Christopher G. Piecuch

Inverted barometer contributions to recent sea level changes along the northeast coast of North America

Regional sea level (SL) changes reflect dynamic and isostatic ocean effects. Recent works have interpreted accelerated and extreme SL changes along the northeast coast of North America primarily in terms of dynamic changes; however, dedicated study of isostatic changes related to surface atmospheric pressure loading—the inverted barometer (IB) effect—has been lacking. This investigation uses five different atmospheric pressure products to analyze the influence of the IB effect on annual mean SL from tide gauge records. The IB effect explains ∼25% of interannual SL variance and accounts for ∼50% of the magnitude of a recent extreme event of SL rise along Atlantic Canada and New England. Estimated IB effects also amount to ∼10–30% of recent multidecadal SL accelerations over the Mid-Atlantic Bight and Southern New England. These findings reiterate the need for careful estimation and removal of isostatic effects for studies of dynamic SL.

Annual Sea Level Changes on the North American Northeast Coast: Influence of Local Winds and Barotropic Motions

AbstractUnderstanding the relationship between coastal sea level and the variable ocean circulation is crucial for interpreting tide gauge records and projecting sea level rise. In this study, annual sea level records (adjusted for the inverted barometer effect) from tide gauges along the North American northeast coast over 1980–2010 are compared to a set of data-assimilating ocean reanalysis products as well as a global barotropic model solution forced with wind stress and barometric pressure. Correspondence between models and data depends strongly on model and location. At sites north of Cape Hatteras, the barotropic model shows as much (if not more) skill than ocean reanalyses, explaining about 50% of the variance in the adjusted annual tide gauge sea level records. Additional numerical experiments show that annual sea level changes along this coast from the barotropic model are driven by local wind stress over the continental shelf and slope. This result is interpreted in the light of a simple dynamic...

Importance of Circulation Changes to Atlantic Heat Storage Rates on Seasonal and Interannual Time Scales

AbstractOcean heat budgets and transports are diagnosed to elucidate the importance of general circulation changes to Atlantic Ocean heat storage rates. The focus is on low- and midlatitude regions and on seasonal and interannual time scales. An estimate of the ocean state over 1993–2004, produced by a coarse-resolution general circulation model fit to observations via the method of Lagrange multipliers, is used. Meridional heat transports are first decomposed into contributions from time-mean and time-variable velocity and temperature and second from zonally symmetric baroclinic (overturning, including Ekman) and zonally asymmetric (gyre and other spatially correlated) circulations. Heat storage rates are then ascribed to ocean–atmosphere heat exchanges, diffusive mixing, and advective processes related to the various components of the meridional heat transport. Results show that seasonal heat storage changes generally represent a local response to surface heat inputs, but seasonal advective changes are ...

Interannual Bottom Pressure Signals in the Australian–Antarctic and Bellingshausen Basins

AbstractAnalyses of large-scale (>750 km) ocean bottom pressure pb fields, derived from the Gravity Recovery and Climate Experiment (GRACE) and from an Estimating the Circulation & Climate of the Ocean (ECCO) state estimate, reveal enhanced interannual variability, partially connected to the Antarctic Oscillation, in regions of the Australian–Antarctic Basin and the Bellingshausen Basin, with pb magnitudes comparable to those of sea level and good correlation between the GRACE and ECCO pb series. Consistent with the theory of Gill and Niiler, the patterns of stronger pb variability are partly related to enhanced local wind curl forcing and weakened gradients in H/f, where H is ocean depth and f is the Coriolis parameter. Despite weaker H/f gradients, motions against them are sufficiently strong to play a role in balancing the local wind input. Topographic effects are as or more important than changes in f. Additionally, and contrary to the dominance of barotropic processes at subannual time scales, barocl...

Mechanisms of Global-Mean Steric Sea Level Change

AbstractGlobal-mean sea level change partly reflects volumetric expansion of the oceans because of density change, otherwise known as global-mean steric sea level change. Owing to nonlinearities in the equation of state of seawater, the nature of processes contributing to recent observed global-mean steric sea level changes has not been well understood. Using a data-constrained ocean state estimate, global-mean steric sea level change over 1993–2003 is revisited, and contributions from ocean transports and surface exchanges are quantified using closed potential temperature and salinity budgets. Analyses demonstrate that estimated decadal global-mean steric sea level change results mainly from a slight, time-mean imbalance between atmospheric forcing and ocean transports over the integration period: surface heat and freshwater exchanges produce a trend in global-mean steric sea level that is mainly offset by the redistribution of potential temperature and salinity through small-scale diffusion and large-sc...

Annual Cycle in Southern Tropical Indian Ocean Bottom Pressure

AbstractThe seasonal monsoon drives a dynamic response in the southern tropical Indian Ocean, previously observed in baroclinic Rossby wave signatures in annual sea level and thermocline depth anomalies. In this paper, monthly mass grids based on Release-05 Gravity Recovery and Climate Experiment (GRACE) data are used to study the annual cycle in southern tropical Indian Ocean bottom pressure. To interpret the satellite data, a linear model of the ocean’s response to wind forcing—based on the theory of vertical normal modes and comprising baroclinic and barotropic components—is considered. The model is evaluated using stratification from an ocean atlas and winds from an atmospheric reanalysis. Good correspondence between model and data is found over the southern tropical Indian Ocean: the model explains 81% of the annual variance in the data on average between 10° and 25°S. Model solutions suggest that, while the annual baroclinic Rossby wave has a seafloor signature, the annual cycle in the deep sea gene...

Mechanisms underlying recent decadal changes in subpolar North Atlantic Ocean heat content

The subpolar North Atlantic (SPNA) is subject to strong decadal variability, with implications for surface climate and its predictability. In 2004–2005, SPNA decadal upper ocean and sea-surface temperature trends reversed from warming during 1994–2004 to cooling over 2005–2015. This recent decadal trend reversal in SPNA ocean heat content (OHC) is studied using a physically consistent, observationally constrained global ocean state estimate covering 1992–2015. The estimates physical consistency facilitates quantitative causal attribution of ocean variations. Closed heat budget diagnostics reveal that the SPNA OHC trend reversal is the result of heat advection by midlatitude ocean circulation. Kinematic decompositions reveal that changes in the deep and intermediate vertical overturning circulation cannot account for the trend reversal, but rather ocean heat transports by horizontal gyre circulations render the primary contributions. The shift in horizontal gyre advection reflects anomalous circulation acting on the mean temperature gradients. Maximum covariance analysis (MCA) reveals strong covariation between the anomalous horizontal gyre circulation and variations in the local wind stress curl, suggestive of a Sverdrup response. Results have implications for decadal predictability.

Vertical Structure of Ocean Pressure Variations with Application to Satellite-Gravimetric Observations

AbstractThe nature of ocean bottom pressure () variability is considered on large spatial scales and long temporal scales. Monthly gridded estimates from the Gravity Recovery and Climate Experiment (GRACE) Release-05 and the new version 4 bidecadal ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO) are used. Estimates of from GRACE and ECCO are generally in good agreement, providing an independent measure of the quality of both products. Diagnostic fields from the state estimate are used to compute barotropic (depth independent) and baroclinic (depth dependent) components. The relative roles of baroclinic and barotropic processes are found to vary with latitude and time scale: variations in at higher latitudes and shorter periods are affected by barotropic processes, whereas fluctuations at lower latitudes and longer periods can be influenced by baroclinic effects, broadly consistent with theoretical scaling arguments. Wind-driven Rossby waves and couplin...

Buoyancy-Driven Interannual Sea Level Changes in the Tropical South Atlantic

AbstractLinear models of dynamical ocean adjustment to wind field changes, local atmospheric driving, and eastern boundary forcing are often invoked to explain observed patterns of interannual regional sea level variability. While skillful in some regions, these processes alone cannot explain low levels of interannual sea level variability observed in the tropical Atlantic. In this study, through a set of modeling approaches, interannual sea level changes in the tropical South Atlantic are attributed and the dynamical influence of buoyancy forcing is elucidated. Similar to recent findings in the southeast tropical Pacific, sea level patterns in the tropical South Atlantic (as estimated from a data-constrained ocean general circulation model) are found to result from the action of both surface wind and buoyancy forcing; in addition to static local effects, the buoyancy-driven changes comprise important nonlocal ocean dynamical processes. It is shown that the buoyancy-driven sea level changes can be underst...

Nonseasonal mass fluctuations in the midlatitude North Atlantic Ocean

Measurements from the Gravity Recovery and Climate Experiment reveal spatially coherent nonseasonal fluctuations in bottom pressure pb in the midlatitude North Atlantic Ocean. The spatial structure of the pb anomalies is centered on the North Atlantic Current along the intergyre region separating subtropical and subpolar ocean basins, and their temporal behavior is tied to indices of climate variability, namely, the North Atlantic Oscillation and the Arctic Oscillation. Correspondence to variations in wind stress curl suggests an interpretation of the midlatitude North Atlantic pb variability in terms of a barotropic Sverdrup balance. Anomalous mass in the midlatitude North Atlantic Ocean covaries with mass anomalies in abutting marginal seas, including the Mediterranean Sea and the Arctic Ocean as well as the North Sea and Hudson Bay. Results have implications for diagnosing ocean meridional heat transports due to depth-independent gyre circulation changes.