Kristina Ahlnäs

Eddies in the Kamchatka Current

Abstract Visible images from the National Oceanic and Atmospheric Administration (NOAA) satellites show intense vortices, which we believe to be oceanic eddies, in the Kamchatka Current in winter. Infra-red images from the same satellites show eddies in the Kamchatka Current, and trains of eddies covering much of the western Bering Sea, in autumn. Such thermal features, which were not explicitly taken into account in previous oceanographic studies of this region, might cause the exceptionally high space and time variability in computed geostrophic transports and indicate a need for a new approach in attempts to clarify the nature of the net circulation and its seasonal and year-to-year variability.

Application of satellite visible band data to high latitude oceans

Abstract Remote sensing of high latitude oceans differs from oceans of tropical and subtropical latitudes because of the decoupling of the waters temperature structure from the flow dynamics at low water temperatures. Sea surface thermal patterns are useful here as tracers of the flow only, without a direct link to the circulation dynamics. Oceanic flow responds to water density contrasts and at high latitudes the density is controlled by salinity distributions. Though density contrasts are not ordinarily detectable with remote sensors, freshwater discharge in the Northeast Pacific contains very fine sediments (glacial flour) that can be detected using Landsat Thematic Mapper (TM) and Multi Spectral Scanner (MSS) imagery. From the TM data we have discovered several new coastal circulation features in the northern Gulf of Alaska. Flow around a coastal island produces a myriad of dipole eddies which are important to the cross-shelf transport of salt and heat. They also serve as sites to test numerical models to allow a better understanding of coastal flows and eddy formation elsewhere. Detection of these dipole eddies from their sediment distributions has allowed the comparison of different spectral bands from six imaging satellite sensors for their abilities to describe these features. We conclude that, in addition to the TM sensors capability, the sediments can be detected with the MSS and Coastal Zone Color Scanner (CZCS) sensors, but not with AVHRR or Defense Meteorological Satellite Program (DMSP). As a consequence of this comparison of satellite visible data, we have confirmed that the dipole eddies here exist under low flow conditions ( 3 /s).

The plume of the Yukon River in relation to the oceanography of the Bering Sea

Abstract Physical and biological oceanography of the northern Bering Sea including the plume of the Yukon River were studied using satellite data in conjunction with shipboard measurements. The satellite data recorded by the NOAA Very High Resolution Radiometer (VHRR) and Advanced Very High Resolution Radiometer (AVHRR), and the Landsat Multispectral Scanner (MSS) and Thematic Mapper (TM) sensors were used to detect sea surface temperatures and suspended sediments. Shipboard measurements of temperature, salinity, and chlorophyll were acquired by the Inner Shelf Transfer and Recycling (ISHTAR) project and were compared to digitally enhanced and archived satellite images. Sea surface temperatures derived from satellite data were generally higher than field measurements. This difference was the likely result of microscale stratification of the water column, although other factors could have been involved also. Satellite data confirmed the known distribution of water masses in the region (Coachman et al., 1975). Upwelled water of oceanic origin (Anadyr Water) was visible as a cold surface plume running from Anadyr Strait north through western Bering Strait. In the east, warm Alaskan Coastal Water was prominent along the Alaskan coast including Norton Sound and the southeastern Chukchi Sea. Areal patterns of temperature, salinity, and phytoplankton distribution, determined from field measurements, agreed reasonably well with patterns of water mass distribution obtained from satellite images. Archived satellite images (1974–1978) were used to investigate the variability of the distribution of sea surface temperature and of the turbid plume of the Yukon River. Alaskan Coastal Water (ACW) first warms near the coast in June and the process extends seaward as summer progresses. Turbid water associated with discharge of the Yukon River progresses in the same fashion, extending northward across the entrance to Norton Sound. Maximum extent of the plume occurs in October. Anadyr Water flows north through Anadyr Strait past St. Lawrence Island; its extent is variable depending on mesoscale pressure and local wind fields.

Evaluation of the ability of various remote sensors to map distributions of suspended sediments in the Gulf of Alaska

Abstract The coastal areas of the Gulf of Alaska have been routinely surveyed by satellite since 1974. Through the use of suspended sediments as tracers, a new feature has been detected recently in the coastal flow near Kayak Island. The three visible channels of the Landsat Thematic Mapper (TM) revealed mushroom shaped dipole eddies in a nearshore current on 22 April 1985. Five of these double eddies were well defined. Three faced ocean-ward east of Kayak Island, and two faced shore-ward west of the island. Dipole eddies have been seen elsewhere before, but never as numerously as those around Kayak Island. These eddies are also unusual in that they do not have a thermal signature. The features were seen in detail in bands 1 and 2 of the TM and in band 1 of the Landsat Multispectral Scanner (MSS). The outlines of the eddies were seen in TM Band 3 and in MSS Band 2. The size of the eddies (diameter around 20 km) is large enough that they could be seen by the more frequently flying sensors of lower resolution satellites such as the NOAA-Advanced Very High Resolution Radiometer (AVHRR). However, the radiometric resolution of the visible band of the AVHRR was not sensitive enough to distinguish them. The purpose of this study is to investigate the potential of various satellite imaging systems to detect surface suspended sediments. The overlapping spectral ranges of several imaging satellites are compared for their ability to detect low concentrations of suspended sediments.