921 Journal of Oceanography, Vol. 59, pp. 921 to 929, 2003 Keywords: ⋅ Subarctic region, ⋅ skewed occurrence frequency, ⋅ quality control, ⋅ range check, ⋅ water intrusion. * Corresponding author. E-mail: oguma@mirc.jha.or.jp Copyright © The Oceanographic Society of Japan. Skewed Occurrence Frequency of Water Temperature and Salinity in the Subarctic Regions SACHIKO OGUMA 1 *, TORU SUZUKI 1 , SYDNEY LEVITUS 2 and YUTAKA NAGATA 1 1 Marine Information Research Center, JHA, 7-15-4 Ginza, Chuo-ku, Tokyo 104-0061, Japan 2 World Data Center for Oceanography, NODC/NOAA, Silver Spring, MD 20910-3282, U.S.A. (Received 10 May 2003; in revised form 18 August 2003; accepted 19 August 2003) In a previous paper (Oguma and Nagata, 2002), it was shown that frequency distri- butions of temperature and salinity in the sea off Sanriku Coast, Japan are skewed, and sometimes observed values exceed m + 5σ (m = mean, σ = standard deviation). This means that, if we apply a 3σ criterion for a range check, many real data would be lost. We have expanded our analysis to the subarctic North Pacific, the subarctic North Atlantic and their surrounding areas, by computing the distributions of skewness and kurtosis. It is found that the region of high positive skewness extends in an east- north-east direction in the Mixed Water Region from off Sanriku, and reaches to about 155°E. A high negative skewness zone is recognized along the southern margin of the Kuroshio Extension. These are thought to be generated by the breaking of the meander of the Kuroshio Extension and subsequent ejection of warm and cold eddies to the north and south, respectively. Other high positive skewness areas are found to the south of Kuril Islands and in the Japan Sea. These are generated due to very sharp vertical gradients of temperature and salinity. The situation in the North At- lantic is very similar to the North Pacific, though the detailed nature is changed due to differences of oceanographic condition. The effect of grid size on the skewed na- ture of the distribution is also discussed. Therefore, if we apply a 3σ criterion, and delete the data which lie outside of the range m ± 3σ, many of the real data will be lost. We represent the non-normal nature of observed fre- quency distributions by using skewness and kurtosis, and expand our region of analysis to the subarctic North Pa- cific Ocean and subarctic North Atlantic Ocean and their surrounding areas. 2. Data Used and Representation of Skewed Nature of Frequency Distributions The Marine Information Research Center (MIRC) of the Japan Hydrographic Association conducted detailed quality checks on the oceanic data in the Northwestern North Pacific, and compiled the MIRC Ocean Dataset 2001 (MODS-2001: MIRC, 2001). This dataset was used for analysis in the Western North Pacific. For the analy- sis in the North Atlantic Ocean, World Ocean Database 2001 (WOD-2001: Ocean Climate Laboratory, NODC, 2002) was used. We used data interpolated to standard depths. In WOD-2001, a range check was done by using a 3σ criterion before the interpolation procedure. As the 1. Introduction By analyzing the data obtained by the Iwate Fisher- ies Technology Center in the sea off Sanriku Coast, Ja- pan, we found that frequency distributions of observed temperature and salinity in the Mixed Water Region be- tween the Kuroshio and Oyashio Fronts are very skewed, and the usual techniques of quality control, such as a range check, are hard to apply (Oguma and Nagata, 2002). For example, some of the temperature values observed at 300 m depth exceed m + 5σ (m is the mean and σ the standard deviation), as shown in Fig. 1. Though the frequency of high temperature values is low, they are shown to be real. These extreme values were found when pure, unmodi- fied Kuroshio Water intrudes into the region due to very specific events such as the approach of Large Warm Ed- dies or an abnormal northward intrusion of the Kuroshio along the Sanriku Coast (Oguma and Nagata, 2002).