JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, C04013, doi:10.1029/2005JC003080, 2006 Seasonal to interannual variations of the western boundary current of the subarctic North Pacific by a combination of the altimeter and tide gauge sea levels Osamu Isoguchi1 and Hiroshi Kawamura1 Received 31 May 2005; revised 30 November 2005; accepted 20 January 2006; published 28 April 2006. [1] Seasonal to interannual variations of the East Kamchatka Current (EKC) and the Oyashio are examined by focusing on their barotropic response to wind forcing by a combined use of altimeter-derived and tide gauge sea levels. An empirical orthogonal function (EOF) analysis is performed on the 9-year altimeter sea level maps with thermosteric signals removed. A second EOF (EOF2) shows a spin-up and spin-down of the subarctic gyres, and its temporal variation is almost accounted for by the time- dependent Sverdrup balance. Tide gauge sea levels at Petropavlovsk-Kamchatsky (PK) agree with EOF2 and the Sverdrup transports in terms of not only the seasonal variation but also its year-to-year variability in winter when the subarctic gyre is spun up most. We also detect two types of EKC/Oyashio variations from the altimeter data: drifting velocities of sea level disturbances and geostrophic velocity anomalies. These two EKC/ Oyashio temporal variations are also accounted for by the Sverdrup balance and agree with the PK sea levels and EOF2. The results imply that the PK sea levels can be a good representative of the subarctic gyre and EKC/Oyashio variations. On the basis of this relation, interannual variations during winter are discussed. The 44-year wintertime sea levels at PK correlate with the wintertime Sverdrup transport and springtime sea surface temperature off the northeastern coast of Japan with decadal-scale variability. This demonstrates that EKC/Oyashio is primarily explained by a barotropic response to large- scale atmospheric forcing and fluctuates on a decadal timescale almost in phase with atmospheric changes and influences the oceanic condition east of Japan. Citation: Isoguchi, O., and H. Kawamura (2006), Seasonal to interannual variations of the western boundary current of the subarctic North Pacific by a combination of the altimeter and tide gauge sea levels, J. Geophys. Res., 111, C04013, doi:10.1029/2005JC003080. 1. Introduction which is characterized by a salinity minimum in the North Pacific subtropical gyre [e.g., Talley, 1993; Talley et al., [2] The East Kamchatka Current (EKC), which is a part of 1995; Yasuda et al., 1996, 2001]. The North Pacific Inter- western boundary currents of the subarctic North Pacific, mediate Water is the deepest and densest water mass flows southwestward along the Kamchatka Peninsula and ventilating the North Pacific, and have a great impact on the Kuril Islands. A part of EKC enters the Okhotsk Sea, long-term climate changes. Understanding and predicting establishes circulation and then exits into the North Pacific. the variation of the subarctic gyre and EKC/Oyashio are thus This water forms the Oyashio by mixing with the water that important for not only societal aspects but also oceano- has directly flown southwestward along the Kuril Islands. graphic and climatic interests. The Oyashio carries cold, fresh, oxygen and nutrient-rich [3] The Oyashio sometimes intrudes anomalously south- water toward the Pacific coast of Japan. Its behavior has a ward along the Sanriku coast with cold water from winter to great impact on fishing grounds [e.g., Yasuda and Watanabe, late spring and directly influences coastal weather and 1994; Yasuda and Kitagawa, 1996], as well as agriculture oceanic conditions. Figure 1 shows a sea surface tempera- through weather conditions in northeastern Japan. Addition- ture (SST) field on 10 April 2003, from the New Generation ally, it has been reported that isopycnal mixing between the SST data (http://www.ocean.caos.tohoku.ac.jp/merge/ cold and fresh Oyashio water and warm and salty Kuroshio sstbinary/actvalbm.cgi?eng=1). These data have been qual- water is a possible mechanism of the formation of North ity controlled, and generated on a high spatial resolution Pacific Intermediate Water [e.g., Sverdrup et al., 1942], (0.05° Â 0.05°), on daily basis by objectively merging satellite SST observations from infrared radiometers (Ad- 1Center for Atmospheric and Oceanic Studies, Graduate School of vanced Very High Resolution Radiometers (AVHRR) and Science, Tohoku University, Miyagi, Japan. Moderate Resolution Imaging Spectroradiometer (MODIS)) and microwave radiometer (Advanced Microwave Scanning Copyright 2006 by the American Geophysical Union. 0148-0227/06/2005JC003080$09.00 Radiometer–Earth Observing System (AMSR-E)) [Guan C04013 1of17 C04013 ISOGUCHI AND KAWAMURA: WIND-DRIVEN EKC/OYASHIO VARIATIONS C04013 Figure 1. Sea surface temperature (SST) image from the New Generation Sea Surface Temperature (NGSST) product on 10 April 2003. Schematic illustration of the surface currents (East Kamchatka Current (EKC) and Oyashio), geographical features cited in this study, and bathymetric lines are superimposed every 2000 m. Grids used for computation of sea level anomaly (SLA) maps along the western boundary are also shown with gray dots. and Kawamura, 2004]. A southward intrusion of cold water observations [Uehara et al., 1997; Kono and Kawasaki, appears along the Sanriku coast. In early spring when the 1997] and satellite altimeter measurements [Isoguchi et al., Oyashio advances more southward, the mixed layer deepens 1997; Qiu, 2002b; Ito et al., 2004]. EKC/Oyashio strength- up to 100 m; hence SST fields can be used as a good ens in winter and weakens in summer and fall, which is representative of temperature at that depth, which has been approximately explained by the time-dependent Sverdrup conventionally used to define the Oyashio water. Many dynamics [Isoguchi et al., 1997; Qiu, 2002b]. Altimeter- studies focusing on the year-to-year variation of the derived large-scale and low-frequency (>30 days) sea level Oyashio southward intrusion have been conducted using variations with the steric height component removed have an index based on temperature fields at a depth of 100 m also been described by the time-dependent Sverdrup bal- [Sekine, 1988, 1999; Hanawa, 1995]. Sekine [1988] showed ance in the subarctic North Pacific [Fu and Davidson, 1995; that the anomalous southward intrusion of the Oyashio is Isoguchi et al., 1997; Stammer, 1997; Vivier et al., 1999]. linked to development of the prominent Aleutian low. [5] Tide gauge sea level records collected for over several Moreover, Sekine [1999] successfully simulated the anom- decades all over the world are very useful data set for long- alous southward intrusions by using a two-layer model, and term studies. Sea level varies intensively in regions of pointed out these could be explained by a barotropic strong currents. Indeed, many studies have been conducted response to wind fields related to the development of the by using the tide gauge records along the Kuroshio path to Aleutian low. Hanawa [1995] pointed out that there exists estimate and describe its variations and volume transport an agreement between annual mean southernmost latitudes [e.g., Kawabe, 1980, 2001]. In the subarctic region, on the of the Oyashio intrusions and the wintertime Sverdrup other hand, only a few studies have been done probably transports, and proposed a simple prediction model for because of following reasons. (1) There exist only a few tide these intrusions. gauges but no stations exist on both sides of a current axis, [4] With the accumulation of observations with a finer and (2) the EKC/Oyashio-induced signals seem to have a resolution in time and space, we began to investigate lower variability. Kashiwai [1991] established a relation seasonal to interannual variations of EKC/Oyashio. Distinct between tide gauge sea levels along EKC/Oyashio and other seasonality of the Oyashio variation has been revealed by indices related to the variation of the subarctic gyre, that is, direct current measurements and repeated hydrographic the southernmost latitude of the Oyashio intrusions and 2of17 C04013 ISOGUCHI AND KAWAMURA: WIND-DRIVEN EKC/OYASHIO VARIATIONS C04013 coastal SSTs. He showed a significant negative correlation and Altimeter Combination System (SSALTO/DUACS) between the sea level at the Petropavlovsk-Kamchatsky Delayed Time Sea Level Anomalies (DT-MSLA)’’, which (PK; see Figure 1) and the monthly zonal index for the is a project that produces maps of sea level anomaly (SLA) Far East, which is defined as a difference of 500 hPa height obtained from the ocean TOPography Experiment for 90°–170°E between 40° and 60°N and which describes (TOPEX)/Poseidon and European Remote Sensing satellite the strength of winter monsoon and the meridional shifts of (ERS) observations; there are available every 7 days for the westerlies; however he showed no further linkage with the period from October 1992 to February 2002 (10 years). EKC/Oyashio variation. Isoguchi and Kawamura [2002] No ERS data were used between January 1994 and March showed that monthly sea levels at PK were in agreement, 1995, corresponding to the 168 day repeat cycle geodetic on a seasonal timescale, with the time series of a first mission of ERS-1 (phases E and F). The grid interval is a empirical orthogonal function (EOF) calculated from 6-year Mercator 1/3°, ranging from 37 km at the equator to 18.5 km altimeter-derived sea level anomaly (SLA) maps north of at 60°N/S. Details of data processing