Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data

Transcription

1 Journal of Oceanography Vol. 52, pp. 389 to Short Contribution Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data TAKAYUKI ISHIKAWA 1 *, JINRO UKITA 1 **, KAY I. OHSHIMA 1, MASAAKI WAKATSUCHI 1, TAKASHI YAMANOUCHI 2 and NOBUO ONO 2 1 The Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido 060, Japan 2 National Institute of Polar Research, Itabashi-ku, Tokyo 173, Japan (Received 14 March 1995; in revised form 27 November 1995; accepted 29 November 1995) Coastal polynyas off East Queen Maud Land in Antarctica are examined using NOAA AVHRR infrared data. From image analyses, two locations of coastal polynyas in this region are identified; one in Breid Bay and the other along the shelf break. The areal coverage of the Breid Bay polynya is significantly related to the strength of katabatic winds, which maintain their strength over the coastal sea due to land topography favoring for their confluence, thereby being capable of removing newly formed ice. Land fast ice in the eastern part of the bay also plays an additional role in the formation mechanism. It is also found that the areal coverage of coastal polynyas in this region fluctuate coherently. Moreover, these fluctuations correspond to the synoptic index, which measures the strength of the offshore wind, with their peaks closely associated with the areal peaks. These facts strongly suggest the influence of synoptic scale weather on the formation and maintenance of polynyas in this region. 1. Introduction Polynyas are areas that remain either partially or totally ice-free in otherwise ice-covered ocean. Due to the exposure to the cold air, polynyas are the sites of active heat and gas exchange between the atmosphere and ocean, often resulting in heavy ice production (Zwally et al., 1985; Gordon and Comiso, 1988). Thus the knowledge of formation and maintenance of polynyas is essential to understand the role that sea ice plays in the physical, chemical, and biological processes in high latitudes (Smith et al., 1990). For classification, the one which occurs between coast and pack ice is referred to as a shore polynya, and the one between fast and pack ice is referred to as a flaw polynya (World Meteorological Organization, 1970). Both types of polynyas are simply referred to as coastal polynyas in this paper. Coastal polynyas are generally formed and maintained by continual removal of newly formed ice by winds and/or currents. In the Antarctic coastal regions, the prevailing ocean currents and winds are westward, thereby forming polynyas on the west side of capes and/or along the eastern part of protected bays in the Antarctic (Knapp, 1972). They can be also maintained by the combined effects of katabatic wind and topography. Kurtz and Bromwich *Present address: Toshiba Co., Shimoishigami 1385, Ohtawara, Tochigi 224, Japan. **Present address: Department of Earth and Planetary Physics, Faculty of Science, The University of Tokyo 113, Japan.

2 390 T. Ishikawa et al. (1983) and Bromwich and Kurtz (1984) showed that the Terra Nova Bay polynya is maintained by katabatic wind and the blocking effect by the Drygalski Ice Tongue, which prohibits ice to drift into the bay. Because of difficulties associated with in-situ observations in high latitudes, remote sensing has been used for polynya studies. For example, Cavalieri and Martin (1985) and Zwally et al. (1985), using the microwave radiometer data, examined the variability of the Antarctic coastal polynyas. As the typical width of coastal polynyas is O (1 km 10 km), the microwave data is only marginally effective to resolve them. Despite the operational limitation imposed by cloud conditions, the Advanced Very High Resolution Radiometer (AVHRR) can provide information about polynyas with its high resolution. This paper analyzes coastal polynyas off East Queen Maud Land, using the infrared (Channel 4, µm) NOAA AVHRR data received daily at Syowa Station during the Japanese Antarctic Climate Research Project ( ) (Yamanouchi and Seko, 1992). 2. Calculation for Ice Concentration Figure 1 shows a typical infrared image used in this study with topography, which consists of pixels. Brightness temperature for each pixel, which is equivalent to approximately an area of 2.2 km by 2.2 km (precise only at a center of image due to curvature effects), provides mixed information about sea ice and open water. Then by inverting the relationship among the brightness temperature, ice concentration, and surface temperature, we can compute ice concentration from an observed brightness temperature (see Ishikawa, 1991, for detailed discussion on the method and its validity). Suppose that (1) one pixel contains sea ice of uniform properties and open water, with ice concentration M, (2) surface temperature of sea ice for the domain is uniform at T ice, (3) temperature of open water is at the freezing point, T f = 1.8 C, and (4) emissivities of sea ice and water are unity (brightness and physical temperatures are identical). The last assumption is reasonable for the polar regions because of low water vapor contents (Tanaka et al., 1985). For the second assumption, the actual size of domain for analyses is less than 70 km, thus having a better chance for a homogeneous surface temperature field. There is also some supporting evidence that the brightness temperature of first-year land fast ice is close to that of pack ice in this region (the difference is less than 5 degrees, which result in an approximately 10 percent error in ice concentration at 75 percent concentration, i.e. Yamanouchi and Wada, 1992). Under these conditions, observed brightness temperature, T obs, is related to ice concentration and surface temperatures by Thus, once T ice is known, M can be computed from T obs = MT ice + ( 100 M)T f. ( 1) 100 M = 100 ( T obs T f ). ( 2) T ice T f We apply this simple algorithm, which has been used for passive microwave images (Zwally et al., 1983), to the AVHRR data.

3 Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data 391 Fig. 1. Infrared AVHRR image (Channel 4) taken on July 18, 1989 with both land and ocean topography. It shows the locations of Syowa Station, Asuka Station, and Station L0. Five areas A E used in the analysis are indicated by square brackets. A and D corresponds to Breid Bay and Otone-Suiro, respectively. Each area is defined by a square of 30 by 30 pixels. Blue and yellow tones indicate higher and lower brightness temperatures, respectively. 3. Polynyas over the Shelf Break It has been known that a flaw polynya frequently occurs off Prince Olav Coast (called Otone-Suiro by early Japanese Antarctic expeditions). Figure 1 shows an infrared image over this region. In the figure, blue and yellow tones are associated with higher and lower brightness temperatures, respectively. Note that a band with high brightness temperature lies over the shelf break off Prince Olav Coast at the depth between m. This narrow band is sandwiched by pack ice on the offshore side and land fast ice on the onshore side, indicating it as a flaw polynya. An image analysis shows that the temperature of the band is lower than the freezing temperature, indicating a partial ice coverage. According to other images from 1987 to 1990, the same feature was also observed. In fact, there is an evidence that the land fast ice margin is located along the shelf break almost year-round, except during a short period from late summer to fall when the margin retreats (Yamanouchi and Seko, 1992).

4 392 T. Ishikawa et al. 4. Breid Bay Polynya It is also found in Fig. 1 that a polynya occurs in Breid Bay (hereafter referred to as the Breid Bay polynya ), which will be closely examined on the basis of infrared images by the algorithm described in Section 2. As mentioned previously, the determination of T ice is crucial in this algorithm. It is assumed that T ice is given by the surface temperature of land fast ice in the eastern Fig. 2. Ice concentration around Breid Bay for five occasions in 1989, with the coastline map. Note that for the analysis on Breid Bay, an area defined by 20 by 20 pixels is used.

5 Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data 393 part of the bay (indicated by a white arrow in the middle-left plot of Fig. 2) based upon two reasons. First, NOAA satellite images for the previous summer reveal that land fast ice at this particular location is actually made of first year ice, and thus thought of having the similar surface temperature to that of the surrounding drifting ice (Yamanouchi and Wada, 1992). More importantly, this particular choice for T ice gives a reasonable distribution on ice concentration; i.e. the maximum concentration for an ice region should not grossly exceed 100%. To see the seasonal variability, Fig. 2 presents ice concentration of Breid Bay and its surrounding area on five different occasions in There persistently appear areas of low ice concentration in the southern part of the bay, and of high ice concentration corresponding with land fast ice. The mean current off Breid Bay is westward along the bottom topography (Ohshima et al., in press). Thus, land fast ice in the eastern part of the bay (see the high concentration area on the image of May 26 in Fig. 2) blocks off drifting ice from entering the bay. This blocking mechanism seems to contribute to maintain this polynya, similar to the Terra Nova Bay polynya. Next, we consider effects by oceanic heat and winds on this polynya. First, even in the summer, an observed water temperature in Breid Bay is near the freezing point throughout its water column, except in the very top surface layer (Iwanami and Tohju, 1987). From this observation, it is likely that during the winter the temperature for the entire water column is dropped to the freezing point. This excludes a possibility that this polynya is maintained by oceanic heat. Secondly, we observe an area with high brightness temperature on the western slope of a valley on the land just south of Breid Bay (see Fig. 1). It is known that katabatic winds tend to be intensified over such a topographic feature from adiabatic warming. The analyses on the pattern of Sastrugi (Kikuchi and Ageta, 1989) and infrared image with a high-pass filter (Yamanouchi and Seko, 1992) also provide strong evidence for the confluence of katabatic winds toward Breid Bay. Bromwich and Kurtz (1984) discussed a similar condition occurred in Terra Nova Bay, and argued that in a confluence area katabatic winds do not easily die down from hydraulic jumps, thus capable of removing newly formed ice for a large area off the coast. Fig. 3. A scatter plot for the area covered by the Breid Bay polynya and the wind speed at Station L0. The polynya coverage is defined as an area with ice concentration less than 50% within the domain marked by a dashed square in Fig. 2. A solid line is a regression line computed by the least square method.

6 394 T. Ishikawa et al. This hypothesis is directly tested for the Breid Bay polynya, using an area estimate for the polynya from images and wind data acquired at an unmanned weather station on the plateau near Breid Bay (marked as L0 in Figs. 1 and 2). An analysis shows a southerly (katabatic) wind prevails throughout the year with an average speed of 12 m/s. Figure 3 shows the relationship between the area of the Breid Bay polynya and the wind speed at the station. Here, the Breid Bay polynya is identified as an area with low ice concentration (<50%) within the domain marked by the dashed square in Fig. 2. Each cross in Fig. 3 represents one day sample. A solid line indicates a regression line given by the least square method, suggesting significant dependence of the polynya area on katabatic wind (the significance level for the presence of a linear relationship is over 95%). In summary, the combined effects of katabatic winds and land fast ice, in a manner similar to the one for the Terra Nova Bay polynya, influence the formation and maintenance of the Breid Bay polynya; katabatic winds remove newly formed ice northward while land fast ice prevents drifting pack ice from entering the bay. Fig. 4. (a) Synoptic index (the pressure at Asuka Station minus the pressure at Syowa Station with a height correction) and (b) the areal coverage of polynyas in A (crosses), B (squares), C (triangles), and D (circles) for the period between September 9 and September 19. Black squares indicate days without observations due to cloudy conditions. Likewise, no observation was available for E during this period.

7 Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data Relationship with Synoptic Scale Wind Here, we examine the spatial and time scales that might exist in the relationship between the areal coverage of polynyas and atmospheric forcing, using five different areas marked by square brackets in Fig. 1. Note that Areas A and D correspond to the Breid Bay polynya and Otone- Suiro, respectively. Atmospheric forcing is measured by the synoptic index, which is defined by the pressure at Asuka Station minus the pressure at Syowa Station (see Fig. 1 for their locations) with a height correction. Positive and negative index values are closely associated with offshore and onshore geostrophic winds. In what follows, we analyze synoptic influence on the polynya formation on the basis of three different periods in Figure 4 shows (a) synoptic index, and (b) the areas of low ice concentrations in A to D during the period between September 9 19, with black squares indicating days without observations due to cloudy conditions. During this 11-day period, the weather condition had been exceptionally Fig. 5. (a) Synoptic index (the pressure at Asuka Station minus the pressure at Syowa Station with a height correction) and (b) the areal coverage of polynyas in A (crosses), C (triangles), D (circles), and E (diamonds) for the period between October and November 6, There was no observation for B during this period due to cloud conditions. Likewise, the days marked by black squares were without observations.

8 396 T. Ishikawa et al. favorable for satellite observations. Note that in spite of distance, the four patterns of the areal coverage from different locations fluctuate coherently. Given the distance between A and D (approximately 700 km), this coherence suggests the influence of a forcing mechanism with a scale larger than 700 km, perhaps the effect by the synoptic scale atmospheric forcing, on the polynya formation. In fact, this point is evidenced by the correspondence between synoptic and areal peaks. The synoptic peak of September (upper figure) matches with the areal peaks of September 15 for A and of September 16 for B to D (lower figure). This is interpreted that the areal peak is associated with offshore winds, which likely remove ice and create a polynya. Figure 5 presents the same relationship between the synoptic index and areas of low ice concentration for the period between October 26 and November 6. Also observed in this figure are the coherence of areal fluctuations from different areas and the correspondence between synoptic and areal peaks. The synoptic peak of November 1 is followed by the areal peak on November 2. Finally, Fig. 6 illustrates the same relationship for June, where the areal peak around June 8 follows the synoptic peak on June 7. Fig. 6. (a) Synoptic index (the pressure at Asuka Station minus the pressure at Syowa Station with a height correction) and (b) the areal coverage of polynyas in A (crosses) and D (circles) for the month of June Black squares indicate days without observations due to cloudy conditions.

9 Coastal Polynyas off East Queen Maud Land Observed from NOAA AVHRR Data 397 The results from the above three periods are consistent: (1) the areal fluctuations from different locations are coherent, (2) the areal coverage of polynyas is related to the synoptic index, and (3) areal peaks occur on the same or the following day after synoptic peaks. From these, it is clear that the areal fluctuations of polynyas are influenced by synoptic scale winds. 6. Summary and Discussion Using infrared NOAA AVHRR images, we investigate the characteristics of the coastal polynyas off East Queen Maud Land. From the information about sea ice concentration, two coastal polynyas are identified, one in Breid Bay and the other along the shelf break. It is found that the areal coverage of the Breid Bay polynya is significantly correlated with the strength of katabatic winds. Like the Terra Nova Bay polynya, the Breid Bay polynya seems to be formed by katabatic winds, which are reinforced by the confluence near the bay. Land fast ice in the eastern part of Breid Bay also plays the blocking role against drifting pack ice that is moving westward. It is also found that the areal coverage of coastal polynyas fluctuate coherently over this region. Moreover, these fluctuations correspond to the pressure field. These facts strongly suggest the influence of synoptic scale weather on the formation of polynyas. For the Breid Bay polynya, the influence by katabatic winds is prominent. On the other hand, it has been argued that synoptic scale weather also affects katabatic winds (Yasunari and Kodama, 1993). Thus it is difficult to assess the extent to which synoptic conditions contribute to the polynya formation as relative to katabatic winds themselves. Even though the relationship between synoptic scale winds and shelf break polynyas is identified, we have not yet understood why they appear along the shelf break, or why the fast ice margin occurred along the shelf break? It is plausible to answer these questions by considering some ocean processes that are affected by bottom topography. From the observation on the water temperature, melting by oceanic heat has been ruled out as a possible mechanism (Ushio et al., 1993). That leaves processes with a dynamics origin such as current shear as possible mechanisms for those phenomena. According to satellite and aircraft observations, the fast ice margin appears fairly straight, suggesting it as a shear crack. If current shear is intensified along the shelf break by topographic effects, fast ice cracks and its margin are likely kept along the shelf. Again, precise understanding needs both current measurements and more theoretical interpretation of shelf dynamics coupled with ice dynamics. Finally, it must be pointed out that there may be some bias in polynya estimates by the AVHRR data because an ice field can be seen only on cloud-free conditions. In Figs. 4 to 6, cloudfree (less cloudy) days are generally associated with the positive synoptic index. For example, for June, the mean synoptic index from days with satellite observations is 3.7, which is significantly larger than 1.45 that is from days without observations (the significance level is over 95%). Because the cloud-free condition is associated with offshore winds, the image analysis possibly overestimates the magnitude of polynyas. To minimize such a bias, the combined use of the SSM/I and/or SAR data in addition to data AVHRR data seems necessary for future investigation. Acknowledgements This work is based on the M. Sci. thesis of TI while he was at Hokkaido University in We would like to thank K. Seko and H. Kakegawa for providing NOAA data, and T. Endoh and T. Kikuchi for unmanned weather data. Thanks are also extended to T. Takizawa and S. Ushio for their providing fruitful discussion. The original manuscript was written while KO was a

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