Michael G. G. Foreman

North Pacific internal tides from the Aleutian Ridge: Altimeter observations and modeling

Internal tides radiating into the North Pacific from the Aleutian Ridge near Amukta Pass are examined using 7 years of Topex/Poseidon altimeter data. The observations show coherent southward phase propagation at the M 2 frequency over a distance of at least 1100 km into the central Pacific. Barotropic and baroclinic models are applied to study this internal tidal signal. Results from the barotropic model show that the strongest cross-slope volume and energy fluxes occur in the vicinity of Amukta Pass, helping to establish this region as an important site for baroclinic energy conversion along the eastern half of the ridge. A two-dimensional version of the Princeton Ocean Model is used to simulate internal tide generation and propagation. A comparison between the altimeter data south of the ridge and the sea-surface signature of the internal tide signal of the model shows good agreement for the phase, both close to the source and well into the far field. Comparison of the phase between model and data also provides evidence for wave refraction. This occurs due to the slow modulation of wavelength associated with the variation in the Coriolis parameter encountered as the internal tide propagates southward. The model results suggest that the net rate of conversion of barotropic to baroclinic energy is about 1.8 GW in the vicinity of Amukta Pass. This represents about 6% of the local barotropic energy flux across the ridge and perhaps 1% of global baroclinic conversion.

Multi‐year meanders and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter

The birth, life and decay of long-lived anticyclonic eddies, or meanders, in the Alaskan Stream are observed from TOPEX/Poseidon (T/P) satellite. Their surface height anomalies approach 72 cm, average diameter is about 160km and they live for 1 to 3 years, propagating along the Stream with mean speed of 2.5 km d−1. Of the six observed, the last three were formed after the first T/P observations in September 1992. Of these, two evolved from eddies near Alaskan Panhandle in winter, drifting across the northern Gulf of Alaska before entering the Alaskan Stream, while another formed in the Stream west of Shelikof Strait.

Versatile Harmonic Tidal Analysis: Improvements and Applications

Abstract New computer software that permits more versatility in the harmonic analysis of tidal time series is described and tested. Specific improvements to traditional methods include the analysis of randomly sampled and/or multiyear data; more accurate nodal correction, inference, and astronomical argument adjustments through direct incorporation in the least squares matrix; multiconstituent inferences from a single reference constituent; correlation matrices and error estimates that facilitate decisions on the selection of constituents for the analysis; and a single program that analyzes one- or two-dimensional time series. This new methodology is evaluated through comparisons with results from old techniques and then applied to two problems that could not have been accurately solved with older software. They are (i) the analysis of ocean station temperature time series spanning 25 yr, and (ii) the analysis of satellite altimetry from a ground track whose proximity to land has led to significant data d...

Global M2 internal tide and its seasonal variability from high resolution ocean circulation and tide modeling

[1] The present study describes a model simulation, where ocean tide dynamics are simulated simultaneously with the ocean circulation. The model is forced by a lunisolar tidal forcing described by ephemerides and by daily climatological wind stress, heat, and fresh water fluxes. The horizontal resolution is about 0.1 and thus, the model implicitly resolves meso-scale eddies and internal waves. In this model simulation the global M2 barotropic to baroclinic tidal energy conversion amounts to 1.2 TW. We show global maps of the surface signature of the M2 baroclinic tide and compare it with an estimate obtained from 19 years of satellite altimeter data. Further, the simulated seasonality in the low mode internal tide field is presented and, as an example, the physical mechanisms causing the non-stationarity of the internal tide generated in Luzon Strait are discussed. In general, this study reveals the impact of inter-annual changes of the solar radiative forcing and wind forced ocean circulation on the generation and propagation of the low mode internal tides. The model is able to simulate non-stationary signals in the internal tide field on global scales which have important implications for future satellite altimeter missions. Citation: Muller, M., J. Y. Cherniawsky, M. G. G. Foreman, and J.-S. von Storch (2012), Global M2 internal tide and its seasonal variability from highresolution ocean circulation and tide modeling, Geophys. Res. Lett., 39, L19607, doi:10.1029/2012GL053320.

Ocean Tides from TOPEX/Poseidon Sea Level Data

Abstract Tidal constants are computed from TOPEX/Poseidon sea level data in the northeast Pacific Ocean for the period of September 1992–December 1997. The method used is harmonic analysis. Tidal constituents are also calculated at track crossover locations, where twice as many observations are available. The crossover constituents are compared with nearest alongtrack constituents and with constituents at 57 pelagic sea level stations in the northeast Pacific. Examples of small-scale features in the alongtrack constituents are presented. These include a strong evidence of internal M2 tides propagating away from the Aleutian Islands and of K1 and O1 shelf waves in the northern Gulf of Alaska. Good agreement was found between the shelf waves observed in altimetry data and those calculated by a numerical tidal model.

Dynamic ocean topography for the northeast Pacific and its continental margins

[1] Estimates of dynamic ocean (or sea surface) topography based on satellite altimetry and gravity observations generally become degraded as they approach land. In this study, dynamic ocean topography for the northeast Pacific Ocean is computed independently of satellite observations using a high resolution model and seasonal climatologies of temperature, salinity, and wind stress. Comparisons with estimates based on satellite gravity and altimetry measurements show reasonable agreement in the deep ocean but are poor on continental margins where the ocean model estimates not only reveal significant seasonal differences, but are also shown to be reasonably accurate when compared with satellite altimetry and coastal tide gauge measurements. The dynamic ocean topography estimates provided here will permit more accurate calculations of the geoid and satellite altimeter absolute heights and currents.

Coastal Oyashio South of Hokkaido, Japan

Abstract Coastal Oyashio Water (COW), the cold low-salinity water lying along the southeast coast of Hokkaido, sometimes flows into Funka Bay on the southwest coast in winter and spring. Because this water is low in density, a density current called the “Coastal Oyashio” was assumed to carry COW along these coasts. However, water property maps for April 1989, February 1992, February 1994, and February 1989 show that only for the latter month did COW extend as far west as Cape Erimo on the southwest coast. Steady-state flows were calculated using a baroclinic model and the density data collected in April 1989. Model results show that the Oyashio carries COW southwestward along the continental slope southeast of Hokkaido. It bifurcates south of Cape Erimo with a coastal portion turning around the cape but not always reaching the southwest coast because of the density structure along that coast. When the Oyashio crosses isobaths, the joint effect of baroclinicity and bottom relief (JEBAR) causes a barotropic...

Seasonal variation of the M2 tide

The seasonal cycle of the main lunar tidal constituent M2 is studied globally by an analysis of a high-resolution ocean circulation and tide model (STORMTIDE) simulation, of 19 years of satellite altimeter data, and of multiyear tide-gauge records. The barotropic seasonal tidal variability is dominant in coastal and polar regions with relative changes of the tidal amplitude of 5–10 %. A comparison with the observations shows that the ocean circulation and tide model captures the seasonal pattern of the M2 tide reasonably well. There are two main processes leading to the seasonal variability in the barotropic tide: First, seasonal changes in stratification on the continental shelf affect the vertical profile of eddy viscosity and, in turn, the vertical current profile. Second, the frictional effect between sea-ice and the surface ocean layer leads to seasonally varying tidal transport. We estimate from the model simulation that the M2 tidal energy dissipation at the sea surface varies seasonally in the Arctic (ocean regions north of 60°N) between 2 and 34 GW, whereas in the Southern Ocean, it varies between 0.5 and 2 GW. The M2 internal tide is mainly affected by stratification, and the induced modified phase speed of the internal waves leads to amplitude differences in the surface tide signal of 0.005–0.0150 m. The seasonal signals of the M2 surface tide are large compared to the accuracy demands of satellite altimetry and gravity observations and emphasize the importance to consider seasonal tidal variability in the correction processes of satellite data.

Modeling Parasite Dynamics on Farmed Salmon for Precautionary Conservation Management of Wild Salmon

Conservation management of wild fish may include fish health management in sympatric populations of domesticated fish in aquaculture. We developed a mathematical model for the population dynamics of parasitic sea lice (Lepeophtheirus salmonis) on domesticated populations of Atlantic salmon (Salmo salar) in the Broughton Archipelago region of British Columbia. The model was fit to a seven-year dataset of monthly sea louse counts on farms in the area to estimate population growth rates in relation to abiotic factors (temperature and salinity), local host density (measured as cohort surface area), and the use of a parasiticide, emamectin benzoate, on farms. We then used the model to evaluate management scenarios in relation to policy guidelines that seek to keep motile louse abundance below an average three per farmed salmon during the March–June juvenile wild Pacific salmon (Oncorhynchus spp.) migration. Abiotic factors mediated the duration of effectiveness of parasiticide treatments, and results suggest treatment of farmed salmon conducted in January or early February minimized average louse abundance per farmed salmon during the juvenile wild salmon migration. Adapting the management of parasites on farmed salmon according to migrations of wild salmon may therefore provide a precautionary approach to conserving wild salmon populations in salmon farming regions.

Flow and temperature models for the Fraser and Thompson Rivers

Abstract High river temperatures have been linked to pre‐spawning mortalities in salmon returning to their natal streams. As a first step in predicting these temperatures, flow and temperature models were developed for the Fraser and Thompson Rivers in British Columbia. The flow model is essentially pre‐calibrated while the temperature model was calibrated against data collected in 1993 and then verified against data from 1994. Root mean square differences between measured and calculated temperatures were found to be 0.70° C at thirteen stations in 1993 and 0.60°C at fifteen stations in 1994. As a purely speculative exercise, the models were then used to investigate the feasibility of using cooler waters from lakes to reduce the warm temperatures recorded in 1994.