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Kim, SY, Cornuelle BD.  2015.  Coastal ocean climatology of temperature and salinity off the Southern California Bight: Seasonal variability, climate index correlation, and linear trend. Progress in Oceanography. 138:136-157.   10.1016/j.pocean.2015.08.001   AbstractWebsite

A coastal ocean climatology of temperature and salinity in the Southern California Bight is estimated from conductivity-temperature-depth (CTD) and bottle sample profiles collected by historical California Cooperative Oceanic Fisheries Investigation (CalCOFI) cruises (1950-2009; quarterly after 1984) off southern California and quarterly/monthly nearshore CTD surveys (within 30 km from the coast except for the surfzone; 1999-2009) off San Diego and Los Angeles. As these fields are sampled regularly in space, but not in time, conventional Fourier analysis may not be possible. The time dependent temperature and salinity fields are modeled as linear combinations of an annual cycle and its five harmonics, as well as three standard climate indices (El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), North Pacific Gyre Oscillation (NPGO)), the Scripps Pier temperature time series, and a mean and linear trend without time lags. Since several of the predictor indices are correlated, the indices are successively orthogonalized to eliminate ambiguity in the identification of the contributed variance of each component. Regression coefficients are displayed in both vertical transects and horizontal maps to evaluate (1) whether the temporal and spatial scales of the two data sets of nearshore and offshore observations are consistent and (2) how oceanic variability at a regional scale is related to variability in the nearshore waters. The data-derived climatology can be used to identify anomalous events and atypical behaviors in regional-scale oceanic variability and to provide background ocean estimates for mapping or modeling. (C) 2015 Elsevier Ltd. All rights reserved.

Fujii, Y, Cummings J, Xue Y, Schiller A, Lee T, Balmaseda MA, Remy E, Masuda S, Brassington G, Alves O, Cornuelle B, Martin M, Oke P, Smith G, Yang XS.  2015.  Evaluation of the Tropical Pacific Observing System from the ocean data assimilation perspective. Quarterly Journal of the Royal Meteorological Society. 141:2481-2496.   10.1002/qj.2579   AbstractWebsite

The drastic reduction in the number of observation data from the Tropical Atmospheric Ocean (TAO)/Triangle Trans-Ocean Buoy Network (TRITON) array since 2012 has given rise to a need to assess the impact of those data in ocean data assimilation (DA) systems. This article provides a review of existing studies evaluating the impacts of data from the TAO/TRITON array and other components of the Tropical Pacific Observing System (TPOS) on current ocean DA systems used for a variety of operational and research applications. It can be considered as background information that can guide the evaluation exercise of TPOS. Temperature data from TAO/TRITON array are assimilated in most ocean DA systems which cover the tropical Pacific in order to constrain the ocean heat content, stratification, and circulation. It is shown that the impacts of observation data depend considerably on the system and application. The presence of model error often makes the results difficult to interpret. Nevertheless there is consensus that the data from TAO/TRITON generally have positive impacts complementary to Argo floats. In the equatorial Pacific, the impacts are generally around the same level or larger than those of Argo. We therefore conclude that, with the current configuration of TPOS, the loss of the TAO/TRITON data is having a significant detrimental impact on many applications based on ocean DA systems. This conclusion needs to be kept under review because the equatorial coverage by Argo is expected to improve in the future.

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.

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.

Hoteit, I, Cornuelle B, Heimbach P.  2010.  An eddy-permitting, dynamically consistent adjoint-based assimilation system for the tropical Pacific: Hindcast experiments in 2000. Journal of Geophysical Research-Oceans. 115   10.1029/2009jc005437   AbstractWebsite

An eddy-permitting adjoint-based assimilation system has been implemented to estimate the state of the tropical Pacific Ocean. The system uses the Massachusetts Institute of Technology's general circulation model and its adjoint. The adjoint method is used to adjust the model to observations by controlling the initial temperature and salinity; temperature, salinity, and horizontal velocities at the open boundaries; and surface fluxes of momentum, heat, and freshwater. The model is constrained with most of the available data sets in the tropical Pacific, including Tropical Atmosphere and Ocean, ARGO, expendable bathythermograph, and satellite SST and sea surface height data, and climatologies. Results of hindcast experiments in 2000 suggest that the iterated adjoint-based descent is able to significantly improve the model consistency with the multivariate data sets, providing a dynamically consistent realization of the tropical Pacific circulation that generally matches the observations to within specified errors. The estimated model state is evaluated both by comparisons with observations and by checking the controls, the momentum balances, and the representation of small-scale features that were not well sampled by the observations used in the assimilation. As part of these checks, the estimated controls are smoothed and applied in independent model runs to check that small changes in the controls do not greatly change the model hindcast. This is a simple ensemble-based uncertainty analysis. In addition, the original and smoothed controls are applied to a version of the model with doubled horizontal resolution resulting in a broadly similar "downscaled'' hindcast, showing that the adjustments are not tuned to a single configuration (meaning resolution, topography, and parameter settings). The time-evolving model state and the adjusted controls should be useful for analysis or to supply the forcing, initial, and boundary conditions for runs of other models.

Schneider, N, Cornuelle BD.  2005.  The forcing of the Pacific decadal oscillation. Journal of Climate. 18:4355-4373.   10.1175/jcli3527.1   AbstractWebsite

The Pacific decadal oscillation (PDO), defined as the leading empirical orthogonal function of North Pacific sea surface temperature anomalies, is a widely used index for decadal variability. It is shown that the PDO can be recovered from a reconstruction of North Pacific sea surface temperature anomalies based on a first-order autoregressive model and forcing by variability of the Aleutian low, El Nino-Southern Oscillation (ENSO), and oceanic zonal advection anomalies in the Kuroshio-Oyashio Extension. The latter results from oceanic Rossby waves that are forced by North Pacific Ekman pumping. The SST response patterns to these processes are not orthogonal, and they determine the spatial characteristics of the PDO. The importance of the different forcing, processes is frequency dependent. At interannual time scales, forcing from ENSO and the Aleutian low determines the response in equal parts. At decadal time scales, zonal advection in the Kuroshio-Oyashio Extension, ENSO, and anomalies of the Aleutian low each account for similar amounts of the PDO variance. These, results support the hypothesis that the PDO is not a dynamical mode, but arises from the superposition of sea surface temperature fluctuations with different dynamical origins.