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mature phase of the El Niño. From boreal fall to winter, the divergent easterly wind anomalies prevent the de- velopment of the climatological equatorial westerly.
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SOLA, 2005, Vol. 1, 089 092, doi: 10.2151/sola. 2005 024

Basin-wide Warming in the Equatorial Indian Ocean Associated with El Niño Masamichi Ohba and Hiroaki Ueda Graduate School of Life and Environmental Sciences, University of Tsukuba, Japan

A physical process of basin-wide warming in the equatorial Indian Ocean (IO) is examined in view of the seasonally different coupling processes between the monsoon circulation and the modulated Walker circulation associated with major El Niño. In the composite analysis of six warm episodes, the reversed Walker circulation along the equator appears over the tropical IO through the Pacific Ocean, resulting in anomalous surface easterlies over the equatorial IO prior to the mature phase of the El Niño. From boreal fall to winter, the divergent easterly wind anomalies prevent the development of the climatological equatorial westerly. Using a reduced gravity ocean model, it is shown that the modulated surface flow over the equatorial IO has a potential to contribute to the basin-wide warming through the heat exchange of the ocean surface. These results imply that the strong seasonality involved in monsoon is important to understand the seasonal phaselocked feature of interannual SST anomalies.

1. Introduction The impact of atmosphere-ocean variability in the Indian Ocean (IO) is closely related to anomalous weather and climate conditions in the surrounding regions. The interannual variations of sea surface temperature (SST) over the IO, including the modulation of monsoon circulation, have been documented by numerous investigators (e.g., Hastenrath 2002). It is widely recognized that the dominant interannual variation of SST in the IO is basin-wide warming (cooling) which lags a few months behind the mature phase of El Niño (La Niña) (e.g., Klein et al. 1999; Lau and Nath 2003). The results of Venzke et al. (2000) suggest that anomalous surface heat fluxes due to weakened wind speed and suppressed convective activity play an important role in warming SST in the IO. Very recently, several studies have suggested that coupled atmosphere-ocean dynamics play a significant role in the formation and maintenance of these zonal SST contrasts in the IO (e.g., Saji et al. 1999; Webster et al. 1999). Xie et al. (2002) revealed the important role of ocean dynamics, especially subsurface thermocline variability associated with the oceanic Rossby wave, in the modulation of SST over the south IO. Moreover, the cross-equatorial oceanic heat transports forced by the monsoon circulation are essential to keep the upper ocean heat balance within bounds (Loschnigg and Webster 2000). Thus, the ocean dynamics and monsoon circulation over the IO are important factors to regulate the seasonal and interannual SST variations. There have been few studies on the precise seasonal Corresponding author: Masamichi Ohba, Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennohdai, Tsukuba, Ibaraki, 305-8572, Japan. E-mail: [email protected]. ©2005, the Meteorological Society of Japan.

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Fig. 1. Evolution of the composite monthly mean Niño3.4 SST anomalies (solid: 5°S 5°N, 120° 170°W) and the Indian Ocean SST anomalies (dashed: 20°S 20°N, 50° 100°E) during the mature phase of the six warmest episodes. The base period is 1950 to 1999.

coupling processes between the monsoon circulation and external forcing in view of basin-wide warming. Thus, the primary goal of this paper is to investigate the characteristics of the physical process involved in basinwide warming in the IO. We conduct SST sensitivity experiments by using the 1.5-layer ocean model. Data and methods are described in Section 2. The El Niño Southern Oscillation (ENSO)-monsoon coupling with regard to the model experiments are presented in Section 3. A summary of the major findings of this study is presented in the final section.

2. Data and methods We utilize monthly atmospheric data obtained from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis on Gaussian grids of T62. Reconstructed monthly SST datasets on 2-degree grids (Smith et al. 1996) are also utilized as a complementary dataset for comparison with the model data. These data are for the period from 1950 to 1999. Composite analyses are performed of the remote impact of El Niño on the low-level monsoon flows over the tropical IO. We pick up the most prominent warm El Niño episodes (1957, 1965, 1972, 1982, 1991, and 1997) and define months in the El Niño onset year as (0) and those in the succeeding year as (1). The composites are subjected to the standard t test for statistical significance, and only when at the 95% significant level is retained in plots. Figure 1 displays the time series of composites Niño-3.4 (5°S 5°N, 120° 170°W) and IO (20°S 20°N, 50° 100°E) SST anomalies around the warm peaks of the major ENSO events. It is clearly seen that the peak composite SST anomalies in the IO lag behind those in the Pacific by about 3 months.

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3. Equatorial westerly and IO warming Several investigations have indicated that atmospheric responses to the El Niño over the IO are accompanied by notable anomalies in surface wind (e.g., Lau and Nath 2003; Tokinaga and Tanimoto 2004). To seek physical interpretations that explain the influence of El Niño on the wind modulation over the tropical Pacific Ocean through the equatorial IO, we present in Fig. 2 the composite anomalies of the Walker circulation between October (0) and December (0) corresponding to the mature phase of the El Niño. During this period, convective activity in the western equatorial Pacific shifts eastward to the central and eastern Pacific. This shift in convection leads to a modulation of the Walker circulation, with anomalous rising motion over the central-eastern Pacific and subsidence over the maritime continent (e.g., Ueda and Matsumoto 2000). Of particular interest of the tropical IO, in relation to the divergence over the western Pacific, is that easterly anomalies are generated and intrude on the western IO in the lower troposphere. These atmospheric cells, extending from the tropical IO through the Pacific Ocean, are manifested as the reversed Walker circulation (hereafter RWC). It is worth mentioning that El Niño events are often accompanied by positive IO dipole/zonal mode events such as in 1972, 1982, and 1997. Thus, combination of the SST anomalies in the IO and Pacific Ocean are responsible for formation of the RWC. For a more detailed analysis of the time variation of the El Niño-related divergent easterly wind anomalies (hereafter DEW) over the equatorial IO through the maritime continent, we present the longitude-time sections of composite anomalies of the surface wind velocity with wind vector (Fig. 3). The light and dark shadings in Fig. 3 denote a region of weakened wind, which is expected to contribute to the increase in SST through a reduction of vertical mixing in the subsurface layer of the ocean and evaporative cooling from the ocean surface. The important feature of the wind anomalies is that the evolution of DEW is located over the eastern and central IO from October (0) to February (1) in conjunction with the mature phase of the El Niño (Fig. 3). The most enhanced DEW, in excess of 2 m s 1 ,

occur around November (0), about 1 month prior to the warm peak of the Niño-3.4 SST (Fig. 1). Corresponding to the peak of the anomalies, the scalar wind is considerably reduced in the boreal fall between 60°E and 85°E. It should be emphasized here that the climatological westerly wind, associated with the eastward surface current in the ocean called Wyrtki Jet, usually prevails in the transition season between summer and winter, especially in May and November, over the equatorial IO (Wyrtki 1973). In this manner, the reduction of wind velocity over the equatorial IO around November (0) can be understood as a result of the weakened climatological westerly in response to the DEW. To save space, we denote the climatological westerly wind as Fall Wyrtki westerly (hereafter FWW). The question we will now address is how these atmospheric changes contribute to the interannual variations of SST in the equatorial IO. It should be emphasized here that the development of DEW over the IO is response to the generation of RWC. Previous investigators (Ueda 2001) have suggested that the influence of the RWC on the underlying SST can be regulated by the strong monsoon seasonality. According to Ueda (2001), the reduction of the FWW causes basin-wide warming through a remarkable reduction in wind speed over the equatorial IO. To understand whether equatorial basin-wide warming is actually due to the remote impact of El Niño in the boreal fall, a simple idealized experiment is conducted that is designed to yield a straightforward answer to this question. We now turn our attention to the effects of the atmospheric modulation over the equatorial IO on the underlying SST and its seasonality, discussed in the previous paragraph. To estimate the variation of equatorial IO SST due to the remote El Niño forcing, a 1.5layer reduced gravity ocean model (Zebiak and Cane 1987) is used. The model grid has a horizontal resolution of 1° latitude by 0.5° longitude. The SST is determined by a balance between the surface heat fluxes, horizontal advection due to imposed winds, horizontal diffusion, and entrainment from below the mixed layer. The

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length of the spin-up period is 3 years. To compute the surface sensible and latent heat fluxes, we use a standard bulk formula. Short- and long-wave radiation data are obtained from the NCEP/NCAR reanalysis data. The distribution of simulated SST, observed SST, and the surface wind field averaged between October and December are shown in Figs. 4a, b. Although the simulated SST off the coast of the Indonesian archipelago is slightly warmer than the observed, the observed SST field is well reproduced, with the meridional temperature gradients in both the Arabian Sea and the southern IO being captured. During the boreal fall, the FWW develops along the equator and is responsible for the calm state in the equatorial IO (Fig. 4b). A time series of simulated SST, observed SST anomalies averaged in the IO and Niño-3.4 SST, is displayed in Fig. 4c. The correlation coefficient between the IO SST anomalies and Niño-3.4 is 0.59 by a 3-month lag, which is consistent with the results of previous studies (e.g., Klein et al. 1999). Although the amplitude of the interannual variability of simulated SST is slightly larger than observation, the time series of the simulated SST and observed SST are similar to each other. Thus, the 1.5-layer ocean model broadly simulates the spatialtemporal variability over the IO, which enables us to conduct an idealized experiment in view of the atmosphere-ocean coupled system. The idealized experiment (EXP1) consists of integrations forced by prescribed pattern of DEW. This experiment is expected to reveal the effect of weakened wind velocity on the IO SST through the coupling between the climatological FWW and the DEW in the fall of the El Niño onset year. To make a reasonable DEW forcing, we pick up the DEW from the 5-month average of the composited zonal wind anomalies from September (0) through January (1) (Fig. 6a). As mentioned above, the FWW is prominent in the three months from October through December. The DEW is imposed for these three months of the El Niño onset year, while monthly varying climatological winds, as obtained from longterm averages of wind field in the NCEP/NCAR reana-

Fig. 5. Schematic diagram of the imposed DEW in the SST sensitivity experiments. +anom denotes DEW and light shading shows climatological wind. DEW is imposed for 3 months from October (July) to December (September) of the El Niño onset year in the EXP1 (EXP2).

lysis data, are imposed during other months (Fig. 5). Monthly varying climatology is used for all other forcing parameters except for the zonal wind component. Meridional wind anomalies, are excluded to estimate only the weakened effect of the FWW. Based on the above forcing scenario, the model is run for 2 years from January (0) through December (1) after a sufficiently long spin-up period. The SST anomaly shown in Fig. 6b is the difference between the model response of EXP1 and the climatology of the 40-year control run in the subsequent January (1). A positive SST anomaly emerges in the equatorial band between 10°S and 10°N. Although an excessively warm SST is seen in the central IO, the result of the model experiment evidently shows basin-scale SST warming in the equatorial region. Thus, it is conceivable that the reduced climatological FWW, resulting from the anomalous Walker circulation, is one of the fundamental factors for a triggering process of equatorial basin-wide warming. We calculate the model SST tendency due to the latent heat flux anomalies in the equatorial IO (10° S 10°N, 50° 100°W) for the OND (0) and ascertain that 72% of the total SST warming is attributed to the reduced latent heat loss (not shown). The findings in this experiment also underscore the critical role of seasonal coupling processes between the monsoon circulation and El Niño forcing in the generation of SST anomalies.

4. Concluding remarks In this study, we have examined basin-wide warming in the equatorial IO during the mature phase of the El Niño, including the possible mechanisms that generate the warm SST anomalies. We have paid attention to the coupling process between the remote El Niño forcing and the monsoon circulation from fall to winter as a trigger or contributor to the anomalous warming in the IO. In the composite analysis of the monthly atmospheric data derived from the NCEP/NCAR reanalysis, the modulated Walker circulation, associated with the El Niño event, results in the anomalous surface easterlies over the equatorial IO and acts to weaken the climatological FWW. Through a reduction of evaporative cooling from the ocean surface, the weakened wind is largely responsible for the anomalous in situ increase of the SST. We have conducted the sensitivity experiment of SST by the reduced gravity ocean model to verify the modulation of SST. The result of the simple experiment indicates that the DEW associated with the warm episode, especially appearing in the boreal fall, significantly reduces the climatological monsoon westerlies and can contribute to the warming of the IO SST in the following winter.

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seasonal phase-locking behavior of the interannual variation of the IO SST. Although the 1.5-layer ocean model does not have a good capability to calculate the amplitude of the SST variability, we can accurately estimate the local effect of the atmospheric change and its polarity. The effects of the basin-wide warming to the atmosphere and changes in cloud cover on the IO in an El Niño year, have not been estimated in this study. More research is needed to further understand the dynamics of basin-wide warming, its persistence, and its link to air-sea interaction in the IO.

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The authors thank the Editor (Prof. Y. Tanimoto) and two anonymous reviewers for constructive comments that lead to improvement of this paper. The model source code is provided by Dr. Yoshiteru Kitamura. This research was supported by Grants-inAid for Young Scientists (B) (15740288) from the Japanese Ministry of Education, Science, Sports and Culture and Core Research for Evolutional Science and Technology (JST/CREST). Finally the authors thank climate research laboratory of Tsukuba University.

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We are not focusing on the off-equatorial SST anomalies that typically reach a maximum deviation over the Southwest IO (Xie et al. 2002). Although we could not simulate the meridional extent of the basinwide warming compared with the observation (e.g. Klein et al. 1999, Fig. 2), this study reconfirmed that the development of the basin-wide warming goes through the regionally different compound processes. We consider that the off-equatorial SST warming is caused by another factor such as modulation of the latent heat flux and the ocean dynamics in relation to meridional wind anomalies, or short wave radiation. For instance, in the Arabian Sea, Murtugudde and Busalacchi (1999) show that the oceanic processes play an important role in interannual variability of SST in addition to the variability of the air-sea fluxes. It is conceivable that the lack of meridional wind anomalies is not enough to reproduce the ocean dynamics and surface heat fluxes in these regions. An additional experiment (EXP2), in which the same external forcing as that used in the EXP1 is imposed in the summer, has been done. We imposed the DEW (Fig. 6a) for three months from July (0) through September (0) in the El Niño onset year (Fig. 5). As a result of this, similar to the positive IO dipole/zonal mode, zonally asymmetric SST anomalies are generated in the following October (0). These two simple experiments imply that the seasonally different coupling processes are a key factor in understanding the evolution of the SST anomalies in the tropical IO. In this regard, strong monsoon seasonality is important to explain the

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