A numerical study of the effects of wind and upstream conditions on the Hawaiian circulation

Andrei Natarov (IPRC, University of Hawaii) and Brian Powell (University of Hawaii)

Abstract

We compare the effects of varying both boundary conditions and wind forcing resolution on a high-resolution Regional Ocean Modeling System (ROMS) designed for the Hawaiian Islands. This regional model uses both Navy Coupled Ocean Model (NCOM) and Hybrid Coordinate Ocean Model (HYCOM) lateral forcing. We find that the resulting circulation is sensitive to both wind and lateral boundary conditions. Higher resolution wind product drives stronger overall circulation in the upper ocean. While the eastward current in the lee of the islands is driven directly by the wind stress curl, westward currents north and south of this flow are enhanced due to mismatch in the velocity prescribed at the western boundary of the domain and the velocity of the wind-driven flow.

Accompanying the differences in the mean circulation patterns are the differences in sea surface temperature (SST), surface salinity (SSS) and surface heat fluxes. Experiments driven by the higher resolution winds have higher heat fluxes and SSTs in the lee of the Islands than lower resolution wind driven counterparts. The surface salinity is lower on the lee side of the Islands, and higher on the windward side for the higher resolution winds. Validation tests consist of comparing the means and variances of SST and SSH fields, as well as temperature and salinity fields with ARGO and Hawaii Ocean Time-Series (HOT) hydrography, and computing standard rms and correlations statistics between the observations and experiments.

The comparison with AVHRR data shows that all four experiments are good at capturing the seasonal cycle; however, there is a warm bias in the monthly means of the model runs during winter months. There is a shift of warmer SST in the Lee of the Big Island with the higher resolution wind forcing, which is different from observations. It is found that HYCOM-driven models generally have a better comparison to Aviso SSH, with higher-resolution winds improving the comparison. The vertical T/S profiles compare favorably with hydrographic observations, especially the temperature profiles. For salinity profiles it is found that runs with NCOM boundary conditions capture salinity profiles in the upper portion of the thermocline very well, while overdiffusing water masses at the base of the thermocline. In contrast, runs with HYCOM boundary conditions do not capture the salinity structure in the uppermost part of the ocean, but capture the stratification at the base of thermocline very well.