On the formation of Subtropical Countercurrent to the west of the Hawaiian Islands

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C5, 3167, doi: /2002jc001366, 2003 On the formation of Subtropical Countercurrent to the west of the Hawaiian Islands Qinyu Liu, Shaoxia Wang, Qi Wang, and Wei Wang Physical Oceanography Laboratory and Ocean-Atmospheric Interaction and Climate Laboratory, Ocean University of China, Qingdao, China Received 27 February 2002; revised 18 December 2002; accepted 3 March 2003; published 29 May [1] Analysis of recent ocean model-assimilation results suggest that the eastward countercurrent in the Northern Subtropical Pacific may be divided into two branches: one in the Northwestern Pacific (19 N 27 N to the west of 170 E) and the other to the west of the Hawaii Islands (to the west of the 158 W) along 19 N 20 N. Two mechanisms contribute to the formation of the eastward countercurrent to the west of Hawaii: First, the Hawaiian Islands blocks the easterly trade winds and generates a dipole of wind stress vorticity with positive wind curl to the north of the Islands (20 N 23 N) and with negative curl to the south (16 N 19 N). This dipole of wind stress vorticity acts to create an eastward current jet to the west of the Hawaii Island, through the Sverdrup relation. Second, the islands can directly block the westward North Equator Current (NEC) to generate an eastward current to the west of the islands. We use a numerical model to test these mechanisms for the generation of the eastward countercurrent to the west of the Hawaii Islands. The results show that while the dipole of the wind stress vorticity is helpful in the forming of the eastward countercurrent, and as the NEC impinges upon the Islands the westward traveling vortex streets formed in the ocean plays a key role in intensifying the eastward current to the west of the Hawaiian Islands. INDEX TERMS: 3339 Meteorology and Atmospheric Dynamics: Ocean/atmosphere interactions (0312, 4504); 4255 Oceanography: General: Numerical modeling; 4520 Oceanography: Physical: Eddies and mesoscale processes; 4532 Oceanography: Physical: General circulation; KEYWORDS: Subtropical Countercurrent, Hawaiian Islands, island barrier effect, dipole of wind stress vorticity, Sverdrup transport, vortex streets, Pacific Citation: Liu, Q., S. Wang, Q. Wang, and W. Wang, On the formation of Subtropical Countercurrent to the west of the Hawaiian Islands, J. Geophys. Res., 108(C5), 3167, doi: /2002jc001366, Introduction [2] In the late 1950s and 1960s, Japanese scientists observed an eastward flow in the western part of the subtropical gyre in the North Pacific (20 N 23 N) and called it the Subtropical Countercurrent (STCC) [Yoshida and Kidokoro, 1967]. In the subsequent 30 years, there have been many observational studies on the STCC, but they have focused on the area west in date line [Yoshida and Kidokoro, 1967; Wyrtki, 1975; Gu, 1999; Lan, 2000], where observations are relatively abundant. Similarly, modeling and theoretical studies have concentrated on explaining the formation mechanism of the STCC in the western North Pacific [Takeuchi, 1984; Kubokawa, 1999]. [3] On the basis of the dynamic height derived from the Levitus climatology, Sea Surface Height (SSH) from the Parallel Ocean Climate Model (POCM) and the SSH anomalies (SSHAs) observed by Topex/Poseidon (T/P) altimeter from October 1992 to October 1996, Liu et al. [2000] has calculated the geostrophic zonal flow in North Pacific and indicated that in the climate mean, the STCC exists in the subtropical circulation region (18 N 25 N Copyright 2003 by the American Geophysical Union /03/2002JC and 130 E 157 W) and is confined to the top 150 m with typical velocity of m/s. This result is similar to geostrophic zonal flow along 145 E [Qiu, 1999, Figure 3]. Qiu et al. [1997] showed that a narrow eastward current, dubbed the Hawaiian Lee Countercurrent (HLCC), exists west of the Hawaii Islands. In the current map derived from long-term drift-buoy observations, there is an elongated pair of cyclonic (in the north) and anti-cyclonic (in the south) eddies to the west of the Hawaiian Islands and between the two eddies there is an eastward flow (19 N, 168W 156 W) [Qiu et al., 1997; Figure 5]. On the basis of the ventilated thermocline theory, the formation mechanism of the HLCC was explored further [Qiu and Durland, 2002]. Mitchum [1995] suggested that passage of the NEC around the Big Islands generates eddies, which then propagate westward as Rossby waves and cause the ocean west of the Big Islands to oscillate with a period of about 90 days. Liu and Wang [1999] conducted a spectral analysis of the T/P SSH data and found strong oscillating signals with periods of days. These features of the intraseasonal oscillations are consistent with those of the long Rossby waves [Liu et al., 2001]. [4] On the basis of an analysis of the new QuikSCAT high-resolution satellite wind data, Xie et al. [2001] reported 36-1

2 36-2 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS far-reaching effects of the Hawaiian Islands, whose wind wake trails behind the Islands westward in Dateline for km. They described an interaction between the ocean current, sea surface temperature, wind and boundarylayer clouds that is responsible for sustaining the island effects for such an unusually long distance. In particular, Xie et al. [2001] proposed that there is an eastward current driven by a dipole wind curl, generated by the Islands blockage of the northeasterly trade winds. Details of the eastward current formation were not explored by Xie et al. [2001]. For example, it is unclear whether the seasonal cycle of the eastward current to west of the Hawaii Islands is consistent with that of the wind-forcing. The ocean barrier effect, island blockage of ocean currents, is generally underrepresented or neglected all together in most of the ocean models and need to be investigated. [5] In the present paper we carry out further analysis on the temporal and spatial features of the STCC (section 2) and the formation mechanism of the eastward countercurrent to the west of the Hawaiian Islands, using a model-data assimilation product (section 3). The Sverdrup relation is applied to estimate the effect of the surface wind on the formation of the subtropical circulation in the North Pacific (section 4). The indirect and direct effect of the Hawaiian Islands on the current in the central North Pacific is described using the 2.5-layer model of McCreary and Yu [1992] (section 5). 2. Temporal and Spatial Characteristics of the STCC [6] Previous research focused on data collected mainly from research vessel cruises on the western North Pacific (west of 170 E). Here we use the National Centers for Environmental Prediction (NCEP) Ocean Data Assimilation System (ODAS) Tropical Pacific Ocean Analyses product [Ji et al., 1995; Ji and Smith, 1995; Derber and Rosati, 1989], which is provided by Data Support Section, Scientific Computing Division, National Center for Atmospheric Research (1996). It contains weekly subsurface temperature and velocity at a horizontal resolution of the (lat. lon.). In addition, there are monthly-mean data of sea surface wind, sea surface pressure and ocean current, temperature, salinity at 27 vertical levels ( m) from 1980 to Figure 1 shows the climate mean of zonal currents in the North Pacific at the depths of 15 m, 35 m, 55 m and 85 m. The strongest eastward currents occur to the north of 30 N, which are the Kuroshio and its extension. The westward current in the south part is the NEC. There is an eastward flow at depths of 15 m, 35 m and 55 m and between 135 E 175 E and 20 N 25 N in the west part of the Subtropical Gyre. The western part (135 E 160 E) and the northern part of this eastward current are separated from the strong Kuroshio and its extension by a westward zonal flow. The eastern part of this eastward current (between 160 E 175 E and 18 N 25 N) is not separated from the Kuroshio extension by a westward flow any more. This eastward countercurrent is one branch of the STCC in west of the North Pacific. It is stronger in the spring and summer than in the autumn and winter. [7] Between 175 E and the Hawaiian Islands along 21 N, another eastward current appears as separated from the northwestern branch by a region of westward or weak currents. This eastward countercurrent, with a depth range of m and stronger in the summer, is another branch of the STCC to west of the Hawaii Islands, which is dubbed as the HLCC in Qiu et al. s [1997] paper. To the east of Hawaii such an eastward current no longer exists. [8] Using an idealized ocean model, Kubokawa [1999] reproduced the STCC in west of the North Pacific. In his model, a low potential-vorticity (PV) water mass is originally formed in the northwestern corner of the Subtropical Gyre where the winter mixed layer is very deep. It then subducts along the thermocline and flows to the southwest. When reaching the middle and western part of the subtropical circulation, such a low PV water mass with different density stacks in the vertical direction and forms a thick body of mode water. This process generates a horizontal density gradient and an eastward flow along the south border of the low PV mode water (recent studies further suggest that such a mode water of a large layer thickness plays an important role in decadal variability of subsurface temperature [Kubokawa and Xie, 2002]. Recently it was verified that the STCC in west of the North Pacific is caused by this convergence of the low PV mode water based on analysis of ocean assimilation data [Li et al., 2003]. The present paper focuses instead on the formation mechanism of the eastward countercurrent to west of the Hawaii Islands. 3. Dipole of Wind Stress Vorticity Near the Hawaiian Islands [9] The Hawaiian Islands are under the effect of the steady northeasterly trade winds. There are six islands in Hawaii with elevations of more than 1 km. They all act to block the northeasterly winds. On the lee of each of these elevated island a wake forms with relatively weak winds [Xie et al., 2001]. On the northern and southern sides of the islands the northeasterly winds are strong but in the wake the winds are weak. Thus the distribution of wind stress vorticity varies significantly in space. [10] The monthly mean wind stress data of the NCEP ODAS Tropical Pacific Ocean Analyses are used in our calculation of seasonal wind stress curl for the entire Pacific (Figure 2). Except in December, January, February, and March, there is an area of positive wind stress vorticity to the north of the Big Island (20 N 23 N 165 W 155 W), whereas to the south an area with negative wind curl appears. The zonal scale of this wind curl dipole is about km based on this data set. The dipole of the wind stress vorticity is strong in the summer and disappears in the winter. Xie et al. [2001] first noted this large-scale wind stress vorticity dipole using new satellite measurements. In the ship observations based on the Comprehensive Ocean-Atmosphere Data Set (COADS; 1 1 ) the dipole of the wind stress vorticity appears almost in the same place with almost the same strength. The annual average vorticity is N/m 3 and N/m 3 for two poles of the dipole, respectively. 4. Sverdrup Relation [11] The wind stress vorticity dipole caused by the Hawaiian Islands would affect the characteristics of the

3 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS 36-3 Figure 1. Zonal velocity (cm/s) at 15 m, 35 m, 55 m, and 85 m depth from NCEP ODAS Tropical Pacific Ocean Analyses, averaged for January 1980~September Contour intervals are 4 cm/s with shade indicating eastward flow. climatic mean circulation of the subtropical region in the North Pacific. On annual mean, in the interior area of the subtropical circulation, the Sverdrup relation should be satisfied to the first order. [12] On the basis of the momentum equation and the continuity equation, the Sverdrup relation in the spherical coordinates can be expressed as bv ¼ t f cos q rr ; ð1þ where V is the meridional velocity for the entire water column, f is the longitude, q is the latitude; r, r and b denote density of the seawater, radius of the Earth and the planetary vorticity gradient; t j and t q are the zonal and meridional wind stresses, respectively. [13] The corresponding zonal velocity for the entire water column is Uðf 1 ; qþ ¼ Z f1 f E t f cos df; where f 1 is any one longitude. [14] Using equations (1) and (2) and the climatic mean wind stress field, we obtain the spatial distribution of the stream function and the zonal current (Figure 3). In the integration, the direct effect of the topography of the Hawaiian Islands (as the solid boundary) on the ocean circulation is neglected. However, the change of the wind ð2þ

4 36-4 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS Figure 2. Monthly mean wind stress vorticity (10 7 N/m 3 ) based on the NCEP ODAS Tropical Pacific Ocean Analyses data. Shade denotes positive wind stress vorticity. stress vorticity caused by the Islands is included in the wind field. In Figure 3b, there is a ridge and a trough in streamfunction to the west of the Hawaiian Islands (16 N 22 N), forced by the dipole of the wind stress vorticity at the same latitude. As shown in Figure 3a, there is an eastward zonal current at 18 N. The largest change of wind stress vorticity in the meridional direction takes place at this latitude.

5 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS 36-5 Figure 3. (a) Zonal velocity (cm/s) and (b) stream function (10 6 m 3 s 1 ), calculated with the Sverdrup relation. [15] The above results indicate that the dipole of the wind stress vorticity near the Hawaiian Islands contributes to the formation of the (Figure 3). On the climatic average, this contribution is on the order of 1 cm/s, which is somewhat small compared to the annual average value of 3 cm/s based on the NCEP ODAS Tropical Pacific Ocean Analyses. The dipole of the wind stress vorticity almost disappears in the winter and is stronger in the summer and therefore, the eastward current to west of the Hawaii Islands is stronger in the summer than in the winter. 5. Direct and Indirect Effects of the Hawaiian Islands on the STCC 5.1. Numerical Model [16] The above Sverdrup calculation showed the indirect effect of the Hawaiian Islands (through wind) on the eastward current to west of the Hawaii Islands. In order to verify this effect under more realistic conditions and to investigate the direct effect of the Hawaiian Islands on the ocean circulation in the Pacific, we conduct a series of numerical experiments with a baroclinic, 2.5-layer model developed by McCreary and Yu [1992] (hereinafter referred to as the MY model). The first (upper) and the second (lower) layer are active while the third layer is at rest and infinitely deep. By examining the ocean climatology, initial layer thickness is set to be 75 m and 225 m for the first and second layers, respectively. The horizontal resolution of the model is 0.4 in latitude and longitude. In the model the upper layer fluid is driven by an ideal surface wind. [17] In the present paper the model domain covers a wide latitude band of 9 N41 N. Therefore, the equatorial b plane in the original MY model is replaced with the spherical coordinates. The biharmonic mixing coefficient v 4 is set to be the same as k 4 = cm 4 s 1, with a maximum damping coefficient g = 1 day 1 [McCreary and Yu, 1992]. The eastern and western boundaries of the model are set at the 200-m isobath of the observed bottom topography. Areas shallower than 200 m are set as land. The no-slip boundary condition is used on the east and west boundaries. The northern and the southern boundaries situate at 9 N and 41 N, respectively. The meridional mass transport through these boundaries is set to be zero because these latitudes are near the zero wind-curl lines. All the other parameters are taken to be the same values as those of McCreary and Yu [1992]. Here we analyze the model output in an area of 10 N 40 N, 130 E 140 W Atmospheric Barrier Effect [18] In order to test and verify the indirect effect of the dipole of the surface wind stress vorticity on the eastward countercurrent to west of the Hawaii Islands, an idealized wind field is used as the driving force of the model. For simplicity, it includes the zonal wind stress but neglects the meridional one. On the basis of the annual mean of the real distribution of the zonal wind stress at 170 E (COADS), the idealized zonal wind stress is expressed as t f ¼ 0:8cosððq 13Þp=90 2:95Þ dyn=cm 2 : ð3þ It is a zonally uniform field, with the maximum easterlies located at 13 N and the maximum westerlies at 43.5 N. [19] In Experiment 1 only this zonal-uniform wind field is used. The barrier effect of the Hawaiian Islands on the ocean circulation is neglected. The model is integrated for Figure 4. Wind stress curl (10 7 N/m 3 ) in Experiment 2.

6 36-6 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS Figure 5. Stream lines of upper layer in Experiment 2 on model day days, after which the model total kinetic energy in the analysis area becomes steady. We believe that the numerical integration of the model reaches its statistical equilibrium. This experiment shows no eastward current. [20] Experiment 2 includes another idealized wind field with an added dipole of the surface wind curl representing the aerodynamical effect of the Hawaiian Islands, t f ¼ 0:8cosððq 13Þp=90 2:95ÞþTdipðq; fþ; ð4þ where Tdipðq; fþ ¼ ð0:2 cosððq 18:5Þp=90 10:2ÞÞ sinððj 201:6Þ=9p þ p=2þ ð5þ for 14:6 N q 22:6 N; 163 W f 154 W and Tdipðq; fþ ¼ 0 ð6þ elsewhere. Figure 4 shows the wind stress curl of equation (4). Here we neglect again the topography effect of the Hawaiian Islands on the ocean circulation. Figure 5 shows the streamlines of the upper layer at the 3795-th modeling day, while Figure 6 shows the long time mean zonal current obtained from Experiment 2. Unlike Experiment 1, there are eddies to the west of the Hawaii Islands in Experiment 2 (Figure 5). There is an eastward flow between the cyclonic and anti-cyclonic eddies. This eastward current is located in the middle part of the subtropical gyre in North Pacific to the west of Hawaii (between 18 N 21 N and W) that is under the influence of the imposed wind stress vorticity dipole. To its north, there is a broad westward flow that separates it from the Kuroshio Figure 6. Long-time mean of the zonal velocity in Experiment 2.

7 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS 36-7 Figure 7. Stream lines of upper layer on model days (top) 3795 and (bottom) 3990, obtained from Experiment 3. extension. Therefore this eastward current to the west of Hawaii Islands is a countercurrent in Subtropical North Pacific. [21] This eastward countercurrent is located between 175 E and the Hawaiian Islands (18 21 N) in Experiment 2, similar to that in the NCEP ODAS Tropical Pacific Ocean Analyses (Figure 1). In the numerical Experiment 2 the velocity at the center of the countercurrent is only 1 cm/s, smaller than the annually averaged value of 3 cm/s in the NCEP ODAS Tropical Pacific Ocean Analyses. This is consistent with the estimate from the Sverdrup relation in section 4, quantifying the important role of wind stress vorticity dipole. It is necessary to indicate that from the idealized wind field (equation (3)) there is an additional pair of the dipole of the wind stress vorticity in the area of 14.6 N 22.6 N and 163 W 154 W (equation (4)) Oceanic Barrier Effect [22] The above analysis shows that the wind stress vorticity dipole, a result of the blocking effect of the Hawaiian Island on the trade wind, affects the ocean circulation and induces an eastward current on the order of about 1.0 cm s 1, in the absence of the ocean circulation effects. Godfrey [1989] deduced an Island Rule and determined the round island transportation of the ocean circulation under the influence of the large-scale wind field. This rule has later been improved and applied to the study of the north Hawaiian ridge current (NHRC [Qiu et al., 1997]). Liu et al. [1999] suggested that the interaction between the

8 36-8 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS Figure 8. Long-term mean zonal velocity of the upper layer in Experiment 3. Kelvin waves and the Rossby waves, which pass the tropical islands, produce the long Rossby waves from the west coast of the islands. However, the eastward current is not discussed in their paper. In order to investigate the direct effect of the Hawaiian Islands on the eastward countercurrent, we conduct Experiment 3, where wind stress given by equation (3) is used and the Hawaiian Islands are treated as solid boundaries that block the ocean currents. [23] Figure 7 shows the streamlines of the upper current field on modeling days 3795 and 3990, after the model reaches its equilibrium. With the Hawaiian Islands as solid boundaries, there are a series of eddies to west of the islands that move westward constantly. The series of clockwise (anticlockwise) eddies at the southern (north) end of the Hawaiian Islands cannot be the results of the local wind because the wind stress vorticity dipole is removed in this experiment. The northern and southern eddies seem to be paired together with their centers shifted by one half of the eddy size in the zonal direction. [24] In the vortex streets to the west of Hawaii, the eddies at the northern (southern) street are cyclonic (anti-cyclonic). The distance between the two vortex streets is about 4 in latitude and the zonal distance between two adjacent eddies in either street is about 12 in longitude. Between the two vortex streets, there is an obvious eastward current. The vortex streets can be traced westward to about 150 E. This phenomenon is similar to that of flow-passing a cylinder as studied extensively in fluid dynamics, although the beta effect and irregular topographic may modify the result. In Experiment 3, when the biharmonic horizontal mixing v 4 and k 4 are changed from cm 4 s 1 to cm 4 s 1, the vortex shedding period is changed from 180 days to 160 days. Therefore a 25% change in mixing coefficient leads to 10% change in the oscillation period, indicating the importance of the appropriate mixing coefficient to simulate the vortex shedding process in this simulation. [25] In our model, the eastward current between the vortex streets extends westward to 150 E. In the north of the cyclonic eddies, there is a westward flow that separates the eastward flow to the west of Hawaii and the Kuroshio extension. Thus the eastward current between the vortex streets becomes an independent one. Its velocity is about 5cms 1 (Figure 8) in Experiment 3. Here, the extent of the STCC and its velocity are both greater than those in Experiment 2, indicating the importance of the ocean circulation blocking and subsequent vortex street formation as a mechanism for the eastward countercurrent to west of the Hawaii Island. 6. Conclusion [26] On the basis of the analysis of the NCEP ODAS Tropical Pacific Ocean Analyses data, it is revealed that on the climatological average, there is an eastward flow in the middle part of the North Pacific. It occurs in a depth of not more than 150 m to the west of the Hawaiian Islands. Its velocity is about m/s. It is strong in the summer while weak in the winter. It is found that there exists a dipole of the positive and negative wind stress vorticity near the Hawaiian islands, which are produced by the blockage of the Hawaiian Islands on the surface wind. The center of the positive curl of the dipole is located at 20 N 23 N and 166 W 155 W, to the north of the Hawaiian Islands, and its intensity is more than Nm 3. The center of the negative curl is located in the area of 16 N 19 N and 165 W 155 W, to the south of the Hawaiian Islands, with an intensity of more than Nm 3. The windcurl dipole is stronger in the summer than in the winter. [27] By using the Sverdrup relation and a simple ocean baroclinic model, the effect of the dipole of the sea surface wind stress vorticity on the mean ocean circulation in the subtropical North Pacific has been estimated. Under this effect, an eastward zonal flow appears to the west of the Hawaiian Islands. In other words, the mountains of the Hawaiian Islands modify the wind field, and this aerodynamical effect produces the wind-curl dipole that further enhances the eastward countercurrent in Subtropical North Pacific, which is other branch from STCC in west of the Subtropical North Pacific. This wind-driving mechanism

9 LIU ET AL.: SUBTROPICAL COUNTERCURRENT TO THE WEST OF THE HAWAIIAN ISLANDS 36-9 well explains the seasonal variations of this branch of the STCC. In addition to this indirect effect of the Hawaiian Islands, we showed that the Hawaiian Islands have a direct effect on the eastward countercurrent as an oceanic barrier. Under this direct blockage of the westward NEC, pairs of eddies westward propagate to the west and form the vortex streets. The eastward flow between these eddy streets is the STCC over long-term average. [28] Finally, it should be pointed out that the abovementioned dynamical processes could affect the temperature distribution in the 2.5-layer model and the air-sea interaction in the area of the STCC. These effects are beyond the capabilities of the simple baroclinic model with idealized conditions as presented herein. Further research is needed to verify these mechanisms quantitatively with observation data. [29] Acknowledgments. This study benefited from the National Nature Science Foundation of China (grant and ) and the Ministry of Science and Technology of China (National Key Program for Developing Basic Science G ). We thank S.-P. Xie, R. X. Huang, J. S. Godfrey, Z. Y. Liu, and B. Qiu for helpful discussions. References Derber, J. D., and A. Rosati, A global oceanic data assimilation system, J. Phys. Oceanogr., 19, , Godfrey, J. S., A Sverdrup model of the depth-integrated flow for the world ocean allowing for island circulations, Geophys. Astrophys. Fluid Dyn., 45, , Gu, Y. L., The subtropical countercurrent profile at 137 E (in Chinese), J. Oceanogr., 21, 22 30, Ji, M., and T. M. Smith, Ocean model response to temperature data assimilation and varying surface wind stress: Intercomparisons and implications for climate forecast, Mon. Weather Rev., 123, , Ji, M., A. Leetmaa, and J. Derber, An ocean analysis system for seasonal to interannual climate studies, Mon. Weather Rev., 123, , Kubokawa, A., Ventilated thermocline strongly affected by a deep mixed layer: A theory for subtropical countercurrent, J. Phys. Oceanogr., 29, , Kubokawa, A., and S.-P. Xie, Steady response of a ventilated thermocline to enhanced Ekman pumping, J. Oceanogr., 58, , Lan, S. H., The main hydrological features at 155 E of the subtropical region in the North Pacific, J. Oceanogr., 1, 8 17, Li, W., H. L. Liu, and Q. Y. Liu, Two branches of the eastward countercurrent in the subtropical North Pacific (in Chinese), Sci. Atmos., 27 in press, Liu, Q. Y., and Q. Wang, The distribution characteristics of interseasonal oscillation of sea surface height in the tropical Pacific (in Chinese), J. Oceanogr., 29, , Liu, Q. Y., H. J. Yang, and W. Li, The climatic characteristics of Subtropical Countercurrent in the North Pacific (in Chinese), Science Atmos., 24, , Liu, Q. Y., S. X. Wang, Z. Y. Liu, and H. J. Yang, The dynamical characteristics of the long Rossby waves of the subtropical countercurrent in the North Pacific, (in Chinese), J. Geophys., 44, 28 38, Liu, Z., L. Wu, and H. Hurlbuet, Rossby wave-coastal Kelvin wave interaction in the extratropics: II. Formation of island circulation, J. Phys. Oceanogr., 29, , McCreary, J. P., and Z. Yu, Equatorial dynamics in a 2 1/2layer model, Prog. Oceanogr., 29, , Mitchum, G. T., The source of 90day oscillations at Wake Island, J. Geophys. Res., 100, , Qiu, B., Seasonal eddy field modulation of the North Pacific subtropical countercurrent: TOPEX/Poseidon observation and theory, J. Phys. Oceanogr., 29, , Qiu, B., and T. S. Durland, Interaction between an island and the ventilated thermocline: Implications for the Hawaiian Lee Countercurrent, J. Phys. Oceanogr., 32, , Qiu, B., D. Koh, C. Lumpkin, and P. Flament, Existence and formation mechanism of the North Hawaiian ridge current, J. Phys. Oceanogr., 27, , Takeuchi, K., Numerical study of the subtropical front and subtropical countercurrent, J. Oceanogr. Soc. Jpn., 40, , Wyrtki, K., Fluctuation of the dynamic topography in the Pacific Ocean, J. Phys. Oceanogr., 15, , Xie, S.-P., W. T. Liu, Q. Y. Liu, and M. Nonaka, Far-reaching effects of the Hawaiian Islands on the Pacific ocean-atmosphere system, Science, 292, , Yoshida, K., and T. Kidokoro, A subtropical countercurrent in the North Pacific An eastward flow near the subtropical convergence, J. Oceanogr. Soc. Jpn., 23, 88 91, Q. Liu, Q. Wang, S. Wang, and W. Wang, Physical Oceanography Laboratory and Ocean-Atmospheric Interaction and Climate Laboratory, Ocean University of China, Number 5, Yushan Road, Qingdao , China. (liuqy@ouc.edu.cn; wangqi@ouc.edu.cn; soniaxia@sina.com; wei@ouc.edu.cn)