Oyashio seasonal intensification and its effect on subsurface temperature variation off the Sanriku coast

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111,, doi: /2006jc003628, 2006 Oyashio seasonal intensification and its effect on subsurface temperature variation off the Sanriku coast Osamu Isoguchi 1 and Hiroshi Kawamura 2 Received 11 April 2006; revised 10 June 2006; accepted 6 July 2006; published 10 October [1] The Oyashio seasonal intensification and its effect on subsurface temperature fields off the Sanriku coast are investigated, based on indices of the Oyashio current. These indices include altimeter-based eddy drifting velocities (EDV) and tide gauge sea levels at Petropavlovsk-Kamchatsky (PK), where the Oyashio upstream region is located. The annual cycles of EDV, the PK sea levels and the Sverdrup transport for the range of N (SV) show similar seasonal features that have (1) maximums in early winter and (2) small secondary local peaks in early summer. These indicate that the Oyashio seasonal variation is mainly governed by the Sverdrup dynamics. The early winter intensification recognized as the general Oyashio seasonal variation involves a large and rapid change from late autumn to early winter, which is roughly explained by the southward shift of westerlies. Its onset timing, which undergoes large year-to-year variability, is indeed determined by the timing of the southward migration of the westerly jet. The early summer intensification can also be explained by a barotoropic response to the intraseasonal evolution of atmospheric circulation in the Baiu (rainy) season, which is observed characteristically in East Asia and East Siberia. The seasonal cycle of subsurface temperatures off the Sanriku coast, where the Oyashio water meets the Kuroshio water, shows (1) a minimum in April and (2) a secondary local minimum in July. The cooling cycle is qualitatively explained by lateral heat advection by the Oyashio current. Citation: Isoguchi, O., and H. Kawamura (2006), Oyashio seasonal intensification and its effect on subsurface temperature variation off the Sanriku coast, J. Geophys. Res., 111,, doi: /2006jc Introduction [2] The Oyashio, a part of the western boundary currents of the subarctic North Pacific, flows southwestward along the southern Kuril Islands, carrying cold, fresh water. It meets the warm, saline Kuroshio water off the Sanriku coast (see Figure 1), where a complicated frontal structure is formed. The behavior of the thermal structure has a great impact on fishing grounds [e.g., Fukushima, 1979; Yasuda and Watanabe, 1994], as well as agriculture, through weather changes in northeastern Japan. Therefore clear understanding of the Oyashio variation and its related thermal structures are of great importance. Some previous studies on the Oyashio variation have used historical subsurface temperature fields off the Sanriku coast, which were intensively observed due to commercial concerns. These observations revealed that the Oyashio water generally penetrated most southward in spring and its interannual variation was approximately explained by the intensity and position of the Aleutian low and the related Sverdrup transport in wintertime [Sekine, 1988; Hanawa, 1995]. 1 Earth Observation Research and Application Center, Japan Aerospace Exploration Agency, Tokyo, Japan. 2 Center for Atmospheric and Oceanic Studies, Graduate School of Science, Tohoku University, Miyagi, Japan. Copyright 2006 by the American Geophysical Union /06/2006JC [3] Mooring and hydrographic sections conducted repeatedly off the southeastern Hokkaido coasts have revealed the Oyashio s seasonal variation, which shows stronger (weaker) current and volume transport in winter and spring (summer and autumn) [Kono and Kawasaki, 1997; Uehara et al., 1997; Kawasaki and Kusaka, 2003; Ito et al., 2004]. Moreover altimeter-derived sea level observations have showed that the distinct Oyashio seasonality is caused by the time-dependent Sverdrup circulation. Indeed many previous studies have shown that the subarctic circulation of the North Pacific was mainly dominated by a barotropic response to atmospheric wind forcing. They used a variety of geophysical records, altimeter-derived sea levels [Fu and Davidson, 1995; Isoguchi et al., 1997; Stammer, 1997; Vivier et al., 1999; Qiu, 2002], moored current meter data [Isoguchi et al., 1997; Isoguchi and Kawamura, 2003], drifting velocities of altimeter-derived eddies, and tide gauge sea levels [Isoguchi and Kawamura, 2003, 2006]. [4] Isoguchi and Kawamura [2003] have demonstrated by tracking sea level anomaly (SLA) maps that eddies along the western boundary of the subarctic North Pacific are advected by time-dependent wind-driven flows on a seasonal time scale. This means that an eddy drifting velocity (EDV) can be an index of the Oyashio current variation. Isoguchi and Kawamura [2006] (hereinafter referred to as IK06) have detected the Sverdrup sea level variation over the North Pacific, as a second mode of an empirical orthogonal function (EOF) of 10-year SLA maps. Its time 1of13

2 Figure 1. Schematic illustration of the surface currents (EKC: East Kamchatka Current, and Oyashio), geographical features cited in this study, and bathymetric lines every 2000m, which are superimposed on a sea surface temperature (SST) image from the New Generation Sea Surface Temperature (NGSST) product on April 10, 2003 [Guan and Kawamura, 2004]. Grids used for computation of sea level anomaly (SLA) maps along the western boundary are also shown with gray dots. Subsurface temperature variation off the Sanriku coast (Figure 11) is calculated within the region shown with a red frame. coefficient (hereinafter, EOF2) correlated well with the Sverdrup transports at N (hereinafter, SV), showing not only well-defined seasonality with maximum (minimum) in winter (autumn) but also its year-to-year variability. IK06 also showed that tide gauge sea levels at Petropavlovsk-Kamchatsky (PK; N, E), where the Oyashio upstream region is located, corresponded with EOF2, SV and the aforementioned altimeter-derived EDV. This suggested that the PK sea levels and EDV could be a good index of the western boundary current variation. Using these newly adopted parameters directly connected with the Oyashio variation, IK06 demonstrated that the interannual Oyashio variation in winter was primarily explained by a barotropic response to large-scale atmospheric forcing on a decadal time scale and had an impact on springtime sea surface temperature fields off the Sanriku coast. However, detailed discussions about the seasonal cycle and related atmospheric conditions have not been held by IK06. In particular, although the existence of a little secondary local peak in early summer (June) was pointed out in the annual cycles of these parameters, its interpretation is still remained unsolved. These seasonal and intraseasonal variations influence the migration of thermal fronts off the Sanriku coast, which is important for fish migration and fishing ground formation [e.g., Fukushima, 1979; Yasuda and Watanabe, 1994]. Meanwhile, the mechanism of the front migration has not fully been resolved as it has not been reproduced with numerical models [Yasuda, 2003]. [5] In this study, we investigate in detail the seasonal and intraseasonal variations of the Oyashio current as a barotoropic response to the evolution of the atmospheric fields, by using the Oyashio-related indices proposed by IK06. Focused on are (1) an abrupt intensification from late autumn to early winter and the year-to-year variation of its timing and (2) the early summer local peak. In addition to that, we discuss the seasonal and intraseasonal variations of subsurface temperature fields off the Sanriku coast, based on the Oyashio seasonal variation including the early summer secondary peak. The remainder of this paper is as follows: Data including the Oyashio-related indices are given in 2 of 13

3 section 2. In sections 3, the seasonal Oyashio variation, including the early winter and early summer intensification, and the related subsurface temperature variation are presented. Section 4 gives summary and conclusive remarks. 2. Data 2.1. Eddy Drifting Velocity (EDV) [6] EDV are calculated from altimeter data as one of the indices of the Oyashio current variation. The altimeter data used have been generated by Segment Sol multimissions d ALTimétrie, d Orbitographie et de localisation précise/ Data Unification and Altimeter Combination System (SSALTO/DUACS) Delayed Time Sea Level Anomalies (DT-MSLA). These were constructed for the period from October 1992 to January 2005 ( 12 years) with a 7-day interval, by merging the sea level anomaly (SLA) observations of the ocean TOPography Experiment (TOPEX)/ Poseidon and European Remote Sensing satellite (ERS)-1/2 or those of Jason-1 and ENVISAT. No ERS data were used between January 1994 and March 1995, corresponding to the 168 day repeat cycle geodetic mission of ERS-1 (phases E and F). The grid interval is a Mercator 1/3, ranging from 37 km at the equator to 18.5 km at 60 N/S. Details of data processing are described by Ducet et al. [2000]. [7] Following IK06, the SLA maps are interpolated onto a 25km 25km grid over the Japan Trench and the Kuril- Kamchatka Trench, which covers 2250 km in length by 200 km in width (Figure 1). EDV are then derived every 7 days from October 1992 to January 2005, from the alongtrench SLA plot (Figure 2). The derived EDV are low-pass filtered with a 35-day (7-day 5) running mean. Figure 3 shows their temporal anomalies normalized with the standard deviation of 1.1cm/s. Positive indicates southwestward. EDV here efficiently represent the migration of the several strong positive SLAs in Figure 2, which indicates propagation of anticyclonic eddies over the trenches Atmospheric Parameters [8] We use ocean surface winds obtained from the National Centers for Environmental Prediction/the National Center for Atmospheric Research (NCEP/NCAR) Reanalysis ( The daily mean 10-m wind is first converted to wind stress using the wind speed dependent drag coefficients of Kondo [1975]. From the 7-day mean wind stress curls the Sverdrup transports (SV) are derived for N by integrating the wind stress curl from the eastern boundary of the Pacific to the Kuril- Kamchatka Trench. Their temporal anomalies normalized with the standard deviation are shown in Figure 3. The long-term monthly Sverdrup transports ( ) are also derived to construct their climatology. In addition, to investigate the general intraseasonal evolution of atmospheric fields, the climatologies of geopotential heights, zonal winds at 500 hpa, and ocean surface winds are constructed with 5-day resolution based on the daily NCEP/NCAR Reanalysis Tide Gauge Sea Levels [9] Tide gauge sea level records at PK are provided by the Permanent Service for Mean Sea Level (PSMSL), hosted by the Proudman Oceanographic Laboratory (POL) [Woodworth, 1991]. The monthly sea levels cover the period from July 1957 to December 2002 (45 and a half year). A pressure correction is applied for them based on the assumption of the inverted barometer by using monthly sea level pressure data obtained from the NCEP/NCAR reanalysis. Following IK06, upper ocean thermosteric signals estimated from the NCEP/NCAR surface heat flux data are eliminated from the pressure-corrected PK sea levels. The residuals are hereinafter used to discuss the Oyashio variation. Their temporal anomalies normalized with the standard deviation are shown for the period from October 1992 to December 2002 in Figure Subsurface Temperature Data [10] Subsurface temperature data at depths of 100m, 200m, and 400m are obtained from the Subarctic Gyre Experiment project (SAGE) [Japan Meteorological Agency, 2001], which contain monthly mean analyzed temperatures for the period from January 1990 to December 2000 on a 1.0 longitude by 0.5 latitude grid over the Pacific ocean. The monthly mean values are determined by using a functional fitting method based on subsurface temperature observation by research vessels, voluntary observing ships, and ocean data buoys [Japan Meteorological Agency, 1990]. We construct the monthly climatological fields from the 11-year data. 3. Results 3.1. Wintertime Abrupt Intensification [11] The annual cycles of the three parameters derived in the previous section are shown in Figure 4. Each annual cycle is repeated twice side by side. Those of the PK sea levels (hereinafter TIDE) and SV are constructed from the 45-year monthly time series, and that of EDV is from 12-year data (the altimeter period). All three cycles have the same seasonal feature with maximum (minimum) in winter (autumn). The seasonality is consistent with the general knowledge of the Oyashio current variation, which has been derived from in situ observations [e.g., Kono and Kawasaki, 1997; Uehara et al., 1997; Kawasaki and Kusaka, 2003] and from altimeter data [Isoguchi et al., 1997; Qiu, 2002; Ito et al., 2004]. The wintertime Oyashio intensification representing the spinning-up of the subarctic gyre is accounted for by the intensification or the southward shift of westerlies, which accompanies the change of the Aleutian low. In particular, an abrupt shift from late autumn to early winter is prominent, which appears more sharply in the time series data (Figure 3). This characteristic phenomenon can also be seen in Figure 2, where the strong positive SLAs (anticyclonic eddies) rapidly change direction around early winter each year, forming a sharp peak to right-hand sides. This structure is remarkable for the period from 1994 to 1998 (see black arrows in Figure 2). [12] The pentad climatological atmospheric fields are used to investigate this characteristic change. The latitudetime plot of the annual cycles of the zonal mean wind stress curl (WSC) averaged for the range of 160 E 140 W and its latitudinal mean averaged for the range of N are shown in Figure 5. The annual cycles at each latitude are smoothed with a hanning filter to reduce noisy structure. In 3of13

4 Figure 2. SLAs along the Japan Trench and the Kuril-Kamchatka Trench as the function of time and along-trench distance. The SLAs are normalized with a standard deviation. The reference point of the x-axis is the intersection of the Japan Trench and the Kuril-Kamchatka Trench. The timing of the onset of the wintertime Oyashio intensification is shown with black arrows. The periods when the early summer local peaks appear are enclosed by red boxes. addition to that, the monthly WSC maps along with the surface wind climatologies are shown from October to March in Figure 6. A WSC zero line, which corresponds roughly with the center of the westerlies, undergoes a large southward shift from 47 N to 35 N within 2 months (from November to January) (Figure 5a). This results in cyclonic spinning-up over the subarctic region (Figure 6). Another contributing factor on the abrupt shift from late autumn to early winter may be the intensification of westerly jet itself in late autumn: the westerlies intensified during October and November make WSC stronger at both sides of its axis. In November, because the axis still stays in the northern region, large parts of the subarctic region are covered with stronger negative WSC (anticyclonic spinning-up). The intensification and southward shift of the westerlies thus could produce the abrupt shift from November to January and could determine the onset of the Oyashio wintertime intensification. [13] The above discussion is based on climatology. The actual timing of the early winter onset is, on the other hand, expected to shift back and forth every year, depending on atmospheric conditions. An anomalous early onset is confirmed in late autumn All three parameters, EDV, SV, and the PK sea levels, have local minimums in September 4 of 13

5 Figure 3. Time series of (upper) 7-day eddy drifting velocity anomalies calculated from Figure 2, (middle) the 7-day Sverdrup transport anomalies averaged N and (lower) monthly tide gauge sea level anomalies at PK after removing steric signals. They are normalized with their standard deviations. The Sverdrup transport and tide gauge sea levels are added 5 and 10, respectively. June 2002 and November 2002 cited in section 3 are shaded and turn positive in November (Figure 3) against the climatological negative values in November (Figure 4). The WSC and surface wind maps in November 2002 and their anomalies from the climatologies are shown in Figure 7. The westerly jet is somewhat strong and shifts more southward than the climatology. The WSC zero line is located around 43 N (Figure 7a), being about 5 south of the climatology (Figure 6), which produces anomalous positive WSC in the zonal region between 40 N and 50 N (Figure 7b). The North Pacific Index (NPI) [Trenberth and Hurrell, 1994], which represents the activity of the Aleutian low, is a useful parameter to judge the degree of anomalous atmospheric conditions in the northern North Pacific. The NPI of November 2002 is 6.28 mbar (2.08 times of its standard deviation) lower than the November mean, implying highly anomalous low-pressure condition. In fact, the blowing of strong cold air due to the meandering and southward shift of the westerlies results in colder air temperatures than usual over Japan. Thus, in November 2002, the anomalous southward shift of the westerlies gives rise to the anomalous early onset of the Oyashio wintertime intensification Secondary Intensification in the Baiu Season [14] Another characteristic feature seen in the annual cycles of the three parameters (Figure 4) is a small local peak in early summer (June). This peak may not necessarily be significant against the standard deviation. Nevertheless the fact that all three parameters have the same tendency suggests the existence of some meaningful current signals. In Figure 3, the early summer peaks following local minimums in spring appear in 1995, 1999, 2001 and 2002 in both EDV and SV data. The PK sea levels show the clear peaks in 2001 and 2002 only. The early summer intensification of EDV is expressed as the positive SLA turning to the left-hand side (southwestward propagation) in the aforementioned years (see the areas enclosed by red boxes in Figure 2). The possible reason that EDV captures more often the early summer peaks is that a barotropic response is effectively reflected onto the EDV signals due to the vertical deep structure of the eddies. On the other hand, because the PK sea levels probably include upper layer signals with stronger variability and/or local phenomena such as eddies, the summer peak signals with smaller amplitudes than the winter peak might be contaminated by other signals. A lump in early summer has also been pointed out based on NPI [Trenberth and Hurrell, 1994]. The NPI is directly related to the Sverdrup transport averaged over the region of N because it is defined as the area-weighted sea level pressure over the region of N, 160 E 140 W. Kawasaki and Kusaka [2003] have reported from repeated hydrographic sections off Hokkaido that the Oyashio summer volume transport is larger than the spring one when it is offset with reference to concurrently measured mooring currents, or when the barotropic component is considered. [15] The pentad climatologies of the atmospheric fields are again investigated to trace their intraseasonal evolution during spring and summer. The latitude-time plot of the zonal mean WSC (Figure 5) shows that the WSC zero line around 36 N in early March gradually moves northward and reaches up to 46 N in mid-may, entailing the weakening of the wind and WSC during this period. Then, it suddenly changes direction, moving southward until early June. After the local southernmost peak in early June, it moves northward again during June and July (Figure 5a). The time series of the area-averaged WSC over the region of 160 E 140 W, N show the seasonal evolution more clearly (Figure 5b). It decreases rapidly in March and 5of13

6 Figure 4. Annual cycles of (a) the monthly tide gauge sea level anomalies at PK after removing steric signals, (b) the Sverdrup transport anomalies averaged N, and (c) the eddy drifting velocity anomalies. Vertical lines represent their standard deviations. The each annual cycle is repeated twice side by side. fluctuates around zero until mid-may. It shifts to an increase phase until early June and decreases again after the local peak. It is somewhat noisy. Nevertheless the general feature with the local peak in early June is consistent with the case of NPI [Trenberth and Hurrell, 1994]. Though the southward shift of the WSC zero line during the increase phase from late May to early June is only 2 3, it seems to crucially generate the characteristic intraseasonal evolution. The result suggests that the Oyashio intensification in early summer can be accounted for by the time-dependent Sverdrup circulation responding to the atmospheric intraseasonal variation. [16] Here we discuss ambient atmospheric circulations related to the aforementioned intraseasonal variation. The pentad climatologies of the geopotential height and zonal wind at 500 hpa are shown for the period from April 2 to July 1 with a 30-day interval (Figure 8a). Also shown in Figure 8b is the latitude-time plot of the 5-day mean 500 hpa zonal wind averaged over Siberia (110 E 150 E) for the period from March to September. The westerlies, represented as the dense contours of the geopotential height, weaken gradually from April to July. Meanwhile they split into two branches through late-may to July, one over Siberia (65 N 75 N) and the other in midlatitudes (shown with black arrows in the upper right panel of Figure 8a and dark shadings where the wind speed > 6m/s in Figure 8b). As a result, the southern one shifts southward over Japan. At the same instance, the trough spreading from Bering Sea to Gulf of Alaska seems to push it to the south in the central and east Pacific. In July, in addition to the decline of the jet, its axis moves northward, particularly over the northeastern Pacific due to the development of the subtropical high. The 6of13

7 Figure 5. (a) A latitude-time plot of zonal mean (160 E 140 W) wind stress curl climatology. The time interval is 5 days. (b) An annual cycle of area-averaged wind stress curl (160 E 140 W, N). They are repeated twice side by side. Positive (negative) contours are drawn with solid (dashed) lines. Positive wind stress curl areas are gradually shaded. Figure 6. Climatologies of monthly surface wind and wind stress curl fields for the period from October to March. A contour interval is N/m 3. Positive wind stress curl areas are gradually shaded. 7of13

8 Figure 7. (a) The surface wind and wind stress curl fields in November 2002 and (b) their anomalies relative to the climatologies. A contour interval is N/m 3. Positive wind stress curl areas are gradually shaded. latitude-time plot of the 500 hpa zonal wind averaged for 160 E 140 W (Figure 9) demonstrates the intraseasonal migration of the westerlies over the North Pacific: the jet s axis first moves northward during March and April and turns around at mid-may, propagating southward until early June. It again moves to the north after the local peak in early June and suddenly decays in July, which corresponds to the ending of the Baiu (rainy) season. A possible cause for the jet s bifurcation after late-may has been considered [Nakamura and Fukamachi, 2004]. The land north of the Okhotsk Sea is heated during summer up to 5 10 C warmer than the Okhotsk Sea, where the sea surface temperature is relative cold throughout the year. The resultant inversion of meridional temperature gradient over the northern Okhotsk Sea brings about the weakening of the westerlies at the upper troposphere and the eventual bifurcation. At the lower troposphere, the Baiu front is formed under the subtropical jet [e.g., Nakamura and Fukamachi, 2004], bringing a lot of precipitation in midlatitudes. [17] An anomalous case of the early summer intensification is confirmed in June The NPI in June 2002 is 1.29 mbar lower than its climatology (1.12 times of its standard deviation) and is the lowest after Moreover, the Oyashio-related three indices in Figure 3 represent the early summer intensification in The monthly mean 500 hpa geopotential heights in June 2002, along with their anomalies are shown in Figure 10. The lower anomalies over N suggest the anomalous southward shift of the westerlies. Thus the anomalous southward shift must have intensified the Oyashio current in early summer of The results derived here suggest that the intraseasonal atmospheric change typical to East Asia and East Siberia, which is formed by land-ocean-atmosphere interaction, can induce the Oyashio intensification in the Baiu season Subsurface Temperature Variation off the Sanriku Coast [18] Here we discuss subsurface temperature variations off the Sanriku coast, where the Oyashio water meets the Kuroshio water, on the relationship with the seasonal intensification of the Oyashio current. The annual cycle of the 200m temperature for the range of E, N (see Figure 1), where the amplitude of the annual cycle and horizontal temperature gradients are large, is shown with its standard deviation in Figure 11a. It shows large wintertime cooling from the maximum in December to the minimum in April. After springtime heating from April to June, it again undergoes a small, short-term cooling from June to July. After the local minimum in July, it increases gradually by the December maximum. The local minimum in July is not necessarily significant against its standard deviation. Nevertheless, a similar annual cycle with the July s local minimum as well as the April s minimum has been detected with the monthly southernmost latitude of the First Oyashio Intrusion, a southward cold intrusion in the Pacific water off the Sanriku coast [Ogawa, 1989]. They were determined from monthly temperature charts at a depth of 100m from 1964 through 1987, by applying the indicative isotherms proposed by Kawai [1972]. These isotherms are different from month-to-month with the range between 5 C from March to May and 8 C from November to December, due to the influences of surface warming and 8of13

9 Figure 8. (a) Climatology of 5-day geopotential height (contours) and zonal wind fields (shadings) at 500 hpa every 30 days for the period from April to July. Contour and shading intervals are 50m and 5m/s, respectively. Black arrows in the right upper panel indicate the bifurcation of the jet. (b) A latitude-time plot of zonal mean (110 E 150 E) climatological zonal wind fields at 500 hpa. A contour interval is 3 m/s. Figure 9. A latitude-time plot of zonal mean (160 E 140 W) climatological zonal wind fields at 500 hpa. The latitudes with the maximum zonal wind are plotted with a white thick line. 9of13

10 Figure 10. The 500 hpa geopotential height in June 2002 and its anomaly relative to the climatology. The anomalies higher (lower) than 15m are shaded lightly (heavily). A shading interval is 15m. cooling. The annual cycle of the First Oyashio Intrusion is cited in Figure 11b. Kawai [1955] and Fukushima [1979] have described the intraseasonal evolution of surface temperature fields off the Sanriku coast from spring to summer, by classifying the following 4 seasons. (1) In spring, after the temperature reached its minimum, it increases rapidly from mid-april until the beginning of the Baiu season, when warm waters penetrate up to subsurface layers. (2) In the Baiu season, the temperature warming stagnates or cooling occurs. (3) In early summer, the surface temperature rises rapidly again from the end of the Baiu season. (4) In midsummer, it reaches the maximum. This seasonal evolution of the surface temperature is roughly consistent with that of the subsurface (200m) temperature in this study. Furthermore, Kawai [1955] suggested the possibility that temporal cooling in the Baiu season was induced by heat advection because the cooling extended to the subsurface layer. [19] We demonstrated the Oyashio intensification in the Baiu season as a barotropic response to atmospheric forcing in the previous section. Here we make a rough estimate of the annual cycle of meridional velocities by assuming that the 200m temperature variation off the Sanriku coast (Figure 11a) is fully induced by meridional heat ; where T is the temperature at 200m depth, and v is meridional current velocity. from Figure 11a and the monthly climatologies of the 200m temperature field at the red box in Figure 1, respectively, yields the annual cycle of v (Figure 11c), where positive indicates southward velocities. The v s cycle estimated, which shows not only the maximum southward current in winter but also the secondary local peak in early summer, is roughly consistent with those of the Oyashio indices; EDV, SV, and the PK sea levels (Figure 4). The similar seasonal cycle of v is also derived based on the 400m temperature climatology. These results suggest that the seasonal ð1þ variation of the subsurface temperature off the Sanriku coast is qualitatively explained by the lateral heat advection by the western boundary currents, including the Oyashio current. [20] It should be noted that the above result requires consistent northward currents during the rising temperature periods in spring and from the end of the Baiu season to December, represented as negative values in Figure 11c. In particular, it is interesting that the subsurface temperature keeps rising during autumn (October December) against the cooling of the surface temperature. This may be the phenomenon that corroborates the existence of the consistent northward currents. In fact, because the WSC zero lines are located further north of the Sanriku area, the linear Sverdrup theory can qualitatively explain the northward western boundary currents in these two warming seasons (see Figure 4). In addition, the anticyclonic eddies (positive SLAs) over the Japan Trench tend to propagate northward during these warming seasons, in particular, they show rapid northward drift in autumn (see Figure 2). Assuming fully advected drifting, it also indicates the northward currents and its intensification in autumn. However no evidence has been found for the existence of explicit northward current and its seasonal variation by direct measurements. Many eddies existing off the Sanriku coast probably make it difficult to detect mean current fields and their variability. For further understanding of the current structure off the Sanriku coast, high-resolution numerical modeling as well as data analyses would be required in the future. 4. Summary and Conclusive Remarks [21] The seasonal evolutions of the Oyashio current and their effects on subsurface temperature variations are examined based on the current-related indices. The Oyashio seasonal variation has been characterized as a barotropic response to the wintertime spinning-up [Sekine, 1988; Hanawa, 1995]. The southernmost latitude of the First Oyashio Intrusion off the Sanriku coast was used as an index of the Oyashio variation. Meanwhile it was difficult to discuss the Oyashio seasonal variation in detail because it is not necessarily the only index that is directly related 10 of 13

11 Figure 11. Annual cycles of (a) subsurface (200m) temperatures off the Sanriku coast (see Figure 1), (b) the southernmost latitude of the First Oyashio Intrusion cited from Ogawa [1989], and (c) meridional velocities estimated from the subsurface temperature fields by assuming lateral advection. Vertical lines in Figures 11a and 11b represent their standard deviations. Each annual cycle is repeated twice side by side. to the Oyashio current. Isoguchi and Kawamura [2006] have demonstrated that the tide gauge sea levels at Petropavlovsk-Kamchatsky (PK) and the eddy drifting velocity (EDV) over the Japan Trench and Kuril-Kamchatka Trench can be the efficient indices of the Oyashio current. Based on their result, we use the three parameters, the PK sea levels, EDV, and the Sverdrup transport (SV), as indices of the Oyashio currents. All their annual cycles show the same characteristic seasonality that has (1) the maximum (minimum) in early winter (late autumn) and (2) the secondary local peak in early summer. [22] The early winter maximum is consistent with the general knowledge about the Oyashio seasonal intensification that it strengthens (weakens) in winter to spring (summer to autumn). In this study, we put emphasis on the rapid change from late autumn to early winter. The pentad climatology of wind stress curl (WSC) demonstrates that the southward shift of the WSC zero line from 47 N to 36 N accounts for the rapid intensification for the period from November to January. The timing of its onset is different every year, depending on atmospheric circulations. A case in November 2002 is an anomalous example of the early onset. The North Pacific Index (NPI) of November 11 of 13

12 2002 is 6.28 mbar (2.08 times of the standard deviation) lower than its climatology, being an extremely anomalous low-pressure condition. Actually, the PK sea levels, EDV and SV all turn positive in November 2002 and the WSC map shows the anomalous southward shift of westerlies, which generates the positive WSC over the subarctic region contrary to climatological negative one. The results suggest that the onset of the Oyashio intensification is mostly determined by the southward shift of the westerlies. [23] The early summer peak of the Oyashio current is firstly suggested as a distinct intraseasonal phenomenon in this study. This variation can also be explained by the intraseasonal evolution of the atmospheric circulation. The zero line of WSC shifts northward rapidly from 36 N to 46 N during spring from March to mid-may. It moves southward once, reaching to 34 N in early June, and it propagates northward again after that. This intraseasonal meridional migration of the westerlies results in the early summer peak of the Sverdrup transport over the subarctic region. 500 hpa geopotential height fields are used to investigate ambient atmospheric circulations related to the intraseasonal variation. The bifurcation of the westerlies over East Siberia and Okhotsk Sea occurs during June and July, which results in the southward shift of its axis and the WSC zero line at ocean surface. At that time, the quasistationary Baiu front is formed under the subtropical jet. A possible mechanism for the bifurcation has been reported to be due to the meridional temperature inversion between the Okhotsk Sea and the northern land area [Nakamura and Fukamachi, 2004]. Thus it is found that the Oyashio current is expected to be intensified in the Baiu season as a barotoropic response to the atmospheric circulations specific to East Asia and East Siberia. [24] The seasonal variation of subsurface temperatures off the Sanriku coast is investigated in the relationship with the Oyashio seasonal intensification. Its annual cycle shows a minimum in April and a secondary local minimum in July, which is consistent with the southernmost latitude of the First Oyashio Intrusion defined by the subsurface temperature fields. By assuming that the above temperature cycle is fully caused by meridional heat advection, we estimate meridional current variations off the Sanriku coast. The estimated annual cycle has not only wintertime maxima but also an early summer local peak. This roughly corresponds with those of the Oyashiorelated indices, the PK sea levels, EDV, and SV, suggesting that the subsurface temperature variation off the Sanriku coast is qualitatively explained by the heat advection by the western boundary currents, including the Oyashio currents. [25] In this study, only a barotropic response was considered as the Oyashio variation. The fact that SV shows the same annual cycle with the early summer peak as those of the PK sea levels and EDV, certainly implies the significant contribution of the barotropic component to the Oyashio annual variation. Nevertheless the baroclinic Rossby waves reaching the western boundary influence the Oyashio variation in particular on a low frequency scale, as pointed out by Qiu [2002]. Its contribution to the Oyashio annual variation should be addressed in the future by using repeated hydrographic observations and numerical simulations. [26] Acknowledgments. The authors thank the editor and the two anonymous reviewers for constructive comments and English editing, which were helpful in improving our paper. The altimeter products used in this study were produced by the CLS Space Oceanography Division as part of the Environment and Climate EU ENACT project (EVK2- CT ) and with support from CNES. 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