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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.

Furue, R, Jia YL, McCreary JP, Schneider N, Richards KJ, Muller P, Cornuelle BD, Avellaneda NM, Stammer D, Liu CY, Kohl A.  2015.  Impacts of regional mixing on the temperature structure of the equatorial Pacific Ocean. Part 1: Vertically uniform vertical diffusion. Ocean Modelling. 91:91-111.   10.1016/j.ocemod.2014.10.002   AbstractWebsite

We investigate the sensitivity of numerical model solutions to regional changes in vertical diffusion. Specifically, we vary the background diffusion coefficient, kappa(b), within spatially distinct subregions of the tropical Pacific, assess the impacts of those changes, and diagnose the processes that account for them. Solutions respond to a diffusion anomaly, delta kappa(b), in three ways. Initially, there is a fast response (several months), due to the interaction of rapidly propagating, barotropic and gravity waves with eddies and other mesoscale features. It is followed by a local response (roughly one year), the initial growth and spatial pattern of which can be explained by one-dimensional (vertical) diffusion. At this stage, temperature and salinity anomalies are generated that are either associated with a change in density ("dynamical" anomalies) or without one ("spiciness" anomalies). In a final adjustment stage, the dynamical and spiciness anomalies spread to remote regions by radiation of Rossby and Kelvin waves and by advection, respectively. In near equilibrium solutions, dynamical anomalies are generally much larger in the latitude band of the forcing, but the impact of off equatorial forcing by delta kappa(b) on the equatorial temperature structure is still significant. Spiciness anomalies spread equator ward within the pycnocline, where they are carried to the equator as part of the subsurface branch of the Pacific Subtropical Cells, and spiciness also extends to the equator via western-boundary currents. Forcing near and at the equator generates strong dynamical anomalies, and sometimes additional spiciness anomalies, at pycnocline depths. The total response of the equatorial temperature structure to delta kappa(b) in various regions depends on the strength and spatial pattern of the generation of each signal within the forcing region as well as On the processes of its spreading to the equator.

Subramanian, AC, Miller AJ, Cornuelle BD, Di Lorenzo E, Weller RA, Straneo F.  2013.  A data assimilative perspective of oceanic mesoscale eddy evolution during VOCALS-REx. Atmospheric Chemistry and Physics. 13:3329-3344.   10.5194/acp-13-3329-2013   AbstractWebsite

Oceanic observations collected during the VOCALS-REx cruise time period, 1-30 November 2008, are assimilated into a regional ocean model (ROMS) using 4DVAR and then analyzed for their dynamics. Nonlinearities in the system prevent a complete 30-day fit, so two 15-day fits for 1-15 November and 16-30 November are executed using the available observations of hydrographic temperature and salinity, along with satellite fields of SST and sea-level height anomaly. The fits converge and reduce the cost function significantly, and the results indicated that ROMS is able to successfully reproduce both large-scale and smaller-scale features of the flows observed during the VOCALS-REx cruise. Particular attention is focused on an intensively studied eddy at 76 degrees W, 19 degrees S. The ROMS fits capture this eddy as an isolated rotating 3-D vortex with a strong subsurface signature in velocity, temperature and anomalously low salinity. The eddy has an average temperature anomaly of approximately -0.5 degrees C over a depth range from 50-600 m and features a cold anomaly of approximately -1 degrees C near 150 m depth. The eddy moves northwestward and elongates during the second 15-day fit. It exhibits a strong signature in the Okubo-Weiss parameter, which indicates significant non-linearity in its evolution. The heat balance for the period of the cruise from the ocean state estimate reveals that the horizontal advection and the vertical mixing processes are the dominant terms that balance the temperature tendency of the upper layer of the ocean locally in time and space. Areal averages around the eddies, for a 15-day period during the cruise, suggest that vertical mixing processes generally balance the surface heating. Although, this indicates only a small role for lateral advective processes in this region during this period, this quasi-instantaneous heat budget analysis cannot be extended to interpret the seasonal or long-term upper ocean heat budget in this region.

Zhang, XB, Cornuelle B, Roemmich D.  2012.  Sensitivity of Western Boundary Transport at the Mean North Equatorial Current Bifurcation Latitude to Wind Forcing. Journal of Physical Oceanography. 42:2056-2072.   10.1175/jpo-d-11-0229.1   AbstractWebsite

The bifurcation of the North Equatorial Current (NEC) plays an important role in the heat and water mass exchanges between the tropical and subtropical gyres in the Pacific Ocean. The variability of western boundary transport (WBT) east of the Philippine coast at the mean NEC bifurcation latitude (12 degrees N) is examined here. A tropical Pacific regional model is set up based on the Massachusetts Institute of Technology general circulation model and its adjoint, which calculates the sensitivities of a defined meridional transport to atmospheric forcing fields and ocean state going backward in time. The adjoint-derived sensitivity of the WBT at the mean NEC bifurcation latitude to surface wind stress is dominated by curl-like patterns that are located farther eastward and southward with increasing time lag. The temporal evolution of the adjoint sensitivity of the WBT to wind stress resembles wind-forced Rossby wave dynamics but propagating with speeds determined by the background stratification and current, suggesting that wind-forced Rossby waves are the underlying mechanism. Interannual-to-decadal variations of the WBT can be hindcast well by multiplying the adjoint sensitivity and the time-lagged wind stress over the whole model domain and summing over time lags. The analysis agrees with previous findings that surface wind stress (especially zonal wind stress in the western subtropical Pacific) largely determines the WBT east of the Philippines, and with a time lag based on Rossby wave propagation. This adjoint sensitivity study quantifies the contribution of wind stress at all latitudes and longitudes and provides a novel perspective to understand the relationship between the WBT and wind forcing over the Pacific Ocean.

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.

Song, H, Hoteit I, Cornuelle BD, Subramanian AC.  2010.  An Adaptive Approach to Mitigate Background Covariance Limitations in the Ensemble Kalman Filter. Monthly Weather Review. 138:2825-2845.   10.1175/2010mwr2871.1   AbstractWebsite

A new approach is proposed to address the background covariance limitations arising from undersampled ensembles and unaccounted model errors in the ensemble Kalman filter (EnKF). The method enhances the representativeness of the EnKF ensemble by augmenting it with new members chosen adaptively to add missing information that prevents the EnKF fromfully fitting the data to the ensemble. The vectors to be added are obtained by back projecting the residuals of the observation misfits from the EnKF analysis step onto the state space. The back projection is done using an optimal interpolation (OI) scheme based on an estimated covariance of the subspace missing from the ensemble. In the experiments reported here, the OI uses a stationary background covariance matrix, as in the hybrid EnKF-three-dimensional variational data assimilation (3DVAR) approach, but the resulting correction is included as a new ensemble member instead of being added to all existing ensemble members. The adaptive approach is tested with the Lorenz-96 model. The hybrid EnKF-3DVAR is used as a benchmark to evaluate the performance of the adaptive approach. Assimilation experiments suggest that the new adaptive scheme significantly improves the EnKF behavior when it suffers from small size ensembles and neglected model errors. It was further found to be competitive with the hybrid EnKF-3DVAR approach, depending on ensemble size and data coverage.

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.