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Zhang, XB, Cornuelle B, Roemmich D.  2011.  Adjoint sensitivity of the Nino-3 surface temperature to wind forcing. Journal of Climate. 24:4480-4493.   10.1175/2011jcli3917.1   AbstractWebsite

The evolution of sea surface temperature (SST) over the eastern equatorial Pacific plays a significant role in the intense tropical air-sea interaction there and is of central importance to the El Nino-Southern Oscillation (ENSO) phenomenon. Effects of atmospheric fields (especially wind stress) and ocean state on the eastern equatorial Pacific SST variations are investigated using the Massachusetts Institute of Technology general circulation model (MITgcm) and its adjoint model, which can calculate the sensitivities of a cost function (in this case the averaged 0-30-m temperature in the Nino-3 region during an ENSO event peak) to previous atmospheric forcing fields and ocean state going backward in time. The sensitivity of the Nino-3 surface temperature to monthly zonal wind stress in preceding months can be understood by invoking mixed layer heat balance, ocean dynamics, and especially linear equatorial wave dynamics. The maximum positive sensitivity of the Nino-3 surface temperature to local wind forcing usually happens similar to 1-2 months before the peak of the ENSO event and is hypothesized to be associated with the Ekman pumping mechanism. In model experiments, its magnitude is closely related to the subsurface vertical temperature gradient, exhibiting strong event-to-event differences with strong (weak) positive sensitivity during La Nina (strong El Nino) events. The adjoint sensitivity to remote wind forcing in the central and western equatorial Pacific is consistent with the standard hypothesis that the remote wind forcing affects the Nino-3 surface temperature indirectly by exciting equatorial Kelvin and Rossby waves and modulating thermocline depth in the Nino-3 region. The current adjoint sensitivity study is consistent with a previous regression-based sensitivity study derived from perturbation experiments. Finally, implication for ENSO monitoring and prediction is also discussed.

Sutton, P, Roemmich D.  2011.  Decadal steric and sea surface height changes in the Southern Hemisphere. Geophysical Research Letters. 38   10.1029/2011gl046802   AbstractWebsite

Sea surface height (SSH) changes result from changes in steric height (SH) and mass. We investigate total SH and mass from co-located measurements of SSH and SH in the upper 1500 dbar (SH(0-1500)). SSH changes are decomposed into SH(0-1500) and 'other' contributions, where 'other' includes SH changes below 1500 dbar and mass changes. This is done using satellite altimeter measurements of SSH available since late 1992 in combination with WOCE-era hydrography and Argo. A hemispheric analysis of co-located WOCE and Argo profiles gives robust Delta SH/DSSH relationships, varying with latitude. The Delta SH/Delta SSH ratio together with satellite SSH yields an estimate of decadal SH increase. It is found that similar to 0.5 of the hemispheric decadal SSH rise is steric, with this proportion increasing southwards. The relatively large rate of SSH increase south of 30 degrees S, the high proportion attributable to SH (i.e., ocean warming) and the great area of the southern ocean, mean the total heat gain south of 20 degrees S is comparable to estimates of global 0-700 m heat gain for this period. Citation: Sutton, P., and D. Roemmich (2011), Decadal steric and sea surface height changes in the Southern Hemisphere, Geophys. Res. Lett., 38, L08604, doi: 10.1029/2011GL046802.

Douglass, E, Roemmich D, Stammer D.  2010.  Interannual variability in North Pacific heat and freshwater budgets. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 57:1127-1140.   10.1016/j.dsr2.2010.01.001   AbstractWebsite

Transports of volume, heat, and freshwater in the North Pacific Ocean from 1992 to 2004 are analyzed using a long-term high resolution expendable bathythermography (XBT) dataset and output from a data-assimilating model. Estimates of geostrophic transport from the data are compared with the model transport to close the volume budget north of the trans-Pacific XBT track. Advective transport from both model output and data are combined with surface fluxes to determine budgets of heat and freshwater in the closed region. The northward heat transport across the XBT track is estimated to be 0.74 +/- 0.1 pW, and has variability of almost 0.5 pW on 3-4 year time scales, while freshwater transport is estimated to be -0.1 +/- 0.06 Sv. The balance between northward advective heat transport and surface heat flux gives a time-varying estimate of heat storage that compares well with observations. A similar balance is found between model estimates of advective freshwater transport and surface freshwater flux. Despite a scarcity of observations and uncertainties in all components, this analysis results in nearly closed budgets of volume, heat, and freshwater. Mean estimates of advective transport of both heat and freshwater agree with previous estimates. An analysis of each component of the heat budget with latitude indicates that a relative lack of time-variability of the surface component is consistent throughout the North Pacific. The dominant advective component is driven by changes in the wind stress curl field. For both heat and freshwater storage, strong signals occur concurrently at all latitudes. This behavior could indicate that these signals are controlled by large-scale dynamics, rather than small-scale disturbances from which signals would need to propagate to be widely felt. The analysis demonstrates the value of bringing models and data together, resulting in budgets that are consistent with observations, yet provide a comprehensive look at the variability of North Pacific heat and freshwater storage that would be unavailable from data alone. (C) 2010 Elsevier Ltd. All rights reserved.

Wilson, WS, Roemmich D, 22 co-authors.  2010.  Observing systems needed to address sea-level rise and variability. Understanding sea-level rise and variability. ( Church J, Woodworth P, Aarup T, Wilson S, Eds.).:376-401., Chichester, West Sussex; Hoboken, NJ: Wiley-Blackwell Abstract


Church, J, Roemmich D, 13 co-authors.  2010.  Ocean temperature and salinity contributions to global and regional sea-level change. Understanding sea-level rise and variability. ( Church J, Woodworth P, Aarup T, Wilson S, Eds.).:143-176., Chichester, West Sussex; Hoboken, NJ: Wiley-Blackwell Abstract
Roemmich, D, Argo Steering T.  2009.  Argo: the challenge of continuing 10 years of progress. Oceanography. 22:46-55.   10.5670/oceanog.2009.65   AbstractWebsite

In only 10 years, the Argo Program has grown from an idea into a functioning global observing system for the subsurface ocean. More than 3000 Argo floats now cover the world ocean. With these instruments operating on 10-day cycles, the array provides 9000 temperature/salinity/depth profiles every month that are quickly available via the Global Telecommunications System and the Internet. Argo is recognized as a major advance for oceanography, and a success for Argo's parent programs, the Global Ocean Data Assimilation Experiment and Climate Variability and Predictability, and for the Global Earth Observation System of Systems. The value of Argo data in ocean data assimilation (ODA) and other applications is being demonstrated, and will grow as the data set is extended in time and as experience in using the data set leads to new applications. The spatial coverage and quality of the Argo data set are improving, with consideration being given to sampling under seasonal ice at higher latitudes, in additional marginal seas, and to greater depths. Argo data products of value in ODA modeling are under development, and Argo data are being tested to confirm their consistency with related satellite and in situ data. Maintenance of the Argo Program for the next decade and longer is needed for a broad range of climate and oceanographic research and for many operational applications in ocean state estimation and prediction.

Le Traon, PY, Larnicol G, Guinehut S, Pouliquen S, Bentamy A, Roemmich D, Donlon C, Roquet H, Jacobs G, Griffin D, Bonjean F, Hoepffner N, Breivik LA.  2009.  Data assembly and processing for operational oceanography: 10 years of achievements. Oceanography. 22:56-69.   10.5670/oceanog.2009.66   AbstractWebsite

Data assembly and processing centers are essential elements of the operational oceanography infrastructure. They provide data and products needed by modeling and data assimilation systems; they also provide products directly usable for applications. This paper discusses the role and functions of the data centers for operational oceanography. It describes some of the main data assembly centers (Argo and in situ data, altimetry, sea surface temperature) developed during the Global Ocean Data Assimilation Experiment. An overview of other data centers (wind and fluxes, ocean color, sea ice) is also given. Much progress has been achieved over the past 10 years to validate, intercalibrate, and merge altimeter data from multiple satellites. Accuracy and timeliness of products have been improved, and new products have been developed. The same is true for sea surface temperature data through the Global High-Resolution Sea Surface Temperature Pilot Project. A breakthrough in processing, quality control, and assembly for in situ data has also been achieved through the development of the real-time and delayed-mode Argo data system. In situ and remote-sensing data are now systematically and jointly used to calibrate, validate, and monitor over the long term the quality and consistency of the global ocean observing system. Main results are illustrated. There is also a review of the development and use of products that merge in situ and remote-sensing data. Future issues and main prospects are discussed in the conclusion.

Douglass, E, Roemmich D, Stammer D.  2009.  Data Sensitivity of the ECCO State Estimate in a Regional Setting. Journal of Atmospheric and Oceanic Technology. 26:2420-2443.   10.1175/2009jtecho641.1   AbstractWebsite

The Estimating the Circulation and Climate of the Ocean (ECCO) consortium provides a framework in which the adjoint method of data assimilation is applied to a general circulation model to provide a dynamically self-consistent estimate of the time-varying ocean state, which is constrained by observations. In this study, the sensitivity of the solution to the constraints provided by various datasets is investigated in a regional setting in the North Pacific. Four assimilation experiments are performed, which vary by the data used as constraints and the relative weights associated with these data. The resulting estimates are compared to two of the assimilated datasets as well as to data from two time series stations not used as constraints. These comparisons demonstrate that increasing the weights of the subsurface data provides overall improvement in the model-data consistency of the estimate of the state of the North Pacific Ocean. However, some elements of the solution are degraded. This could result from incompatibility between datasets, possibly because of hidden biases, or from errors in the model physics made more evident by the increased weight on subsurface data. The adjustments to the control parameters of surface forcing and initial conditions necessary to obtain the more accurate fit to the data are found to be within prior error bars.

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.

Cai, W, Pan A, Roemmich D, Cowan T, Guo X.  2009.  Argo profiles a rare occurrence of three consecutive positive Indian Ocean Dipole events, 2006-2008. Geophysical Research Letters. 36   10.1029/2008gl037038   Abstract

During 2006-2008, the Indian Ocean (IO) experienced a rare realization of three consecutive positive IO Dipoles (pIODs), including an unusual occurrence with a La Nina in 2007. Common to all three pIODs is an early excitation of equatorial easterly anomalies. Argo profiles reveal that for the 2008 and 2006 pIODs the wind anomalies are generated by the following process: upwelling Rossby waves propagating into the western IO and their subsequent reflection as equatorial upwelling Kelvin waves enhance the seasonal upwelling, changing sea surface temperature (SST) gradients. For the 2007 pIOD, coastal upwelling Kelvin waves off the Sumatra-Java coast associated with the 2006 pIOD/El Nino, radiate into the IO as upwelling Rossby waves. They curve sharply equatorward to arrive at the central equatorial IO, inducing easterly anomalies, upwelling Kelvin waves, and the unusual pIOD. Our results suggest that real-time Argo observations, when assimilated into predictive systems, will enhance IOD forecasting skills.

Uehara, H, Kizu S, Hanawa K, Yoshikawa Y, Roemmich D.  2008.  Estimation of heat and freshwater transports in the North Pacific using high-resolution expendable bathythermograph data. Journal of Geophysical Research-Oceans. 113   10.1029/2007jc004165   AbstractWebsite

The mean heat and freshwater transports in the North Pacific subtropical gyre during 1998-2002 are estimated. High-resolution expendable bathythermograph/expendable conductivity-temperature-depth (XBT/XCTD) transects (PX-40, Honolulu to Japan; PX-37, San Francisco to Honolulu; PX-10, Honolulu to Guam; PX-44, Guam to Taiwan/Hong Kong) are used to calculate geostrophic transport across each of the ship tracks. Ekman transport is estimated from satellite-scatterometer wind stress. The mean heat and freshwater transport convergences into the northern box bounded by the PX-40/37 transects and the Tsushima and Bering Straits are 0.26 +/- 0.16 pW (pW = 10(15) W) and -0.26 +/- 0.11 Sv (Sv = 10(6) m(3)/s), respectively. Heat and freshwater transport convergences into the western box bounded by the PX-40/10/44 transects and the Tsushima Strait are estimated to be 0.32 +/- 0.17 pW and 0.08 +/- 0.07 Sv, respectively. In both boxes, warmer waters transported inward by the Ekman flow and by the Kuroshio are compensated by the export of waters at cooler temperatures, whose peaks are found in the temperatures of the mode waters formed in the North Pacific. The salt budget is also described to consider the mechanisms of freshwater transport. Since the western box includes the region with the strongest heat loss to the atmosphere and is possibly a key region for climatic decadal variation, it is necessary to continue the high-resolution XBT/XCTD measurement and to make an effort at improving the estimation of heat and freshwater transports in order to contribute to advancing climate studies.

Ueno, H, Oka E, Suga T, Onishi H, Roemmich D.  2007.  Formation and variation of temperature inversions in the eastern subarctic North Pacific. Geophysical Research Letters. 34   10.1029/2006gl028715   AbstractWebsite

Hydrographic data from profiling float observations for 2001 - 2005 and from expendable bathythermograph observations for 1993 - 2005 were analyzed to study the formation and variation of temperature inversions (T-inversions) in the eastern subarctic North Pacific ( SNP). The formation and variation of T-inversions differed significantly between the northern and southern regions of the eastern SNP. In the northern region, the temperature minimum (T-min) at the top of T-inversions outcropped to the sea surface and was cooled in the mixed layer nearly every winter. This process caused a seasonal cycle in the magnitude of T-inversions (DT), with a maximum in winter. In the southern region, the winter T-min outcropped relatively infrequently and the DT did not exhibit a significant maximum in winter during most years. The T-min in the southern region was likely to outcrop upstream near the date line roughly one year earlier and was then advected to the southern region.

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.

Maes, C, Ando K, Delcroix T, Kessler WS, McPhaden MJ, Roemmich D.  2006.  Observed correlation of surface salinity, temperature and barrier layer at the eastern edge of the western Pacific warm pool. Geophysical Research Letters. 33   10.1029/2005gl024772   AbstractWebsite

Recent theory suggests that ocean-atmosphere interactions in the western Pacific warm pool are of fundamental importance to interannual variations associated with El Nino and the Southern Oscillation (ENSO). The warm pool encompasses the highest mean sea surface temperatures (SSTs) in the world ocean, intense atmospheric deep convection and heavy rainfall, and the formation of thick salt-stratified barrier layers that help to sustain the high SSTs. This study shows that the eastern edge of the warm pool is characterized by a strong zonal salinity front throughout 2002-2004. The analysis also indicates a tighter empirical relationship than previously observed between the eastern edge of the warm pool, high SSTs, the presence of barrier layers, and the fetch of westerly wind bursts. These results suggest that such a frontal region is a critical in controlling ocean-atmosphere interactions in the western Pacific warm pool and highlight the importance of the upper ocean salinity in climate variability.

Bowen, MM, Sutton PJH, Roemmich D.  2006.  Wind-driven and steric fluctuations of sea surface height in the southwest Pacific. Geophysical Research Letters. 33   10.1029/2006gl026160   AbstractWebsite

Large fluctuations in sea surface height (SSH) occurred in the southwest Pacific between New Zealand and Fiji in the late 1990s. A model of SSH including steric heating and wind-driven Rossby waves explains more than 40% of the observed SSH variance in the region over the 12 years of satellite measurements. The modelled SSH also agrees with dynamic height calculated from subsurface temperatures measured along an expendable bathythermograph (XBT) line between New Zealand and Fiji. The model simulations show a large high in SSH was created by anomalous downward Ekman pumping during 1998 when the seasonal change to upwelling failed to occur. The 50-year wind record shows other downwelling events have occurred in the region predominantly during El Nino conditions.

Douglass, E, Roemmich D, Stammer D.  2006.  Interannual variability in northeast pacific circulation. Journal of Geophysical Research-Oceans. 111   10.1029/2005jc003015   AbstractWebsite

[1] Interannual variability of the circulation in the northeast Pacific Ocean is explored through a joint analysis of expendable bathythermograph (XBT) and expendable conductivity-temperature-depth (XCTD) data, satellite altimetry, and output from a model that was constrained by ocean data. XBT temperature profiles with high spatial resolution are available in the eastern North Pacific along two repeated transects. These ship tracks, along with the coast of North America, define a closed "box'' which is used to study the time-mean circulation and its variability on interannual timescales. Geostrophic velocities from XBT data are compared with geostrophic velocities from model output as well as the full model velocity fields. Correlations in variability on interannual timescales between transport in the subpolar gyre and in the subtropical gyre are present in both model output and data. The nature of the variability, and its relation to the changes of the strength of the North Pacific Current (NPC), which supplies the water for both gyres, are explored. Interannual variability in gyre transport is found to be related to both the bifurcation of the NPC, resulting in an anticorrelation in transport between the two gyres, and to variations in NPC strength, resulting in simultaneous changes in the two gyres. The dominant signal is found to be a long-term increase in the NPC, which results in a strengthening of the subtropical gyre. Possible connections with local-scale wind stress changes and with the El Nino/Southern Oscillation phenomenon are also explored.

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.

Sutton, PJH, Bowen M, Roemmich D.  2005.  Decadal temperature changes in the Tasman sea. New Zealand Journal of Marine and Freshwater Research. 39:1321-1329. AbstractWebsite

Ocean temperature changes between 1991 and 2005 in the eastern Tasman Sea were analysed. This area was chosen because of a combination of data availability, low mesoscale variability and because of its importance in determining the climate of the downwind New Zealand landmass. A large warming extending to the full depth of the water column (c. 800 m) was found to have occurred between 1996 and 2002. This warming was seen in measurements by expendable bathythermographs and also in satellite sea surface temperature and sea surface height products, and has a clear impact on New Zealand's terrestrial temperature. The nature of the wan-ning is discussed, together with likely forcing mechanisms. No local forcing mechanisms are consistent with the observed warming, leading to the conclusion that the signal seen in the Tasman Sea is part of a larger South Pacific-wide phenomenon.

Roemmich, D, Riser S, Davis R, Desaubies Y.  2004.  Autonomous profiling floats: Workhorse for broad-scale ocean observations. Marine Technology Society Journal. 38:21-29. AbstractWebsite

The autonomous profiling float has been a revolutionary development in oceanography enabling global broad-scale ocean observations of temperature, salinity, velocity, and additional variables. The Argo gloat array applies this new technology to provide unprecedented measurements of the global upper ocean in near real time, with no period of exclusive use. It builds on its predecessors, the upper ocean thermal networks of the 1970s to 1990s- extending the spatial domain and depth range, improving the accuracy, and adding salinity and velocity. Precision satellite measurements of sea surface height, as made by the Jason-1 altimeter, combine with Argo data in a dynamically complementary description of sea level variability and its subsurface causes. The broad-scale Argo float array is a central element in the international infrastructure for ocean research. A comprehensive ocean observing system can be constructed from floats, together with satellite measurements, improved measurements of air-sea fluxes, moored time-series in the tropics and other special locations, shipboard hydrography, and high resolution measurements in fronts, eddies and boundary currents from autonomous gliders. One of the primary objectives of the observing system is to close the oceanic budgets of momentum, heat, and freshwater on seasonal and longer time-scales in order to understand the role of the ocean in the climate system.

Willis, JK, Roemmich D, Cornuelle B.  2004.  Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales. Journal of Geophysical Research-Oceans. 109   10.1029/2003jc002260   AbstractWebsite

[1] Satellite altimetric height was combined with approximately 1,000,000 in situ temperature profiles to produce global estimates of upper ocean heat content, temperature, and thermosteric sea level variability on interannual timescales. Maps of these quantities from mid-1993 through mid-2003 were calculated using the technique developed by Willis et al. [ 2003]. The time series of globally averaged heat content contains a small amount of interannual variability and implies an oceanic warming rate of 0.86 +/- 0.12 watts per square meter of ocean (0.29 +/- 0.04 pW) from 1993 to 2003 for the upper 750 m of the water column. As a result of the warming, thermosteric sea level rose at a rate of 1.6 +/- 0.3 mm/yr over the same time period. Maps of yearly heat content anomaly show patterns of warming commensurate with ENSO variability in the tropics, but also show that a large part of the trend in global, oceanic heat content is caused by regional warming at midlatitudes in the Southern Hemisphere. In addition to quantifying interannual variability on a global scale, this work illustrates the importance of maintaining continuously updated monitoring systems that provide global coverage of the world's oceans. Ongoing projects, such as the Jason/TOPEX series of satellite altimeters and the Argo float program, provide a critical foundation for characterizing variability on regional, basin, and global scales and quantifying the oceans' role as part of the climate system.

Willis, JK, Roemmich D, Cornuelle B.  2003.  Combining altimetric height with broadscale profile data to estimate steric height, heat storage, subsurface temperature, and sea-surface temperature variability. Journal of Geophysical Research-Oceans. 108   10.1029/2002jc001755   AbstractWebsite

A new technique is demonstrated for combining altimetric height (AH) and sea-surface temperature (SST) with in situ data to produce improved estimates of 0/800 m steric height (SH), heat content, and temperature variability. The technique uses a linear regression onto AH to construct an initial guess for the subsurface quantity. This guess is then corrected toward the in situ data creating an estimate with substantially less error than could be achieved using either data set alone. Inclusion of the SST data further improves the estimates and illustrates how the procedure can be generalized to allow inclusion of additional data sets. The technique is demonstrated over a region in the southwestern Pacific enclosing the Tasman Sea. Nine-year time series of heat storage and temperature variability, averaged over 4degrees latitude and longitude and 1 year in time, are calculated. The estimates have RMS errors of approximately 4.6 W/m(2) in heat storage, 0.10degreesC in subsurface temperature and 0.11degreesC in surface temperature, and fractional errors of 20, 28, and 18%, respectively, relative to the total variance overall spatial and temporal scales considered. These represent significant improvements over previous estimates of these quantities. All the time series show strong interannual variability including the El Nino event of 1997. Application of these techniques on a global scale could provide new insight into the variability of the general circulation and heat budget of the upper ocean.

Roemmich, D, Gould WJ.  2003.  The future of climate observations in the global ocean. Sea Technology. 44:10-15. AbstractWebsite
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.

Sutton, PJH, Roemmich D.  2001.  Ocean temperature climate off north-east New Zealand. New Zealand Journal of Marine and Freshwater Research. 35:553-565. AbstractWebsite

The ocean temperature field off the north-east coast of New Zealand is studied to quantify the annual cycle and reveal the intra- and inter-annual variability. The data used are repeat expendable bathythermograph (XBT) sections between Auckland and either Suva or Honolulu which have been collected quarterly since 1986. These sections give temperature measurements between the surface and 800 m and Auckland and 30 degreesS from 1986 to August 1999. The mean and annual cycle are compared with those from the NOAA World Ocean Atlas (WOA98). The results are similar; however WOA98 lacks the horizontal resolution to fully discern the East Auckland Current and North Cape Eddy, while the XBT analysis lacks the temporal resolution to discern higher frequency intra-annual signals. The temperature variability in the mixed layer is dominated by the annual cycle, which accounts for 80-90% of the variance. The amplitude of the annual cycle diminishes rapidly with depth, from 2.8 degreesC at the surface, to c. 0.1 degreesC at 180 m. The phase of the annual cycle is retarded with depth, with peak temperatures occurring in February at the surface and in June/July at 180 m. Removing the annual cycle from the time series reveals the more subtle inter- and intra-annual variability. This variability is of the order of 1 degreesC in the upper 50 m, decreasing to 0.3 degreesC at 400-500 m. The surface layer was cold between 1991 and 1994 (c. 0.7 degreesC cooler than average), and 0.7 degreesC warmer than average in 1999. The deeper ocean shows a different signal, being up to 0.3 degreesC cooler in 1990-92, 0.3 degreesC warmer in 1998, and c. 0.2 degreesC warmer than average in 1999. The inter-annual mixed layer variability is highly correlated with the Southern Oscillation Index and also with inter-annual terrestrial air temperature and wind measurements from northern New Zealand. In contrast, at higher intra-annual frequencies, the mixed layer variability is not correlated with air and wind measurements. At these higher frequencies, the air temperature is better correlated with the sea surface temperature (SST) than with the bulk mixed layer temperature.