M. ; Girotto, Manuela ; Hammond, William C. ; Hock, Regine ; Holschuh, Nicholas ; Kopp, Robert E. ; Landerer, Felix ; Larour, Eric ; Menemenlis, Dimitris ; Merrifield, Mark ; Mitrovica, Jerry X. ; Nerem, R. Steven ; Nias, Isabel J. ; Nieves, Veronica ; Nowicki, Sophie ; Pangaluru, Kishore ; Piecuch, Christopher G. ; Ray, Richard D. ; Rounce, David R. ; Schlegel, Nicole‐Jeanne ; Seroussi, Helene ; Shirzaei, Manoochehr ; Sweet, William V. ; Velicogna, Isabella ; Vinogradova, Nadya ; Wahl, Thomas ; Wiese, David N. ; Willis, Michael J.Global sea level provides an important indicator of the state of the warming climate, but changes in regional sea level are most relevant for coastal communities around the world. With improvements to the sea‐level observing system, the knowledge of regional sea‐level change has advanced dramatically in recent years. Satellite measurements coupled with in situ observations have allowed for comprehensive study and improved understanding of the diverse set of drivers that lead to variations in sea level in space and time. Despite the advances, gaps in the understanding of contemporary sea‐level change remain and inhibit the ability to predict how the relevant processes may lead to future change. These gaps arise in part due to the complexity of the linkages between the drivers of sea‐level change. Here we review the individual processes which lead to sea‐level change and then describe how they combine and vary regionally. The intent of the paper is to provide an overview of the current state of understanding of the processes that cause regional sea‐level change and to identify and discuss limitations and uncertainty in our understanding of these processes. Areas where the lack of understanding or gaps in knowledge inhibit the ability to provide the needed information for comprehensive planning efforts are of particular focus. Finally, a goal of this paper is to highlight the role of the expanded sea‐level observation network—particularly as related to satellite observations—in the improved scientific understanding of the contributors to regional sea‐level change.
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ArticleContributions of different sea-level processes to high-tide flooding along the US coastline(American Geophysical Union, 2022-07-14) Li, Sida ; Wahl, Thomas ; Barroso, Amanda ; Coats, Sloan ; Dangendorf, Sönke ; Piecuch, Christopher G. ; Sun, Qiang ; Thompson, Philip R. ; Liu, LintaoCoastal communities across the United States (U.S.) are experiencing an increase in the frequency of high-tide flooding (HTF). This increase is mainly due to sea-level rise (SLR), but other factors such as intra- to inter-annual mean sea level variability, tidal anomalies, and non-tidal residuals also contribute to HTF events. Here we introduce a novel decomposition approach to develop and then analyze a new database of different sea-level components. Those components represent processes that act on various timescales to contribute to HTF along the U.S. coastline. We find that the relative importance of components to HTF events strongly varies in space and time. Tidal anomalies contribute the most along the west and northeast coasts, where HTF events mostly occur in winter. Non-tidal residuals are most important along the Gulf of Mexico and mid-Atlantic coasts, where HTF events mostly occur in fall. We also quantify the minimum number of components that were required to cause HTF events in the past and how this number changed over time. The results highlight that at present, due to SLR, fewer components are needed to combine to push water levels above HTF thresholds, but tidal anomalies alone are still not sufficient to reach HTF thresholds in most locations. Finally, we explore how co-variability between different components leads to compounding effects. In some places, positive correlation between sea-level components leads to significantly more HTF events than would be expected if sea-level components were uncorrelated, whereas in other places negative correlation leads to fewer HTF events.
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ArticleInfluence of nonseasonal river discharge on sea surface salinity and height(American Geophysical Union, 2022-01-18) Chandanpurkar, Hrishikesh A. ; Lee, Tong ; Wang, Xiaochun ; Zhang, Hong ; Fournier, Séverine ; Fenty, Ian ; Fukumori, Ichiro ; Menemenlis, Dimitris ; Piecuch, Christopher G. ; Reager, John T. ; Wang, Ou ; Worden, JohnRiver discharge influences ocean dynamics and biogeochemistry. Due to the lack of a systematic, up-to-date global measurement network for river discharge, global ocean models typically use seasonal discharge climatology as forcing. This compromises the simulated nonseasonal variation (the deviation from seasonal climatology) of the ocean near river plumes and undermines their usefulness for interdisciplinary research. Recently, a reanalysis-based daily varying global discharge data set was developed, providing the first opportunity to quantify nonseasonal discharge effects on global ocean models. Here we use this data set to force a global ocean model for the 1992–2017 period. We contrast this experiment with another experiment (with identical atmospheric forcings) forced by seasonal climatology from the same discharge data set to isolate nonseasonal discharge effects, focusing on sea surface salinity (SSS) and sea surface height (SSH). Near major river mouths, nonseasonal discharge causes standard deviations in SSS (SSH) of 1.3–3 practical salinity unit (1–2.7 cm). The inclusion of nonseasonal discharge results in notable improvement of model SSS against satellite SSS near most of the tropical-to-midlatitude river mouths and minor improvement of model SSH against satellite or in-situ SSH near some of the river mouths. SSH changes associated with nonseasonal discharge can be explained by salinity effects on halosteric height and estimated accurately through the associated SSS changes. A recent theory predicting river discharge impact on SSH is found to perform reasonably well overall but underestimates the impact on SSH around the global ocean and has limited skill when applied to rivers near the equator and in the Arctic Ocean.
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ArticleHigh-tide floods and storm surges during atmospheric rivers on the US West Coast(American Geophysical Union, 2022-01-18) Piecuch, Christopher G. ; Coats, Sloan ; Dangendorf, Sönke ; Landerer, Felix ; Reager, John T. ; Thompson, Philip R. ; Wahl, ThomasAtmospheric rivers (ARs) cause inland hydrological impacts related to precipitation. However, little is known about coastal hazards associated with these events. We elucidate high-tide floods (HTFs) and storm surges during ARs on the US West Coast during 1980–2016. HTFs and ARs cooccur more often than expected from chance. Between 10% and 63% of HTFs coincide with ARs on average, depending on location. However, interannual-to-decadal variations in HTFs are due more to tides and mean sea-level changes than storminess variability. Only 2–15% of ARs coincide with HTFs, suggesting that ARs typically must cooccur with high tides or mean sea levels to cause HTFs. Storm surges during ARs reflect local wind, pressure, and precipitation forcing: meridional wind and barometric pressure are primary drivers, but precipitation makes secondary contributions. This study highlights the relevance of ARs to coastal impacts, clarifies the drivers of storm surge during ARs, and identifies future research directions.
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ArticleWhat caused recent shifts in tropical pacific decadal sea-level trends?(American Geophysical Union, 2019-10-31) Piecuch, Christopher G. ; Thompson, Philip R. ; Ponte, Rui M. ; Merrifield, Mark ; Hamlington, Benjamin D.Satellite altimetry reveals substantial decadal variability in sea level 𝜁 across the tropical Pacific during 1993–2015. An ocean state estimate that faithfully reproduces the observations is used to elucidate the origin of these low-frequency tropical Pacific 𝜁 variations. Analysis of the hydrostatic equation reveals that recent decadal 𝜁 changes in the tropical Pacific are mainly hermosteric in nature, related to changes in upper-ocean heat content. A forcing experiment performed with the numerical model suggests that anomalous wind stress was an important driver of the relevant heat storage and thermosteric variation. Closed budget diagnostics further clarify that the wind-stress-related thermosteric 𝜁 variation resulted from the joint actions of large-scale ocean advection and local surface heat flux, such that advection controlled the budget over shorter, intraseasonal to interannual time scales, and local surface heat flux became increasingly influential at longer decadal periods. In particular, local surface heat flux was important in contributing to a recent reversal of decadal 𝜁 trends in the tropical Pacific. Contributions from local surface heat flux partly reflect damping latent heat flux tied to wind-stress-driven sea-surface-temperature variations.
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ArticleThe relationship between U.S. East Coast sea level and the Atlantic Meridional Overturning Circulation: a review(American Geophysical Union, 2019-08-09) Little, Christopher M. ; Hu, Aixue ; Hughes, Chris W. ; McCarthy, Gerard D. ; Piecuch, Christopher G. ; Ponte, Rui M. ; Thomas, Matthew D.Scientific and societal interest in the relationship between the Atlantic Meridional Overturning Circulation (AMOC) and U.S. East Coast sea level has intensified over the past decade, largely due to (1) projected, and potentially ongoing, enhancement of sea level rise associated with AMOC weakening and (2) the potential for observations of U.S. East Coast sea level to inform reconstructions of North Atlantic circulation and climate. These implications have inspired a wealth of model‐ and observation‐based analyses. Here, we review this research, finding consistent support in numerical models for an antiphase relationship between AMOC strength and dynamic sea level. However, simulations exhibit substantial along‐coast and intermodel differences in the amplitude of AMOC‐associated dynamic sea level variability. Observational analyses focusing on shorter (generally less than decadal) timescales show robust relationships between some components of the North Atlantic large‐scale circulation and coastal sea level variability, but the causal relationships between different observational metrics, AMOC, and sea level are often unclear. We highlight the importance of existing and future research seeking to understand relationships between AMOC and its component currents, the role of ageostrophic processes near the coast, and the interplay of local and remote forcing. Such research will help reconcile the results of different numerical simulations with each other and with observations, inform the physical origins of covariability, and reveal the sensitivity of scaling relationships to forcing, timescale, and model representation. This information will, in turn, provide a more complete characterization of uncertainty in relevant relationships, leading to more robust reconstructions and projections.
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ArticleMechanisms controlling global mean sea surface temperature determined from a state estimate(John Wiley & Sons, 2018-04-13) Ponte, Rui Vasques de Melo ; Piecuch, Christopher G.Global mean sea surface temperature ((T) over bar) is a variable of primary interest in studies of climate variability and change. The temporal evolution of (T) over bar) can be influenced by surface heat fluxes ((F) over bar)) and by diffusion ((D) over bar)) and advection ((A) over bar)) processes internal to the ocean, but quantifying the contribution of these different factors from data alone is prone to substantial uncertainties. Here we derive a closed (T) over bar) budget for the period 1993-2015 based on a global ocean state estimate, which is an exact solution of a general circulation model constrained to most extant ocean observations through advanced optimization methods. The estimated average temperature of the top (10-m thick) level in the model, taken to represent (T) over bar), shows relatively small variability at most time scales compared to (F) over bar), (D) over bar), or (A) over bar), reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in (T) over bar) is mostly determined by small imbalances between (F) over bar) and (D) over bar), with negligible contributions from (A) over bar). While (D) over bar) seems to simply damp (F) over bar) at the annual period, a different dynamical role for (D) over bar) at semiannual period is suggested by it being larger than (F) over bar). At periods longer than annual, (A) over bar) contributes importantly to (T) over bar) variability, pointing to the direct influence of the variable ocean circulation on (T) over bar) and mean surface climate. Plain Language Summary Global mean sea surface temperature (T) over bar) is a key metric when defining the Earth's climate. Determining what controls the evolution of (T) over bar )T is thus vital for understanding past climate variability and predicting its future evolution. Processes that control (T) over bar) involve forcing surface heat fluxes, as well as advection and diffusion of heat internal to the ocean, but their relative contributions are poorly known and difficult to assess from observations alone. Here we use advanced methods to combine models and data and derive a closed budget for (T) over bar) variability in terms of the forcing, advection, and diffusion processes. The estimated (T) over bar) shows relatively small variability compared to surface forcing, advection, or diffusion, reflecting the tendency for largely balancing effects from all the latter terms. The seasonal cycle in (T) over bar) is mostly determined by small imbalances between forcing and diffusion, with negligible contributions from advection. Diffusion does not always act as a simple damping of forcing surface fluxes, however. In addition, at periods longer than annual, advection contributes importantly to (T) over bar) variability. The results point to the direct influence of the variable ocean circulation on (T) over bar) and the Earth's surface climate.
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ArticleA preindustrial sea-level rise hotspot along the Atlantic Coast of North America(American Geophysical Union, 2020-02-13) Gehrels, W. Roland ; Dangendorf, Sönke ; Barlow, Natasha L. M. ; Saher, Margot H. ; Long, Antony J. ; Woodworth, Philip L. ; Piecuch, Christopher G. ; Berk, KevinThe Atlantic coast of North America north of Cape Hatteras has been proposed as a “hotspot” of late 20th century sea‐level rise. Here we test, using salt‐marsh proxy sea‐level records, if this coast experienced enhanced sea‐level rise over earlier multidecadal‐centennial periods. While we find in agreement with previous studies that 20th century rates of sea‐level change were higher compared to rates during preceding centuries, rates of 18th century sea‐level rise were only slightly lower, suggesting that the “hotspot” is a reoccurring feature for at least three centuries. Proxy sea‐level records from North America (Iceland) are negatively (positively) correlated with centennial changes in the North Atlantic Oscillation. They are consistent with sea‐level “fingerprints” of Arctic ice melt, and we therefore hypothesize that sea‐level fluctuations are related to changes in Arctic land‐ice mass. Predictions of future sea‐level rise should take into account these long‐term fluctuating rates of natural sea‐level change.
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ArticleThe mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses(American Geophysical Union, 2019-11-06) Jackson, Laura ; Dubois, Clotilde ; Forget, Gael ; Haines, Keith ; Harrison, Matthew ; Iovino, Doroteaciro ; Toyoda, Takahiro ; Kohl, Armin ; Mignac, Davi ; Masina, Simona ; Peterson, K. Andrew ; Piecuch, Christopher G. ; Roberts, Chris ; Robson, Jon ; Storto, Andrea ; Toyoda, Takahiro ; Valdivieso, Maria ; Wilson, Christopher G. ; Wang, Yiguo ; Zuo, HaoThe observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal time scale changes across the North Atlantic including in heat content, heat transport, and the circulation. However, there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However, the ability of the reanalyses to represent the dynamics must also be assessed. We use an ensemble of global ocean reanalyses to examine the time mean state and interannual‐decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular, we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50°N from the mid‐1990s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5°N since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5°N since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved.