Robert W. Helber

Satellite-Derived Surface Current Divergence in Relation to Tropical Atlantic SST and Wind

The relationships between tropical Atlantic Ocean surface currents and horizontal (mass) divergence, sea surface temperature (SST), and winds on monthly-to-annual time scales are described for the time period from 1993 through 2003. Surface horizontal mass divergence (upwelling) is calculated using surface currents estimated from satellite sea surface height, surface vector wind, and SST data with a quasi-linear, steadystate model. Geostrophic and Ekman dynamical contributions are considered. The satellite-derived surface currents match climatological drifter and ship-drift currents well, and divergence patterns are consistent with the annual north–south movement of the intertropical convergence zone (ITCZ) and equatorial cold tongue evolution. While the zonal velocity component is strongest, the meridional velocity component controls divergence along the equator and to the north beneath the ITCZ. Zonal velocity divergence is weaker but nonnegligible. Along the equator, a strong divergence (upwelling) season in the central/eastern equatorial Atlantic peaks in May while equatorial SST is cooling within the cold tongue. In addition, a secondary weaker and shorter equatorial divergence occurs in November also coincident with a slight SST cooling. The vertical transport at 30-m depth, averaged across the equatorial Atlantic Ocean between 2°S and 2°N for the record length, is 15(6) 10 6 m 3 s 1 . Results are consistent with what is known about equatorial upwelling and cold tongue evolution and establish a new method for observing the tropical upper ocean relative to geostrophic and Ekman dynamics at spatial and temporal coverage characteristic of satellite-based observations.

Equatorial upwelling in the western Pacific warm pool

Vertical velocity on the equator in the western Pacific warm pool is investigated using data from the Coupled Ocean-Atmosphere Response Experiment enhanced monitoring array (EMA) centered at 0°, 156°E. The data consist of hourly subsurface horizontal velocity time series from August 1991 until April 1994. Vertical velocity is calculated using horizontal velocity components and the application of the continuity equation. During the first year, from March 1992 until February 1993, data are available from five moorings of the EMA and thus provide nine different combinations of moorings from which to calculate vertical velocity. Four moorings were available during the remaining time period. Random errors are found to be <10−5 m s−1, while systematic errors (finite difference error, systematic instrument error, and error due to surface extrapolation) may be larger. It is suggested that errors, including finite difference errors, are not larger than the vertical velocity estimate. The estimates of vertical velocity are valid on spatial scales the size of the array (∼400 km) and timescales longer than a few days. They reveal a seasonal cycle manifested during a moderate El Nino. Results indicate upwelling, on average from 70 m down to 250 m over the 2 year time period, being slightly stronger in 1992 coincident with the stronger El Nino year. The divergence of horizontal velocity components, resulting in positive vertical velocity, is due to geostrophic divergence on the equator produced from a westward directed zonal pressure gradient force. Meridional divergence and zonal wind stress are uncorrelated, suggesting that Ekman convergence due to local westerly winds is only of partial influence. Consequently, downwelling is not found near the surface, where contributions from local winds and geostrophic divergence are in opposition. This estimate of vertical velocity indicates that water is upwelled in the warm pool from much deeper than but with comparable magnitude to the central and eastern Pacific.

Evaluating the Impact of the Number of Satellite Altimeters Used in an Assimilative Ocean Prediction System

Abstract The impact of the number of satellite altimeters providing sea surface height anomaly (SSHA) information for a data assimilation system is evaluated using two comparison frameworks and two statistical methodologies. The Naval Research Laboratory (NRL) Layered Ocean Model (NLOM) dynamically interpolates satellite SSHA track data measured from space to produce high-resolution (eddy resolving) fields. The Modular Ocean Data Assimilation System (MODAS) uses the NLOM SSHA to produce synthetic three-dimensional fields of temperature and salinity over the global ocean. A series of case studies is defined where NLOM assimilates different combinations of data streams from zero to three altimeters. The resulting NLOM SSHA fields and the MODAS synthetic profiles are evaluated relative to independently observed ocean temperature and salinity profiles for the years 2001–03. The NLOM SSHA values are compared with the difference of the observed dynamic height from the climatological dynamic height. The syntheti...

Overview of U.S. Navy Operational Oceanographic Models in Support of Acoustic Applications

The Naval Oceanographic Office operational global 1/8° Navy Coastal Ocean Model assimilates satellite and in‐situ data to produce daily 72‐hr forecasts. Output includes 3D fields of temperature, salinity, u‐ and v‐components of ocean currents at standard depth levels, and these support derived fields including sound speed and sonic layer depth. The global model provides initial/boundary conditions for nested regional models, primarily relocatable NCOM. The relocatable NCOM modeling system can be set up quickly for areas of interest, includes river and tidal forcing, and is forced with a high‐resolution atmospheric mesoscale model. Local and remote observations are incorporated into the models through the Navy Coupled Ocean Data Assimilation system, which assimilates sea surface temperature data from satellite, ships and buoys, profile data from floats and gliders, xbts, CTDs, fixed and drifting buoys as well as altimeter‐derived sea surface heights and ice concentration. In this presentation we will dis...