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A
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.

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Bograd, SJ, Chereskin TK, Roemmich D.  2001.  Transport of mass, heat, salt, and nutrients in the southern California Current System: Annual cycle and interannual variability. Journal of Geophysical Research-Oceans. 106:9255-9275.   10.1029/1999jc000165   AbstractWebsite

Net fluxes of mass, heat, salt, nutrients, oxygen, and chlorophyll into a control volume within the southern California Current System (CCS) were computed from data collected on 55 cruises over a 14 year period (1984-1997). This analysis builds on an earlier work [Roemmich, 1989] by using an additional 39 cruises over 10 years, allowing for reliable estimates of the temporal variability in the fluxes on seasonal and interannual timescales and a reduction in the corresponding error budgets. A close balance was found between geostrophic convergence and Ekman divergence for the 14 year, seasonal, and interannual cruise subsets using three different wind products. Wind data taken concomitantly with the hydrographic sampling provided the best balance and hence the best flux estimates. The southern CCS was found to be a region with higher evaporation over precipitation and net heat gain by the ocean from the atmosphere (86 W m(-2) in the 14 year mean) in all seasons. Significant variability in both the Ekman and geostrophic transports and the net property fluxes was found to be related to low-frequency (interpentadal and El Nino - Southern Oscillation timescale) changes in the dominant wind and circulation patterns in the CCS. Variability in primary productivity, estimated from the derived nutrient fluxes, accompanied the environmental changes. Application of this model to the ongoing data collection will further reduce the error bars on the fluxes and will allow for continued monitoring of changes in the physical and biological structure of the southern CCS.

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.

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.

Bryden, HL, Roemmich DH, Church JA.  1991.  Ocean heat transport across 24°N in the Pacific. Deep-Sea Research Part a-Oceanographic Research Papers. 38:297-324.   10.1016/0198-0149(91)90070-v   AbstractWebsite

Ocean heat transport across 24-degrees-N in the North Pacific is estimated to be 0.76 X 10(15) W northward from the 1985 transpacific hydrographic section. This northward heat transport is due half to a zonally averaged vertical meridional circulation cell and half to a horizontal circulation cell. The vertical meridional cell is a shallow one, in which the northward Ekman transport of warm surface waters returns southward only slightly deeper and colder, all within the upper 700 m of the water column. In terms of its meridional heat transport, the horizontal circulation cell is also shallow with effectively all of its northward heat transport in the upper 700 m of the water column. Previous estimates of North Pacific heat transport at subtropical latitudes had ranged between 1.14 X 10(15) W northward and 1.17 X 10(15) W southward. The error in this new direct estimate of Pacific heat transport is approximately 0.3 X 10(15) W. In addition, it is suggested that the annual variation in poleward heat transport across 24-degrees-N in the Pacific is of order 0.2 X 10(15) W, as long as the deep circulation below 1000 m exhibits little variation in water mass transport. Together, the Pacific and Atlantic transoceanic sections essentially close off the global ocean north of 24-degrees-N so that the total ocean heat transport across 24-degrees-N is estimated to be 2.0 X 10(15) W northward. This ocean heat transport is larger than the northward atmospheric energy transport across 24-degrees-N of 1.7 X 10(15) W. The ocean and atmosphere together transport 3.7 X 10(15) W of heat across 24-degrees-N, which is in reasonable agreement with classic values of 4.0 X 10(15) W derived from consideration of the Earth's radiation budget but which is markedly less than the 5.3 X 10(15) W required by recent satellite radiation budget determinations.

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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.

Cazenave, A, Meyssignac B, Ablain M, Balmaseda M, Bamber J, Barletta V, Beckley B, Benveniste J, Berthier E, Blazquez A, Boyer T, Caceres D, Chambers D, Champollion N, Chao B, Chen JL, Cheng LJ, Church JA, Chuter S, Cogley JG, Dangendorf S, Desbruyeres D, Doll P, Domingues C, Falk U, Famiglietti J, Fenoglio-Marc L, Forsberg R, Galassi G, Gardner A, Groh A, Hamlington B, Hogg A, Horwath M, Humphrey V, Husson L, Ishii M, Jaeggi A, Jevrejeva S, Johnson G, Kolodziejczyk N, Kusche J, Lambeck K, Landerer F, Leclercq P, Legresy B, Leuliette E, Llovel W, Longuevergne L, Loomis BD, Luthcke SB, Marcos M, Marzeion B, Merchant C, Merrifield M, Milne G, Mitchum G, Mohajerani Y, Monier M, Monselesan D, Nerem S, Palanisamy H, Paul F, Perez B, Piecuch CG, Ponte RM, Purkey SG, Reager JT, Rietbroek R, Rignot E, Riva R, Roemmich DH, Sorensen LS, Sasgen I, Schrama EJO, Seneviratne SI, Shum CK, Spada G, Stammer D, van de Wal R, Velicogna I, von Schuckmann K, Wada Y, Wang YG, Watson C, Wiese D, Wijffels S, Westaway R, Woppelmann G, Wouters B, Grp WGSLB.  2018.  Global sea-level budget 1993-present. Earth System Science Data. 10:1551-1590.   10.5194/essd-10-1551-2018   AbstractWebsite

Global mean sea level is an integral of changes occurring in the climate system in response to unforced climate variability as well as natural and anthropogenic forcing factors. Its temporal evolution allows changes (e.g.,acceleration) to be detected in one or more components. Study of the sea-level budget provides constraints on missing or poorly known contributions, such as the unsurveyed deep ocean or the still uncertain land water component. In the context of the World Climate Research Programme Grand Challenge entitled "Regional Sea Level and Coastal Impacts", an international effort involving the sea-level community worldwide has been recently initiated with the objective of assessing the various datasets used to estimate components of the sea-level budget during the altimetry era (1993 to present). These datasets are based on the combination of a broad range of space-based and in situ observations, model estimates, and algorithms. Evaluating their quality, quantifying uncertainties and identifying sources of discrepancies between component estimates is extremely useful for various applications in climate research. This effort involves several tens of scientists from about 50 research teams/institutions worldwide (www.wcrp-climate.org/grand-challenges/gc-sea-level, last access: 22 August 2018). The results presented in this paper are a synthesis of the first assessment performed during 2017-2018. We present estimates of the altimetry-based global mean sea level (average rate of 3.1 +/- 0.3mm yr(-1) and acceleration of 0.1 mm yr(-2) over 1993-present), as well as of the different components of the sea-level budget (http://doi.org/10.17882/54854, last access: 22 August 2018). We further examine closure of the sea-level budget, comparing the observed global mean sea level with the sum of components. Ocean thermal expansion, glaciers, Greenland and Antarctica contribute 42%, 21%, 15% and 8% to the global mean sea level over the 1993-present period. We also study the sea-level budget over 2005-present, using GRACE-based ocean mass estimates instead of the sum of individual mass components. Our results demonstrate that the global mean sea level can be closed to within 0.3 mm yr(-1) (1 sigma). Substantial uncertainty remains for the land water storage component, as shown when examining individual mass contributions to sea level.

Chereskin, TK, Roemmich D.  1991.  A comparison of measured and wind-derived Ekman transport at 11°N in the Atlantic-Ocean. Journal of Physical Oceanography. 21:869-878.   10.1175/1520-0485(1991)021<0869:acomaw>2.0.co;2   AbstractWebsite

A comparison of measured and wind-derived ageostrophic transport is presented from a zonal transect spanning the Atlantic Ocean along 11-degrees-N. The transport per unit depth shows a striking surface maximum that decays to nearly zero at a depth of approximately 100 m. We identify this flow in the upper 100 m as the Ekman transport. The sustained values of wind stress and the penetration depth of the Ekman transport reported here are considerably greater than in previous observations, which were made in conditions of light winds. The transport of 12.0 +/- 5.5 x 10(6) m3 s-1, calculated from the difference of geostrophic shear and shear measured by an acoustic Doppler current profiler, is in good agreement with that estimated from the shipboard winds, 8.8 +/- 1.9 x 10(6) m3 s-1, and from climatology, 13.5 +/- 0.3 x 10(6) m3 s-1. Qualitatively, the horizontal distribution of the wind-driven flow was best predicted by the shipboard winds. The cumulative transport increased linearly over the western three-fourths of the basin, where the winds were large and spatially uniform, and remained constant over the eastern fourth where the easterly stress was uncharacteristically low. The mean depth of the Ekman transport extended below the mixed layer depth, which varied from 25 to 90 m. The profile of ageostrophic transport does not appear consonant with slablike behavior in the mixed layer, even when spatial variations in mixed layer depth are taken into account.

Chiswell, SM, Roemmich D.  1998.  The East Cape Current and two eddies: a mechanism for larval retention? New Zealand Journal of Marine and Freshwater Research. 32:385-397. AbstractWebsite

Current meters deployed near East Cape, New Zealand, for 1 year show large differences between the offshore flow and that inshore of the 1000 m isobath. Offshore, mean flows are to the south-east, and are the East Cape Current. Inshore, mean flows are to the north-east, indicating that there may be a persistent inshore counter current. Comparison between the offshore current meter measurements and geostrophic currents inferred from TOPEX/Poseidon altimetry shows good agreement. We use this agreement to argue that passive drifter trajectories can be modelled using the altimeter data. By simulating drifter releases into the region, and modelling their trajectories, we develop a statistical picture of likely retention times for passive drifters. Drifters can get retained in one or other of two permanent eddies: the East Cape and Wairarapa Eddies, and retention within the system can be as high as 2-3 years, but depends on release time and location. If weak-swimming larvae such as rock lobster larvae behave as passive drifters, retention and recirculation within the eddies may provide a mechanism allowing them to survive within the system long enough to recruit as juvenile lobsters.

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
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Cornuelle, BD, Morris MY, Roemmich DH.  1993.  An objective mapping method for estimating geostrophic velocity from hydrographic sections including the equator. Journal of Geophysical Research-Oceans. 98:18109-18118.   10.1029/93jc01729   AbstractWebsite

Objective mapping can remove the equatorial singularity from the problem of estimating geostrophic shear from noisy density measurements. The method uses the complete thermal wind relation, so it is valid uniformly on and off the equator. Errors in the thermal wind balance are due to neglected terms in the momentum balance, which are treated as noise in the inverse problem. The question of whether the geostrophic balance holds near the equator is restated as a need to estimate the size of the ageostrophic noise in the thermal wind equation. Objective mapping formalizes the assumptions about the magnitudes and scales of the geostrophic currents and about the magnitudes and scales of the ageostrophic terms and measurement errors. The uncertainty of the velocity estimates is calculated as part of the mapping and depends on the signal to noise ratio (geostrophic density signal to ageostrophic ''noise'') in the data, as well as the station spacing and the scales assumed for the geostrophic velocities. The method is used to map zonal velocity from a mean Hawaii-Tahiti Shuttle density section. These are compared with previous velocity estimates for the same dataset calculated using other techniques. By choosing appropriate scales, the objective map can duplicate previous results. New temperature data are presented from a repeating, high-resolution expendable bathythermograph section crossing the equator at about 170-degrees-W with four cruises a year between 1987-1991. There appear to be significant differences between this mean temperature and the shuttle mean temperature. Temperature is converted to density with the aid of a mean T-S relation and geostrophic velocity maps are calculated for the 4-year mean. The mean geostrophic undercurrent obtained from our sections is weaker than in the shuttle estimate and is centered slightly north of the equator. Enforcing symmetry about the equator removes the offset of the current, giving a stronger, but narrow undercurrent. The density field apparently includes significant (O(0.5 kg M-3)) large-scale ageostrophic variability which makes velocity estimates from single cruises poorly determined near the equator.

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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.

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.

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.

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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.

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.

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.

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).

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.

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.

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.

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.

H
Hautala, SL, Roemmich DH.  1998.  Subtropical mode water in the Northeast Pacific Basin. Journal of Geophysical Research-Oceans. 103:13055-13066.   10.1029/98jc01015   AbstractWebsite

A new type of mode water in the upper thermocline of the eastern subtropical North Pacific is identified and examined using data from World Ocean Circulation Experiment high-resolution repeat expendable bathythermograph (XBT) section PX37 and archives of historical XBT data. This water mass (labeled Eastern Subtropical Mode Water) is characterized by a subsurface potential vorticity minimum and is located east of Hawaii (Northeast Pacific Basin) in a density range of 24-25.4 sigma(theta). It is a distinct water mass from the classical subtropical mode water (STMW) of the western Pacific. Eastern STMW is formed as a relatively deep late-winter mixed layer, associated with the subtropical/subpolar water mass boundary near 25 degrees-30 degrees N, 135 degrees-140 degrees W, and is capped and subducted into the permanent thermocline. Along a section between San Francisco and Honolulu, Eastern STMW production is seen in every year for which there is adequate data. In this section the volume of Eastern STMW formed each winter and the temperature of the potential vorticity minimum are similar during the periods 1970-1979 and 1991-1997.

Hautala, SL, Roemmich DH, Schmitz WJ.  1994.  Is the North Pacific in Sverdrup balance along 24°N? Journal of Geophysical Research-Oceans. 99:16041-16052.   10.1029/94jc01084   AbstractWebsite

Hydrographic data from a zonal section along approximately 24-degrees-N is used to demonstrate that basin-scale baroclinic geostrophic transport agrees with the Sverdrup relation in the subtropical North Pacific. Moreover, profiles of vertical velocity, as derived from the linear vorticity equation, are consistent with constraints upon deeper flows. Sverdrup balance from the eastern boundary to about 137-degrees-E is in accord with most other elements of the general circulation to the extent that they are known. In the Philippine Sea, west of 137-degrees-E, the results suggest that a mean northward flow of 5-10 Sv occurs. In the northeast Pacific Basin, a significant subbasin-scale deviation of about 7 Sv from Sverdrup balance is revealed; this deviation is distinguished by excess shallow southward flow near the water mass boundary between the subtropical gyre and the subpolar waters adjacent to the American coast.

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.