SST FRONTS OF THE PACIFIC COASTAL AND MARGINAL SEAS I. Belkin, P. Cornillon Graduate School of Oceanography, University of Rhode Island (URI), USA Email: [email protected] The Pathfinder AVHRR SST data from 1985–1996 are used to survey thermal fronts of the Pacific coastal and marginal seas. The SST fields are declouded and fronts are detected with the Cayula-Cornillon algorithms developed at the URI. In this work we describe newly found, persistent frontal features and their seasonal variability as well as interannual variability. Major climatic fronts are identified and compared with literature data. The ocean-wide frontal pattern consists of several types of fronts. Western boundary currents (Kuroshio, Oyashio, and the East Australian Current) are associated with well-defined fronts. Eastern boundary fronts are also prominent, especially in the North Pacific (California Current System). In the Tasman Sea, a quasi- stationary bifurcation of the Tasman Front is identified at 35°S, 165°E; the Subtropical Front has been traced from south of Tasmania eastward up to the Southland Current off New Zealand. A well-defined double Subtropical Frontal Zone is confirmed east of New Zealand. A new front is observed in the tropical-subtropical Southeast Pacific. Shelf-slope fronts are observed over the shelf break/upper slope almost everywhere. Being strictly controlled by topography, these fronts are quasi-stationary and therefore readily located from satellite data. Inner shelf fronts are observed in the Bering Sea, Eastern China Seas, and South China Sea. Coastal upwelling fronts are observed west of the American continent. These fronts are seasonally dependent, being best seen in winter. Vast frontal zones, apparently associated with coastal upwelling, extend much farther offshore than anticipated, up to O(1000km) into the open ocean. INTRODUCTION by “Western and eastern boundary currents” that details boundary currents of the Northwest Pacific, Ocean fronts are relatively narrow zones that separate Northeast Pacific, Southwest Pacific, and Southeast broader zones with different stratification types and/or Pacific. Fronts of the Pacific marginal seas are described different water masses; the fronts are almost always in “Marginal Seas”. Principal results and conclusions are accompanied by enhanced horizontal gradients of summarized in the last Section. The References section temperature, salinity, density, nutrients and other contains an extensive bibliography; the space properties (Fedorov, 1986; Belkin, 2003). Fronts and limitations, however, forced us to provide only the the associated currents are critically important in heat minimum number of references for each front and and salt transport, ocean-atmosphere interaction and region to serve as starting points for the interested ecosystem functioning. reader. Detailed regional studies of fronts will be Satellite-retrieved sea surface temperature (SST) data published elsewhere, complete with exhaustive have been used to study the Pacific Ocean fronts since bibliographies (e.g. Hickox et al., 2000; Belkin and the 1970s (e.g. Legeckis, 1978). Earlier studies have Cornillon, 2003; Belkin et al., 2004). been focused mainly on fronts associated with western boundary currents such as Kuroshio (Qiu et al., 1990) DATA AND METHOD and East Australian Current (Nilsson and Cresswell, Fronts are high-gradient zones; therefore most 1980). Fronts associated with eastern boundary objective computer-based approaches to front currents and coastal upwellings have been studied in identification are based on gradient computations such areas as the California Current (Strub et al., (Kazmin and Rienecker, 1996; Yuan and Talley, 1991; Strub and James, 1995) and Peru-Chile Current 1996; Nakamura and Kazmin, 2003). Our approach is (Brink et al., 1983; Fonseca, 1989). based on histogram analysis. Since every front The above studies, being very important in elucidating separates two relatively uniform water bodies, physics and geography of regional features, have frequency histograms of any oceanographic utilized different methods; most studies were based on characteristic, e.g. SST, in the vicinity of the front data sets of relatively limited duration. The present work should have two frequency modes that correspond to is based on a unifying approach developed at the URI, the water masses separated by the front, while the consistently applied to a global data set of thoroughly latter corresponds to the frequency minimum between calibrated measurements. We took advantage of and the modes. The front detection and tracking is greatly benefited from availability of (1) advanced performed at three levels: window, image and a algorithms for front detection and cloud screening sequence of overlapping images. An optimum window developed earlier at the URI, and (2) the Pathfinder data size determined experimentally is 32 by 32 pixels set, both briefly described in Section “Data and (Cayula and Cornillon, 1992). The edge (front) Method”. A general outline of the Pacific detection algorithm uses all pixel-based SST values coastal/marginal seas’ frontal pattern is given in within each window to compute a SST frequency “General pattern of surface thermal fronts”, followed histogram for the given window. For each window 90 • PAPERS • PHYSICAL OCEANOGRAPHY PACIFIC OCEANOGRAPHY, Vol. 1, No. 2, 2003 SST FRONTS OF THE PACIFIC COASTAL AND MARGINAL SEAS that contains a front, the corresponding SST histogram Australian Current in the South Pacific. The would have a frequency minimum identified with the Kuroshio front south of Japan is best seen in front. winter. The Oyashio front is prominent NNE of Japan year-round. The East Australian Current This basic idea has been implemented by Cayula et al. front is better seen in the austral winter and spring. (1991), Cayula and Cornillon (1992, 1995, 1996) and • Eastern boundary fronts. These fronts are related Ullman and Cornillon (1999, 2000, 2001); the reader to the wind-induced coastal upwelling; they are is referred to these works for pertinent details. The best defined off Washington-Oregon-California fronts were derived from the NOAA/NASA coasts, off Central America gulfs (Tehuantepec, Pathfinder SST fields (Vazquez et al., 1998) for the Papagayo, and Panama), and off Peru-Chile coasts. period 1985–1996. These fields were obtained from • Marginal seas fronts. The semi-enclosed seas east the Advance Very High Resolution Radiometer of Asia (Bering, Okhotsk, Japan, Bohai, Yellow, (AVHRR) Global Area Coverage data stream (two East China, and South China seas) feature 9.28 km resolution fields per day) and are available numerous well-defined fronts located either over the shelf break or within the shelf area. The shelf from the Jet Propulsion Laboratory. SST fronts were break and inner shelf fronts are strongly seasonal. obtained from the cloud-masked SST fields with the The Sea of Japan is also crossed by the multi-image edge detection algorithm (Cayula and westernmost extension of the trans-ocean North Cornillon, 1996; Ullman and Cornillon, 1999, 2000, Pacific Polar Front. 2001). The cloud masking and front detection algorithms were applied to each of the 8364 SST WESTERN AND EASTERN BOUNDARY images in the 12 year data set. The frontal data were CURRENTS aggregated monthly (e.g. 12 Januaries taken together) and seasonally (e.g., the winter climatology is Northwest Pacific. The northernmost western obtained from all Januaries, Februaries, and Marches boundary current of the NW Pacific originates in the taken together). Two basic types of frontal maps are western Bering Sea off the Koryak Coast of Siberia. used in the analysis: long-term frequency maps and After exiting the Bering Sea, the current continues east quasi-synoptic composite maps. The long-term of Kamchatka Peninsula as the East Kamchatka frequency maps show the pixel-based frequency F of Current with the associated East Kamchatka Front fronts normalized on cloudiness: For each pixel, (EKF). The EKF (and the associated current) is likely F = N/C, where N is the number of times the given a major source of the Polar Front; the latter has pixel contained a front, and C is the number of times recently been tracked across the entire North Pacific the pixel was cloud-free. Thus, the frequency maps are (Belkin et al., 2002). Farther downstream, off Kuril best suited for displaying most stable fronts. At the Islands, the Polar Front is associated with the Kuril same time, frontal frequency maps understate some Current, termed the Kuril Front (KurF). Off the fronts associated with time-varying meandering southern Kuril Islands and Hokkaido, the Kuril currents. In such cases quasi-synoptic composite maps Current is alternatively called the Oyashio Current, so are most useful since they present synoptic snapshots the Polar Front there is often called the Oyashio Front of “instant” fronts detected in individual SST images (OF). within a given time frame (e.g. week, month, or The East Kamchatka Front (EKF) is observed along season), without any averaging or smoothing. The the east coast of the Kamchatka Peninsula, south of frontal composite maps thus allow one to detect most 55°N. The front is best defined in late winter–early unstable fronts that are not conspicuous in the frontal spring (March–April). The EKF was observed each frequency maps. March and each April from 1985–1996 except for April 1992. From June through August, the EKF GENERAL PATTERN OF SURFACE visibility is poor mainly because of the maize of SST THERMAL FRONTS fronts typical of the summertime. In late summer– General pattern of surface thermal fronts of the Pacific early fall (September–November), the front is again Ocean is illustrated by two long-term frontal visible most of the time. The multi-annual variability frequency maps, for boreal winter and summer of the EKF is noticeable in late fall–winter (Figures 1–2, p. 101). The color scale emphasizes (December–February). From 1985–1989, the front stable fronts shown in hot colors (red, orange and was only visible 20% of the time, whereas from 1990– yellow). Except for a few open oceans fronts – 1996, the front was present 70% of the time.