Chinese Journal of Oceanology and LimnologyVol. 30 No. 4, P. 675-683, 2012http://dx.doi.org/10.1007/s00343-012-1155-2

Sea surface height variations in the Mindanao Dome region in response to the northern tropical Pacifi c winds*

SONG Dan (宋丹) 1, 2 , HU Dunxin (胡敦欣) 1, 2, ** , ZHAI Fangguo (翟方国) 1, 2, 3 1 Key Laboratory of Ocean Circulation and Waves, Chinese Academy of Sciences, Qingdao 266071, China 2 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China 3 Graduate University of Chinese Academy of Sciences, Beijing 100049, China

Received Jul. 15, 2011; accepted in principle Jul. 30, 2011; accepted for publication Nov. 8, 2011

© Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2012

Abstract Sea surface height (SSH) variability in the Mindanao Dome (MD) region is found to be one of the strong variations in the northern Pacifi c. It is only weaker than that in the Kuroshio Extension area, and is comparable to that in the North Pacifi c Subtropical Countercurrent region. Based on a 1.5-layer reduced gravity model, we analyzed SSH variations in this region and their responses to northern tropical Pacifi c winds. The average SSH anomaly in the region varies mainly on a seasonal scale, with signifi cant periods of 0.5 and 1 year, ENSO time scale 2–7 years, and time scale in excess of 8 years. Annual and long-term variabilities are comparably stronger. These variations are essentially a response to the northern tropical Pacifi c winds. On seasonal and ENSO time scales, they are mainly caused by wind anomalies east of the region, which generate westward-propagating, long Rossby waves. On time scales longer than 8 years, they are mostly induced by local Ekman pumping. Long-term SSH variations in the MD region and their responses to local winds are examined and discussed for the fi rst time .

Keyword : 1.5-layer model; Mindanao Dome (MD); sea surface height (SSH); Ekman pumping

1 INTRODUCTION

The Mindanao Dome (MD) is a cyclonic recirculation gyre east of Mindanao Island in the Philippines. Based on results from an ocean general circulation model, Masumoto and Yamagata (1991) indicated that the MD develops in boreal winter, because of local upwelling when a positive wind-stress curl associated with the northeast Asian winter monsoon increases in the region. The MD decays in spring because of the intrusion of downwelling, long Rossby waves generated in winter by trade winds further east, as well as a retreat of the local curl. This seasonality has been recognized by several studies (Tozuka et al., 2002; Suzuki et al., 2005; Kashino et al., 2011).

Wang et al. (2000) discussed semiannual variability of the thermocline adjustment in the tropical Pacifi c. Qu et al. (2008) indicated the existence of semiannual signals in SSH and current variations in the western tropical Pacifi c, based on analysis of altimeter data

together with model results. Tozuka et al. (2002) and Kashino et al. (2011) discovered semiannual signals in MD evolution by analyzing temperature and heat budget variations. All the above works supposed that the semiannual variations result from a combination of local Ekman upwelling and remotely forced, westward-propagating long Rossby waves.

Interannual oceanic variations in the MD region were also addressed. Tozuka et al. (2002) analyzed MD evolution in various El Niño/La Niña years. They suggested the importance of this evolution for heat content variability in the ENSO cycle. Kashino et al. (2011) also studied heat content variations in the MD region, on ENSO time scales.

Except for Qu et al. (2008), previous studies of MD evolution were mostly based on analysis of

* Supported by the National Natural Science Foundation of China (No. 40890151), and the National Basic Research Program of China (973 Program) (No. 2012CB417401) ** Corresponding author: dxhu@qdio.ac.cn

676 CHIN. J. OCEANOL. LIMNOL., 30(4), 2012 Vol.30

temperature and heat content; the SSH was nearly unexamined. As shown in Fig.1, areas of pronounced SSH variability in the northern Pacifi c are primarily in the Kuroshio Extension area east of Japan, the North Pacifi c Subtropical Countercurrent region east of Taiwan Island, and the Mindanao Dome region east of Mindanao Island. The two former areas have been extensively studied (Qiu, 1999; Qiu and Chen, 2005, 2010a, 2010b). The region of SSH variability (root mean square) greater than 8 cm in the MD region is largely within 127°–142°E ， 5°–10°N. This region coincides with the core of the MD climatology shown in Fig.2. Hereafter, we will refer to this region as Box-A.

We examine SSH variations in the MD region and their responses to northern tropical Pacifi c winds,

based on results from a 1.5-layer reduced gravity model. In Section 2, a brief introduction to the model is given, and its outputs are validated. In Section 3, the response of SSH variations to northern tropical Pacifi c winds is discussed. Further discussion and conclusions are presented in Sections 4 and 5, respectively.

2 THE 1.5-LAYER MODEL

2.1 Description

A linear 1.5-layer quasi-geostrophic, reduced gravity model is used to estimate the SSH variations in the MD region. The model is also a wind driven non-dispersive baroclinic long Rossby wave model. The model is expressed by (Meyers, 1979):

∂∂

−∂∂

=η ηtC

xwER ε (1.1)

Here, ƞ represents the SSH anomaly.

Cfg'HR 0=

β2 (1.2)

is the phase speed of a westward-propagating, non-dispersive baroclinic long Rossby wave, where H 0 is the undisturbed upper-layer depth, and

ε = - g'g (1.3)

where g is gravitational acceleration and

Fig.1 SSH variability in terms of root mean square in the north Pacifi c derived from MODAS (Modular Ocean Data Assimilation System) monthly mean SSH in 1993–2010

Values larger than 8 cm are shaded gray and black shaded indicates land and islands.

Fig.2 MODAS sea surface height climatology east of Mindanao Island

Isoline interval is 2 cm. The dotted line encloses Box-A discussed in the text.

677No.4 SONG et al.: SSH variations in MD region in response to winds

g' g=Δρρ0

(1.4)

is the reduced gravitational acceleration. Here, ρ 0 is the sea water density of the upper layer and Δ ρ is the sea water density difference between the two layers. In Eq.1.1,

w kf f

kfE x= ⋅∇× = ⋅∇× +( ) ( )τ

τβτ

ρ ρ0 0

1 (1.5)

is the Ekman pumping velocity anomaly at the interface of the two layers induced by anomalous wind, where τ =< τ x , τ y > is the vector of wind-stress

anomaly, f is the Coriolis parameter and β =∂∂fy

is its

meridional gradient. ∇∇ =∂∂

+∂∂x y

i j, and < i , j, k > are

eastward, northward and upward indicators, respectively.

If we let F= ɛ w E and set ƞ ( x e , y, t )=0 at the eastern boundary (Fu and Qiu, 2002), we obtain the solution of Eq.1.1 as

η( , , ) ( , , ) 'x y tC

F x' y t x x'C

xx

x

e= +

−∫

1

R R

d (1.6)

If only the local Ekman pumping is considered, and ƞ 0 is initialized to zero, we calculate the local wind-induced SSH anomaly as

ηE t

tx y t F x y t t( , , ) ( , , )= ∫ d

0

(1.7)

Here, we set ɛ =-0.003 and calculate the phase speed C R according to Eq.1.2 along different latitudes, such as C R =0.62 m/s at 5°N and C R =0.15 m/s at 10°N. With regard to the wind-stress, we calculate the drag coeffi cient according to Mellor (2004).

2.2 Validation

The NCEP (National Centers for Environmental Prediction) Reanalysis-2 daily wind is used to force the model, and the MODAS daily SSH, a high-quality 0.25-degree altimetry reanalysis (Boebel and Barron, 2003; Barron et al., 2009), is used to validate model results.

Equation 1.6 is applied at various latitudes in Box-A to acquire time series of SSH anomalies at each grid point. Correlation coeffi cients between time series of modeled and MODAS SSH anomalies from 1993–2010 are shown in Fig.3. It is clearly seen that correlation coeffi cients are all larger than 0.6, with smallest values near the western boundary, which

may result from nonlinear processes. The correlation coeffi cients increase northward, which may be caused by the choices of model parameters, especially C R . In any event, the results agree suffi ciently with altimetry reanalysis data, and will not strongly infl uence the following discussions.

Figure 4 compares the time series of modeled and MODAS SSH anomalies, averaged over the period 1993–2010 in Box-A. The fl uctuations of the two are nearly identical (especially for the fi rst 10 years), with correlation coeffi cients of 0.78 for monthly mean and 0.76 for the 13-month running mean. This consistency suggests that SSH variations in the MD region are mainly responding to the northern tropical Pacifi c wind anomalies, both locally and more remotely to the east. The remote wind anomalies generate westward-propagating, long Rossby waves.

3 WIND-INDUCED SSH VARIATIONS

As shown in Fig.5, the SSH anomaly average over the MD region varies principally on a seasonal scale, with signifi cant periods of 0.5 and 1 year, ENSO time scale 2–7 years, and time scale longer than 8 years.

Fig.3 Correlation coeffi cient between modeled and MODAS SSH anomalies during 1993–2010

Upper panel: monthly mean; Lower panel: 13-month running mean.

678 CHIN. J. OCEANOL. LIMNOL., 30(4), 2012 Vol.30

Fig.4 Time series of modeled (solid) and MODAS (dashed) sea surface height anomalies averaged in Box-A Upper panel: monthly mean; Lower panel: 13 months running mean.

Fig.5 Power spectrums of modeled SSH anomalies averaged in Box-A in 1979-2010 Left panel: monthly mean; Right panel: 13 months running mean.

679No.4 SONG et al.: SSH variations in MD region in response to winds

The annual and longer-than-8-year variabilities are especially strong. We will now assess the effects of local winds, from which we will approximate the effects of the remote winds. We defi ne local winds as those that induce local Ekman pumping in Box-A.

As shown in Eq.1.5, the Ekman pumping velocity

can be divided into two terms, 1

0ρ fcurlτ and

βρ

τ0

2f x,

which are induced by the wind-stress curl and zonal winds (or β effect; Tozuka et al., 2002), respectively. Hereafter, these are designated the curl and zonal terms. Time series of these terms and total Ekman pumping velocities averaged over Box-A are shown in Fig.6. As a whole, the local wind-stress curl induces a positive Ekman pumping velocity, while the local zonal wind induces a negative velocity. The mean value of total vertical velocity is 1.09×10 -6 m/s,

Fig.6 Monthly mean Ekman pumping velocity averaged over Box-A during 1993–2010, induced by local wind-stress curl (solid) and local zonal wind (dashed), respectively

Fig.7 Left panel: annual cycle of Ekman pumping velocity averaged over Box-A, induced by local wind-stress curl (solid), local zonal wind (dashed), and sum of those two terms (dotted); Right panel: power spectra of monthly mean Ekman pumping velocity

680 CHIN. J. OCEANOL. LIMNOL., 30(4), 2012 Vol.30

indicating strong upwelling in the MD region, as suggested by historical observations east of the Philippines (Udarbe-Walker and Villanoy 2001).

It is clear that the vertical velocities caused by both curl and zonal terms fl uctuate seasonally. However, as shown in the left panel of Fig.7, their seasonalities are in opposite phase. Consequently, the annual variability of total local wind-induced Ekman pumping is very slight. This indicates that when the positive local wind-stress curl induces upwelling, the westward local zonal wind causes water mass convergence and generates simultaneous downwelling. This agrees with the Asian monsoon evolution shown by Masumoto and Yamagata (1991). In winter, southwestward winds produce a positive local wind-stress curl, whereas in summer the northeastward monsoon causes the positive curl to become more negative in August. As shown in the right panel of Fig.7, the semiannual peak of total Ekman pumping velocity variability averaged over Box-A is more pronounced than its annual counterpart. Furthermore, there are signifi cant peaks corresponding to periods longer than 8 years.

Fig.8 shows time series of monthly mean SSH anomalies averaged over Box-A, induced by local winds. Corresponding power spectra are shown in Fig. 9. It is evident that an annual cycle is intrinsic to the SSH variations induced by both curl and zonal terms, whereas the total local wind-induced SSH anomaly shows little seasonality. Instead, lower-frequency fl uctuation of SSH induced by local winds is clearly the strongest. This is also seen in the right panel of Fig.9, with a peak around 9 years.

The existence of strong long-term SSH variability in the MD region has been demonstrated, as shown in Fig.5. However, is this caused by local winds? To answer this question, a 9-year harmonic analysis is done. The amplitude distribution of 9-year harmonics of SSH anomalies in the northern tropical Pacifi c is displayed in Fig.10. First, the strongest variability is situated east of the Philippines, and is a local phenomenon. Second, amplitudes in the MD region are mostly larger than 8 cm, comparable to the local wind-induced SSH variability shown in Fig.8. Therefore, the long-term SSH variations in the MD region are presumed to be mainly a response to local winds.

4 DISCUSSION Aside from the seasonal and longer-than-8-year

variations, those on ENSO time scales of 2–7 years are also signifi cant, as shown in Fig.5. Here, we compare the 13-month running mean SSH anomaly averaged over Box-A to the Nino-4 SST anomaly index. The latter is calculated from weekly mean data available from a website of the Climate Prediction Center of NCEP. As shown in Fig.11, the two time series are signifi cantly correlated, with a negative coeffi cient of -0.89. Therefore, SSH in the MD region varies in response to ENSO events on an interannual time scale. The SSH is lower than normal during El Niño years, and higher during La Niña years.

It makes sense that ENSO signals are strongly related to westward-traveling ocean long waves (Wang and Picaut, 2004). The 1.5-layer model indicates that in the MD region, these signals

Fig.8 Time series of monthly mean SSH anomalies averaged in Box-A during 1993–2010, induced by the curl term (solid), the zonal term (dashed) and the sum of these two terms (marked), respectively

681No.4 SONG et al.: SSH variations in MD region in response to winds

Fig.9 Power spectra of time series shown in Fig.8 Left panel: curl term; Middle panel: zonal term; Right panel: sum of the two terms.

Fig.10 Amplitude of 9-year harmonics of sea surface height anomalies in the northern tropical Pacifi c Values larger than 4 cm are shaded gray and black shaded indicates land and islands.

Fig.11 Indices of 13-month running mean of Nino-4 SST and Box-A SSH anomalies

682 CHIN. J. OCEANOL. LIMNOL., 30(4), 2012 Vol.30

propagate westward in the form of non-dispersive, baroclinic long Rossby waves, which are generated by Ekman pumping anomalies east of the domain. We therefore conclude that SSH variations in the MD region on various time scales are mainly a response to the northern tropical Pacifi c winds.

We believe that the semiannual variability of SSH is mainly a response to remote winds east of the MD region. This seems to be inconsistent with Qu et al. (2008). Figs.7 (right panel) and 9 (right panel) show a semiannual peak emerging in the local Ekman pumping, but disappearing in the local wind-induced SSH variations. We argue that the SSH variability is an integrated result of Ekman pumping velocity. Therefore, short-term variations will be weakened and long-term variations will be enhanced.

The lower-frequency (time scale longer than 8 years) SSH variations in the MD region and their responses to local winds are discussed for the fi rst time in the present work. The MD region is unique, because of the following fi ndings. Short-term SSH anomalies caused by local wind-stress curl and local zonal winds, which are both associated with the Asian monsoon, vary with opposite phase. Local wind-induced long-term SSH variations are signifi cant. We suggest that the lower-frequency variations in local winds may result from some larger-scale mechanism.

It is interesting that local winds induce only slight SSH variation on seasonal time scales in the MD region, yet these winds are important in the seasonal evolution of the MD (Masumoto and Yamagata, 1991). This indicates that the SSH anomalies do not always accompany MD strength variations. We assume that the annual MD evolution is caused by the local wind-stress curl, rather than by local zonal winds.

5 CONCLUSION

The MD region is an area of the northern Pacifi c in which SSH strongly varies. The SSH variability in this region is second only to that in the Kuroshio Extension area, and is comparable to that in the northern Pacifi c Subtropical Countercurrent region. SSH variability (in terms of root mean square) greater than 8 cm is located mainly in the region 127°E–142°E, 5°N–10°N. This region is called Box-A. The SSH average over Box-A varies mainly with signifi cant periods 0.5 and 1 year, 2–7 years, and longer than 8 years. The greatest amplitude of variability is on the long-term.

The SSH variations in the MD region, as estimated by a 1.5-layer reduced gravity model, are a response to northern tropical Pacifi c winds. On seasonal and ENSO time scales, the regional SSH variations are mainly caused by wind anomalies well east of the region, which generate westward-propagating, long Rossby waves. On interannual time scales, SSH is lower than normal during El Niño years, and higher during La Niña years. On time scales longer than 8 years, SSH is basically induced by local wind variations.

6 ACKNOWLEDGMENT

The authors are grateful to Drs. Zhiliang LIU, Junqiao FENG, Fujun WANG, and other research team members for inspiring discussions.

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