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Chris W. Hughes - One of the best experts on this subject based on the ideXlab platform.

  • Detecting trends in Bottom Pressure measured using a tall mooring and altimetry
    Journal of Geophysical Research: Oceans, 2015
    Co-Authors: Joanne Williams, Chris W. Hughes, Mark E. Tamisiea
    Abstract:

    Stable, accurate measurements of ocean Bottom Pressure would be valuable for a range of purposes, including ocean circulation monitoring and measurement of the mass component of the changing sea level budget. Geographic variability of Bottom Pressure is in general smaller than variability of sea level, particularly at equatorial sites. However existing Bottom Pressure recorder technology suffers from drift of several cm/yr, too much for practical realization of these purposes. Therefore we investigate the use of a tall hydrographic mooring to detect trends in ocean Bottom Pressure, using data from the Rapid experiment in the North Atlantic. The accuracy of the method is dependent on the number of instruments on the mooring, and we demonstrate how an ocean model (in our case NEMO) can be used to provide an estimate of accuracy of this technique and hence guide mooring design. We also show how it is also dependent on the operational calibration of instruments. We find that, together with altimetry and sea-surface temperatures, such a mooring can be used to provide Bottom Pressure variations to within about 1 mbar (1 cm sea-level). We estimate that an optimally calibrated mooring in the North Atlantic could detect a trend in Bottom Pressure to an accuracy of ±1 mm/year after approximately 12 years of operation.

  • Boundary wave communication of Bottom Pressure and overturning changes for the North Atlantic
    Journal of Geophysical Research, 2008
    Co-Authors: Vassil Roussenov, Chris W. Hughes, Richard G. Williams, Rory J. Bingham
    Abstract:

    [1] The relationship between changes in sea-surface height, Bottom Pressure, and overturning is explored using isopycnal model experiments for the North Atlantic. Changes in high-latitude forcing are communicated rapidly over the basin through boundary wave propagation along the continental slope, involving a hybrid mixture of Kelvin and topographic Rossby waves, as well as spreading more slowly through advection along the western boundary. This wave communication leads to coherent signals in sea-surface height and Bottom Pressure variability extending for several thousand kilometers along the continental slope. The model results are in broad agreement with altimetric diagnostics, and the patterns only alter in detail with the realism of the topography. The adjustment in Bottom Pressure is directly linked to a change in overturning since west-east contrasts in Bottom Pressure are associated with a zonal integral in the meridional geostrophic flow. Correlation patterns reveal that temporal changes in overturning are primarily connected to the vertical contrast in Bottom Pressure, across the shelf and continental slope, along the western boundary.

  • The relationship between sea‐level and Bottom Pressure variability in an eddy permitting ocean model
    Geophysical Research Letters, 2008
    Co-Authors: Rory J. Bingham, Chris W. Hughes
    Abstract:

    We investigate the relationship between sea-level (after application of an inverse-barometer correction) and ocean Bottom Pressure, in an eddy-permitting ocean model. We find the presence of eddies can disrupt this relationship even on timescales as short as 10–20 days, but only in the regions of most energetic eddy variability. Away from eddies, the relationship is similar to that seen in a coarserresolution model, with a tight relationship between sea-level and Bottom Pressure at high frequencies, but with significant correlations between sea-level and Bottom Pressure at interannual timescales seen only in shelf sea regions. In the deep ocean, regions where sea-level and Bottom Pressure remain related out to the longest timescales are in the Arctic Ocean and regions of the Southern Ocean, where particularly large amplitude barotropic fluctuations are found but where the mesoscale signal is weak

  • the relationship between sea level and Bottom Pressure variability in an eddy permitting ocean model
    Geophysical Research Letters, 2008
    Co-Authors: Rory J. Bingham, Chris W. Hughes
    Abstract:

    We investigate the relationship between sea-level (after application of an inverse-barometer correction) and ocean Bottom Pressure, in an eddy-permitting ocean model. We find the presence of eddies can disrupt this relationship even on timescales as short as 10–20 days, but only in the regions of most energetic eddy variability. Away from eddies, the relationship is similar to that seen in a coarserresolution model, with a tight relationship between sea-level and Bottom Pressure at high frequencies, but with significant correlations between sea-level and Bottom Pressure at interannual timescales seen only in shelf sea regions. In the deep ocean, regions where sea-level and Bottom Pressure remain related out to the longest timescales are in the Arctic Ocean and regions of the Southern Ocean, where particularly large amplitude barotropic fluctuations are found but where the mesoscale signal is weak

  • Three forms of variability in Argentine Basin ocean Bottom Pressure
    Journal of Geophysical Research, 2007
    Co-Authors: Chris W. Hughes, Vladimir N. Stepanov, B. Barnier, G. W. Hargreaves
    Abstract:

    Ocean Bottom Pressure data are examined from a year-long deployment of two Bottom Pressure recorders, separated by 417 km, in the Argentine Basin. Three forms of variability are found. At high frequencies ( periods shorter than 2 hours) the signal appears to be due to gravity wave propagation with the waves preferentially coming from the west. At intermediate frequencies ( periods between 2 hours and 12 days), the two records are highly coherent and in phase, reflecting large-scale coherent fluctuations which are well reproduced in a barotropic ocean model. However, the dominant mode is at a period of 20 days, with a phase lag of about 90 - 100 degrees between the two records, consistent with the a mode of variability previously inferred from satellite altimetry and current meters, although the slightly shorter period is consistent with the suggestion that more than one mode contributes to the altimeter observations. Comparison with altimetry demonstrates that aliasing of higher-frequency signals is not a significant source of error and confirms that a previously used altimetry mapping technique reduces the apparent amplitude of the mode. The Pressure records show a transport change of about 280 Sv over one 12-day period, and altimetry suggests that regions of higher variability exist. The mode is only weakly excited in barotropic models but is strongly excited in a baroclinic model with a realistic eddy field, suggesting that it is intrinsically linked to interactions between eddies, mean flow, and topography

Rui M. Ponte - One of the best experts on this subject based on the ideXlab platform.

  • Interannual Bottom Pressure Signals in the Australian–Antarctic and Bellingshausen Basins
    Journal of Physical Oceanography, 2014
    Co-Authors: Rui M. Ponte, Christopher G. Piecuch
    Abstract:

    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...

  • satellite derived interannual ocean Bottom Pressure variability and its relation to sea level
    Geophysical Research Letters, 2013
    Co-Authors: Christopher G. Piecuch, Katherine J. Quinn, Rui M. Ponte
    Abstract:

    [1] Knowledge of the relationship between Bottom Pressure pb and sea level ζ is important for understanding ocean circulation and climate. We use recent Gravity Recovery and Climate Experiment (GRACE) Release-05 data along with altimetry to investigate the relationship between ζ and pb over long periods (>1 year) and large scales (>750 km). Elevated pb signals are observed over deep extratropical regions (e.g., Southern Ocean basins) and shallow or semi-enclosed areas (e.g., Indonesian and Nordic seas). In these places, considerable ζ variance is explained by pb variance. Correlation between ζ and pb is significant in many regions, including instances of significant negative correlation suggestive of active baroclinic processes. Results exemplify the good quality of GRACE Release-05 data and demonstrate that contemporary regional ζ variability cannot always be interpreted in terms of steric changes alone.

  • Estimating high frequency ocean Bottom Pressure variability
    Geophysical Research Letters, 2011
    Co-Authors: Katherine J. Quinn, Rui M. Ponte
    Abstract:

    [1] Knowledge of variability in ocean Bottom Pressure (pb) at periods < 60 days is essential for minimizing aliasing in satellite gravity missions. We assess how well we know such rapid, non-tidal pb signals by analyzing in-situ Bottom Pressure recorder (BPR) data and available global estimates from two very different modeling approaches. Estimated pb variance is generally lower than that measured by the BPRs, implying the presence of correlated model errors. Deriving uncertainties from differencing the model estimates can thus severely underestimate the aliasing errors. Removing estimated series from BPR data tends to reduce the variance by up to ∼5 cm2 but residual variance is still ∼5–20 cm2 and not negligible relative to expected variance in climate pb signals. The residual pb variability can be correlated over hundreds of kilometers. Results indicate the need to improve estimates of rapid pb variability in order to minimize aliasing noise in current and future satellite-based pb observations.

  • Bottom Pressure changes around Antarctica and wind-driven meridional flows
    Geophysical Research Letters, 2009
    Co-Authors: Rui M. Ponte, Katherine J. Quinn
    Abstract:

    [1] Spatially-averaged Bottom Pressure anomalies near Antarctica (south of 60°S) calculated from GRACE data are well correlated with those produced by the ECCO project using least-squares optimization methods to fit an ocean model to most available data. Both GRACE and ECCO results indicate mass exchange primarily between the Southern Ocean and the Pacific and the importance of zonal wind stress to this exchange. The ECCO flow fields show that the near-surface meridional Ekman transport, directly driven by the zonal wind stress, is nearly balanced by return flows below the shallowest topography at 60°S (∼1300 m at this Drake Passage latitude), with the return transport being slightly lagged in time relative to the Ekman transport. Such time lags, which may result from geostrophic adjustment at depth, cause the small associated net transport across 60°S to be ∼90° out of phase with the wind. This in turn can explain why zonal wind stress and Bottom Pressure anomalies around Antarctica tend to be anticorrelated in both GRACE and ECCO results.

  • Bottom Pressure changes around Antarctica and wind-driven
    Geophysical Research Letters, 2009
    Co-Authors: Rui M. Ponte, Katherine J. Quinn
    Abstract:

    [1] Spatially-averaged Bottom Pressure anomalies near Antarctica (south of 60°S) calculated from GRACE data are well correlated with those produced by the ECCO project using least-squares optimization methods to fit an ocean model to most available data. Both GRACE and ECCO results indicate mass exchange primarily between the Southern Ocean and the Pacific and the importance of zonal wind stress to this exchange. The ECCO flow fields show that the near-surface meridional Ekman transport, directly driven by the zonal wind stress, is nearly balanced by return flows below the shallowest topography at 60°S (~1300m at this Drake Passage latitude), with the return transport being slightly lagged in time relative to the Ekman transport. Such time lags, which may result from geostrophic adjustment at depth, cause the small associated net transport across 60°S to be ~90° out of phase with the wind. This in turn can explain why zonal wind stress and Bottom Pressure anomalies around Antarctica tend to be anticorrelated in both GRACE and ECCO results.

Katherine J. Quinn - One of the best experts on this subject based on the ideXlab platform.

  • satellite derived interannual ocean Bottom Pressure variability and its relation to sea level
    Geophysical Research Letters, 2013
    Co-Authors: Christopher G. Piecuch, Katherine J. Quinn, Rui M. Ponte
    Abstract:

    [1] Knowledge of the relationship between Bottom Pressure pb and sea level ζ is important for understanding ocean circulation and climate. We use recent Gravity Recovery and Climate Experiment (GRACE) Release-05 data along with altimetry to investigate the relationship between ζ and pb over long periods (>1 year) and large scales (>750 km). Elevated pb signals are observed over deep extratropical regions (e.g., Southern Ocean basins) and shallow or semi-enclosed areas (e.g., Indonesian and Nordic seas). In these places, considerable ζ variance is explained by pb variance. Correlation between ζ and pb is significant in many regions, including instances of significant negative correlation suggestive of active baroclinic processes. Results exemplify the good quality of GRACE Release-05 data and demonstrate that contemporary regional ζ variability cannot always be interpreted in terms of steric changes alone.

  • Estimating high frequency ocean Bottom Pressure variability
    Geophysical Research Letters, 2011
    Co-Authors: Katherine J. Quinn, Rui M. Ponte
    Abstract:

    [1] Knowledge of variability in ocean Bottom Pressure (pb) at periods < 60 days is essential for minimizing aliasing in satellite gravity missions. We assess how well we know such rapid, non-tidal pb signals by analyzing in-situ Bottom Pressure recorder (BPR) data and available global estimates from two very different modeling approaches. Estimated pb variance is generally lower than that measured by the BPRs, implying the presence of correlated model errors. Deriving uncertainties from differencing the model estimates can thus severely underestimate the aliasing errors. Removing estimated series from BPR data tends to reduce the variance by up to ∼5 cm2 but residual variance is still ∼5–20 cm2 and not negligible relative to expected variance in climate pb signals. The residual pb variability can be correlated over hundreds of kilometers. Results indicate the need to improve estimates of rapid pb variability in order to minimize aliasing noise in current and future satellite-based pb observations.

  • Bottom Pressure changes around Antarctica and wind-driven meridional flows
    Geophysical Research Letters, 2009
    Co-Authors: Rui M. Ponte, Katherine J. Quinn
    Abstract:

    [1] Spatially-averaged Bottom Pressure anomalies near Antarctica (south of 60°S) calculated from GRACE data are well correlated with those produced by the ECCO project using least-squares optimization methods to fit an ocean model to most available data. Both GRACE and ECCO results indicate mass exchange primarily between the Southern Ocean and the Pacific and the importance of zonal wind stress to this exchange. The ECCO flow fields show that the near-surface meridional Ekman transport, directly driven by the zonal wind stress, is nearly balanced by return flows below the shallowest topography at 60°S (∼1300 m at this Drake Passage latitude), with the return transport being slightly lagged in time relative to the Ekman transport. Such time lags, which may result from geostrophic adjustment at depth, cause the small associated net transport across 60°S to be ∼90° out of phase with the wind. This in turn can explain why zonal wind stress and Bottom Pressure anomalies around Antarctica tend to be anticorrelated in both GRACE and ECCO results.

  • Bottom Pressure changes around Antarctica and wind-driven
    Geophysical Research Letters, 2009
    Co-Authors: Rui M. Ponte, Katherine J. Quinn
    Abstract:

    [1] Spatially-averaged Bottom Pressure anomalies near Antarctica (south of 60°S) calculated from GRACE data are well correlated with those produced by the ECCO project using least-squares optimization methods to fit an ocean model to most available data. Both GRACE and ECCO results indicate mass exchange primarily between the Southern Ocean and the Pacific and the importance of zonal wind stress to this exchange. The ECCO flow fields show that the near-surface meridional Ekman transport, directly driven by the zonal wind stress, is nearly balanced by return flows below the shallowest topography at 60°S (~1300m at this Drake Passage latitude), with the return transport being slightly lagged in time relative to the Ekman transport. Such time lags, which may result from geostrophic adjustment at depth, cause the small associated net transport across 60°S to be ~90° out of phase with the wind. This in turn can explain why zonal wind stress and Bottom Pressure anomalies around Antarctica tend to be anticorrelated in both GRACE and ECCO results.

Christopher G. Piecuch - One of the best experts on this subject based on the ideXlab platform.

  • Interannual Bottom Pressure Signals in the Australian–Antarctic and Bellingshausen Basins
    Journal of Physical Oceanography, 2014
    Co-Authors: Rui M. Ponte, Christopher G. Piecuch
    Abstract:

    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...

  • Dynamics of satellite-derived interannual ocean Bottom Pressure variability in the western tropical North Pacific
    Journal of Geophysical Research: Oceans, 2013
    Co-Authors: Christopher G. Piecuch
    Abstract:

    [1] Ocean Bottom Pressure variability, derived from Release-05 Gravity Recovery and Climate Experiment time-variable gravity coefficients over the ocean, is investigated along the tropical North Pacific for the case of long time scales (>1 yr) and large space scales (>750 km). To interpret the observations, a linear model of the Bottom Pressure response to interior wind stress curl is derived on the basis of normal vertical modes; the adjustment comprises contributions from barotropic Sverdrup dynamics as well as first baroclinic mode Rossby waves. Model solutions are evaluated numerically using time-mean ocean stratification from the Ocean Comprehensible Atlas and time-varying surface wind stress from the European Centre for Medium-Range Weather Forecasts ERA-Interim reanalysis. In the western tropical North Pacific, model and data compare favorably; simulated and observed time series are significantly correlated, and the model generally explains more than half of the data variance; the good correspondence between model and data speaks to the good quality of the satellite-derived fields. In the central and eastern tropical North Pacific, findings are more ambiguous; model and data time series are mostly not significantly correlated, and simulations generally explain less than half of the data variance; discrepancies between model and data could point to physics absent from the model, for example, signals generated at the eastern boundary. Results provide observational demonstration that baroclinic contributions to Bottom Pressure changes can be important at low latitudes and low frequencies; findings hint at a basin-scale influence of tropical climate modes on the ocean Bottom Pressure field.

  • satellite derived interannual ocean Bottom Pressure variability and its relation to sea level
    Geophysical Research Letters, 2013
    Co-Authors: Christopher G. Piecuch, Katherine J. Quinn, Rui M. Ponte
    Abstract:

    [1] Knowledge of the relationship between Bottom Pressure pb and sea level ζ is important for understanding ocean circulation and climate. We use recent Gravity Recovery and Climate Experiment (GRACE) Release-05 data along with altimetry to investigate the relationship between ζ and pb over long periods (>1 year) and large scales (>750 km). Elevated pb signals are observed over deep extratropical regions (e.g., Southern Ocean basins) and shallow or semi-enclosed areas (e.g., Indonesian and Nordic seas). In these places, considerable ζ variance is explained by pb variance. Correlation between ζ and pb is significant in many regions, including instances of significant negative correlation suggestive of active baroclinic processes. Results exemplify the good quality of GRACE Release-05 data and demonstrate that contemporary regional ζ variability cannot always be interpreted in terms of steric changes alone.

Detlef Stammer - One of the best experts on this subject based on the ideXlab platform.

  • Ocean Bottom Pressure variations estimated from gravity, nonsteric sea surface height and hydrodynamic model simulations
    Journal of Geophysical Research, 2011
    Co-Authors: Frank Siegismund, V. Romanova, Armin Köhl, Detlef Stammer
    Abstract:

    [1] Ocean Bottom Pressure variability is analyzed from three monthly products available from (1) the Gravity Recovery and Climate Experiment (GRACE), (2) sterically corrected altimetry, and (3) from a forward run of the German part of the Estimating the Circulation and Climate of the Ocean (GECCO-2) model. Results lead to an approximate error estimate for each of the ocean Bottom Pressure (OBP) maps under the assumption of noncorrelated errors among the three products. The estimated error maps are consistent with the misfits of individual fields against OBP sensor data, with the caveat that a general underestimation of the signal strength, as a common, correlated error in all products, cannot be recovered by the method. The signal-to-noise ratio (SNR) increases in all products, when a 3 month running mean filter is applied. Using this filter, we estimate globally averaged errors of 8.6, 11.1, and 5.7 mm of equivalent water height for GRACE, nonsteric altimetry, and GECCO2, respectively. Based on resulting uncertainties, a new OBP product is being produced by merging all three data sets. When validated with Bottom Pressure observations this new OBP product has a 20% increased SNR compared to the best individual product (GECCO2-ref). Estimated total ocean mass variations explain a considerable part of OBP variability with a SNR above 1 in most of the ocean. In some regions the nonuniform part is weaker than the estimated error. However, most dynamic ocean models are designed to reproduce only the nonuniform, dynamic, OBP variability, but do not accurately describe total mass variability.

  • Comparison of GRACE and Model-Based Estimates of Bottom Pressure Variations Against In Situ Bottom Pressure Measurements
    Advanced Technologies in Earth Sciences, 2010
    Co-Authors: Detlef Stammer, Armin Koehl, V. Romanova, Frank Siegismund
    Abstract:

    Results from several numerical ocean models were used in combination with available ocean Bottom Pressure data to quantitatively examine the skill of ocean models in simulating fast and slow Bottom Pressure variations and to test the quality of GRACE monthly fields of Bottom Pressure variations. The comparison between model simulations and Bottom Pressure data does reveal a substantial agreement between models and Pressure measurements on high frequencies, but also some clear differences on longer time scales (> 1 year) that need to be corrected in order to improve estimates of the barotropic circulation. We also find a good agreement between monthly GRACE solutions and ECCO/GECCO syntheses that encourage us now to use the GRACE fields as constraints in ocean syntheses efforts. Differences of the order of a few centimeters appear consistent with previously estimated uncertainties provided by Quinn and Ponte (2008). There appears to be a large potential for assimilating GRACE data into ocean circulation models and thereby correct the seasonally varying barotropic circulation in the models, but results also highlight remaining uncertainties in the GRACE data.

  • Relation between sea level and Bottom Pressure and the vertical dependence of oceanic variability
    Geophysical Research Letters, 2007
    Co-Authors: Nadya T. Vinogradova, Rui M. Ponte, Detlef Stammer
    Abstract:

    [1] The relation between large-scale sea level and Bottom Pressure variability is studied using long (50-yr) simulations of a general circulation model under realistic forcing. Admittance and coherence analyses are used to characterize the Bottom Pressure and sea level relationship as a function of period, horizontal spatial scale, and location. At the model grid scale (1°), Bottom Pressure is found to be essentially equivalent to sea level at periods 60°) and in shallow depths (

  • relation between sea level and Bottom Pressure and the vertical dependence of oceanic variability
    Geophysical Research Letters, 2007
    Co-Authors: Nadya T. Vinogradova, Rui M. Ponte, Detlef Stammer
    Abstract:

    [1] The relation between large-scale sea level and Bottom Pressure variability is studied using long (50-yr) simulations of a general circulation model under realistic forcing. Admittance and coherence analyses are used to characterize the Bottom Pressure and sea level relationship as a function of period, horizontal spatial scale, and location. At the model grid scale (1°), Bottom Pressure is found to be essentially equivalent to sea level at periods 60°) and in shallow depths (<200 m). Elsewhere, Bottom Pressure and sea level fields can differ significantly. Results indicate an increase of the importance of baroclinic signals with decreasing latitude and spatial scale, with significant baroclinic signals at intra-seasonal and longer periods present in many subtropical and mid-latitude regions. Variability is clearly baroclinic at inter-annual periods, regardless of location and spatial scale. Results have broad implications for the interpretation and processing of both satellite altimetry and gravity data and for their assimilation into numerical models.

  • Role of ocean currents and Bottom Pressure variability on seasonal polar motion
    Journal of Geophysical Research: Oceans, 1999
    Co-Authors: Rui M. Ponte, Detlef Stammer
    Abstract:

    Changes in the ocean angular momentum (OAM) components about the equatorial axes, either due to fluctuations in currents or Bottom Pressure (mass redistribution), can induce movements of the Earth's pole of rotation, commonly referred to as polar motion or wobble. Output from a 1° resolution ocean model is used to calculate the effective equatorial OAM functions χ1O and χ2O, corresponding to polar motion excitation about the equatorial axis pointing to the Greenwich and 90°E meridians, respectively. Time series of χO are combined with similar atmospheric series χA, computed from the National Centers for Environmental Prediction/National Center for Atmospheric Research reanalyses, to interpret the observed low-frequency polar motion excitation for the period 1985–1996. Results indicate that the oceans are a very important excitation source for the Chandler (∼433 days), annual, and semiannual wobbles, providing for much better amplitude and phase agreement with the observed excitation at these periods, in comparison with what is obtained when only the atmosphere is considered. Both oceanic mass and motion terms are found to be important but with mass signals having somewhat larger amplitudes. The role of regional variability in ocean currents and Bottom Pressure in contributing to χO signals is quantified. Midlatitude regions (∼30°–70°) figure prominently as places of strong local oceanic excitation signals. The North Pacific basin is found to be generally important for χ1O excitation, while the Southern Ocean is important for both χ1O and χ2O. The largest positive covariances of local with global χ1O signals occur in the Kuroshio region near the western boundary of the North Pacific for χ1O and southwest of Australia for χ2O.