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Holte, J, Talley LD, Gilson J, Roemmich D.  2017.  An Argo mixed layer climatology and database. Geophysical Research Letters. 44:5618-5626.   10.1002/2017gl073426   AbstractWebsite

A global climatology and database of mixed layer properties are computed from nearly 1,250,000 Argo profiles. The climatology is calculated with both a hybrid algorithm for detecting the mixed layer depth (MLD) and a standard threshold method. The climatology provides accurate information about the depth, properties, extent, and seasonal patterns of global mixed layers. The individual profile results in the database can be used to construct time series of mixed layer properties in specific regions of interest. The climatology and database are available online at . The MLDs calculated by the hybrid algorithm are shallower and generally more accurate than those of the threshold method, particularly in regions of deep winter mixed layers; the new climatology differs the most from existing mixed layer climatologies in these regions. Examples are presented from the Labrador and Irminger Seas, the Southern Ocean, and the North Atlantic Ocean near the Gulf Stream. In these regions the threshold method tends to overestimate winter MLDs by approximately 10% compared to the algorithm.

Roemmich, D, Gilson J, Sutton P, Zilberman N.  2016.  Multidecadal change of the South Pacific gyre circulation. Journal of Physical Oceanography. 46:1871-1883.   10.1175/jpo-d-15-0237.1   AbstractWebsite

Multidecadal trends in ocean heat and freshwater content are well documented, but much less evidence exists of long-term changes in ocean circulation. Previously, a 12-yr increase, 1993 to 2004, in the circulation of the South Pacific Subtropical Gyre interior was described. That analysis was based on differences between early Argo and 1990s hydrographic data and changes in sea surface height. Here, it is shown that the trend of increasing circulation continues through 2014, with some differences within the Argo decade (2005 to 2014). Patterns that indicate or are consistent with increasing equatorward transport in the eastern portion of the South Pacific Gyre are seen in Argo temperature and steric height, Argo trajectory velocity, altimetric sea surface height, sea surface temperature, sea level pressure, and wind stress. Between 2005 and 2014 the geostrophic circulation across 35 degrees S, from 160 degrees W to South America, was enhanced by 5 Sv (1 Sv 10(6) m(3) s(-1)) of added northward flow. This was countered by a southward transport anomaly between the date line and 160 degrees W. Corresponding temperature trends span the full 2000-m depth range of Argo observations. The 22-yr trend, 1993 to 2014, in sea surface height at 35 degrees S, 160 degrees W is 8 cm decade(-1). Trends in sea surface temperature over 34 yr, 1981 to 2014, show a similar spatial pattern to that of sea surface height, with an increase of 0.5 degrees C decade(-1) at 35 degrees S, 160 degrees W. These multidecadal trends support the interpretation of the 40 degrees S maximum in global ocean heat gain as resulting from anomalous wind forcing and Ekman convergence.

Gasparin, F, Roemmich D, Gilson J, Cornuelle B.  2015.  Assessment of the upper-ocean observing system in the equatorial Pacific: The role of Argo in resolving intraseasonal to interannual variability*. Journal of Atmospheric and Oceanic Technology. 32:1668-1688.   10.1175/jtech-d-14-00218.1   AbstractWebsite

Using more than 10 years of Argo temperature and salinity profiles (2004-14), a new optimal interpolation (OI) of the upper ocean in the equatorial Pacific is presented. Following Roemmich and Gilson's procedures, which were formulated for describing monthly large-scale anomalies, here every 5 days anomaly fields are constructed with improvements in the OI spatial covariance function and by including the time domain. The comparison of Argo maps with independent observations, from the TAO/TRITON array, and with satellite sea surface height (SSH), demonstrates that Argo is able to represent around 70%-80% of the variance at intraseasonal time scales (periods of 20-100 days) and more than 90% of the variance for the seasonal-to-longer-term variability. The RMS difference between Argo and TAO/TRITON temperatures is lower than 1 degrees C and is around 1.5 cm when the Argo steric height is compared to SSH. This study also assesses the efficacy of different observing system components and combinations, such as SSH, TAO/TRITON, and Argo, for estimating subsurface temperature. Salinity investigations demonstrate its critical importance for density near the surface in the western Pacific. Objective error estimates from the OI are used to evaluate different sampling strategies, such as the recent deployment of 41 Argo floats along the Pacific equator. Argo's high spatial resolution compared with that of the moored array makes it better suited for studying spatial patterns of variability and propagation on intraseasonal and longer periods, but it is less well suited for studying variability on periods shorter than 20 days at point locations. This work is a step toward better utilization of existing datasets, including Argo, and toward redesigning the Tropical Pacific Observing System.

Roemmich, D, Church J, Gilson J, Monselesan D, Sutton P, Wijffels S.  2015.  Unabated planetary warming and its ocean structure since 2006. Nature Climate Change. 5:240-245.   10.1038/nclimate2513   AbstractWebsite

Increasing heat content of the global ocean dominates the energy imbalance in the climate system(1). Here we show that ocean heat gain over the 0-2,000 m layer continued at a rate of 0.4-0.6 W m(-2) during 2006-2013. The depth dependence and spatial structure of temperature changes are described on the basis of the Argo Program's(2) accurate and spatially homogeneous data set, through comparison of three Argo-only analyses. Heat gain was divided equally between upper ocean, 0-500 m and 500-2,000 m components. Surface temperature and upper 100 m heat content tracked interannual El Nino/Southern Oscillation fluctuations(3), but were offset by opposing variability from 100-500 m. The net 0-500 m global average temperature warmed by 0.005 degrees C yr(-1). Between 500 and 2,000m steadier warming averaged 0.002 degrees C yr(-1) with a broad intermediate-depth maximum between 700 and 1,400 m. Most of the heat gain (67 to 98%) occurred in the Southern Hemisphere extratropical ocean. Although this hemispheric asymmetry is consistent with inhomogeneity of radiative forcing(4) and the greater area of the Southern Hemisphere ocean, ocean dynamics also influence regional patterns of heat gain.

Abraham, JP, Baringer M, Bindoff NL, Boyer T, Cheng LJ, Church JA, Conroy JL, Domingues CM, Fasullo JT, Gilson J, Goni G, Good SA, Gorman JM, Gouretski V, Ishii M, Johnson GC, Kizu S, Lyman JM, Macdonald AM, Minkowycz WJ, Moffitt SE, Palmer MD, Piola AR, Reseghetti F, Schuckmann K, Trenberth KE, Velicogna I, Willis JK.  2013.  A review of global ocean temperature observations: Implications for ocean heat content estimates and climate change. Reviews of Geophysics. 51:450-483.   10.1002/rog.20022   AbstractWebsite

The evolution of ocean temperature measurement systems is presented with a focus on the development and accuracy of two critical devices in use today (expendable bathythermographs and conductivity-temperature-depth instruments used on Argo floats). A detailed discussion of the accuracy of these devices and a projection of the future of ocean temperature measurements are provided. The accuracy of ocean temperature measurements is discussed in detail in the context of ocean heat content, Earth's energy imbalance, and thermosteric sea level rise. Up-to-date estimates are provided for these three important quantities. The total energy imbalance at the top of atmosphere is best assessed by taking an inventory of changes in energy storage. The main storage is in the ocean, the latest values of which are presented. Furthermore, despite differences in measurement methods and analysis techniques, multiple studies show that there has been a multidecadal increase in the heat content of both the upper and deep ocean regions, which reflects the impact of anthropogenic warming. With respect to sea level rise, mutually reinforcing information from tide gauges and radar altimetry shows that presently, sea level is rising at approximately 3mmyr(-1) with contributions from both thermal expansion and mass accumulation from ice melt. The latest data for thermal expansion sea level rise are included here and analyzed.

Roemmich, D, Gould WJ, Gilson J.  2012.  135 years of global ocean warming between the Challenger expedition and the Argo Programme. Nature Climate Change. 2:425-428.   10.1038/nclimate1461   AbstractWebsite

Changing temperature throughout the oceans is a key indicator of climate change. Since the 1960s about 90% of the excess heat added to the Earth's climate system has been stored in the oceans(1,2). The ocean's dominant role over the atmosphere, land, or cryosphere comes from its high heat capacity and ability to remove heat from the sea surface by currents and mixing. The longest interval over which instrumental records of subsurface global-scale temperature can be compared is the 135 years between the voyage of HMS Challenger(3) (1872-1876) and the modern data set of the Argo Programme(4) (2004-2010). Argo's unprecedented global coverage permits its comparison with any earlier measurements. This, the first global-scale comparison of Challenger and modern data, shows spatial mean warming at the surface of 0.59 degrees C +/- 0.12, consistent with previous estimates(5) of globally averaged sea surface temperature increase. Below the surface the mean warming decreases to 0.39 degrees C +/- 0.18 at 366m (200 fathoms) and 0.12 degrees C +/- 0.07 at 914 m (500 fathoms). The 0.33 degrees C +/- 0.14 average temperature difference from 0 to 700 m is twice the value observed globally in that depth range over the past 50 years(6), implying a centennial timescale for the present rate of global warming. Warming in the Atlantic Ocean is stronger than in the Pacific. Systematic errors in the Challenger data mean that these temperature changes are a lower bound on the actual values. This study underlines the scientific significance of the Challenger expedition and the modern Argo Programme and indicates that globally the oceans have been warming at least since the late-nineteenth or early-twentieth century.

Roemmich, D, Gilson J.  2011.  The global ocean imprint of ENSO. Geophysical Research Letters. 38   10.1029/2011gl047992   AbstractWebsite

The ENSO-related spatial patterns and global averages of ocean temperature, salinity, and steric height are estimated from over 7 years of Argo data, 2004-2011. Substantial extratropical variability is seen in all variables in addition to familiar tropical ENSO signals. Surface layer (0-100 dbar) and subsurface (100-500 dbar) temperature variations are both important in determining steric height and sea surface height patterns. For the two years prior to the 2009 El Nino, the upper 100 dbar of the ocean gained 3.3 x 10(22) J yr(-1) of heat, while the 100-500 dbar layer lost a similar amount. The ENSO-related vertical redistribution of globally-averaged heat content between surface and subsurface layers, occurring throughout the record, is due primarily to changes in the east-west tilting of the equatorial Pacific thermocline. The large temperature changes in the individual layers mask the smaller vertically-averaged temperature change, in which the ocean loses heat when the surface layer is anomalously warm and gains heat when the surface layer is cool. Citation: Roemmich, D., and J. Gilson (2011), The global ocean imprint of ENSO, Geophys. Res. Lett., 38, L13606, doi:10.1029/2011GL047992.

Auad, G, Roemmich D, Gilson J.  2011.  The California Current System in relation to the Northeast Pacific Ocean circulation. Progress in Oceanography. 91:576-592.   10.1016/j.pocean.2011.09.004   AbstractWebsite

The California Current System is described in its regional setting using two modern datasets. Argo provides a broadscale view of the entire eastern North Pacific Ocean for the period 2004-2010, and the High Resolution XBT Network includes transects from Honolulu to San Francisco (1991-2010) and to Los Angeles (2008-2010). Together these datasets describe a California Current of 500-800 km width extending along the coast from 43 degrees N to 23 degrees N. The mean southward transport of the California Current is about 5 Sv off Central and Southern California, with about 2.5 Sv of northward flow on its inshore side. Interannual variations are 50% or more of the mean transports. The salinity minimum in the core of the California Current is supplied by the North Pacific Current and by freshwater from the northern continental shelf and modified by alongshore geostrophic and across-shore Ekman advection as well as eddy fluxes and air-sea exchange. The heat and freshwater content of the California Current vary in response to the fluctuating strength of the alongshore geostrophic flow. On its offshore side, the California Current is influenced by North Pacific Intermediate Waters at its deepest levels and by Eastern Subtropical Mode Waters on shallower density surfaces. In total, the sources of the California Current, its alongshore advection, and its strong interactions with the inshore upwelling region and the offshore gyre interior combine to make this a rich and diverse ecosystem. The present work reviews previous contributions to the regional oceanography, and uses the new datasets to paint a spatially and temporally more comprehensive description than was possible previously. Published by Elsevier Ltd.

Church, J, Roemmich D, Domingues CM, Willis JK, White NJ, Gilson JE, Stammer D, Kohl A, Chambers DP, Landerer FW, Martozke J, Gregory JM, Suzuki T, Cazenave A, Le Traon PY.  2010.  Ocean temperature and salinity contributions to global and regional sea-level change. Understanding sea-level rise and variability. ( Church J, Woodworth PL, Aarup T, Wilson WS, Eds.).:143-176., Chichester, West Sussex; Hoboken, NJ: Wiley-Blackwell Abstract
Roemmich, D, Johnson GC, Riser S, Davis R, Gilson J, Owens WB, Garzoli SL, Schmid C, Ignaszewski M.  2009.  The Argo Program: Observing the global ocean with profiling floats. Oceanography. 22:34-43.   10.5670/oceanog.2009.36   AbstractWebsite

The Argo Program has created the first global array for observing the subsurface ocean. Argo arose from a compelling scientific need for climate-relevant ocean data; it was made possible by technology development and implemented through international collaboration. The float program and its data management system began with regional arrays in 1999, scaled up to global deployments by 2004, and achieved its target of 3000 active instruments in 2007. US Argo, supported by the National Oceanic and Atmospheric Administration and the Navy through the National Oceanographic Partnership Program, provides half of the floats in the international array, plus leadership in float technology, data management, data quality control, international coordination, and outreach. All Argo data are freely available without restriction, in real time and in research-quality forms. Uses of Argo data range from oceanographic research, climate research, and education, to operational applications in ocean data assimilation and seasonal-to-decadal prediction. Argo's value grows as its data accumulate and their applications are better understood. Continuing advances in profiling float and sensor technologies open many exciting possibilities for Argo's future, including expanding sampling into high latitudes and the deep ocean, improving near-surface sampling, and adding biogeochemical parameters.

Roemmich, D, Gilson J.  2009.  The 2004-2008 mean and annual cycle of temperature, salinity, and steric height in the global ocean from the Argo Program. Progress in Oceanography. 82:81-100.   10.1016/j.pocean.2009.03.004   AbstractWebsite

The Argo Program has achieved 5 years of global coverage, growing from a very sparse global array of 1000 profiling floats in early 2004 to more than 3000 instruments from late 2007 to the present. Using nearly 350,000 temperature and salinity profiles, we constructed an upper-ocean climatology and monthly anomaly fields for the 5-year era, 2004-2008. A basic description of the modern upper ocean based entirely on Argo data is presented here, to provide a baseline for comparison with past datasets and with ongoing Argo data, to test the adequacy of Argo sampling of large-scale variability, and to examine the consistency of the Argo dataset with related ocean observations from other programs. The Argo 5-year mean is compared to the World Ocean Atlas, highlighting the middle and high latitudes of the southern hemisphere as a region of strong multi-decadal warming and freshening. Moreover the region is one where Argo data have contributed an enormous increment to historical sampling, and where more Argo floats are needed for documenting large-scale variability. Globally, the Argo-era ocean is warmer than the historical climatology at nearly all depths, by an increasing amount toward the sea surface; it is saltier in the surface layer and fresher at intermediate levels. Annual cycles in temperature and salinity are compared, again to WOA01, and to the National Oceanography Center air-sea flux climatology, the Reynolds SST product, and AVISO satellite altimetric height. These products are consistent with Argo data on hemispheric and global scales, but show regional differences that may either point to systematic errors in the datasets or their syntheses, to physical processes, or to temporal variability. The present work is viewed as an initial step toward integrating Argo and other climate-relevant global ocean datasets. (C) 2009 Elsevier Ltd. All rights reserved.

Willis, JK, Lyman JM, Johnson GC, Gilson J.  2009.  In situ data biases and recent ocean heat content variability. Journal of Atmospheric and Oceanic Technology. 26:846-852.   10.1175/2008jtecho608.1   AbstractWebsite

Two significant instrument biases have been identified in the in situ profile data used to estimate globally integrated upper-ocean heat content. A large cold bias was discovered in a small fraction of Argo floats along with a smaller but more prevalent warm bias in expendable bathythermograph (XBT) data. These biases appear to have caused the bulk of the upper-ocean cooling signal reported by Lyman et al. between 2003 and 2005. These systematic data errors are significantly larger than sampling errors in recent years and are the dominant sources of error in recent estimates of globally integrated upper-ocean heat content variability. The bias in the XBT data is found to be consistent with errors in the fall-rate equations, suggesting a physical explanation for that bias. With biased profiles discarded, no significant warming or cooling is observed in upper-ocean heat content between 2003 and 2006.

Roemmich, D, Gilson J, Davis R, Sutton P, Wijffels S, Riser S.  2007.  Decadal spinup of the South Pacific subtropical gyre. Journal of Physical Oceanography. 37:162-173.   10.1175/jpo3004.1   AbstractWebsite

An increase in the circulation of the South Pacific Ocean subtropical gyre, extending from the sea surface to middepth, is observed over 12 years. Datasets used to quantify the decadal gyre spinup include satellite altimetric height, the World Ocean Circulation Experiment ( WOCE) hydrographic and float survey of the South Pacific, a repeated hydrographic transect along 170 W, and profiling float data from the global Argo array. The signal in sea surface height is a 12-cm increase between 1993 and 2004, on large spatial scale centered at about 40 S, 170 W. The subsurface datasets show that this signal is predominantly due to density variations in the water column, that is, to deepening of isopycnal surfaces, extending to depths of at least 1800 m. The maximum increase in dynamic height is collocated with the deep center of the subtropical gyre, and the signal represents an increase in the total counterclockwise geostrophic circulation of the gyre, by at least 20% at 1000 m. A comparison of WOCE and Argo float trajectories at 1000 m confirms the gyre spinup during the 1990s. The signals in sea surface height, dynamic height, and velocity all peaked around 2003 and subsequently began to decline. The 1990s increase in wind-driven circulation resulted from decadal intensification of wind stress curl east of New Zealand - variability associated with an increase in the atmosphere's Southern Hemisphere annular mode. It is suggested ( based on altimetric height) that midlatitude gyres in all of the oceans have been affected by variability in the atmospheric annular modes on decadal time scales.

Willis, JK, Lyman JM, Johnson GC, Gilson J.  2007.  Correction to “Recent cooling of the upper ocean”. Geophysical Research Letters. 34:L16601.   10.1029/2007GL030323   AbstractWebsite
Roemmich, D, Gilson J, Willis J, Sutton P, Ridgway K.  2005.  Closing the time-varying mass and heat budgets for large ocean areas: The Tasman Box. Journal of Climate. 18:2330-2343.   10.1175/jcli3409.1   AbstractWebsite

The role of oceanic advection in seasonal-to-interannual balances of mass and heat is studied using a 12-yr time series of quarterly eddy-resolving expendable bathythermograph (XBT) surveys around the perimeter of a region the authors call the Tasman Box in the southwestern Pacific. The region contains the South Pacific's subtropical western boundary current system and associated strong mesoscale variability. Mean geostrophic transport in the warm upper ocean (temperature greater than 12 degrees C) is about 3.8 Sv (1 Sv equivalent to 10(6) m(3) s(-1)) southward into the box across the Brisbane, Australia-Fiji northern edge. Net outflows are 3.3 Sv eastward across the Auckland, New Zealand-Fiji edge, and 2.7 Sv southward across Sydney, Australia-Wellington, New Zealand. Mean Ekman convergence of 2.2 Sv closes the mass budget. Net water mass conversions in the upper ocean consist of inflow of waters averaging about 26 degrees C and 35.4 psu balanced by outflow at about 18 degrees C and 35.7 psu, and reflect the net evaporation and heat loss in the formation of South Pacific Subtropical Mode Water. The mean heat balance shows good agreement between ocean heat flux convergence (42.3 W m(-2)), heat loss to the atmosphere from the NCEP-NCAR reanalysis (39.2 W m(-2)), and heat storage calculated from data in the box interior (1.3 W m(-2)). On interannual time scales, volume transport through the box ranges from about I to 9 Sv, with heat flux convergence ranging from about 20 to 60 W m(-2). An interannual balance in the heat budget of the warm layer is achieved to within about 10 W m(-2) (or 6 W m(-2) for the upper 100 m alone). Maxima in the advective heat flux convergence occurred in 1993, 1,997, and 1999-2000, and corresponded to maxima in air-sea heat loss. The evolution of surface-layer temperature in the region is the residual of nearly equal and opposing effects of ocean heat flux convergence and air-sea exchange. Hence, ocean circulation is a key element in the interannual heat budget of the air-sea climate system in the western boundary current region.

Gilson, J, Roemmich D.  2002.  Mean and temporal variability in Kuroshio geostrophic transport south of Taiwan (1993-2001). Journal of Oceanography. 58:183-195.   10.1023/a:1015841120927   AbstractWebsite

Observations of the Kuroshio south of Taiwan have been carried out on a quarterly basis since late 1992 as part of the basin-wide High Resolution expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) network. Mean geostrophic transport in the Kuroshio, 0-800 m, from 34 cruises is 22.0 Sv +/- 1.5, consistent with previous results from moorings and geostrophic calculations in the upstream Kuroshio region. The mean core of the current has speed about 90 cm (-1)(S) and is located close to Taiwan. At this location the Kuroshio appears to be confined mainly to the upper 700 m, and there is no evident tight recirculation of the current. Eddy variability is substantial, and large eddies can be seen propagating westward for thousands of kilometers in TOPEX/Poseidon altimetric data, impinging on the current and altering its structure and transport. The annual range in transport is about 8 Sv +/- 6, with maximum in summer. Interannual variability is about 12 Sv +/- 6, with transport maxima in 1995 and 2000 and a minimum in 1997-1998. Interannual variability in the upstream Kuroshio may be uncorrelated with that in the downstream region south of Japan, where the transport is much greater. Our quarterly sampling aliases high frequency variability of the current, and an improved boundary-current observation program would include more frequent transects and occasional deeper measurements.

Roemmich, D, Gilson J, Cornuelle B, Weller R.  2001.  Mean and time-varying meridional transport of heat at the tropical subtropical boundary of the North Pacific Ocean. Journal of Geophysical Research-Oceans. 106:8957-8970.   10.1029/1999jc000150   AbstractWebsite

Ocean heat transport near the tropical/subtropical boundary of the North Pacific during 1993-1999 is described, including its mean and time variability. Twenty-eight trans-Pacific high-resolution expendable bathythermograph (XBT)/expendable conductivity-temperature-depth (XCTD) transects are used together with directly measured and operational wind estimates to calculate the geostrophic and Ekman transports. The mean heat transport across the XBT transect was 0.83 +/- 0.12 pW during the 7 year period. The large number of transects enables a stable estimate of the mean field to be made, with error bars based on the known variability. The North Pacific heat engine is a shallow meridional overturning circulation that includes warm Ekman and western boundary current components flowing northward, balanced by a southward flow of cool thermocline waters (including Subtropical Mode Waters). A near-balance of geostrophic and Ekman transports holds in an interannual sense as well as for the time mean. Interannual variability in geostrophic transport is strikingly similar to the pattern of central North Pacific sea level pressure variability (the North Pacific Index). The interannual range in heat transport was more than 0.4 pW during 1993-1999, with maximum northward values about 1 pW in early 1994 and early 1997. The ocean heat transport time series is similar to that of European Centre for Medium-Range Weather Forecasts air-sea heat flux integrated over the Pacific north of the XBT line. The repeating nature of the XBT/XCTD transects, with direct wind measurements, allows a substantial improvement over previous heat transport estimates based on one-time transects. A global system is envisioned for observing the time-varying ocean heat transport and its role in the Earth's heat budget and climate system.

Roemmich, D, Gilson J.  2001.  Eddy transport of heat and thermocline waters in the North Pacific: A key to interannual/decadal climate variability? Journal of Physical Oceanography. 31:675-687.   10.1175/1520-0485(2001)031<0675:etohat>;2   AbstractWebsite

High-resolution XBT transects in the North Pacific Ocean, at an average latitude of 22 degreesN, are analyzed together with TOPEX/Poseidon altimetric data to determine the structure and transport characteristics of the mesoscale eddy field. Based on anomalies in dynamic height, 410 eddies are identified in 30 transects from 1991 to 1999, including eddies seen in multiple transects over a year or longer. Their wavelength is typically 500 km, with peak-to-trough temperature difference of 2.2 degreesC in the center of the thermocline. The features slant westward with decreasing depth, by 0.8 degrees of longitude on average from 400 m up to the sea surface. This tilt produces a depth-varying velocity/temperature correlation and hence a vertical meridional overturning circulation. In the mean, 3.9 Sv (Sv equivalent to 10(6) m(3) s(-1)) of thermocline waters are carried southward by the eddy field over the width of the basin, balanced mainly by northward flow in the surface layer. Corresponding northward heat transport is 0.086 +/- 0.012 pW. The eddy field has considerable variability on seasonal to interannual timescales. For the 8-yr period studied here, eddy variability was the dominant mechanism for interannual change in the equatorward transport of thermocline waters, suggesting a potentially important forcing mechanism in the coupled air-sea climate system.

Gilson, J, Roemmich D, Cornuelle B, Fu LL.  1998.  Relationship of TOPEX/Poseidon altimetric height to steric height and circulation in the North Pacific. Journal of Geophysical Research-Oceans. 103:27947-27965.   10.1029/98jc01680   AbstractWebsite

TOPEX/Poseidon altimetric height is compared with 20 transpacific eddy-resolving realizations of steric height. The latter are calculated from temperature (expendable bathythermograph (XBT)) and salinity (expendable conductivity and temperature profiler (XCTD)) profiles along a precisely repeating ship track over a period of 5 years. The overall difference between steric height and altimetric height is 5.2 cm RMS. On long wavelengths (lambda < 500 km), the 3.5 cm RMS difference is due mainly to altimetric measurement errors but also has a component from steric variability deeper than the 800 m limit of the XBT. The data sets are very coherent in the long wavelength band, with coherence amplitude of 0.89. This band contains 65% of the total variance in steric height. On short wavelengths (lambda > 500 km), containing 17% of the steric height variance, the 3.0 cm RMS difference and lowered coherence are due to the sparse distribution of altimeter ground tracks along the XBT section. The 2.4 cm RMS difference in the basin-wide spatial mean appears to be due to fluctuations in bottom pressure. Differences between steric height and altimetric height increase near the western boundary, but data variance increases even more, and so the signal-to-noise ratio is highest in the western quarter of the transect. Basin-wide integrals of surface geostrophic transport from steric height and altimetric height are in reasonable agreement. The 1.9 x 10(4) m(2) s(-1) RMS difference is mainly because the interpolated altimetric height lacks spatial resolution across the narrow western boundary current. A linear regression is used to demonstrate the estimation of subsurface temperature from altimetric data. Errors diminish from 0.8 degrees C at 200 m to 0.3 degrees C at 400 m. Geostrophic volume transport, 0-800 m, shows agreement that is similar to surface transport, with 4.8 Sverdrup (Sv) (10(6) m(3) s(-1)) RMS difference. The combination of altimetric height with subsurface temperature and salinity profiling is a powerful tool for observing variability in circulation and transport of the upper ocean. The continuing need for appropriate subsurface data for verification and for statistical estimation is emphasized. This includes salinity measurements, which significantly reduce errors in specific volume and steric height.