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2018
Zilberman, NV, Roemmich DH, Gille ST, Gilson J.  2018.  Estimating the velocity and transport of Western Boundary current systems: A case study of the East Australian Current near Brisbane. Journal of Atmospheric and Oceanic Technology. 35:1313-1329.   10.1175/jtech-d-17-0153.1   AbstractWebsite

Western boundary currents (WBCs) are highly variable narrow meandering jets, making assessment of their volume transports a complex task. The required high-resolution temporal and spatial measurements are available only at a limited number of sites. In this study a method is developed for improving estimates of the East Australian Current (EAC) mean transport and its low-frequency variability, using complementary modern datasets. The present calculation is a case study that will be extended to other subtropical WBCs. The method developed in this work will reduce uncertainties in estimates of the WBC volume transport and in the interannual mass and heat budgets of the meridional overturning circulations, improving our understanding of the response of WBCs to local and remote forcing on long time scales. High-resolution expendable bathythermograph (HR-XBT) profiles collected along a transect crossing the EAC system near Brisbane, Australia, are merged with coexisting profiles and parking-depth trajectories from Argo floats, and with altimetric sea surface height data. Using HR-XBT/Argo/altimetry data combined with Argo trajectory-based velocities at 1000 m, the 2004-15 mean poleward alongshore transport of the EAC is 19.5 +/- 2.0 Sv (1 Sv 10(6) m(3) s(-1)) of which 2.5 +/- 0.5 Sv recirculate equatorward just offshore of the EAC. These transport estimates are consistent in their mean and variability with concurrent and nearly collocated moored observations at 27 degrees S, and with earlier moored observations along 30 degrees S. Geostrophic transport anomalies in the EAC system, including the EAC recirculation, show a standard deviation of +/- 3.1 Sv at interannual time scales between 2004 and 2015.

2017
Jayne, SR, Roemmich DH, Zilberman NV, Riser SC, Johnson KS, Johnson GC, Piotrowicz SR.  2017.  The Argo Program: Present and future. Oceanography. 30:18-28.   10.5670/oceanog.2017.213   Abstract

The Argo Program has revolutionized large-scale physical oceanography through its contributions to basic research, national and international climate assessment, education, and ocean state estimation and forecasting. This article discusses the present status of Argo and enhancements that are underway. Extensions of the array into seasonally ice-covered regions and marginal seas as well as increased numbers of floats along the equator and around western boundary current extensions have been proposed. In addition, conventional Argo floats, with their 2,000 m sampling limit, currently observe only the upper half of the open ocean volume. Recent advances in profiling float technology and in the accuracy and stability of float-mounted conductivity-temperature-depth sensors make it practical to obtain measurements to 6,000 m. The Deep Argo array will help observe and constrain the global budgets of heat content, freshwater, and steric sea level, as well as the full-depth ocean circulation. Finally, another extension to the Argo Program is the addition of a diverse set of chemical sensors to profiling floats in order to build a Biogeochemical-Argo array to understand the carbon cycle, the biological pump, and ocean acidification.

Gasparin, F, Roemmich D.  2017.  The seasonal march of the equatorial Pacific upper-ocean and its El Nino variability. Progress in Oceanography. 156:1-16.   10.1016/j.pocean.2017.05.010   AbstractWebsite

Based on two modern data sets, the climatological seasonal march of the upper-ocean is examined in the equatorial Pacific for the period 2004-2014, because of its large contribution to the total variance, its relationship to El Nino, and its unique equatorial wave phenomena. Argo provides a broadscale view of the equatorial Pacific upper-ocean based on subsurface temperature and salinity measurements for the period 2004-2015, and satellite altimetry provides synoptic observations of the sea surface height (SSH) for the period 1993-2015. Using either 11-year (1993-2003/2004-2014) time-series for averaging, the seasonal Rossby waves stands out clearly and eastward intraseasonal Kelvin wave propagation is strong enough in individual years to leave residuals in the 11-year averages, particularly but not exclusively, during El Nino onset years. The agreement of altimetric SSH minus Argo steric height (SH) residuals with GRACE ocean mass estimates confirms the scale-matching of in situ variability with that of satellite observations. Surface layer and subsurface thermohaline variations are both important in determining SH and SSH basin-wide patterns. The SH/SSH October-November maximum in the central-eastern Pacific is primarily due to a downward deflection of the thermocline (similar to 20 m), causing a warm subsurface anomaly (>1 degrees C), in response to the phasing of downwelling intraseasonal Kelvin and seasonal Rossby waves. Compared with the climatology, the stronger October-November maximum in the 2004-2014 El Nino composites is due to higher intraseasonal oscillations and interannual variability. Associated with these equatorial wave patterns along the thermocline, the western warm/fresh pool waters move zonally at interannual timescales through zonal wind stress and pressure gradient fluctuations, and cause substantial fresh (up to 0.6 psu) and warm (similar to 1 degrees C higher than the climatology) anomalies in the western central Pacific surface-layer during the El Nino onset year, and of the opposite sign during the termination year. These El Nino-related patterns are then analyzed focusing on the case of the onset of the strong 2015/2016 episode, and are seen to be around two times larger than that in the 2004-2014 El Nino composites. The present work exploits the capabilities of Argo and altimetry to update and improve the description of the physical state of the equatorial Pacific upper-ocean, and provides a benchmark for assessing the accuracy of models in representing equatorial Pacific variability. (C) 2017 Elsevier Ltd. All rights reserved.

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.

Zilberman, NV, Roemmich DH, Gille ST.  2017.  The East Pacific Rise current: Topographic enhancement of the interior flow in the South Pacific Ocean. Geophysical Research Letters. 44:277-285.   10.1002/2016gl069039   AbstractWebsite

Observations of absolute velocity based on Argo float profiles and trajectories in the ocean interior show evidence for an equatorward current, the East Pacific Rise current, between 42 degrees S and 30 degrees S, along the western flank of the East Pacific Rise. The East Pacific Rise current carries predominantly intermediate water masses, including Subantarctic Mode Water and Antarctic Intermediate Water, and deeper waters, from the southern edge of the subtropical gyre toward the Equator. The 2004 to 2014 mean East Pacific Rise current transport extrapolated through the 0-2600m depth range is 8.11.6 sverdrup (Sv) (1 Sv=10(6)m(3)s(-1)), consistent with a wind-driven interior transport influenced by the East Pacific Rise topography. While deep ocean mixing and geothermal heating can both create pressure gradients that support geostrophic flows in the deep ocean, this study indicates that about half of the East Pacific Rise current transport is associated with topographic steering of the deep flow over the East Pacific Rise.

National Academies of Sciences, Engineering, Medicine.  2017.  Sustaining ocean observations to understand future changes in earth’s climate. :150., Washington, DC: The National Academies Press   10.17226/24919   Abstract

The ocean is an integral component of the Earth’s climate system. It covers about 70% of the Earth’s surface and acts as its primary reservoir of heat and carbon, absorbing over 90% of the surplus heat and about 30% of the carbon dioxide associated with human activities, and receiving close to 100% of fresh water lost from land ice. With the accumulation of greenhouse gases in the atmosphere, notably carbon dioxide from fossil fuel combustion, the Earth’s climate is now changing more rapidly than at any time since the advent of human societies. Society will increasingly face complex decisions about how to mitigate the adverse impacts of climate change such as droughts, sea-level rise, ocean acidification, species loss, changes to growing seasons, and stronger and possibly more frequent storms. Observations play a foundational role in documenting the state and variability of components of the climate system and facilitating climate prediction and scenario development. Regular and consistent collection of ocean observations over decades to centuries would monitor the Earth’s main reservoirs of heat, carbon dioxide, and water and provides a critical record of long-term change and variability over multiple time scales. Sustained high-quality observations are also needed to test and improve climate models, which provide insights into the future climate system. Sustaining Ocean Observations to Understand Future Changes in Earth’s Climate considers processes for identifying priority ocean observations that will improve understanding of the Earth’s climate processes, and the challenges associated with sustaining these observations over long timeframes.

2016
Nagano, A, Kizu S, Hanawa K, Roemmich D.  2016.  Heat transport variation due to change of North Pacific subtropical gyre interior flow during 1993-2012. Ocean Dynamics. 66:1637-1649.   10.1007/s10236-016-1007-2   AbstractWebsite

Applying segment-wise altimetry-based gravest empirical mode method to expendable bathythermograph temperature, Argo salinity, and altimetric sea surface height data in March, June, and November from San Francisco to near Japan (30(ay) N, 145(ay) E) via Honolulu, we estimated the component of the heat transport variation caused by change in the southward interior geostrophic flow of the North Pacific subtropical gyre in the top 700 m layer during 1993-2012. The volume transport-weighted temperature (T (I)) is strongly dependent on the season. The anomaly of T (I) from the mean seasonal variation, whose standard deviation is 0.14(ay)C, was revealed to be caused mainly by change in the volume transport in a potential density layer of 25.0-25.5 sigma (oee integral) . The anomaly of T (I) was observed to vary on a decadal or shorter, i.e., quasi-decadal (QD), timescale. The QD-scale variation of T (I) had peaks in 1998 and 2007, equivalent to the reduction in the net heat transport by 6 and 10 TW, respectively, approximately 1 year before those of sea surface temperature (SST) in the warm pool region, east of the Philippines. This suggests that variation in T (I) affects the warm pool SST through modification of the heat balance owing to the entrainment of southward transported water into the mixed layer.

Yan, XH, Boyer T, Trenberth K, Karl TR, Xie SP, Nieves V, Tung KK, Roemmich D.  2016.  The global warming hiatus: Slowdown or redistribution? Earths Future. 4:472-482.   10.1002/2016ef000417   AbstractWebsite

Global mean surface temperatures (GMST) exhibited a smaller rate of warming during 1998-2013, compared to the warming in the latter half of the 20th Century. Although, not a "true" hiatus in the strict definition of the word, this has been termed the "global warming hiatus" by IPCC (2013). There have been other periods that have also been defined as the "hiatus" depending on the analysis. There are a number of uncertainties and knowledge gaps regarding the "hiatus." This report reviews these issues and also posits insights from a collective set of diverse information that helps us understand what we do and do not know. One salient insight is that the GMST phenomenon is a surface characteristic that does not represent a slowdown in warming of the climate system but rather is an energy redistribution within the oceans. Improved understanding of the ocean distribution and redistribution of heat will help better monitor Earth's energy budget and its consequences. A review of recent scientific publications on the "hiatus" shows the difficulty and complexities in pinpointing the oceanic sink of the "missing heat" from the atmosphere and the upper layer of the oceans, which defines the "hiatus." Advances in "hiatus" research and outlooks (recommendations) are given in this report.

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.  2016.  The strong freshwater anomaly during the onset of the 2015/2016 El Nino. Geophysical Research Letters. 43:6452-6460.   10.1002/2016gl069542   AbstractWebsite

The strong freshwater anomaly in the equatorial Pacific is investigated during the onset of the 2015/2016 El Nino using oceanic observational data sets and atmospheric reanalyzes. The 2015 salinity patterns are marked by a large equatorial freshwater anomaly whose extensive spatial and large amplitude characteristics have not previously been captured in the 2004-2014 Argo record. As the main contributors of the freshwater budget, zonal advection and surface forcing have similar amplitude but with maxima located at different longitudes around the dateline. The comparison of the substantial rainfall and westerly winds observed in 2015 with the 2009 and 1997 El Nino onset years shows that 2015 characteristics combine both typical salinity-related patterns of Central and Eastern Pacific El Nino. Dynamically, this large freshwater anomaly causes a positive steric height anomaly in the western Pacific and increases eastward surface acceleration at the SSS front due to the zonal pressure gradient.

2015
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.

2014
Bowen, M, Sutton P, Roemmich D.  2014.  Estimating mean dynamic topography in boundary currents and the use of Argo trajectories. Journal of Geophysical Research-Oceans. 119:8422-8437.   10.1002/2014jc010281   AbstractWebsite

A Mean Dynamic Topography (MDT) is required to estimate mean transport in the ocean, to combine with altimetry to derive instantaneous geostrophic surface velocities, and to estimate transport from shipboard hydrography. A number of MDTs are now available globally but differ most markedly in boundary currents and the Antarctic Circumpolar Current. We evaluate several MDTs in two boundary currents off New Zealand (in the subtropical western boundary current system east of the country and in the Subantarctic Front to the south) using satellite, hydrographic, and Argo observations near two altimeter tracks. Argo float trajectories are combined with estimates of shear to produce new MDTs along both altimeter tracks: sufficiently high numbers of Argo floats travel in both boundary currents to allow a useful estimate of the mean flow at 1000 m depth and conservation of potential vorticity is used to account for more realistic flow paths. In finding a MDT, we show the uncertainties in the estimates of velocity differences between 1000 m and the surface from density climatologies, while often not estimated, need to be considered. The MDT computed from the Argo trajectories is generally consistent with the CLS09 MDT in both boundary currents and, in some locations, distinctly different from the MDT using a level of no motion assumption. The comparison suggests velocities from Argo trajectories can be usefully combined with other observations to improve estimates of flows and MDT in boundary currents.

Giglio, D, Roemmich D.  2014.  Climatological monthly heat and freshwater flux estimates on a global scale from Argo. Journal of Geophysical Research-Oceans. 119:6884-6899.   10.1002/2014jc010083   AbstractWebsite

The global pattern of climatological monthly heat and freshwater fluxes at the ocean surface is estimated using Argo temperature and salinity profile data for the period 2004-2013. Temperature or salinity changes are calculated in a volume of water above an isopycnal that is below the mixed layer and not subject to mixed-layer entrainment. Horizontal advection components from geostrophic velocity and from Ekman transport, based on wind stress, are also included. The climatological monthly heat or freshwater flux at the ocean surface is estimated as the sum of advective and time tendency contributions. The air-sea flux estimates from Argo are described in global maps and basin-wide integrals, in comparison to atmospheric reanalysis data and to air-sea flux products based on observations. This ocean-based estimate of surface fluxes is consistent with property variations in the subsurface ocean and indicates greater amplitude for the climatological monthly heat flux values in the subtropics compared to other products. Similarly, the combination of Argo freshwater flux and reanalysis evaporation, suggests greater amplitude for climatological monthly precipitation in the tropics.

Zilberman, NV, Roemmich DH, Gille ST.  2014.  Meridional volume transport in the South Pacific: Mean and SAM-related variability. Journal of Geophysical Research-Oceans. 119:2658-2678.   10.1002/2013jc009688   AbstractWebsite

The large increase in upper-ocean sampling during the past decade enables improved estimation of the mean meridional volume transport in the midlatitude South Pacific, and hence of the climatically important Meridional Overturning Circulation. Transport is computed using Argo float profile data for geostrophic shear and trajectory data for reference velocities at 1000 m. For the period 2004-2012, the mean geostrophic transport across 32 degrees S is 20.66.0 Sv in the top 2000 m of the ocean. From west to east, this includes the southward East Australian Current (23.32.9 Sv), its northward recirculation (16.33.6 Sv), the broad interior northward flow (18.4 +/- 4.1 Sv), and the net northward flow (9.2 +/- 2.2 Sv) in opposing currents in the eastern Pacific. The basin-integrated geostrophic transport includes 7.3 +/- 0.9 Sv of surface and thermocline waters, 4.9 +/- 1.0 Sv of Subantarctic Mode Water, and 4.9 +/- 1.4 Sv of Antarctic Intermediate Water. Interannual variability in volume transport across 32 degrees S in the South Pacific shows a Southern Annual Mode signature characterized by an increase during the positive phase of the Southern Annular Mode and a decrease during the negative phase. Maximum amplitudes in geostrophic transport anomalies, seen in the East Australian Current and East Australian Current recirculation, are consistent with wind stress curl anomalies near the western boundary. Key Points Improved meridional volume transport in the South Pacific using Argo EAC transport variability tied to the SAM

2013
Giglio, D, Roemmich D, Cornuelle B.  2013.  Understanding the annual cycle in global steric height. Geophysical Research Letters. 40:4349-4354.   10.1002/grl.50774   AbstractWebsite

Steric variability in the ocean includes diabatic changes in the surface layer due to air-sea buoyancy fluxes and adiabatic changes due to advection, which are dominant in the subsurface ocean. Here the annual signal in subsurface steric height (eta' below 200 db) is computed on a global scale using temperature and salinity profiles from Argo floats. The zonal average of over a season (e.g., eta'(March) - eta'(December)) is compared to the wind-forced vertical advection contribution (Delta eta'(w)) both in the global ocean and in different basins. The results show agreement that extends beyond the tropics. The estimate of Delta eta'(w) is based on the Ekman pumping and assumes that the seasonal vertical velocity is constant over the depth range of interest. This assumption is consistent with annual isopycnal displacements inferred from Argo profiles. The contribution of horizontal advection to Delta eta' is significant in some regions and consistent with differences between Delta eta' and Delta eta'(w).

Zilberman, NV, Roemmich DH, Gille ST.  2013.  The mean and the time variability of the shallow meridional overturning circulation in the tropical south pacific ocean. Journal of Climate. 26:4069-4087.   10.1175/jcli-d-12-00120.1   AbstractWebsite

The meridional transport in the Pacific Ocean subtropical cell is studied for the period from 2004 to 2011 using gridded Argo temperature and salinity profiles and atmospheric reanalysis surface winds. The poleward Ekman and equatorward geostrophic branches of the subtropical cell exhibit an El Nino-Southern Oscillation signature with strong meridional transport occurring during La Nina and weak meridional transport during El Nino. At 7.5 degrees S, mean basinwide geostrophic transport above 1000 dbar is 48.5 +/- 2.5 Sv (Sv 10(6) m(3) s(-1)) of which 30.3-38.4 Sv return to the subtropics in the surface Ekman layer, whereas 10.2-18.3 Sv flow northward, feeding the Indonesian Throughflow. Geostrophic transport within the subtropical cell is stronger in the ocean interior and weaker in the western boundary during La Nina, with changes in the interior dominating basinwide transport. Using atmospheric reanalyses, only half of the mean heat gain by the Pacific north of 7.5 degrees S is compensated by oceanic heat transport out of the region. The National Oceanography Centre at Southampton air-sea flux climatology is more consistent for closing the oceanic heat budget. In summary, the use of Argo data for studying the Pacific subtropical cell provides an improved estimate of basinwide mean geostrophic transport, includes both interior and western boundary contributions, quantifies El Nino/La Nina transport variability, and illustrates how the meridional overturning cell dominates ocean heat transport at 7.5 degrees S.

2012
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.

Giglio, D, Roemmich D, Gille ST.  2012.  Wind-driven variability of the subtropical North Pacific Ocean. Journal of Physical Oceanography. 42:2089-2100.   10.1175/jpo-d-12-029.1   AbstractWebsite

The Argo array provides a unique dataset to explore variability of the subsurface ocean interior. This study considers the subtropical North Pacific Ocean during the period from 2004 to 2011, when Argo coverage has been relatively complete in time and space. Two distinct patterns of Argo dynamic height transport function ((DH) over cap) are observed: in 2004/05, the gyre is stronger, and in 2008/09 it is weaker. The orientation of the subtropical gyre also shifts over time: the predominantly zonal major axis shifts to a more northwest-southeast orientation in 2004/05 and to a more southwest-northeast orientation in 2008/09. The limited temporal range of the Argo observations does not allow analysis of the correlation of ocean transport and wind forcing in the basin for the multiyear time scale (6-8-yr period) typical of the dominant gyre patterns. The meridional geostrophic transport anomaly between 180 degrees and 150 degrees E is computed both from Argo data (0-2000 db) and from the Sverdrup relation (using reanalysis winds): similarities are observed in a latitude-time plane, consistent with local forcing playing an important role. A forcing contribution from the eastern subtropics will also reach the region of interest, but on a longer time scale, and it is not analyzed in this study. Compared with the 8-yr Argo record, the longer 19-yr time series of satellite altimetry shows a similar but somewhat modified pattern of variability. A longer Argo record will be needed to observe the decadal-scale fluctuations, to separate interannual and decadal signals, and to ensure statistical confidence in relating the wind forcing and the oceanic response.

2011
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.

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.

2010
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

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