[1] The impact of global Navy Layered Ocean Model (NLOM) sea surface height (SSH) on global Navy Coastal Ocean Model (NCOM) nowcasts of ocean currents is investigated in a series of experiments. The studies focus on two primary aspects: the role of NLOM horizontal resolution and the role of differences between the SSH means in NLOM and the Modular Ocean Data Assimilation System (MODAS) climatology. To evaluate the impact of changes to the assimilation system, we compare observed drifter trajectories with trajectories simulated using global NCOM over 7-day timescales. The results indicate general improvement in NCOM currents as a result of increasing NLOM horizontal resolution. The effects of accounting for the differences between NLOM and MODAS mean that SSH is less clear, with some regions showing a decline in simulation skill while others show improvement or little impact. These outcomes supported recommendations for operational transition to 1/32°global NLOM with continued refinement to methods accounting for differences in mean SSH. (2007), Evaluation of ocean models using observed and simulated drifter trajectories: Impact of sea surface height on synthetic profiles for data assimilation,

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Global ocean climatology Temperature and salinity observations Since the 1970s, the U.S. Navy has maintained a state-of-the-art gridded global monthly full-depth climatology of temperature and salinity and their standard deviations called the Generalized Digital Environmental Model (GDEM). The present document describes the development and evaluation of GDEM4, the newest version of GDEM. As part of the evaluation of GDEM4, comparisons are made in this report to GDEM3 and to four other ocean climatologies: the NODC World Ocean Atlas (WOA2005), the NODC 1/4 degree resolution climatology, the WOCE Global Hydrographic Climatology (WGHC), and the NRLMODAS 2D (MODAS2D) surface temperature climatology. GDEM4 combines the methodology originally developed for GDEM3 with an expanded observation data set. In the evaluations performed, the GDEM4 climatology has been shown to be a significant improvement over GDEM3 and to compare favorably and in many ways improve on fields from the other climatologies used in the evaluations.

In the NW Atlantic, the temporal mean circulation results in large amplitude surface topography precluding the reasonable use of mean altimetric surfaces or profiles for precise estimation of the geoid. We examine a procedure which (1) uses airborne expendable bathythermograph (AXBT) sections collected along altimeter ground tracks to estimate the single‐pass or instantaneous surface dynamic topography, (2) employs concurrent or simultaneous satellite altimeter overflights to provide a measure of instantaneous sea level, and (3) arrives at a precise estimate of the alongtrack geoid profile by differencing the instantaneous altimetric sea level and the AXBT‐derived dynamic topography. This technique is applied to several Geosat Exact Repeat Mission (ERM) ground tracks in the area of the NW Atlantic Regional Energetics Experiment (REX) using AXBT survey data collected in April and July 1987. Geoid profile estimates are repeatable between these two independent data sets to within 10–20 cm rms. This is near the estimated noise level due to barotropic variability in the Gulf Stream region. These AXBT/Geosat‐derived profiles provide a more consistent estimate of the alongtrack geoid gradient than do available gravimetric geoids. Analysis of the statistics of the ensemble of collinear passes provides an estimate of the geographically uncorrelated orbit error of approximately 1–2 m. The total surface topography derived as the difference between adjusted instantaneous sea level profiles and the AXBT/Geosat‐derived geoid profiles does not suffer significant contamination by either geographically uncorrelated or correlated orbit error.

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