Bifurcation of the North Equatorial Current derived from altimetry in the Pacific Ocean*

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Journal of Hydrodynamics Ser.B, 2006,18(5): 620-626

sdlj.chinajournal.net.cn

BIFURCATION OF THE NORTH EQUATORIAL CURRENT DERIVED FROM ALTIMETRY IN THE PACIFIC OCEAN* WANG Qing-ye Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Graduate School of the Chinese Academy of Sciences, Beijing 100039, China,E-mail: [email protected] HU Dun-xin Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China

(Received March 29, 2006) ABSTRACT: The gridded (1/3°*1/3°) altimetry data from October 1992 through December 2004 were analyzed to study the seasonal and interannual variabilities of the bifurcation of the North Equatorial Current (NEC) at the surface in the western North Pacific Ocean. Calculations show that on annual average the bifurcation occurs at about 13.4°N at the surface. The geostrophic flow derived from Sea Surface Height (SSH) data shows that the southernmost latitude of the NEC bifurcation at the surface is about 12.9°N in June and the northernmost latitude is about 14.1°N in December. Correlation analyses between the bifurcation latitude and the Southern Oscillation Index (SOI) suggest that the bifurcation latitude is highly correlated with the El Niño/Southern Oscillation (ENSO) events. During the El Niño years the bifurcation of the NEC takes place at higher latitudes and vice versa. KEY WORDS: bifurcation of the North Equatorial Current, altimetry data, Pacific Ocean, North Equatorial Current (NEC),El Niño/Southern Oscillation (ENSO) events

1. INTRODUCTION In the western North Pacific, the North Equatorial Current (NEC) bifurcates into the northward flowing Kuroshio and the southward flowing Mindanao Current as it encounters the Philippine coast[1-3]. The bifurcation latitude has direct influences on the partition of the NEC mass and heat transport between the poleward and equatorward flows and on the dynamical structure of the low-latitude western boundary currents[4,5] as well to some evtent. Due to the importance of the eastern-Asia monsoon [6,7] and

the El Niño/Southern Oscillation (ENSO) events for the bifurcation area of the NEC, the seasonal and interannual changes of the NEC bifurcation latitude were generally noticed in many studies[8-10]. Using hydrographic data obtained in 1934, Nitani[2] claimed that the NEC bifurcated at about 11°N and with a few cruise data taken in the 1960s he estimated the NEC northernmost bifurcation at about 14.5°N. The authors also showed that the bifurcation shifted to the north with increasing depth. On the basis of analyzing the water mass distribution along the western boundary, Toole et al.[11] estimated the NEC bifurcation at the latitude around 12°N, while with two hydrographic surveys in September 1987 and April 1988, respectively, the NEC was observed to bifurcate near the latitude 13°N[3]. In general, at the surface the NEC should bifurcate along the line of zero wind curl (curl τ = 0 ) according to the Sverdrup theory, when the wind is steady. However, in reality, the winds are changing with both time and space, resulting in significant impact on the NEC bifurcation in terms of local and remote effects. Anyway, the Sverdrup theory can not accommodate itself to the depth-dependence of the NEC bifurcation latitude. So far, about the northward shift of the NEC bifurcation with the depth, there were two explanations, i.e., (1) it results from the interaction between the Sverdrup transport and boundary conditions on density[12] and (2) it results from the balance between the layer thickness and the planetary vorticity[3]. Later on, using

* Project supported by the National Natural Science Foundation of China (Grants Nos: D06-40552002, 40576016) and by the Qingdao Municipal Bureau of Science and Technology (Grant No: 02-KJYSH-03). Biography: WANG Qing-ye (1977-),Male, Ph.D., Lecturer

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the time-dependent linear Sverdrup theory and high-resolution nonlinear reduced-gravity model, Qiu and Lukas[8] showed that the bifurcation of the NEC occurred at its southernmost position in February and northernmost position in October. However, Qu et al.[13] and Qu and Lukas[9] constructed the climatology of temperature and salinity with historical data and found that the NEC bifurcation latitude shifted from 13.3°N at the surface to about 20°N at 1000 m level with the vertical average of the northernmost and southernmost positions of 14.8°N in July and 17.2°N in December, respectively. They concluded that the local Ekman pumping associated with monsoonal winds was important for the seasonal bifurcation of the NEC. With the numerical result of JAMSTEC, Kim et al. [10] suggested that the NEC bifurcation point on the vertical average moved equatorward during summer and poleward during winter. At the surface, there are two different results on the bifurcation latitude of NEC. Qu and Lukas [9] suggested that near the sea surface (i.e., the distance from the sea surface being less than 100 m), the NEC bifurcated at its southernmost (13.4°N) in June and its northernmost (14.8°N) in November. However, Kim et al.[10] showed the NEC bifurcation at the surface happened at its southernmost position (13.2°N) in May and its northernmost position (15.1°N) in September. Due to the data limitation, only a few studies focused on the interannual variability of the NEC bifurcation. With a numerical model, Qiu and Lukas[8] claimed that it took place at its highest latitudes a year after the El Niño and at its lowest latitude during the La Niña years. Recently, with their high-resolution OGCM results, Kim et al.[10] suggested that the meridional migration of the NEC bifurcation was strongly correlated to the ENSO (i.e., the Southern Oscillation Index (SOI)) with the correlation coefficient greater than 0.8 at the depths around the thermocline. That means that the NEC bifurcation occurs at its northernmost position during the El Niño years and at its southernmost position during the La Niña years. That result is quite different from that obtained by Qiu and Lukas[8], who showed the bifurcation followed the El Niño events with one year time lag. It can be seen from the comparison of the above results that the seasonal variations of the bifurcation claimed by Qu and Lukas[9] and Kim et al.[10] are consistent except those near the surface, but greatly different from those given by Qiu and Lukas[8], who suggested its southernmost position in winter (February), instead of summer. Concerning the apparent different conclusions, Miyama et al. [14] conducted several experiments with a 1.5-layer model and revealed that the results of Qiu and Lukas[8] were greatly influenced by their artificial northern boundary

in the model. Their Pacific Ocean in model is closed along 38°N with increased horizontal viscosity from 30°N to the north boundary 38°N. If the model ocean is closed along 60°N in the experiments, then the seasonal variation of the bifurcation latitude is consistent with Qu and Lukas’s results[9]. In addition, the JAMSTEC model results[10] confirmed Qu and Lukas[9]’s conclusion except near the surface. However, concerning the bifurcation latitude of the NEC at the surface, there appeared obvious difference between the results obtained by Qu and Lukas[9] and by Kim et al.[10]. In addition, there was also not doubtless view on the interannual variability of the NEC bifurcations predicted by Qiu and Lukas[8] and by Kim et al.[10]. Therefore, the above two problems need to be clarified. Since October 1992, there had been more than 12 years altimetry data before 2004 with the horizontal resolution 1/3°*1/3°. These data were obtained with much finer resolution than historical temperature and salinity data (such as those by Qu and Lukas[9]) to resolve finer structure and are more convincible for studying the interannual variability than the ocean model data,such as those used by Kim et al. [10]. Therefore, this article is intended to study in detail the seasonal and interannual variabilities of the NEC bifurcation at the surface by using the merged gridded altimetry data, especially to solve the two problems mentioned above.

2. DATA AND METHOD OF ANALYSIS The altimetry data used for this study were resulted from the Aviso products. The gridded Sea Level Anomalies (SLA) with the horizontal resolution 1/3°*1/3° on a Mercator grid in the period of October 1992 through December 2004 were used, which were derived by simultaneously using the altimeters Topex/Poseidon, Jason-1, ERS-1/2 and Envisat. They were generated with the method of the orbit error reduction processing and the time interval was 7 d. In addition, the data also consisted of the Mean Dynamic Topography (MDT, h ), which had the same horizontal resolution as the SLA data. The MDT is a part of mean sea surface height due to steady currents, which corresponds to the mean sea surface height minus geoid. The sea level anomalies was first averaged for each month and form a long time series of 147 months, from October 1992 through December 2004. Then the climatology of the sea level anomalies were derived from the long time series, and finally the surface geostrophic flow was calculated according to the geostrophic relation to be mentioned below. By assuming the geostrophy, the anomalous surface geostrophic velocity is related to the SLA ( h' ) by

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u 'g =

g k × ∇h ' f

(1)

where k is a unit vector in the vertical direction, f is the Coriolis parameter, and g is the gravity constant. Let

ug =

g k × ∇h f

(2)

be the mean surface geostrophic velocity and h be the MDT (Fig. 1). So, the Absolute Dynamic Topography ADT = SLA + MDT = h' + h . Therefore,

u g = u g + u' g =

g k × ∇ ADT is the surface f

geostrophic velocity. °N30

20

10

120

130

140

150

160°E

Fig. 1 Mean surface dynamic topography (m)

In the past, the bifurcation latitude of the NEC was defined to be the latitude at which the meridional velocity averaged within a given wide band off the continent is zero (cf. Qiu and Lukas[8]; Qu and Lukas[9]). For example, Qu and Lukas[9] averaged the meridional velocity in a 5° band off the coast, owing to the sparsity of the hydrographic data, while Qiu and Lukas[8] and Kim et al.[10] averaged in a 2° band with numerical model results. Obviously, 5° band for averaging the meridional velocity to resolve the NEC bifurcation is too coarse and the model results with finer resolution are hard to be validated with hydrographic data available. Therefore, to determine the bifurcation latitude, besides using the method presented by previous authors with various bands of 1, 2, 5 to find meridional velocity equal to zero, another method is proposed here to determine a trajectory with the 1/3°*1/3° altimetry data along which any water particles will not finally join either the tropical gyre in the south or the subtropical gyre in the north and then to determine the bifurcation point along the trajectory.

The specific manipulation is based on the method of calculating Lagrangian trajectories proposed by Döös[15], Blanke and Raynaud[16], and Vries and Döös[17]. The calculation of Lagrangian trajectories from the gridded velocity field mainly includes four steps. In this article, it is supposed that the horizontal velocity fields in each month determined by altimetry data do not depend on time. First, a linear interpolation of velocity was made inside the grid box containing the particle. Second, the trajectory in the grid box and the position where the particle left the grid box were determined by a differential equation. Third, in the next grid box where the particle arrived, the above two steps were repeated. Finally, the first two steps were carried out in a series of grid boxes and the whole trajectory was found. In the present articla, the procedure of calculating operation is as follows. Along each meridional line in a grid, a point can be easily found first, from which water particle starts to move but finally will fit into neither the tropical gyre nor the subtropical gyre, and then connect those points to form a line, which is called the trajectory of the NEC bifurcation. That is to say, a particle starting from the north of the point will finally enter the subtropical gyre, while a particle starting from the south of the point will finally enter the tropical gyre. Obviously, the zonal velocity in the NEC decreases rapidly, while the meridional velocity increases, when the NEC approaches the coast of the Philippine. By examining the gradient of the zonal velocity along the trajectories from January to December defined with the altimetry data of 1992–2004 (not shown here), it is found that before the NEC getting obviously bifurcated, the gradient is apparently less than 0.02 ms-1/o and increases rapidly from 0.03 ms-1/o up to more than 0.1 ms-1/o when the NEC approaches the coast. So along the trajectory of the NEC bifurcation the starting bifurcation point of the NEC is defined by a criterion of the gradient of the zonal velocity equal to 0.03 ms-1/o . Then, the bifurcation latitude of the NEC is determined by averaging the latitudes along the trajectory from the starting bifurcation point to the coast.

3. SEASONAL VARIATION The monthly mean absolute dynamic topography is presented in Fig. 2, from which a subtropical gyre north of 13°N and a tropical gyre south of that appear. The NEC is the boundary between the two gyres. According to the method described in the last section, the NEC bifurcation latitude is calculated and shown in Fig. 3. The bifurcation occurs at its southernmost position (12.9°N) in June and northernmost position (14.1°N) in December. For its annual average, the bifurcation takes place at about

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13.4°N. January

different from those in Fig. 3 but also from those given by Qu and Lukas[9], and its southernmost position is in July.

February

°N20 °N 14

15 10

13.6

5 March

April

°N20

13.2

15

12.8 1

3

5

10

7 Month

9

11

5 May

Fig. 3 Seasonal variation of the NEC bifurcation latitude determined from the surface geostrophic flow

June

N20 15

°N 14

10

13.6

5 July

August

13.2

°N20

12.8

15

12.4 10

1

3

5

7 Month

9

11

5 September

October

Fig. 4 Bifurcation latitude of the NEC calculated by previous method with the different bands of 1° (solid line), 2° (thick solid line) and 5° (dashed line) off the coast of the Philippines

°N20 15 10 5 November

December

°N20 15 10 5 120

125

130

135

140 120

125

130

135

140°E

Fig. 2 Monthly absolute dynamic topography (m)

Besides, the bifurcation latitude of the NEC is also estimated with the method used by previous authors with the different bands of 1°, 2° and 5° off the coast of the Philippines and depicted in Fig. 4. It can be seen from Fig. 4 that the results with 1° and 2° band are nearly the same, showing the southernmost point of the NEC bifurcation in June and northernmost in December with only less than 10% difference in latitude, which are quite similar to those shown Fig. 3. However, the results with 5° band are not only

The range from the southernmost to northernmost latitudes of the NEC bifurcation at the surface is about 1.2° (Fig. 3), which is smaller than 1.4° (13.4ºN-14.8ºN) in Ref.[9] near the sea surface (< 100 m) and 1.9o (13.2ºN-15.1ºN) in Ref.[10]. The reason of the difference needs to be further studied. It must be pointed out that in Fig. 3, the bifurcation in July takes place further north by about 0.4° than in June and August, which is statistically significant. To explain this phenomenon, the wind stress curl field is calculated for the same time as the altimetry data. The wind data used are from FSU for the period of 1992 through 2001 and from the Quikscat and NCEP blended wind field for the period from 2002 to 2004 instead. The FSU wind data are interpolated into a 1/2°*1/2° grid to have similar resolution with the blended wind field. Averaged in the region 124°E-130°E, 11.5°N-15.5°N, the averaged wind stress curl in the region is shown in Fig. 5. The local wind stress curl has a maximum in November and a minimum in May, which is about one month ahead of the NEC bifurcation migration shown in Fig. 3. In addition, the local wind stress curl in July is larger than in June and August. Therefore, it can be

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concluded that the effect of local wind field curl could be the main reason for the further north NEC bifurcation in July, because larger wind curl will cause further north bifurcation of the NEC.

Wind stress curl

12 8 4 0 1

3

5

7 Month

9

11

Fig. 5 Wind stress curl (10-8Nm-3) averaged near the Philippine coast (11.5°N-15.5°N, 124°E-130°E)

4. INTERANNUAL VARIABILITY The time series of the bifurcation latitude of the NEC established according to the above method and of the SOI from October 1992 through December 2004 are depicted in Fig.6 with the anomalies obtained by applying a 12-month running mean filter and removing the mean. The two time series are quite highly correlated to each other with a correlation coefficient of 0.70. It is apparent from Fig.6 that the NEC bifurcates at higher latitudes (positive anomaly) in 1992/1993, 1997/1998 and 2002/2003, when the El Niño occurs, and at lower latitudes (negative anomaly) in 1996 and 1999/2000, when the La Niña takes place. The conclusion about the relationship of the NEC bifurcations is consistent with that drawn by Kim et al. [10], but different from the conclusion of Qiu and Lukas[8], who claimed that the NEC bifurcates at its northernmost latitude one year after the El Niño.

Table 1 Month for southernmost and northernmost points of NEC bifurcation at the surface from various authors

Qiu and Lukas[8]

February

October

Qu and Lukas[9]

June (0-100m)

November (0-100m)

[10]

r = -0.70

Northernmost

Kim et al.

May

September

Present paper

June

December

In sum, in terms of seasonality of the NEC bifurcation latitude a comparison of various results can be seen from Table 1. There are two types of studies, with observational data[9] and with numerical models[8,10]. The results of the first two articles are nearly the same in terms of the southernmost and northernmost latitudes, in June and November in Qu and Lukas[9], and in June and December in the present article, respectively, though the data used are different, hydrographic data and altimetry data and the grid bands have the differences of 5° and 2°, respectively. There exists the difference of only one month between each other for the northernmost latitude, which might be caused by errors of data in cold season. The results in the second two articles are quite different in the southernmost and northernmost latitudes, that is, February and October[8] and May and September[10], respectively, which might result from different model specifications or boundary conditions and etc. This indecates that it would be difficult for the numerical model to well represent the real structure of surface current if the model is not validated by observational data.

Anomaly

Southernmost

2

0

-2 1992

1996

Year

2000

2004

Fig. 6 Time series of the NEC bifurcation latitude anomalies (solid line) versus the SOI anomalies (dashed line) with a 12-month running mean filter

To verify the high correlation of the NEC bifurcation change in interannual time scale with the El Niño events, the power spectra for the time sieries of the bifurcation latitude and of the SOI from October 1992 through December 2004 are delineated in Fig.7. The two spectra are very similar to each other with nearly the same peaks at about the 6th, 9th, 12 th months, as well as the 32nd month, the El Niño signal, which indicates the NEC bifurcation, fluctuates well with the SOI. 5. SUMMARY AND CONCLUSIONS Seasonal and interannual migrations of the NEC bifurcation latitude are studied in the present paper with (1) the high resolution (1/3°*1/3°) gridded altimetry data from October 1992 through December 2004 and (2) a new method of finding the trajectory of the NEC bifurcation, along which any water particles will not finally connect the tropical gyre in the south or the subtropical gyre in the north. The main results are as follows.

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Spectral value

4

2

0 100

Spectral

4

10 Period(month) （a)

1

10 Period(month) （b)

1

2

0 100

Fig. 7 Power spectral density distributions (solid line) and 95% confidence line (dashed line)

(1) On annual average, the NEC bifurcates at about 13.4°N at the surface, which seems more reasonable compared with previous results[3]. (2) On its seasonality, the following conclusions have been reached. The NEC bifurcates at its southernmost position (12.9°N) in June and northernmost position (14.1°N) in December, both of which are quite similar to those given by Qu and Lukas[9] from historical hydrographic data except its northernmost position appearing in November, instead of December. The difference might result from the sparsity and lack of the historical hydrographic data in November and December, which were reduced by 50% in summer used by Qu and Lukas[9]. But our results are quite different from those obtained with the numerical models of Qiu and Lukas[8] and Kim et al.[10]. The range of meridional migration of the NEC bifurcation point at the surface is about 1.2o, which is the smallest compared with other results (1.4° given by Qu and Lukas[9], 1.9° by Kim et al.[10]). That needs to be further studies to clarify which is correct. The fact that the bifurcation point in July is further north than in June and August could be explained by the stronger wind curl in that region in July. The comparison made in the above context indicates that the results with observational data are similar to each other[9], but very different with the numerical model results[8,10]. And what is more, the results of the numerical models are very different

from each other. It seems that the modeld need to be further improved, especially to be validated by observations. (3) The correlation analysis shows that the NEC bifurcation latitude fluctuates with the SOI at various frequencies. Especially, it migrates in phase with the ENSO. During the El Niño years, the NEC tends to bifurcate at higher latitudes, while during the La Niña years, the NEC generally bifurcates at lower latitudes, as suggested by Kim et al.[10] but not by Qiu and Lukas[8]. As Miyama et al.[14] pointed out, the problem in the results of Qiu and Lukas[8] might result from their artificial boundary in their model with the dtudied domain only extending northward to 38°N and with the Indonesian Throughflow closed. (4) It seems that the method proposed in this paper to determine the NEC bifurcation point through finding a trajectory works well.

ACKNOWLEDGEMENTS The authors would like to thank Drs. Bai Xue-zhi and Liu Zhi-liang for their invaluable discussions. The gridded altimetry data used in this study were provided by the AVISO/Altimetry web, the surface wind data by Florida State University, and the Quikscat and NCEP blended wind by the University Corporation of Atmospheric Research.

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