A Triggering Process of Indian Ocean Basin-wide Warming
in Relation to ENSO forcing.
* Masamichi Ohba, Hiroaki Ueda
(Graduate School of Life and Environmental Sciences, University of Tsukuba)* Graduate School of Life and Environmental Sciences, University of Tsukuba,
1-1-1 Tennohdai, Tsukuba, Ibaraki, 305-8572, Japan. e-mail: s0430256@ipe.tsukuba.ac.jp
Abstract
The physical process of the Indian Ocean (IO) basin-wide warming is investigated using NCEP/NCAR reanalysis data and1.5 layer ocean model experiments. Composite analysis of six warm episodes of the El Niño illustrates that the reversed
Walker circulation (RWC) associated with El Niño-Southern Oscillation (ENSO) modulate the surface atmosphericcirculation over the IO. During the autumn, the enhanced easterly wind anomalies and reduction of wind velocity are
remarkable over the equatorial IO. This modulation of atmospheric circulation can be explained by the RWC, which
effectively weaken the climatological westerly wind. To elucidate how remote El Niño forcing influences the underlying
SST, we conduct sensitivity experiment. The anomalous zonal wind, assuming the general remote forcing from the El Niño,
is imposed during the boreal autumn, while climatological wind are used during the other season of year in the experiment.
As a result of this experiment, the basin-wide warming in the equatorial IO is reproduced in the succeeding winter throughthe reduction of evaporative cooling. Thus, a coupling process between the RWC associated with the El Niño events and themonsoon circulation during the boreal autumn has a sufficient potential to trigger the IO basin-wide warming.
Keywords: Indian Ocean, basin-wide warming, ENSO, Walker circulation.

1. Introduction
The impacts of atmosphere-ocean variability in theIndian Ocean (IO) are closely related to anomalous weatherand climate condition in the surrounding regions. Theinterannual variation of sea surface temperature (SST) overthe IO, including the modulation of monsoon circulation,
has been documented in numerous investigations (e.g.,Nicholls 1995; Hastenrath et al 2002). It is widely
recognized that the dominant interannual variation of SST
in the IO is basin-wide warming (cooling) which is lagged
behinds to a mature phase of the El Niño (La-Nina) a fewmonths (Nigam and Shen 1993 Tourre and White 1995;
Chambers et al. 1999; Klein et al. 1999; Yu and Rienecker
1999; Lau and Nath 2003). The result in Venzke et al.
(2000) suggests that the anomalous surface heat fluxes dueto the weakened wind speed and the suppressed convective
activity play an important role in warming SST in the IO.
Very recently, several studies have suggested that thecoupled atmosphere-ocean dynamics plays a significant
role in the formation and maintenance of these zonal SST
contrast in the IO (e.g. Saji et al. 1999; Webster et al.
1999). Xie et al. (2002) have revealed the important role ofthe ocean dynamics, especially subsurface thermocline
variability associated with the ocean Rossby wave, inmodulation of SST over the south IO. Moreover, the crossequatorial oceanic heat transports forced by the monsoon
circulation are essential to keeping the upper ocean heatbalance within bounds (Loschnigg and Webster 2000).
Thus, the ocean dynamics and monsoon circulation overthe IO is an important factor that regulates both theseasonal and interannual variation of SST.
While, there have been a small number of studies thatfocus on precise seasonal coupling processes between themonsoon circulation and the external forcing in view thebasin-wide warming. Thus, the primary goal of this paper isto investigate the characteristics of the physical process
involved in the basin-wide warming in the IO. For thispurpose, we conduct the sensitivity experiments on SST byuse of 1.5 layer ocean model. Data and model description
are described in Section 2. The ENSO-monsoon couplingwith regard to the model experiments are presented inSection 3. A summary of the major findings of this study
are presented in the last section.

Fig. 1: Evolution of composite monthly mean Niño-3.4 SST(solid: 5ºS-5ºN, 120º-170ºW) anomalies and Indian Ocean
SST (dashed: 20ºS-20ºN, 50º-100ºE) anomalies during themature phase of the six warmest episodes.

2. Datasets and ocean model
We analyzed monthly atmospheric data derived from theNational Centers for Environmental Prediction (NCEP)/
National Center for Atmospheric Research (NCAR)
reanalysis (Kalnay et al. 1996) on Gaussian grids of T62.
As a complementary dataset for comparing with the model
data, reconstructed monthly SST datasets of 2-degree grid
are also utlized (Smith et al. 1996). These data are prepared
for the period from 1955 to 1999. To make reasonable
composite, we pick out the most prominent warm El Niño
episodes, using a similar method in Wang et al. (2000). Fig.
1 displays the time series of composites Niño3.4 (5ºS-5ºN,120º-170ºW) and IO (20ºS-20ºN, 50º-100ºE) SST
anomalies around the warm peaks of the major ENSO
events. It is clear that the peak of composite SST anomalies
in the IO lags behind those in Pacific by about 3 months.

six warm episodes (1957, 1965, 1972, 1982, 1991 and1997). We define months in the El Niño onset year as (0),those in the following year as (1).

Fig. 2: Climatological (a)simulated SST, (b)observed SST
and surface wind averaged between October and December.
(c)The 3-month running mean of simulated SST (solid),observed SST (dash) over the IO and Niño 3.4 SST (lightshading).
To estimate the variation of SST in the IO due toanomalous external forcing, associated with the ENSO
signal, 1.5-layer reduced gravity ocean model (Zebiak andCane 1987) is used. The physical grid has a horizontal
resolution of 1º latitude × 0.5º longitude. The SST isdetermined by a balance between surface fluxes, horizontal
advection by imposed winds, radiative cooling, horizontal
diffusion, and entrainment from below the mixed layer. To
compute the surface sensible and latent heat fluxes, we use
a standard bulk formula. The radiative forcing including
short wave radiation and long wave radiation are takenfrom the NCEP/NCAR reanalysis data. The distribution ofsimulated SST, observed SST, and flow field averaged
between October and December are shown in Figs. 2a, b.
Overall, observed SST field is well reproduced, with themeridional temperature gradients in both the Arabian Sea
and southern Indian Ocean being captured. During theboreal fall, the equatorial westerly jet, shifted slightly
northward from the equator, develops in response to thesummer monsoon (Wyrtki 1973). Time series of simulated
SST, observed SST anomalies averaged in the IO andNiño-3.4 SST are displayed in Fig. 2c. The correlation
coefficient between the IO SST anomalies and Niño-3.4
exhibits 0.58 by 4 months lag that is consistent withprevious studies (e.g., Klein et al. 1999). The time series
between the simulated SST and observed SST are nearly
similar to each other. Thus, the 1.5-layer ocean model
sufficiently simulate spatial-temporal variability over theIO, which enable us to conduct idealized experiment inview of atmosphere-ocean coupled system.

3. Monsoon-ENSO coupling
We examine the remote impact of El Niño on the lowlevel monsoon flows over the tropical IO. To describe theevolution of anomalous circulation during the El Niño,
composite analyses are performed based on those of major

Fig. 3: Longitude-time section of composite anomalies of(a) surface zonal wind and wind vector from January (0) toMarch (1) for major warm episode of the El Niño year along
latitudinal band between 5ºS and 5ºN. (b) Same as (a)except for wind velocity and law wind vector.
Figure 3 shows the longitude-time sections of composite
anomalies of the surface zonal wind and wind velocity overthe equatorial IO and maritime continent. The vectors
represent the composited wind anomalies and raw wind,respectively. The easterly anomalies over the eastern andcentral IO are recognizable during the boreal autumn inconjunction with a mature phase of the El Niño (Fig. 3a).
The most enhanced easterly anomalies are found to occur
about 2 months prior to the warm peak of Niño3.4 SST(Fig. 1). The scalar wind velocity in the central IO,
centered around 80ºE, is considerably reduced in the boreal
autumn (Fig. 3b). It should be emphasized here that theclimatological westerly wind is prominent around
November in the equatorial IO (Wyrtki 1973). Thus, theclimatological westerly wind in the central IO can beremarkably weakened, especially in the boreal autumn, inresponse to the ENSO-related divergent easterly wind.
To seek physical interpretations that explain theinfluence of El Niño, we present Fig. 4, the composite
anomalies of Walker circulation between October (0) andDecember (0), corresponded to the mature phase of El

Niño. The convective activity in the western equatorial
Pacific shifts eastward to the central and eastern Pacific.
This shift in convection leads to a modulation of theWalker circulation, with anomalous rising over the centraleastern Pacific and sinking over the maritime continent. As
for the tropical IO, in relation to the downward motion, thedivergent easterly anomalies are prominent in the lower
troposphere. These results indicate the presence of thereversed Walker circulation (hereafter RWC) extending
from the tropical IO through Pacific Ocean.

excessively warm SST is seen in the central IO, basin-widewarming is clearly simulated. Therefore, we conclude thatthe remote El Niño forcing in the autumn is thefundamental factor for triggering process of the basin-widewarming.

Fig. 4: Longitude-height section of composite anomalies ofzonal and vertical wind averaged over the latitudinal band
5ºS-5ºN between October (0) and December (0) for major
warm episode of the El Niño. Dark (light) shading denotes
u-component wind less (greater) than -1.5 (+1.5) ms-1.
The question we will now address is how theatmospheric change contributes to the interannual variationof SST in the IO. Ueda (2001) suggests that the influenceof RWC to the underlying SST can be regulated by thestrong monsoon seasonality. The divergent easterly
anomalies during the boreal autumn weaken the
climatological westerly wind. The reduction in wind speed
is expected to induce reduced vertical mixing in the surface
layer of the ocean and reduced evaporation from the ocean
surface. Thus, in the succeeding winter, positive SST
anomalies expand over the tropical IO.
To understand whether the basin-wide warming in the IO
is actually due to the remote impact of El Niño in theboreal autumn, an idealized experiment is conducted that isdesigned to yield a straightforward answer to this question.
We impose the divergent easterly anomalies onto theatmospheric circulation in the tropical IO, and ascertain its
response to the underlying SST. In this experiment, we use
monthly varying climatological surface heat flux. The
composited zonal wind anomalies between September (0)and January (1) is used as a remote impact of the RWC(Fig. 5a). This wind anomaly is imposed for three months
from October (0) through December (0). As stated above,the climatological westerly wind is prominent in thesemonths, which is transition period of the monsoon.
Monthly varying climatological wind is imposed duringother months.
Figure 5b shows the model response to the easterly wind
anomaly in January (1). In the tropical IO, warm SST
anomaly emerges in the subsequent boreal winter. AlthoughFig. 5: (a) Composite anomalies of surface zonal wind
between September (0) and January (1) for the major warm
episodes of the El Niño year. (b) SST anomalies in thesensitivity experiment.

4. Concluding remarks
In this study, we examined the basin-wide warming in theIO during the warm episode of El Niño, including thepossible mechanisms that regulate the SST and its seasonaldependability. We have paid great attention to the changes
in atmospheric circulation and its influence on theunderlying SST. The result of the idealized experimentindicates that the seasonal coupling process between theseasonality of the monsoon circulation and remote El Niño
forcing plays a crucial role in the trigger process of thebasin-wide warming in the IO.
This conclusion is based on following findings. From theanalysis of the atmospheric datasets derived from theNCEP/NCAR reanalysis, it is demonstrated that Walker
circulation over the Pacific and IO is weakened by theENSO forcing. During the boreal autumn, the RWC act toweaken the climatological westerly jet and thus reduce theevaporative cooling around the central IO. The 1.5 layer
ocean model confirms that these atmospheric changes
significantly contribute to the basin-wide warming in theIO. Consequently, it is conceivable that seasonal march of
the climatological wind is the important factor tounderstand the physical process of basin-wide SST
warming in the IO. This is consistent with the inferableopposite mechanism during a cold episode of El Niño thatcauses the basin-wide cooling in response to increased
evaporative cooling through the reinforced Walker
circulation.
Based on this study and those of previous investigators(Klein et al. 1999; Ventzke et al. 2000), it has become clearthat atmospheric change accompanying El Niño induce
surface warming of the IO. The previous investigators also
indicate the significant effect of the change in cloud coverthat contributes to the SST warming. More detailed
analysis is needed to reveal the development process of theIO basin-wide warming.

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