Tomomichi Ogata & Yukio Masumoto

Transcription

1 Ocean Dynamics (2010) 60: DOI /s Interactions between mesoscale eddy variability and Indian Ocean dipole events in the Southeastern tropical Indian Ocean case studies for 1994 and 1997/1998 Tomomichi Ogata & Yukio Masumoto Received: 29 May 2009 /Accepted: 17 May 2010 /Published online: 6 June 2010 # Springer-Verlag 2010 Abstract Interannual modulation of mesoscale eddy activity at the intraseasonal timescale in the southeastern tropical Indian Ocean and its relation to the Indian Ocean dipole mode (IOD) events are investigated using results from a high-resolution ocean general circulation model. The model reproduces observed characteristics of the intraseasonal variability and its interannual modulation fairly well, with large variances of the intraseasonal variability during the 1994 and 1997/1998 IOD events. Large negative temperature anomaly off the coasts of Java and the Lesser Sunda Islands in boreal summer, due to seasonal variation and interannual anomaly, extended further to the east in 1994, and the associated strong Indonesian throughflow enhanced the baroclinic instability in the upper layer, generating anomalously large mesoscale eddy activity. The eddy heat transport, in turn, significantly affected decaying phase of the 1994 IOD event. On the other hand, the development of the cold region off the Java Island associated with the 1997/ 1998 IOD event occurred in boreal winter, causing weaker baroclinic instability and hence weaker eddy activity off Java. This led to little influence on the heat budget in the southeastern tropical Indian Ocean for the 1997/1998 IOD event. Responsible Editor: Hideharu Sasaki T. Ogata (*) International Pacific Research Center, University of Hawaii at Manoa, POST Bldg, Room 412F1680 East-West Road, Honolulu, HI 96822, USA ogatat@hawaii.edu Y. Masumoto Research Institute for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan Keywords Indian Ocean. Interannual variability. Intraseasonal variability. Mesoscale eddy 1 Introduction A far southeastern basin in the tropical Indian Ocean, hereafter referred to as SETIO, between the Lesser Sunda Islands and northwestern Australia east of approximately 100 E is known as a region of strong mesoscale eddy activity (e.g., Quadfasel and Cresswell 1992; Feng and Wijffels 2002; Yu and Potemra 2006). Feng and Wijffels (2002) showed strong intraseasonal variability (ISV) in the observed sea surface height (SSH) anomaly, with distinct seasonal variation in the ISV amplitude, with large amplitude appearing from July to September. They concluded that the ISV is associated with eddy activity caused by baroclinic instability in the South Equatorial Current (SEC)/Indonesian Throughflow (ITF) system in the SETIO, partly because they did not have adequate data to study barotropic instability. Yu and Potemra (2006) investigated the ISV in the same region using a 4.5-layer model forced by climatological monthly-mean wind forcing. They succeeded in reproducing the ISV with larger variability during July to September and attributed it to mixed instability, which was sensitive to transports through three main passages to the Indian Ocean, namely the Lombok, Ombai and Timor passages, in particular the Lombok Strait. In addition to such internal instability within the ocean, the ISV can be directly forced by local wind variability along the southern coast of the Lesser Sunda Islands and generated by eastward propagation of the intraseasonal coastal Kelvin waves, which are remotely forced by the zonal wind variability in the equatorial Indian Ocean (Iskandar et al. 2005; Iskandar 2007).

2 718 Ocean Dynamics (2010) 60: The SETIO is also known as one of the action centers for the Indian Ocean dipole mode (IOD), which is an air sea coupled climate mode at the interannual timescale inherent in the tropical Indian Ocean (Saji et al. 1999; Webster et al. 1999; Murtugudde et al. 2000). Positive IOD is characterized by negative temperature and lower SSH anomalies on the Indonesian side of the SETIO, creating meridional temperature gradient and associated vertical shear in the SEC that are stronger than those under climatological conditions. Thus, together with the El Nino/Southern Oscillation phenomenon in the Pacific Ocean, the IOD in the Indian Ocean is one of the dominant phenomena affecting magnitude of the ITF at the interannual timescale (Meyers 1996; Masumoto 2002; Sprintall et al. 2009). Although such interannual variations in the current system of the SETIO are expected to appear in the interannual modulation of the ISV activity through local instabilities, so far, there is no study on this important link between the two signals in the SETIO. In general, the ISV associated with the instabilities generates net heat transport across the mean current to reduce the temperature gradient. Therefore, the interannual modulation of the ISV activity may affect heat budget in this region at the interannual timescale, hence affecting evolution of interannual climate variability modes in the SETIO. In this study, the ISV and its interannual modulation within the SETIO and their influence on the IOD evolution are investigated using results from a highresolution ocean general circulation model (OGCM). A brief description of the OGCM and dataset used in the following analyses is presented in Section 2. The simulated ISV and the interannual variations of the ISV activity are illustrated and compared to the observed characteristics of the ISV in Section 3. Details of the relation between the ISV activity and the IOD events in 1994/1995 and 1997/ 1998 are shown in Section 4. Conclusions and discussion about the Pacific influence on the ISV activity in the SETIO are given in Section 5. 2 Data and model descriptions In the present study, we mainly utilize results from a highresolution OGCM, called OFES (OGCM for the Earth Simulator). OFES is a three-dimensional, finite difference primitive equation model based on the Modular Ocean Model version 3 (MOM3), developed at Geophysical Fluid Dynamics Laboratory/National Oceanic and Atmospheric Administration (Pacanowski and Griffies 2000). It is tuned and improved to obtain the best performance on the Earth Simulator at Japan Agency for Marine-Earth Science and Technology. The model covers near-global areas from 75 S to 75 N, with a horizontal grid spacing of 0.1 in both longitude and latitude. There are 54 levels in the vertical, of which 20 levels are located in the upper 200-m depth to reproduce the variations within and shallower than the thermocline as realistically as possible. The horizontal mixing process is parameterized by the bi-harmonic scheme, and the K-Profile Parameterization scheme is adopted for the vertical mixing (Large et al. 1994). A high-resolution bottom topography data provided by the OCCAM project is used, with the Partial Cell scheme embedded in MOM3, to represent the coastline and bottom topography as realistically as possible. Note that the major features such as the Maldives island chain and the Ninety East Ridge are well represented in OFES. The model is, at first, forced by climatological monthlymean wind stresses calculated from the NCEP/NCAR reanalysis (Kalnay et al. 1996) for 50 years and then driven by the daily-mean wind stresses of the reanalysis data from 1950 to The surface heat flux is calculated using a bulk formula similar to Rosati and Miyakoda (1988), with necessary data from the NCEP/NCAR reanalysis. For freshwater flux, we utilize a scheme based on precipitation minus evaporation, with the NCEP/NCAR reanalysis precipitation data and the evaporation calculated using the bulk formula. In order to include influences of the river runoff, a term with a timescale of 6 days is added to relax surface salinity to the climatological monthly-mean values of World Ocean Atlas 98 (WOA98; Boyer et al. 1998). Similar relaxations to the climatological monthly-mean temperature and salinity fields are adopted near the boundaries at 75 S and 75 N. For more details of the model description, readers are referred to Masumoto et al. (2004) and Sasaki et al. (2006). In this study, we analyze the results of OFES forced by NCEP/NCAR daily reanalysis data for the period from 1990 to 2003, during which several satellite observations are available for comparison. Note that two positive IOD events, in 1994 and 1997/1998, occurred in this period. The Ocean Topography Experiment/Poseidon (T/P) and European Remote Sensing Satellite-1 and-2 (ERS-1/2) blended dataset (Ducet et al. 2000) is used to evaluate the model results. The combined T/P and ERS-1/2 altimeter data provides global SSH variability with high temporal (7-day interval) and spatial (0.25 in longitude and 0.33 in latitude) resolutions from October 1992 to the present. In order to focus on the ISV and its relation to the interannual variations in the SETIO, the total variability of each variable is separated into intraseasonal, seasonal, and interannual timescales; so for any variable of interest, V, V ¼ V ISV þ V clim þ V anom : ð1þ To separate the variability, climatological seasonal variation (V clim ) is calculated at first and subtracted from

3 Ocean Dynamics (2010) 60: the total variations. Then, the and days bandpass filters are applied to the OFES results and the observed SSH variability, respectively, to extract the ISV or V ISV. Since the variability shorter than about 2 weeks is relatively small in OFES, most of the remaining variability (V anom ) consists of only the interannual variations. In the following analyses, therefore, we call these separated variabilities as the ISV, seasonal variation, and interannual anomaly, respectively. 3 Model validations 3.1 Simulated seasonal to interannual variability in the tropical Indian Ocean Figure 1 compares climatological mean temperature and zonal currents along 120 E in March and September from OFES and WOA98. The 20 C isotherm, which corresponds to the middle of the thermocline, is highlighted by red. OFES resembles well the observed features, including shoaling of the 20 C isotherm toward the Java coast (north of 14 S) and strengthening of the westward current in the upper ocean in boreal summer. However, observed thermocline slope is significantly weaker than that in OFES, due likely to strong smoothing applied to the WOA98 dataset. Figure 2 shows the comparison of the Dipole Mode Index (DMI; Saji et al. 1999) obtained from OFES and Extended Reconstruction sea surface temperature (ERSST; Smith et al. 2008). The DMI is defined as the difference of area-averaged SST anomaly between the western box (50 70 E, 10 S 10 N) and the eastern box ( E, 10 S 0 N). OFES captures positive (negative) IOD events during 1994 and 1997 (1996 and 1998), with a correlation coefficient of 0.66, which is significant at the 95% confidence level. Fig. 1 Latitude-depth section of the seasonal climatology for zonal current (shade) and temperature (contour) along 120 E. Left (right) panels show results of OFES (WOA98) in March and September. Note that current fields in WOA98 are derived by means of the geostrophic calculation that assumes 1,000 m as the depth of no motion

4 720 Ocean Dynamics (2010) 60: Fig. 2 Dipole Mode Index (DMI) from January 1990 to December Black line (red line) indicates the DMI derived from OFES (ERSST) 3.2 Intraseasonal variability in the SETIO and its interannual modulation Figure 3 compares horizontal distribution of the observed and simulated variances of the intraseasonal SSH variability in the SETIO for the period from January 1993 to December Both results exhibit large amplitude variability in a region south of the Java Island (100 E 115 E, 9 S 15 S). The simulated amplitude, however, is slightly larger and shifted southward compared with the observed one, due perhaps to slight difference in location and structure of the SEC between the two results. Intraseasonal SSH variance is also large along the coast of the Sumatra and Java islands, showing a manifestation of intraseasonal coastal Kelvin waves generated by zonal wind variability in the equatorial Fig. 3 Horizontal distributions of the intraseasonal variance of the SSH variability from satellite altimetry observation (14- to 98-day band-passed variability) and OFES (18- to 96-day band-passed variability) for a period from January 1993 to December 2003 (cm (cm Indian Ocean as well as by the local alongshore winds (Iskandar et al. 2005). However, this coastal area of the large amplitude is separated from the offshore maximum variances in both OFES and observations. Figure 4 illustrates the observed and simulated variations of the total SSH along 12 S from January 1994 to December 1995 to show distinct characteristics of the variations. Significant variability at the intraseasonal timescale is clearly seen in both observations and OFES simulation, showing clear westward propagation with a speed of 0.19 m/s and a typical period of days. The amplitude of the variability tends to be large from July to December and to be small from February to June, which is consistent with Feng and Wijffels (2002). In addition, distinct seasonal variation can be observed in both OFES and observations: The SSH becomes larger during August to December and smaller during February to June near 90 E. This annual signal also propagates westward with a speed of 0.15 m/s, suggesting an association with annual forced Rossby waves in the southern tropical Indian Ocean (Masumoto and Meyers 1998). Note that the above characteristics of the intraseasonal and seasonal timescales are basically the same in other years, even though the interannual variability is superposed on the seasonal and intraseasonal variations. As another measure for the ISV activity and its interannual modulation, an eddy kinetic energy (EKE) associated with the 18- to 96-day variability is defined as Ke = ρ(u ISV 2 + v ISV 2 ). Then, its monthly value with 3-month sliding window is calculated and separated into the seasonal climatology (Ke clim ) and remaining interannual variation (Ke anom ) similar to the derivation of Eq. 1. The SSH variation contains both large-scale variations associated with atmospheric intraseasonal disturbances and the mesoscale eddy variability due to internal oceanic processes. On the other hand, the EKE tends to capture mesoscale eddy variability and does not represent the largescale forced variations. In this regard, the EKE anomaly time series is more appropriate for exploring the role of intraseasonal eddy activities associated with oceanic pro-

5 Ocean Dynamics (2010) 60: Fig. 4 Longitude-time sections of the SSH variability from January 1994 to December 1995 at 12 S for satellite observation and OFES (cm) cesses. Figure 5a displays time series of variance of the intraseasonal EKE variability averaged over 95 E 115 E along 12 S. The variance is calculated each month for 3-month time window, and the time series is normalized by the standard deviation of the variance. The observed value of the EKE is calculated from the geostrophic velocity estimated from the T/P and ERS-1/2 SSH variability. The standard deviation of the OFES simulated and observed EKE anomalies are 31.8 and 23.9 g/cm/s 2, respectively. Both the simulated ISV and observed ISV demonstrate significant interannual modulations. The ISV variances are larger (weaker) in 1994 and 1997/1998 (1993 and 1998/ 1999) in both OFES and observations. The correlation coefficient between simulated and observed EKE anomaly is 0.56, which is above the 90% confidence level. There are some differences between the simulated and observed EKE anomalies in 1995/1996 and 1996/1997. The discrepancy in 1995/1996 may be due to the simulated negative heat content anomaly in the SETIO during boreal summer of 1995, while the difference in 1996/1997 are associated with warm anomaly around 14 S, which is propagated from the western tropical Pacific. Detailed analysis on the influence of the Pacific Ocean on the eddy activity in the SETIO is ongoing and will be reported in a separate paper. In order to investigate the relationship between the eddy activities in the SETIO and basin-scale subsurface phenomena in the Indian Ocean, Fig. 5b shows horizontal normalized by Std. ( ) Fig. 5 a Time series of anomaly of the intraseasonal EKE variability averaged over 95 E 115 E along 12 S for satellite observation (black line) and for OFES (green line). DMI derived from ERSST is also superposed (red line). b Regression of the EKE anomaly time series in Fig. 5a onto heat content anomaly in the upper 250-m depth for the Indian Ocean basin. Note that EKE for satellite observation in a is calculated using the geostrophic relation

6 722 Ocean Dynamics (2010) 60: distribution of the heat content anomaly in the upper 250-m depth regressed to the EKE anomaly time series shown in Fig. 5a, with the heat content anomaly leading to the EKE anomaly by 3 months. While negative heat content anomaly occupies in the eastern tropical Indian Ocean with large signal occurring along the coast of Sumatra and Java Islands, positive anomaly appears in the southwestern tropical Indian Ocean. The maximum anomaly exceeds 200 Cm in both positive and negative regions. This structure is generally similar to the spatial distribution of the upper layer heat content anomaly during the positive IOD event (Rao et al. 2002). The time series of the DMI is also plotted in Fig. 5a. The significant positive DMI appeared during 1994 and 1997/ 1998, which corresponds to the positive IOD years. The two time series of the DMI and the OFES simulated EKE anomaly indicate in-phase relation with a correlation coefficient of 0.59, which is above the 90% significant level. The DMI leads the eddy activity by 3 months. The above results suggest that the EKE anomaly, which also corresponds to the ISV anomaly, in the SETIO has strong correlation with the IOD event in the tropical Indian Ocean. In general, the cold temperature anomaly off the Sumatra and Java islands associated with the IOD events enhances the meridional temperature gradient in the far eastern region around 12 S, a situation favorable for increasing the eddy activity through the enhanced baroclinic instability. In the following section, we will explore the eddy development and background conditions during the 1994 and 1997/1998 IOD events. 4 Eddy activity during the 1994 IOD event 4.1 Seasonal anomaly and eddy activity in the SETIO Figure 6 shows interannual anomaly of the heat content in the upper 250-m depth averaged in the SETIO ( E, 12 S 3 N) during January 1994 December The negative temperature anomaly in the SETIO associated with the 1994 IOD matured in boreal summer (August), with the largest decreasing of over 2 C. This temperature anomaly extended along the coast of Sumatra, Java and the Lesser Sunda Islands, with upwelling favorable southeasterly wind field over the SETIO (Fig. 6b). The cold anomaly further penetrates eastward into a region around the Timor Island, suggesting coastal Kelvin wave propagation in response to the upwelling favorable wind anomaly along the coasts. Note that the seasonal variation of the upper layer temperature also indicates its minimum in boreal summer due to enhanced seasonal upwelling along the Sumatra and Java coasts (e.g., Qu and Meyers 2005). The seasonal variation together with the interannual anomaly associated Fig. 6 a Time series of the heat content anomaly in the upper 250-m depth averaged in the SETIO ( E, 12 S 3 N) during January 1994 December b Heat content anomaly in the upper 250-m depth (shade) and the surface wind stress anomaly (vectors) averaged from June to August 1994 with the IOD event generated significantly cold condition in the northern part of the SETIO during boreal summer in In this particular case, a ratio of the interannual anomaly to the seasonal variation is about 1.8. Upper layer currents in the SETIO also went through significant changes associated with the aforementioned temperature anomalies. Figure 7b demonstrates a latitudedepth section of zonal current and the temperature across 120 E where the ITF through the Timor Passage connects to the SEC. This area also corresponds to an upstream region of the active intraseasonal eddy activities (see Fig. 3). The thermocline was located around 150-m depth south of 13 S, and it became shallower to the north to about 70 m around 11 S. Strong westward zonal current in the upper layer was observed between 13 S and 11 S, with significant vertical shear near the thermocline depth. The upper layer current structure in the SETIO is affected by transports through several passages connecting the Indonesian Seas and the SETIO. Interannual anomalies of the transport through the Lombok, Ombai and Timor passages and the averaged current shallower than the depth of 100 m are shown in Fig. 7. The averaged upper ocean current into the SETIO through the Timor passage was

7 Ocean Dynamics (2010) 60: (c) (d) Fig. 7 a Time series of the spatially averaged current in the upper 100-m depth through the Lombok Strait, Ombai Passage, and Timor Passage (c) during January 1994 December Thick (thin) line shows monthly averaged (monthly climatological) transport in a c. d Latitude-depth section of zonal current (shade) and temperature (contour) along 120 E averaged from June to August Note that the positive value in a c means the current is toward the Indian Ocean increased about 10 cm/s (corresponding transport of 1 Sv) during boreal summer, while there was no significant increase in the Lombok and Ombai throughflows during the 1994 IOD event, which is consistent with the subsurface temperature field shown in Fig. 6. It is also noted that there were a weak negative subsurface temperature anomaly associated with an increase in the transport across the Timor passage during boreal summer in 1995 when the eddy activity was increased in OFES. Mechanisms responsible for the strong eddy activity in the SETIO can be clarified by considering the EKE equation. Following Masina et al. (1999), the equation is þ~u rke ¼ r ~u 0 p 0 gr 0 w 0 r 0 ~u 0 ~u 0 r~u þ res:; ð2þ where Ke is the EKE, ~u is the horizontal velocity, w is the vertical current, p is the pressure, ρ is the density perturbation, ρ 0 is the mean density, and is the horizontal gradient operator. The residual term, indicated by res. corresponds to the diffusion terms. The overbar indicates low-frequency variation longer than the 96-day period, while the primed variables represent the intraseasonal variations. The second (third) term on the right-hand side represents baroclinic (barotropic) energy conversion. The horizontal distributions of the meridional temperature gradient, as an index of the baroclinicity for the background condition to the perturbation, the baroclinic and barotropic energy conversion terms averaged over upper 250-m depth in the SETIO during 3 months from June to August 1994 are shown in Fig. 8. Superposed on these variables are the distribution of the simulated surface EKE. The strong meridional temperature gradient existed along 12 S, spanning from 110 E to 125 E, with a relatively weaker gradient extending to the west near 95 E. This strong baroclinicity is associated with the anomalous cooling along the coast of the Lesser Sunda Islands and relatively strong Timor throughflow during the second half of This, in turn, enhances the EKE through the baroclinic energy conversion term. The baroclinic energy conversion appeared to be large in a region centered at 112 E, 12 S, with a zonally elongated shape along the SEC, and the region corresponded to the area of the largest EKE. Westward shift of large baroclinic energy conversion to the strong meridional temperature gradient suggests the importance of downstream advection due to the SEC. The barotropic conversion term, on the other hand, was relatively small over the entire region of the SETIO during the 1994 IOD period, suggesting the baroclinic instability in the SEC as a dominant mechanism for the eddy generation. This result is consistent with Feng and Wijffels (2002), but different from Yu and Potemra (2006) in which the barotropic instability near the Lombok Strait was another dominant energy source for the eddy activity. In addition, a canonical linear stability analysis, using a simple continuously stratified model with the mean conditions derived from the OFES results, demonstrates reasonable values of growth rate and period (Table 1; see Appendix 1 for details). The largest growth rate occurred at around 116 E, 12 S during the 1994 IOD, although the

8 724 Ocean Dynamics (2010) 60: (C/deg) Table 1 Maximum growth rate (e-folding time) and period for the most unstable wave derived from a linear stability analysis for the 1994 and 1997/1998 IOD cases Jun Aug1994 Sep Nov1997 (c) (g*cm (g*cm e-folding time (days) Period (days) The values are averaged over the region from 10 S to 14 S and from 115 E to 120 E anomaly associated with the IOD event. To explore this point during the 1994 IOD event, a heat budget analysis is conducted using the following ¼ Q rc @y wt Þþdiff; ð3þ where T is temperature, Q is the heat flux at sea surface, (u, v, w) are the three components of velocity, ρ is the density, C p is the heat capacity under constant pressure, and H is the upper layer thickness, which is assumed to be constant at 250-m depth in the present study. The last term on the righthand side indicates the diffusion term. In this equation, each variable (T, u, v, w, or Q) is separated into three components: the ISV, the climatological seasonal cycle, and the interannual anomaly, as in Eq. 1. Temperature flux can be expressed as: (d) growth rate of about 77 days is somewhat longer than the value given by Feng and Wijffels (2002). 4.2 Heat budget analysis and heat content variation in the SETIO ( C*cm/s) Fig. 8 Horizontal distributions of meridional temperature gradient, magnitude of the barotropic energy conversion term, magnitude of the baroclinic energy conversion term (c), and meridional eddy heat transport (d) averaged over the upper 250-m depth for a period from June to August The EKE averaged over the upper 250-m depth for a period from August to November 1994 is superposed on each panel using solid contours Large eddy activities generate net meridional heat transport and, therefore, should contribute to heat budget in the SETIO, especially in the region of the negative temperature vt ¼ðv ISV þ v clim þ v anom ÞðT ISV þ T clim þ T anom Þ ¼ v ISV T ISV þ...þ v anom T anom : ð4þ The first term on the right-hand side, v ISV T ISV, is defined as eddy heat transport in the following analysis. The horizontal distribution of the eddy heat transport in the SETIO is shown in Fig. 8d, indicating almost the same distribution with that for the baroclinic conversion term. Time series of each term in the thermodynamic Eq. 3, integrated over the upper layer (0- to 250-m depth) in the SETIO ( E, 12 S 3 N), for the 1994 IOD event is shown in Fig. 9a. During the cooling phase (Feb. to Jun. 1994) prior to the IOD event, the heat divergence associated with seasonal temperature variability and interannual current anomaly was dominant. The IOD in 1994 evolved from August to October and the cold temperature condition continued until December. However, in this developing phase (Jul. to Nov. 1994), strong meridional eddy heat transport overwhelmed heat divergence term and contributed to the heat content increase in the SETIO, which prevented the development of the IOD event. There was another positive peak in the rate of time change of the averaged upper layer temperature from December 1994 to February 1995, corresponding to the decay period of the 1994 IOD event. In this period, the

9 Ocean Dynamics (2010) 60: heat transport (PW) v anom *T anom v anom *T clim v clim *T anom v eddy *T eddy dt/dt thermodynamic equation and the other without the eddy heat transport contribution. In boreal summer to fall 1994, the eddy heat transport term significantly reduced the negative heat content anomaly in the SETIO. On the other hand, the estimated heat content anomaly without the eddy heat transport continued to cool until November, and the heat content did not return to its normal condition even at the end of Eddy activity during the 1997/98 IOD event e e+06-2e+06-3e+06 Feb94 Jul94 Dec94 May95 Oct95 hc_seio hc_seio@without_eddy Feb94 Jul94 Dec94 May95 Oct95 Fig. 9 a Time series of each term in the thermodynamic Eq. 3, integrated over the upper layer (0- to 250-m depth) in the SETIO ( E, 12 S 3 N) during December 1993 December b Time series of the heat content anomaly in the upper 250-m depth. The red solid line indicates the estimate from the full contribution of all the terms in the thermodynamic equation, while the blue dashed line shows the estimate without the eddy heat transport contribution. In a, v anom T anom (red), v anom T clim (green), v clim T anom (blue), v ISV T ISV (black), and Q=r 0 c p H (aqua) are shown Another large positive IOD event occurred in late 1997, in conjunction with the El Nino phenomenon in the Pacific Ocean. It is interesting to investigate in detail the relation between the ISV and the interannual variation in the SETIO for this particular event since there are significant differences between the two conditions in 1994 and The time series of the interannual heat content anomaly in the upper 250-m depth averaged over the SETIO region is shown in Fig. 10a. The temperature anomaly decreased from early 1997 and reached its minimum in November with a negative anomaly of about 2 C. Since the seasonal temperature variation takes its coldest phase in August, the total negative temperature anomaly, which created the favorable condition for the generation of unstable distur- ( ) product of the seasonal temperature variation and the interannual current anomaly played a key role, and the eddy heat transport had a weak negative contribution to the warming. The heat budget result reveals that the heat input through the western side (across 95 E) was dominant for the latter warming peak, and especially the eastward current anomalies centered at 5 S, 80-m depth and at 12 S contributed to the heat transport in the SETIO during this period (not shown). Significant influence of the eddy heat transport during the developing phase of the IOD is also demonstrated by two calculations of the heat content variability in the SETIO. Figure 9b exhibits time series of the heat content anomaly in the upper 250-m depth for two cases: one estimated from the full contribution of all the terms in the Fig. 10 Same as in Fig. 6, except for the 1997/1998 IOD event (

10 726 Ocean Dynamics (2010) 60: (c) 1997 event, while the upper ocean current increased about 10 cm/s (corresponding to transport of 1 Sv) after the event. The upper ocean current through the Lombok Strait, on the other hand, strengthened by 40 cm/s (corresponding to transport of 0.5 Sv) during the period from September to December Note that the easterly wind anomaly in the region south of the Java and the Lesser Sunda Islands was also weaker than that during the 1994 IOD event. The above differences in the background conditions between the two IOD events result in a quite different behavior of the eddy activity in the SETIO. Figure 12 shows horizontal distributions of meridional temperature gradient, baroclinic energy conversion rate, and meridional ( C/deg) (d) 2 3 (g*cm /s ) (c) 2 3 (g*cm /s ) Fig. 11 Same as in Fig. 7, except for the 1997/1998 IOD case bances in 1997, was weaker than that in The horizontal distribution of the interannual heat content anomaly in the upper 250-m depth (Fig. 10b) shows the cold region along the southern coasts of Java and Lesser Sunda Islands. However, in 1997, the anomalous temperature extended only to the Lombok Strait, and there was no significant temperature anomaly east of it. The differences between the 1997 case and the 1994 case are also captured by the velocity fields and the transports in the ITF. The time series of the upper ocean averaged current through the Lombok Strait and Timor Sea during 1997/1998 and the latitude-depth section of the zonal current along 120 E averaged from September to December 1997 are also shown in Fig. 11. The Timor transport did not indicate any significant positive anomaly during the (d) ( C*cm/s) Fig. 12 Same as in Fig. 8, except for the 1997/1998 IOD case. The EKE is averaged from November 1997 to February 1998

11 Ocean Dynamics (2010) 60: e e+06 hc_seio -4e+06 Jan97 Jun97 Dec97 Apr98 Sep98 Fig. 13 Same as in Fig. 9b, except for a period from December 1996 to December 1998, covering the 1997/1998 IOD event eddy heat transport in the SETIO during fall and winter 1997 when the meridional eddy kinetic energy anomaly had its largest value (see Fig. 5a). Because of the mismatch in the phase between the seasonal and interannual temperature evolutions, the degree of the baroclinicity was weaker in 1997 compared to that during the 1994 IOD event, which was reflected by a weaker shoaling of the thermocline toward the Indonesian coast and the associated weaker vertical shear of the zonal current east of the Lombok Strait in This caused a reduction of baroclinic energy conversion rate, eddy kinetic energy, and, hence, the meridional eddy heat transport in It should be noted that although the merdional temperature gradient larger than 1 C/deg occupied almost the same area as that for the 1994 case, the region of strong meridional temperature gradient was significantly small in In addition, the linear stability analysis reveals that the largest growth rate during the 1997/1998 IOD event is about half of the one during the 1994 IOD event (Table 1), which is consistent with the disappearance of the region of strong baroclinicity in the SEC. The area-averaged heat content anomaly in the region south of the Java Island during 1997/1998 IOD event is shown in Fig. 13. As expected from the above results, the time series of the heat content anomaly with and without the eddy heat transport term indicates very similar behaviors, suggesting no significant influence of the eddy term on the evolution of the 1997/1998 IOD event. 6 Summary and discussion In this study, relations between mesoscale eddy activity at the intraseasonal timescale and the interannual IOD events in the SETIO are investigated using the results from the corr@eke_seio-timor corr@eke_seio-ult_timor corr@eke_seio-enso corr@eke_seio-iod 90% significant (0.36) lag (month) Fig. 14 Lagged correlation of the EKE anomaly in the upper 250-m depth averaged over S and E, with Timor throughflow anomaly in the layer from the sea surface down to 1,000-m depth (red solid line), with Timor transport in the upper 100-m depth (green dotted line), with the zonal wind anomaly averaged over the central equatorial Pacific (5 S 5 N, 170 E 170 W) as an index for the ENSO (blue dotted line), and with the zonal wind anomaly averaged over the central equatorial Indian Ocean (5 S 5 N, E) as an index for the IOD (purple dotted line). Negative value indicates that each variable leads to the EKE variability. The black dash-dotted lines indicate 90% significant level

12 728 Ocean Dynamics (2010) 60: high-resolution OGCM, OFES. OFES reproduces well the observed ISV in the SETIO and its interannual modulation, with large variances of the ISV during the 1994 and 1997/ 1998 IOD events. During the 1994 IOD event, the large negative temperature anomaly off the south coasts of Java and the Lesser Sunda Islands developed in boreal summer due to phase matching between the seasonal variation and the interannual anomaly. The eastward extension of the cold region and the associated strong Timor throughflow enhanced the baroclinic instability in the upper layer, generating the anomalous mesoscale eddy activity there. The eddy heat transport, in turn, significantly affected the decay of the 1994 IOD event. On the other hand, the development of the cold region off the Java Island associated with the 1997/1998 IOD event occurred in boreal winter, and the weaker Timor throughflow also prevented large eddy activity off Java. This resulted in little influence of the eddy heat transport on the heat budget in the SETIO for the 1997/1998 IOD event. In Section 3, we show that the interannual modulation of the eddy activity in the SETIO has large correlation with the IOD events and also with the upper layer Timor throughflow. Potemra and Schneider (2007), however, demonstrated that the interannual variability of the ITF transport anomaly in the thermocline layer (between 100 and 500 m) is controlled by divergent winds over the equatorial Pacific and Indian Ocean, suggesting the importance of the variability not only in the Indian Ocean but also in the Pacific Ocean. To explore the possibility of such influence from the Pacific side on the ISV in the SETIO, we calculated lagged correlation between the EKE in the SETIO and indices for the IOD, ENSO, and ITF anomalies (Fig. 14). As expected from the above results, correlation coefficients with the upper layer Timor throughflow anomaly and with the IOD index show its largest minimum correlation with a negative lag of a few months. On the other hand, the correlation coefficient with the ENSO index is almost zero for any of the negative lags, suggesting no direct influence of the ENSO events on the ISV in the SETIO. However, the depth-integrated net transport through the Timor Passage indicates relatively large negative correlation with the eddy activities at a 6-month lag, suggesting that signals in the Pacific Ocean may affect the conditions in the SETIO through changes in the net ITF transport. Appendix 1: Linear stability analysis Governing equations for the linear stability analysis in a quasi-geostrophic potential vorticity framework is þ f 2 2 f 2 U N 2 @z 2 y N 2 ¼ 0; ð5þ ð6þ where U is the mean zonal velocity, f is the Coriolis parameter, N 2 is the Blant Visala frequency, β is the planetary beta term, and y is the stream function. With y ¼ fðzþ cosðpy=2lþexp½ikðx ctþš, Eqs. 5 and 6 are replaced by the equations of fðzþ, ðu cþ k 2 p2 4L 2 þ f 2 N 2 þ 2 f 2 U N 2 fðzþ ð8þ fðzþ fðzþ ¼0; ð7þ For the stability analysis, U(z) and N 2 (z) are derived from the OFES output averaged from 10 S to 14 S, f= s 1, β= m 1 s 1, and meridional scale of eddy L=500 km is assumed. The lower boundary is set at the 750-m depth. Eigenvalues (phase velocity) c are numerically solved for each zonal wavenumber k using an exact diagonalization method by the Linear Algebra PACKage (LAPACK). Acknowledgments We thank two anonymous reviewers for the helpful comments, which greatly improved our original manuscript. We also would like to appreciate suggestions and English editing by Dr. Zuojun Yu, which were very important for the completion of this paper. Dr. Hideharu Sasaki (JAMSTEC/ESC) provided us outputs of OFES runs, and this thesis could not be accomplished without his help. Comments and encouragements by Prof. Kelvin Richards, Prof. Shang-Ping Xie, Prof. Kirk Bryan, Prof. Yukari Takayabu, and Dr. Susan Wijffels were essential to improve this study. Appendix 2: Sensitivity of heat budget analysis to the reference depth In Sections 4.2 and 5, the heat budget analyses in the SETIO are conducted with a bottom of the reference box at 250-m depth. Here, we show influence of this reference depth on the heat budget analysis.

13 Ocean Dynamics (2010) 60: Fig. 15 Time-depth section of the temperature anomaly averaged in the SETIO (12 S 3 N, E). The 20 C isotherm depth (mixed-layer depth) is superposed as a thin line (thick line) ( Time series of the 20 C isotherm depth, the mixed-layer depth, and vertical profiles of the temperature anomaly averaged in the SETIO ( E, 12 S 3 N) are shown in Fig. 15. Large temperature anomaly variation is located between 50- and 150-m depths, and the 20 C isotherm is located around 100-m depth. Furthermore, the mixed-layer thickness is located near m, which is much shallower than the depth of 20 C isotherm. Significantly cold temperature anomalies correspond to the positive IOD events which appeared in 1994 and The 20 C isotherm was shoaling in these periods. All these variabilities occurred in the upper layer shallower than the depth of 200 m. Ratio of time change of the heat content in the upper 50, 100, 150, 200, and 250 m from 1994 to 1998 are shown in Fig. 16. While the time series for the boxes shallower than 50 and 100 m underestimated the tendency term, the ones for the boxes deeper than 150 m showed almost the same amplitude. A much deeper reference depth would provide the same result with the one with using the 250-m reference depth since all the major variability is already included. (PW) dt/dt (50m) dt/dt (100m) dt/dt (150m) dt/dt (200m) dt/dt (250m) Feb94 Dec94 Oct95 Aug96 Jun97 Apr98 Fig. 16 Time series of tendency of the heat content anomaly averaged over the box defined as Fig. 9b, except the reference depth is changed to from 50 to 250 m This result justifies the selection of the bottom of the box at any depth deeper than 150 m in the OFES results. References Boyer TP, Levitus S, Antonov JI, Conkright ME, O Brien T, Stephens C, Trotsenko B (1998) World Ocean Atlas 1998, vol 6: salinity of the Indian Ocean, NOAA Atlas NESDIS 32. US Government Printing Office, Washington Ducet N, Le Traon PY, Reverdin G (2000) Global high-resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2. J Geophys Res 105: Feng M, Wijffels S (2002) Intraseasonal variability in the South Equatorial Current of the East Indian Ocean. J Phys Oceanogr 32: Iskandar I (2007) Intraseasonal oceanic variations in the Southern Tropical Indian Ocean. PhD thesis, The University of Tokyo, p 128 Iskandar I, Mardiansyah W, Masumoto Y, Yamagata T (2005) Intraseasonal Kelvin waves along the southern coast of Sumatra and Java. J Geophys Res 110:C doi: /2004jc Kalnay E, Kanamitsu M, Kistler R, Collins W, Deaven D, Gandin L, Iredell M, Saha S, White G, Woollen J, Zhu Y, Leetmaa A, Reynolds R, Chelliah M, Ebisuzaki W, Higgins W, Janowiak J, Mo K, Ropelewski C, Wang J, Jenne R, Joseph D (1996) The NCEP/NCAR 40-year reanalysis project. Bull Amer Meteor Soc 77: Large WG, McWilliams JC, Doney SC (1994) Oceanic vertical mixing a review and a model with a nonlocal boundary layer parameterization. Rev Geophys 32: Masina S, Philander G, Bush A (1999) An analysis of tropical instability waves in a numerical model of the Pacific Ocean. Part II: generation and energetics. J Geophys Res 104(29): Masumoto Y (2002) Effects of interannual variability in the eastern Indian Ocean on the Indonesian throughflow. J Oceanogr 58: Masumoto Y, Meyers G (1998) Forced Rossby waves in the southern tropical Indian Ocean. J Geophys Res 103(27): Masumoto Y, Sasaki H, Kagimoto T, Komori N, Ishida A, Sasai Y, Miyama T, Motoi T, Mitsudera H, Takahashi K, Sakuma H, Yamagata T (2004) A fifty-year eddy-resolving simulation of the world ocean: preliminary outcomes of OFES (OGCM for the Earth Simulator). J Earth Simulator 1:35 56 Meyers G (1996) Variation of Indonesian throughflow and ENSO. J Geophys Res 101:

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