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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 114, D20119, doi:10.1029/2008JD011441, 2009

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ArticleCross-equatorial influences of submonthly scale southerly surges overthe eastern Indian Ocean during Southern Hemisphere winterYoshiki Fukutomi1 and Tetsuzo Yasunari1,2
Received 8 November 2008; revised 8 July 2009; accepted 6 August 2009; published 31 October 2009.

[1] Lower-tropospheric southerly surges occurring at submonthly timescales overthe eastern Indian Ocean (EIO) during Southern Hemisphere (SH) winter are
extratropical-tropical interaction phenomena involving various processes. This study
examined the physical mechanism driving equatorial wave disturbances associated withsurge-induced cross-equatorial flow over the EIO. Japanese 25-Year Reanalysis (JRA25)
and Tropical Rainfall Measuring Mission Satellite (TRMM) 3B42 data products from June
to August for the 7 years from 1998 to 2004 were analyzed. Composite analyses of
low-level wind and vorticity fields reveal that an equatorial clockwise gyre develops justafter cross-equatorial penetration of southerly surge winds. This gyre is interpreted as
an equatorially trapped Rossby wave that develops asymmetrically around the equator in
response to surge-induced cross-equatorial flow. Eddy vorticity budget analysis
demonstrates the dynamic role of the southerly surge in the genesis of the equatorialclockwise gyre. The result indicates that an injection of negative absolute vorticity into theequatorial tropics due to both advective and convergent processes associated with thesurge winds is primarily responsible for the gyre genesis. Cross-equatorial advection ofbasic state absolute vorticity by the surge (eddy) winds and vortex stretching by surge
wind convergence are most effective in spinning up the equatorial clockwise gyre. Wave
activity flux analysis further emphasizes that Rossby wave energy propagation from theSH extratropics into the equatorial EIO region is caused by the southerly surge. These
dynamic diagnostics suggest that the southerly surge plays a lateral forcing role in exciting
the equatorial wave disturbance over the EIO. Composite analyses of moisture flux and
precipitation anomalies characterize the equatorial clockwise gyre as a convectively
coupled equatorial Rossby wave. Enhanced cross-equatorial moisture transport due to thesoutherly surge appears to be linked to precipitation along the west coast of the southern
Indo-China Peninsula. A case study of the southerly surge event during August 2000
illustrates the formation of the equatorial clockwise gyre and its convectively coupledequatorial Rossby wave signature.
Citation: Fukutomi, Y., and T. Yasunari (2009), Cross-equatorial influences of submonthly scale southerly surges over the eastern
Indian Ocean during Southern Hemisphere winter, J. Geophys. Res., 114, D20119, doi:10.1029/2008JD011441.

1. Introduction[2] Lower-tropospheric southerly wind surges from theextratropics into the tropics regulate the tropical convective
activity over the eastern Indian Ocean (EIO) during Southern
Hemisphere (SH) winter [Fukutomi and Yasunari, 2005,
hereafter referred to as FY05]. FY05 reported strong
evidence that the submonthly (6 – 25 days) southerly surges
are extratropical-tropical interaction phenomena that involvea variety of processes, such as development of tropicalconvection, dry and cold air advection from the extratropics

1
Frontier Research Center for Global Change, Japan Agency for
Marine-Earth Science and Technology, Yokohama, Japan.
2
Hydrospheric Atmospheric Research Center, Nagoya University,
Nagoya, Japan.

Copyright 2009 by the American Geophysical Union.
0148-0227/09/2008JD011441$09.00

into the tropics, and air-sea interaction in the EIO region.
The southerly surges are caused by midlatitude Rossby wave
propagation from the southwestern Indian Ocean through theSouth Pacific. The strong baroclinic development of Rossby
waves in the entrance region of the SH wintertime subtropical westerly jet off the west coast of Australia amplifies the
low-level southerlies, leading to the southerly surges overthe EIO.
[3] FY05 concentrated on the impacts of the southerly
surges on tropical convection. They mainly discussed thephysical processes associated with the local blowup of
convection that subsequently occurs after the surge peak.
Interestingly, they also noticed a cross-equatorial connection between southerly surges and the South Asian monsoonal circulation and convection. Southerly surge enters
the southern Bay of Bengal (BOB), forming transient crossequatorial flow that encompasses the whole tropical EIO
region. The cross-equatorial flow turns clockwise andD201191 of 15

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merges into the monsoon westerly flow over South Asiaaround 5° to 10°N. Coherent strengthening of an equatorialclockwise gyre and westerly winds with the surge event are
seen over the southern Bay of Bengal. The surge-induced
cross-equatorial flow appears to produce the equatorial
wave disturbance. Nevertheless, the underling physicalprocesses for the dynamical impacts exerted by the southerly surges have not been well discussed.
[4] Many previous works have recognized that transientcross-equatorial flows associated with meridional windsurges exert important lateral forcing on tropical disturbances such as westerly wind bursts, convectively coupledequatorial waves, and tropical cyclones in the broad Indian
Ocean-Pacific region. For example, northerly cold surges
from the East Asian extratropics and their related crossequatorial flows play a role in the development of equatorial westerly wind bursts in the western Pacific [e.g.,
Murakami and Sumi, 1981; Love, 1985a, 1985b; Chu, 1988;
Sumathipala and Murakami, 1988; Chu and Frederick,
1990; Kiladis et al., 1994; Meehl et al., 1996; Suppiah andWu, 1998; Yu and Rienecker, 1998; Compo et al., 1999; Yu etal., 2003]. In addition, the northerly surges have effects on
tropical cyclones and other near-equatorial vortex-type disturbances over the Maritime Continent [e.g., Love, 1985b;
Chang et al., 2003, 2005]. As documented by FY05,
southerly surges on various timescales and their related
cross-equatorial flows originating in the Southern Hemisphere extratropics are able to induce equatorial westerly
wind events [e.g., Wang and Murakami, 1987; Shresthaand Murakami, 1988; Murakami, 1988; Murakami and
Sumathipala, 1989; Hartten, 1996; Straub and Kiladis,
2003]. There is also evidence that tropical cyclogenesis is
influenced by cross-equatorial flow from the SH [e.g., Love,
1985a, 1985b].
[5] Several of the above studies discussed the generation
mechanisms of equatorial westerly wind events that arepreceded by the meridional wind surge; specifically, equatorward propagation of surface high-pressure perturbation(i.e., pressure surge) accompanied by the meridional windsurge enhances the zonal (west-east) pressure gradientaround the equator, which then sets up strong westerly
wind events [Love, 1985a; Chu, 1988; Chu and Frederick,
1990; Yu et al., 2003]. In addition, equatorial zonal wind
events are formed as a part of equatorially trapped Rossby
or Kelvin waves excited by the meridional wind surges[e.g., Kiladis et al., 1994; Straub and Kiladis, 2003]. On
the basis of numerical simulation results, Lim and Chang[1981] and Zhang and Webster [1992] gave a theoreticalbasis for equatorial waves forced by cross-equatorial surge
flows that was similar to those observed by the above
studies.
[6] Our current concern is the fundamental mechanismthat drives the equatorial wave disturbances following thesoutherly surges over the EIO. As mentioned above,
spatially and temporally coherent structures in low-level
circulation on the NH side of the EIO-southern Bay of
Bengal region were observed during the surge event. The
purpose of this study is to confirm that the southerly
surges are the forcing for the equatorial disturbances andto explore how the surge-induced cross-equatorial flows
force the equatorial disturbances. We attempt to determine
the role of the southerly surge in the creation of theD20119clockwise equatorial gyre. We first reconfirm the crossequatorial teleconnective nature of the surge by composite
analysis of atmospheric fields. We then analyze theimpacts of the cross-equatorial flow using dynamical
diagnostics. This paper is organized as follows: section 2
briefly describes the data and analysis procedure used.
Basic structures of the surge-related atmospheric fields are
discussed in section 3. Results from dynamical analysis,
conducted to ensure the cross-equatorial effects of the surge,
are presented in section 4. Section 5 describes regionalhydrological impacts, while section 6 analyzes a case ofsurge-induced equatorial disturbance. Finally, section 7
presents a summary and discussion.

2. Data and Methods[7] For this study, we used a global atmospheric reanalysis data set and a satellite-based precipitation data set. The
atmospheric reanalysis data set is the Japanese 25-Year
Reanalysis (JRA25), which covers the period from 1979
through 2004. This data set was recently produced by theJapan Meteorological Agency (JMA) numerical assimilation and forecast system and provides many kinds ofassimilated and forecasted variables available at a 6-hour
temporal resolution and various grid types [Onogi et al.,2005, 2007]. We used the atmospheric horizontal winds,
geopotential height, temperature, and specific humidity on
horizontal 2.5° grids and 12 pressure levels (1000, 925, 850,
700, 600, 500, 400, 300, 250, 200, 150, and 100) from theganl-p25h subset of JRA25 [see Onogi et al., 2007].
Following the method of FY05, we computed stream
function, velocity potential, divergence, vorticity, and divergent winds from horizontal winds using a sphericalharmonic transform method. All the atmospheric variables
were daily averaged before data analysis.
[8] The precipitation data set is the Tropical Rainfall
Measuring Mission satellite (TRMM) 3B42 product[Kummerow et al., 2000] obtained from the National
Aeronautics and Space Administration (NASA)/Goddard
Space Flight Center (GSFC). This data set covers the period
from 1998 through the present on a 0.25° spatial and with a
3-hour temporal resolution. The precipitation data wereregridded on a 1° grid by spatial averaging and then daily
averaged.
[9] The data analysis was carried out during June-JulyAugust (JJA) for a 7-year period from 1998 to 2004 whenboth JRA25 and TRMM data are commonly available. Mostof the data processing and analysis methods employed here
are similar to those used in our previous study (FY05).
Anomaly time series of all variables were computed byremoving the first three harmonics from the original
365-day time series. The anomaly time series were thenfiltered into a 6- to 25-day band using a Butterworth filter[Kaylor, 1977]. To establish the relationships between the
low-level southerly surges and large-scale dynamical
fields, we began by identifying the southerly surge eventsusing the same procedure as FY05. The selection of cases
was based on the surge index. The index was defined
during JJA of each year by the area-averaged 850-hPa
meridional wind time series over the box (87.5 –92.5°E,
17.5– 2.5°S). This grid box encloses the region of maximum
variance of the 6- to 25-day filtered 850-hPa meridional

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wind in the tropics of the EIO (see FY05). The surge eventswere defined by strong positive peaks of this index, withvalues exceeding the criterion of climatological 1.5 standard
deviation (2.2 m s1). In total, 21 surge events were foundduring the seven JJA seasons. These events (surge peakphases) are referred to as the key dates for compositing.
Daily lag composites of various fields were constructed
relative to these key dates, denoted by day 0 phases.
[10] It should be noted here that we confirmed enhanced
activity of submonthly disturbances over the EIO prior tothe composite analysis. We compared several statisticalcharacteristics obtained from JRA25 with those shown by
FY05 (not shown). A spatial pattern of variance of the 6- to
25-day filtered 850-hPa meridional wind was similar to thatin the FY05 results. A variance maximum over the EIO wasobserved also in the present results. In addition, power
spectra of unfiltered anomalous time series of the 850-hPa
meridional wind in the box defined for the surge index had
dominant power in the submonthly range. Hence theseresults must justify our choice of a frequency range fortime filtering.

3. Development of Equatorial Waves AssociatedWith the Southerly Surge
3.1. Basic Circulation Patterns[11] This section examines the space-time evolution of
low-level circulation anomalies in the Indian Ocean-south
Asian region. Composite maps were created from the
21 surge events to highlight the development of the equatorial wave disturbance that might be induced as a responseto the southerly surge. Figure 1 shows a composite sequenceof 850-hPa winds. The most important feature is theevolution of the equatorial clockwise gyre just after crossequatorial penetration of the southerly surge.
[12] From days 2 through 0, the pathway of the southerly surge is well established. Southerly-southeasterly wind
anomalies cross the equator and reach the southern Bay of
Bengal. At day 0, the cross-equatorial southerly flow turns
clockwise and merges with the westerly flow over thesouthern Bay of Bengal-Southeast Asia. This continuous
flow forms an anticyclonic circulation (clockwise gyre:
cyclonic circulation in the SH side) centered on northwestern Sumatra Island. After the penetration of the southerly
flow, this clockwise gyre moves westward along the equatorand develops over the equatorial EIO by day +3. The
equatorial clockwise gyre is mostly strengthened fromday+2 through day +3. Westerlies along the northern flankof the equatorial clockwise gyre are enhanced toward thewestern Pacific. Simultaneously, an off-equatorial cyclonic
circulation (anticlockwise gyre) is also strengthened overthe northern Bay of Bengal. After day +3, a pair of northsouth gyres moves further westward to the western Indian
Ocean-Arabian Sea region with slight decaying. The northsouth double gyre structure during the period from day +1
through day +5 is very similar to that of the 10- to 20-day
monsoon mode [e.g., Chen and Chen, 1993; Goswami and
Ajaya Mohan, 2001]. As pointed out also by FY05, theexcitation of the equatorial clockwise gyre could be a
result of the southerly surge. This clockwise gyre isinterpreted as a kind of equatorially trapped Rossby wave,
which is asymmetric about the equator [e.g., Chatterjee

D20119and Goswami, 2004] and a response to the surge-induced
cross-equatorial flow.
3.2. Vorticity and Divergence Anomalies[13] To trace clearer and sharper structures of the equatorial clockwise gyre, a composite sequence of relative
vorticity anomalies is displayed in Figure 2 together withdivergence anomalies. The region of negative vorticity
anomalies (Figures 2a – 2d) corresponding to the equatorialclockwise gyre (Figure 1) extends southeastward from
northwest of the islands of Sumatra into the equatorial
central Indian Ocean. Initially, an anticyclonic (clockwise)
vortex center is formed east of the Sumatra islands near
5°N, 92.5°E at the surge peak phase (Figure 2a). The vortex
center subsequently moves southwestward and crosses theequator, and then arrives at 2.5°S, 80°E by day +3. At thesame time, the negative anomalies form a zonally elongatedband. Note that the region of negative vorticity anomalies
is equivalent to that of anomalous cyclonic shear on theSH side.
[14] Convergent anomalies (Figures 2e – 2f) also develop
along 5°S just after the surge peak phase. Weak convergent
regions have already appeared south of the equator at day 0.
On the other hand, divergent anomalies prevail in the offequatorial surge region, which reflects a divergent characteristic of the southerly surge flow. A zonally elongatedconvergent band grows from day +1 through day +2 along
the SH side equatorial zone spanning 5°S – 0. The low-level
convergence following the surge peak can be caused by aboundary layer frictional effect on the surge flow as discussed by FY05. The convergent band is aligned with theanomalous negative (cyclonic) vorticity band (Figures 2a –2d) during the period from day +1 through day +3. In
addition, the convergent band appears to lead the evolutionof the negative vorticity band south of the equator. Thisimplies that the surge-induced convergence may contributeto generating the cyclonic shear as a part of the clockwise
gyre through a vortex stretching effect. That matter isdiscussed in section 4.
3.3. Vertical Structure[15] To examine the vertical structure of the equatorialclockwise gyre, longitude-height cross sections of composite relative vorticity anomalies averaged around the equator(2.5°S – 2.5°N) from day +1 to day +4 are displayed in
Figure 3. This 4-day period includes the development andmature stages of the gyre. On day +1, a region of negative
anomaly extends from the lower troposphere to the upper
troposphere between 80° and 100°E, suggesting a deep
equivalent barotropic structure of the gyre throughout thetroposphere. After day +2, a core of negative anomaly shifts
westward, and a region of positive anomaly appears in theupper troposphere. From day +3 to day +4, the verticalstructure exhibits a first baroclinic structure throughout thetroposphere between 60° and 90°E. The equivalent barotropic structure with negative anomalies is still pronounced
in the lower and middle troposphere up to roughly 400 hPa.
On the other hand, a positive anomaly aloft at about
100– 300 hPa is quite obvious. This baroclinic structure is
likely caused by the convective development 2– 4 days
after the surge peak as shown by FY05 (in their Figure 4)
and Figure 8 in the later section.

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D20119Figure 1. Composites of 6- to 25-day filtered 850-hPa wind vector anomalies and stream function
anomalies (contours) from day 2 through day +5 with 1-day interval based on the surge index. Solid(dashed) contours are positive (negative) values. Contour interval is 4.0  105 m2 s1. Zero contours areomitted. Light (dark) shading indicates the positive (negative) stream function anomalies at the 95%
level. gCh represents the center of the equatorial clockwise gyre.

3.4. Westward Propagating Wave Characteristics[16] As aforementioned, the equatorial clockwise gyre ispresumably consistent with an equatorial Rossby wave-type
disturbance. To obtain insight into equatorial Rossby wave
characteristics of the gyre, a longitude-time cross section ofcomposite relative vorticity anomalies along the equator(2.5°S – 2.5°N) in the Indian Ocean region is illustrated in
Figure. 4. The clear westward phase propagation of negative
anomalies is evident after the surge peak phase (day 0). A
core of the negative anomalies which corresponds to thecenter of the equatorial clockwise gyre is formed by day +2.
Then, these anomalies propagates westward by roughly

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D20119Figure 2. Composites of 6- to 25-day filtered 850-hPa (a, b, c, and d: left) relative vorticity anomaliesand (e, f, g, and h: right) divergence anomalies from day 0 through day +3. Solid (dashed) contours are
positive (negative) values. Contour interval of the relative vorticity (divergence) anomalies is 2.0 
106 s1 (0.5  106 s1). Zero contours are omitted. Light (dark) shading indicates positive (negative)
anomalies significant at the 95% level.

day +7. The westward phase speed estimated from this plot
is approximately 7 m s1. This value is somewhat greaterthan the phase speeds estimated by previous works on theequatorial Rossby waves in the tropical Indian Ocean –South Asian Monsoon region [e.g., Chatterjee and Goswami,
2004; Yokoi and Satomura, 2005]. However, our estimate

is adequately within the range of typical phase speed ofequatorial Rossby waves identified by several studies [e.g.,
Kiladis and Wheeler, 1995; Pires et al., 1997]. We
measured also the size of the gyre along the equator by
the relative vorticity composites (Figures 2, 3, and 4). The
zonal scale of the gyre is estimated to be 3000 –4000 km

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D20119the present analysis could be interpreted as an equatorial
Rossby wave.

4. Dynamical Diagnostics for the Equatorial
Wave Genesis
4.1. Vorticity Budget Analysis[17] The composite sequences (Figures 1 and 2) above
reveal the formation and movement of the equatorial clockwise gyre. Transient negative vorticity accumulation in theequatorial EIO region might create the clockwise gyrefollowing the surge event. It is interesting that the clockwise
gyre begins to develop mainly from the NH side (Figure 2a)in conjunction with the anticyclonic turning of the surgeinduced cross-equatorial flow (Figure 1). One can thus
expect that this transient cross-equatorial flow advects
negative vorticity from the SH into the NH side of theequatorial region, which would play a vital role in theincrease of negative eddy vorticity in the region. To understand the physical processes responsible for the growth ofthe equatorial clockwise gyre, we employ vorticity budget
analysis. The equation governing transient eddy vorticity
tendency can be derived as@z 0¼ V0  r z þ f  V  rz 0  D0 z þ f  Dz 0 þ R ð1Þ
@t
DIV2
ADV2
DIV1
ADV1

where z is the vertical component relative vorticity, t is time,
f is the Coriolis parameter, D is the horizontal divergence,
V is the horizontal wind vector, and R is the residual.
Primes denote submonthly (6 – 25 days) eddy components,and overbars refer to low-frequency basic states definedas 25-day low-pass smoothed values.
[18] The first term on the right-hand side is the advectionof basic state absolute vorticity by submonthly eddy winds(ADV1); the second term is the advection of the submonthly eddy vorticity by the basic state winds (ADV2);
the third term is the vortex stretching due to submonthly
eddy divergence (DIV1); and the fourth term is the vortex
stretching due to the basic state divergence (DIV2). The

Figure 3. Longitude-height cross sections of composite of
6- to 25-day filtered relative vorticity anomalies along
2.5°S– 2.5°N from day +1 through day +4. Contour interval
is 0.5  106 s1. Zero contours are omitted. Light (dark)
shading indicates positive (negative) anomalies significant
at the 95% level.
which can be equivalent to a half wavelength of a wave
including the gyre. Therefore we estimate wavelengthsof 6000 – 8000 km (zonal wave numbers 5 – 7). These
values are close to the wavelengths estimated by theprevious studies. Overall, these space-time characteristics
suggest that the equatorial clockwise gyre identified in

Figure 4. Longitude-time cross section of composite of
6- to 25-day filtered 850-hPa relative vorticity anomalies
averaged between 2.5°S and 2.5°N. Contour interval is0.5  106 s1. Zero contours are omitted. Light (dark)
shading indicates positive (negative) anomalies significant
at the 95% level.

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D20119Figure 5. Composites of terms in the eddy vorticity tendency equation at day +1 (see text for details):(a) total advection effect (ADV1 + ADV2), (b) total divergence effect (DIV1 + DIV2), (c) eddy vorticity
tendency, and (d) sum of the two terms (ADV1+DIV1) at 850 hPa. Contour intervals are 2.0  1011 s2.
Zero contours are omitted, and 1.0 unit contours are added. Light (dark) shading indicates positive(negative) anomalies significant at the 95% level.
residual term R contains the twisting, vertical advection,
and all other terms. This formulation of the vorticity
equation is similar to that employed by Yokoi and Satomura[2006]. We directly computed the eddy vorticity tendency
term on the left and the four primary terms on the right(ADV1, ADV2, DIV1, DIV2), and then used these terms toinvestigate the effects of surge-induced eddies in the generation of the equatorial clockwise gyre.
[19] Figure 5 displays composite patterns of the computed
terms in equation (1) on day +1. This phase corresponds tothe development stage of the gyre just after the surge peak,as seen in Figures 1 and 2. We present the total advection
effect (Figure 5a: ADV1+ADV2), the total divergenceeffect (Figure 5b: DIV1+DIV2), the eddy vorticity tendency(Figure 5c), and the sum of ADV1 and DIV1 (Figure 5d).
[20] Off-equatorial negative tendencies south of 5°S overthe EIO are dominated by the advection effect (Figure 5a).
Generation of cyclonic shear (negative vorticity tendencies)
by the advection effect occurs along the surge region.
Negative anomalies are also observed near and northwestof Sumatra Island and southeast of Sri Lanka. These nearequatorial signals had already been observed on day 1through day 0 (not shown) and dominated the initialdevelopment of the clockwise gyre from day 1 throughday +1. It is noteworthy that the distribution of ADV1
(Figure 6d) resembles the distribution of total advection(Figure 5a). In particular, ADV1 indicates larger values
around the equator, suggesting the importance of the direct
effect of the surge winds. Additionally, ADV2 (not shown)is predominantly positive in the region of ADV1 negativearound the equator to the southeast of Sri Lanka. Hence
ADV2 works as a dissipater for the negative vorticity in theregion. ADV1 and ADV2 compensate each other; however,
the absolute magnitude of the negative ADV1 is larger thanthat of the positive ADV2. As a result, the total advection(ADV1 + ADV2) is still negative in the region, as shown in
Figure 5a. In summary, the surge-induced cross-equatorial
flow entering the southern Bay of Bengal tends to inject thebasic state negative vorticity northward into the NH side,which drives the anticyclonic (negative) vorticity tendency
to the north of the equator.
[21] The total divergence effect shapes a zonally elongated narrow band of strong negative values spanning 5°S to
0° (Figure 5b). The band well reflects concurrent negative(cyclonic) tendencies in the same location (Figure 5c). Herethe distribution of the DIV1 term (Figure 6h) resembles thatof the total divergence effect (Figure 5b). In the region of
the negative eddy vorticity tendencies close to the equator(Figure 5c), the magnitude of negative DIV1 values isalmost the same degree as that of the negative total
divergence effect. Thus the DIV1 term dominates theaccumulation of negative eddy vorticity on the SH side
equatorial zone, which results in the generation of eddy
cyclonic vorticity corresponding to the equatorial gyre. Notethat the magnitude of DIV2 was found to be much smaller
than that of DIV1 (not shown). DIV2 does not contribute to
the negative vorticity increase in this region.
[22] The sum of ADV1 and DIV1 indicates comparatively
complex structures (Figure 5d); however, the negativevalues are concentrated in the region of the negative

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D20119Figure 6. As in Figure 5 for (a – d) ADV1 and (e – h) DIV1 from day 2 to day +1.
tendency around the equator (Figure 5c), emphasizing thatcombined effects of these terms facilitate the negative eddy
vorticity increase in the equatorial EIO. In particular, thesoutherly surge winds originate in the SH extratropics, andtheir induced convergence effects (i.e., ADV1 and DIV1)
play significant roles in forcing the Rossby wave over theequatorial EIO.
[23] We also computed the vertical advection term, thetwisting term, and a nonlinear term defined by eddy
vorticity flux divergence included in the R term (notshown). The resulting composite distributions of these terms

are not well organized, and their magnitudes are quite small
compared with the above four terms. Hence the effect of the
R term can be considered unimportant.
[24] By using the same vorticity budget diagnostics, we
additionally document the processes contributing to themaintenance of the off-equatorial cyclonic circulation(anticlockwise gyre) over the Bay of Bengal as seen in theprevious section and FY05. The day +1 phase corresponds tothe developing stage of the cyclonic circulation because apositive vorticity tendency prevails over the Bay of Bengal(Figure 5c). The sum of terms ADV1 and DIV1 indicates

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large positive values over the Bay of Bengal to the north of
10°N (Figure 5d). Besides, ADV1 and DIV1 are predominantly positive in the same region (Figure 6d). Thesefeatures imply that the positive vorticity tendency is mainly
produced by ADV1 and DIV1. However, the total advection(ADV1 + ADV2) is mostly negative over the Bay of Bengal,
which is caused by large negative values of ADV2 (notshown). The positive values of ADV1 are substantiallycanceled by the negative values of ADV2. Consequently,
DIV1 can be the most important process that maintains thecyclonic circulation over the Bay of Bengal. It should be
noted that the magnitude of DIV2 is rather small andnegligible for this discussion.
[25] The contribution of the matured surge flow to theintensification of the equatorial clockwise gyre has been
discussed by focusing the stage after the surge peak.
Another concern is an initial source of the clockwise gyrewhich grows as the equatorial clockwise gyre after the surge
peak. As has been noted in the previous section, the centerof the clockwise gyre already exists in the NH side (near5°N, 90°E) at day 0 (Figure 2). This gyre moves southwestward and develops along the equator in the later stages(as in Figures 1 and 2). We confirmed that this gyre center isformed by day 1 in the vorticity composites (not shown).
In addition, the southerly surge winds begin to blow fromday 2 (as in Figure 1 and FY05). The cross-equatorial
flow formed by the southerly surge extends to the south of
Bay of Bengal at this time. Therefore the developing stageof the southerly surge could be associated with the growthof this initial clockwise gyre in the NH side. A composite
sequence of ADV1 (Figures 6a– 6d) shows that negativevalues are collocated with the zone of the southerly surge
flow in the EIO region (Figure 1 and FY05). From day 2through day 0, a region of the negative values is locatedsouth of the leading edge of the surge (approximately 5 –10°N). This region corresponds to the region of negative
vorticity anomalies (i.e. the clockwise gyre) during the same
period. These features suggest that ADV1 is an importantsource of negative vorticity tendency which generates theinitial clockwise gyre. The negative values of ADV1 during
this period are basically caused by the cross-equatorialsoutherly surge (eddy) winds. On the other hand, DIV1
(Figures 6e– 6h) is not significant in the equatorial EIO
region prior to the surge peak. This indicates that theconvergent forcing as discussed previously does not workin the developing stage of the surge. Besides, DIV2 is muchsmaller than other terms and ADV2 is partly offsets ADV1
at the initial clockwise gyre location (not shown). Thus
ADV1 is a main contributor to the initial clockwise gyreformation. The advection of basic state absolute vorticity by
the southerly surge winds acts to reinforce the initialclockwise gyre prior to the surge peak.
4.2. Wave Activity Flux Analysis[26] To further emphasize the importance of the lateral
forcing due to the southerly surge originating in the SH
extratropics, we employed the barotropic version of
Plumbfs wave activity flux [Plumb, 1986], derived by Sobel
and Bretherton [1999] and Sobel and Maloney [2000]. They
successfully diagnosed the low-level Rossby wave energy
accumulation over the tropical western Pacific using theirwave activity flux definition.

D20119[27] Here the horizontal wave activity flux vector for thetransient eddies MT consists of the radiative part MR andthe advective part hViM; that is,
MT ¼ MR þ hViM ;ð2Þwhere h( )i is a climatological SH-winter average, and M is
the wave activity defined as
M¼

pz 02 cos f
2jrhz a ij

ð3Þ

where hz ai is the SH-winter climatology of absolute
vorticity, p is pressure normalized by a reference pressure(1000 hPa), and f is latitude. The double overbar indicates atime average over a specific period.
[28] The radiative part is equivalent to the barotropic partof the localized E-P flux of Trenberth [1986], except for thepressure factor, which is written as
1
MR ¼ p cos f ðv02  u02 Þ; u0 v0 :2

ð4Þ[29] The MT vectors are parallel to the total groupvelocity of Rossby wave energy propagation, and the MR
vectors are parallel to the group velocity relative to the basic
state flow. In general, the convergence of MT is a goodmeasure of Rossby wave energy accumulation [e.g., Sobel
and Bretherton, 1999; Sobel and Maloney, 2000].
[30] We examined both MT and MR during the surge
period. The transient eddy components of these were
estimated from composite submonthly eddy zonal (u) andmeridional (v) winds, and relative vorticity. Figure 7 displays the MT and MR vectors together with their divergence averaged over the period from day 5 throughday +5. This period includes the development, mature, anddecay stages of the southerly surge event.
[31] The MT vectors (Figure 7a) are directed northward –northwestward with their divergent features extending fromthe subtropical to tropical EIO. A location of these strong
fluxes coincides with that of the southerly surge winds(Figure 1). These fluxes substantially originated over the
midlatitude EIO at 30° to 40°S (not shown). Interestingly,
most of the fluxes to the east of 80°E cross the equator andreach approximately 5°N. Therefore it is reasonable tosuppose that northward propagation of Rossby wave energy
from the SH midlatitudes into the NH equatorial region
occurs along the southerly surge flow. Areas of the northward wave activity flux convergence occur around theequator. In comparison with the vorticity anomaly composites (Figure 2), these convergence spots are collocated withthe region of negative vorticity anomalies corresponding to
the equatorial gyre. This implies that the wave activity fluxand its convergence associated with the southerly surge leadto the growth of the equatorial gyre as a part of tropical
Rossby waves.
[32] The MR vectors (Figure 7b) are used as a measure ofeddy activity and their propagation characteristics during
the surge period. Similar to MT (Figure 7a), a northward
component of the radiative fluxes is enhanced in the surge
region. The angle of MR vectors is somewhat different from

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Figure 7. (a) Wave activity flux MT and its divergence
averaged over day 5 to day +5. (b) As in Figure 7a, except
for radiative component MR of MT. The unit of the wave
activity flux is m2 s2. The divergence (convergence) isrepresented by solid (dashed) contours with 1.0  106 m s2interval. Zero contours are omitted. Areas of the convergenceless than 1.0  106 m s2 are shaded.
that of MT vectors; however, their magnitudes are very
similar, suggesting that the contribution of the radiative partto the total wave activity flux is significantly large. In otherwords, the northward component of the MT vector isdominated by the radiative flux MR over the EIO. This fact
supports the idea that the enhanced eddy activity associatedwith the southerly surge is responsible for the lateral forcing
as a form of Rossby wave energy propagation.

5. Hydrological Impacts of the Surge[33] Next we consider the atmospheric hydrological signatures that are enhanced by the southerly surge. FY05
reported evidence that the convection develops over the SH
side of the tropical EIO a few days later than the surge peak.
However, they gave only a limited description of the crossequatorial hydrological impacts. Here we focus on aspectsof the NH side precipitation increase following the surge
event. Figure 8 gives composite maps of precipitation andvertically integrated moisture flux anomalies. The overall
pattern of the moisture flux anomalies is basically similar tothat of the low-level wind anomalies (Figure 1). A clear
pathway of cross-equatorial moisture transport is createdacross the EIO-southern Bay of Bengal region around thesurge peak (day 1 through day +1). This moisture flow
obviously corresponds to the surge-induced cross-equatorial

D20119flow. The southerly moisture flow curves clockwise into
westerly moisture flow over the southern Bay of BengalAndaman Sea region. This westerly moisture flow bursts
toward the southern Indo-China Peninsula and the Malay
Peninsula (south of 15°N). These features suggest that thesurge-induced cross-equatorial flow plays a role in entraining moisture from the SH into the Bay of Bengal andSoutheast Asian regions.
[34] Another interesting feature is that a localized positive
precipitation anomaly region appears to the west of thesouthern Indo-China Peninsula in association with thewesterly moisture flow burst during day 0 through day +1.
We also examined a composite distribution of the moisture
flux convergence anomalies (not shown) and confirmed
the moisture flow converge in this increased precipitation
signal. This increased precipitation signal might be attributable to the transient moisture flow enhanced by thesurge.
[35] We turn now to the behavior of the westward-moving
equatorial clockwise gyre that was also identified in thecomposite moisture flux anomalies. This moisture flux gyreis apparently identical to the equatorial clockwise gyre seenin Figure 1. A region of positive precipitation anomalies
expands over the EIO during the gyre development, which
can be regarded as the convection organization triggered by
the surge [FY05]. This region of increased precipitation isaccompanied by the low-level convergence band (Figure 2).
A clear structure of a convectively coupled equatorial
Rossby wave is formed by day +3. The increased precipitation region is well embedded in the gyre at day +3.
However, the structure is short lived and not maintained
after day +4. In the following days, the gyre moves
westward into the central-western Indian Ocean; nevertheless, the region of strong positive precipitation anomalies is
not trailed by the gyre. The strong positive precipitation
anomalies develop rather eastward within 80° to 100°E as
the gyre moves westward. The gyre and the region of strong
positive precipitation anomalies (i.e., the convective region)appear to decouple after day +4.

6. August 2000 Case Study[36] This section presents a typical case of the equatorial
wave development following the surge event during earlyAugust 2000. The case illustrates the features in the previous
composite analysis results. Figure 9 shows submonthly
filtered anomalies of 850-hPa winds together with raw
precipitation during the period 4 – 10 August 2000 with
2-day intervals. Note that the southerly surge peak occurs
on 6 August. Two days prior to the surge peak (4 August), a
comparatively weaker counterclockwise gyre is centered on
the equator to the south of Sri Lanka. At the surge peak
(6 August), strong cross-equatorial flow penetrates from theSH extratropics into the NH tropics over the EIO. A
clockwise (anticyclonic) gyre begins to develop to thenorthwest of Sumatra. Two precipitation maxima appear
along the busting westerly flow at the northern flank of thegyre and along the southeasterly surge flow over theequatorial EIO. Two days later (8 August), the equatorialclockwise gyre moves southwestward with marked intensification. A region of enhanced precipitation expands within
the gyre. In particular, the equatorial EIO precipitation

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D20119Figure 8. Composites of moisture flux vector (kg m1 s1) and precipitation (mm day1) anomalies
from day 2 through day +5. The precipitation anomalies are shaded (see color scale bar). Regions
enclosed by black contours indicate significant anomalies at 95% level.
increases along the southern flank of the gyre. A convectively
coupled equatorial Rossby wave signature is clearly evident
at this time. Four days after the surge peak (10 August), thegyre moves westward into the central Indian Ocean. On theother hand, the enhanced precipitation band still remains
over the EIO, although it slightly extends toward the central

Indian Ocean. The gyre does not trail the precipitation band
while it is leaving from the EIO. The decoupling of the gyreand the enhanced precipitation (i.e., convection) region, as
pointed out in the previous composite results, is clearlyobservable in this case.

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D20119[37] The structure and evolution of the equatorial clockwise gyre are represented by vorticity fields in Figure 10.
The sequence of 850-hPa relative vorticity and wind
anomalies (Figures 10a– 10d) well captures the growth andwestward movement of the gyre associated with the surgeinduced cross-equatorial flow. Here a strengthened signatureof the equatorial clockwise gyre is also identified in the raw
relative vorticity and wind fields (Figures 10e – 10h). A
westward movement of the center of the gyre can be traced,as seen in the vorticity anomaly fields (Figures 10a– 10d).
During 6 – 8 August, the surge-induced cross-equatorial flow
is created at the western half of the gyre (75 – 85°E). A zero
absolute vorticity line within this meridian shifts about 5°
northward from its position on 4 August, implying theinjection of negative absolute vorticity from the SH into
the NH due to the cross-equatorial flow.

7. Summary and Discussion

Figure 9. 6- to 25-day filtered wind vector anomalies andstream function anomalies (contours with 1.0  106
interval, zero contours are omitted) at 850 hPa and total
precipitation (shading) for 4 – 10 August 2000 with 2-day
interval. Precipitation values greater than 10 mm day1 areshaded (see color scale bar).

[38] This study examines the cross-equatorial influencesof the southerly surges over the eastern Indian Ocean.
Emphasis is placed on the dynamical role of the surgeinduced cross-equatorial flow in the genesis of the equatorial wave as a form of clockwise gyre. The work was
motivated by the fact that low-level meridional surges are
an important lateral forcing of tropical disturbances invarious locations, especially in the East Asian and western
Pacific monsoon regions. We have tried to confirm that thislateral forcing concept is also applicable to the case of theSH winter southerly surge in the eastern Indian Ocean
region.
[39] The formation of the equatorial clockwise gyre justafter the surge event is well identified in the composites ofwind and vorticity anomalies. We interpret this gyre to be akind of equatorial Rossby wave. To examine a physicalmechanism linking the surge and the equatorial clockwise
gyre genesis, we conducted eddy vorticity budget analysis.
The result suggests the injection of negative absolute
vorticity around the equator due to both advective andconvergent processes associated with the surge winds. The
cross-equatorial advection of the negative basic state absolute vorticity by the surge winds increases negative eddy
vorticity in the NH side equatorial region. In addition, thesurge-wind convergence accumulates negative eddy vorticity to the south of the equator. The combination of these twoprocesses is primarily responsible for the growth of theequatorial clockwise gyre. Furthermore, the wave activity
diagnostics illustrate that the southerly surge that originatesin the SH extratropics facilitates northward propagation of
Rossby wave energy into the tropics, emphasizing that thesoutherly surge is an important extratropical forcing of
tropical disturbances in the EIO region.
[40] We note not only the dynamical roles but also thehydrological roles of the southerly surge. The crossequatorial moisture transport caused by the southerly surge
appears to partly contribute to the enhancement of precipitation along the west coast of southern Indo-China peninsula.
At the peak stage of the equatorial gyre growth, theprecipitation maximum is embedded in the gyre, indicating
a well-defined convectively coupled equatorial Rossby
wave signature. After that, the equatorial gyre moves
westward while the precipitation maximum stays over the

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FUKUTOMI AND YASUNARI: CROSS-EQUATORIAL INFLUENCE OF SOUTHERLY SURGES

Figure 10. As in Figure 9 except for (a, b, c, and d: left) relative vorticity anomalies and wind vector
anomalies and (e, f, g, and h: right) total relative vorticity, total wind vectors, and zero absolute vorticity
contour (thick solid line). Contour intervals are 1.0  105 s1. Zero contours are omitted. Values lessthan 1.0  105 s1 are shaded.
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EIO, which suggests the decoupling of the gyre andconvection.
[41] The case analysis for August 2000 clearly shows thedevelopment of the equatorial clockwise gyre as a convectively coupled equatorial Rossby wave associated with thesoutherly surge event. Overall features in a sequence ofwinds, precipitation, and vorticity are almost consistent withthose in the composites.
[42] The dynamic structure of the equatorial clockwise
gyre that develops after the surge peak accompanies thecounterclockwise (cyclonic) gyre over the Bay of Bengal.
This north-south double-vortex structure is regarded as an
equatorial Rossby wave having an asymmetric pattern withrespect to the equator. The pattern is basically similar to ann = 2 equatorial Rossby wave [Matsuno, 1966]. However,another possible interpretation for this type of wave pattern
has been given by Chatterjee and Goswami [2004]. They
proposed that this pattern was a translated n = 1 Rossby
wave symmetric about the dynamic equator, positioned at
approximately 5 – 10°N. Note that the dynamic equator isdefined by the mean absolute vorticity zero line during
northern summer [e.g., Tomas and Webster, 1997]. Whichinterpretation is most appropriate is still open to discussion.
Further careful investigation is required to compare theobservational patterns and the linear theory for equatorial
waves.
[43] The transient cross-equatorial flow induced by thesoutherly surge injects negative absolute vorticity from theSH side into the NH side equatorial EIO region. This
process may locally enhance inertial unstable conditions
extending from the equator to the dynamic equator [Tomas
and Webster, 1997], also located in this region. Tomas andWebster [1997] pointed out that the Indian Ocean sector
during northern summer satisfies the inertial unstable condition as an average feature. They proposed that such a
potential condition plays an important role in determining
the location of tropical convection in this sector. However, it
is unclear whether inertial instability affects the atmosphericconditions disturbed by the submonthly southerly surges
examined in the present work. Generally, the timescale ofnatural oscillation due to inertial instability is approximately
4 – 5 days around 5°N [Tomas and Webster, 1997]. For
instance, the absolute vorticity zero line in the case analysis(Figures 10e– 10h) migrated northward and southward, inassociation with the southerly surge and the westward
moving equatorial clockwise gyre on a submonthly timescale; however, it did not appear to oscillate over the shortperiod of 4 – 5 days. It may be difficult to detect the effects
of inertial instability on the surge-induced atmosphericconditions because of the dominant submonthly variability
in this region.
[44] One of the possible extensions of this study might bean examination of oceanic responses to the submonthly
southerly surges. It is important to note that several recent
studies have investigated dynamic and thermodynamic
responses of the tropical Indian Ocean to atmosphericsubmonthly variability [e.g., Sengupta et al., 2001, 2004;
Shinoda and Han, 2005; Masumoto et al., 2005; Han et al.,2006; Masumoto et al., 2008; Ogata et al., 2008]. However,most of these studies have not focused on the ocean
circulation and thermal structures that depend on seasonality

D20119and the types of atmospheric submonthly disturbances.
Future work should take these aspects into consideration.
[45] Acknowledgments. We appreciate the comments of three anonymous reviewers, which helped to improve the paper. We thank Bin Wang
and Kazuyoshi Kikuchi for providing insightful discussions. This research
was initiated while Y.F. was a visiting researcher at International PacificResearch Center (IPRC), University of Hawaii at Manoa from October2005 through September 2006. This research was supported in part by the
JAMSTEC-IPRC joint research program (2005 – 2006).

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Y. Fukutomi, Frontier Research Center for Global Change, Japan
Agency for Marine-Earth Science and Technology, 3173-25 Showamachi,
Kanazawa-ku, Yokohama, Kanagawa 236-0033, Japan. (fukutomi@jamstec.
go.jp)
T. Yasunari, Hydrospheric Atmospheric Research Center, Nagoya
University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan.
(yasunari@hyarc.nagoya-u.ac.jp)

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