ARTICLE IN PRESS Deep-Sea Research I 51 (2004) 2123–2136 www.elsevier.com/locate/dsr The Rossby wave as a key mechanism of Indian Ocean climate variability Mark R. Jurya, Bohua Huangb aEnvironmental Science Department, Center for Environmental Studies, University of Zululand, KwaDlangezwa 3886, South Africa bCenter for Ocean Land Atmosphere Studies, Maryland, USA Received 18 November 2003; received in revised form 3 June 2004; accepted 3 June 2004 Available online 7 October 2004 Abstract We analyze the time-longitude structure of composite cases from model-assimilated ocean data in the period 1958–1998, following on from earlier work by Huang and Kinter (J. Geophys. Res. 107(C11) (2002) 3199) that studied east–west thermocline variability in the Indian Ocean. Our analysis focuses on the Rossby wave signal along the thermocline ridge in the tropical SW Indian Ocean (101S, 60–801E), where wind stress curl is important. Anomalous winds in the equatorial east Indian Ocean force successive Rossby waves westward at speeds of 0.1 m sÀ1730%. With a wavelength of 7000 km, the period of oscillation is in the range 1.9–5.2 years. The Indian Ocean Rossby wave is partially resonant with the global influence of the El Nino–Southern Oscillation, except during quasi-biennial rhythm. The presence of the Rossby wave offers potential predictability for east–west atmospheric circulation systems and climate that affect resources in countries surrounding the Indian Ocean. r 2004 Elsevier Ltd. All rights reserved. 1. Introduction Somalia and surface cooling off Madagascar. A zone of maximum convection occurs over the The west Indian Ocean (WIO) is a climatically eastern warm pool (51N–101S, 70–1001E) with important region, recording on average 10 tropical precipitation rates48 mm dayÀ1. Zonal winds in cyclones each year between December and March. the tropical Indian Ocean are weak within the These storms often bring devastating conse- equatorial wave-guide because of the prevailing quences to the Mascarene Islands, Madagascar meridional flow of the monsoon. The equatorial and Mozambique (Naeraa and Jury, 1998). The thermocline lies around 120 m depth and exhibits mean sea surface temperature (SST) in the WIO little east–west slope in contrast with the Pacific varies between 26 and 28 1C, and is cooler than the and Atlantic Oceans (Xie et al., 2002). The mean eastern Indian Ocean, due to upwelling off structure of the thermocline is a ridge–trough pair at 71Sand201S, respectively, with mean tempera- E-mail address: [email protected] (M.R. Jury). tures of 18 and 22 1C for the upper 234 m and 0967-0637/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr.2004.06.005 ARTICLE IN PRESS 2124 M.R. Jury, B. Huang / Deep-Sea Research I 51 (2004) 2123–2136 thermocline depths of 80 and 180 m, respectively. (Murtugudde et al., 2000; Behera and Yamagata, Due to the lack of a zonal gradient in the 2001; Xie et al., 2002). equatorial zone, it was thought that the Indian In the present study, we further investigate the Ocean may not have the appropriate features to mechanisms of ocean variability using model-assimi- develop its own El Nino–Southern Oscillation lated data, following Huang and Kinter (2002).Of (ENSO)-like interannual variability (Latif and particular interest is the nature of sub-tropical Barnett, 1995). Rossbywavesandtheirimpactontheclimate Yet, east–west gradients in the thermocline can system. Toward this end, we analyze in situ ocean develop at times (Hastenrath et al., 1993; Webster data assimilated by a numerical model (Schneider et al., 1999; Murtugudde et al., 2000; Ueda and et al., 1999). Key issues to be addressed include: the Matsumoto, 2000), with atmospheric circulation time-longitude structure of Rossby waves, the and convection varying in a manner consistent characteristics of phase propagation, preferred loca- with Bjerknes (1969) feedback. At the onset of a tions for wave amplification, and the extent of Pacific El Nino event as atmospheric convection coupling with the atmosphere. We conclude that shifts eastward from the warm pool, anomalous much of ocean variability comprising the east–west easterly winds develop near the equator in the east dipole in the Indian Ocean can be attributed to an Indian Ocean. These generate a gradual warming off-equatorial ocean Rossby wave that propagates to the west (Nigam and Shen, 1993; Klein et al., slowly from the east following a surge of wind at the 1999; Lau and Nath, 2000) that will be further onsetofmostElNinoevents.Thepaperisorganized explored here. into sections on data and methods, results sub- Although surface fluxes explain changes in SST divided into wave character and zonal phase over much of the tropical Indian Ocean, this is less propagation (hovmoller analysis), and discussion. so in the thermocline ridge (5–121S, 55–851E) according to the results of Klein et al. (1999), Lau and Nath (2000). Ocean model results 2. Data and methods (Murtugudde and Busalacchi, 1999; Murtugudde et al., 2000) and analysis of sea surface height Upper ocean heat content fields (mean tempera- measurements (Chambers et al., 1999) suggest that ture in the top 234 m) for the period 1958–1998 dynamic processes contribute to SST variability in have been derived from an ocean data assimilation the tropical Indian Ocean. Huang and Kinter system operated at COLA as outlined in Huang (2000, 2001, 2002) have used upper ocean heat and Kinter (2002), hereafter HK02. The data content from data-assimilated ocean analyses to assimilation uses a variational scheme (Derber and identify the westward propagating Rossby wave as Rosati, 1989) to combine temperature observa- a key feature of the near-continuous interannual tions in an ocean general circulation model oscillation of the upper tropical Indian Ocean (oGCM). The analysis uses all observations since the late 1950s. This result substantiated the available in a moving 10-day assimilation window. finding from the TOPEX/Poseidon altimeter The observations are inserted into the first guess measurements during the 1990s. Observational field of the oGCM and discrepancies are iteratively studies of warm and cool events (Reason et al., minimized. The oGCM is a version of the GFDL 2000; Jury et al., 2002) demonstrate that local heat modular ocean model (Huang and Schneider, and radiative fluxes, and vertical entrainment from 1995; Schneider et al., 1999; Huang et al., 2002). wind stress curl account for a significant portion of It is forced by monthly averaged surface wind SST variance during onset phase. However, the stress from NCEP reanalysis available at an surface heat fluxes do not fully explain the irregular grid with a variable resolution of around variations of SST there from fall to next spring. 21 near the equator. The solar flux is prescribed The lack of closure in the budget for local SST (Oberhuber, 1988), surface heat and long-wave changes in the WIO may be attributed to fluxes are parameterized (Philander et al., 1987; horizontal advection via incoming Rossby waves Rosatiand Miyakoda,1988). ARTICLE IN PRESS M.R. Jury, B. Huang / Deep-Sea Research I 51 (2004) 2123–2136 2125 Ocean temperature observations are assimilated more observations along key lines from Perth to from in situ SST measurements from the COADS the NW, from Madagascar to the NE, from archive (Slutz et al., 1985), temperature profile Mauritius to the north, and from Mombassa to (XBT, MBT, CTD, etc.) measurements (Conk- the east (Masumoto and Meyers, 1998), which are right et al., 1998) and, since 1981, with satellite- important for our present study. However, some blended weekly SST fields (Reynolds and Smith, of these, especially in the South Indian Ocean, 1994). The number of temperature profiles is were established with TOGA in 1985. generally 3000–9000 per year within the square Fig. 1a and b shows the spatial distributions of between 301Eand1201E, 301S and 301N during the temperature profiles per degree-square for the the 41 years (see Fig. 1 a and b). In 1967, 1968, and years 1958–1980 and 1981–1998. It is evident that 1979, it surpassed 15000 profiles. In situ SST measurements on major ship tracks were enhanced measurements exceeded 90,000 per year, reaching in the later period in the South Indian Ocean. Even a maximum of 270,000 per year. However, many after this enhancement, the distribution of the sub- of these observations, especially the sub-surface surface temperature observations was still sparse data, are concentrated along the coasts. There are and may be wider than the typical zonal decorr- elation scale in the equatorial ocean (Smith, 1995). Therefore, the assimilated product is inevitably Distribution of Temperature Profiles strongly affected by the ocean model and the Profile Number, 1958-1980 30N surface forcing fields. The model-derived results 25N are considered in this light and we therefore 20N employ analysis techniques that highlight repeti- 15N tious patterns (e.g., EOF, composite). 10N The anomalies of model-analyzed upper ocean 5N 5000 heat content are highly correlated with the EQ 5S 1000 independently derived sea surface height anoma- lies from the TOPEX/Poseidon satellite altimeter 10S 500 15S measurements (HK02) using monthly data. It 20S 300 suggests that the assimilated temperatures are 25S 250 useful for analyzing the interannual variability in 30S 30E 40E 50E 60E 70E 80E 90E 100E 110E this region. One may attribute this high consis- (a) 200 tency to newly established ship data in recent 150 years. However, our EOF analysis of the heat Profile Number, 1981-1998 100 content anomalies (HCa) in different periods 30N 25N 75 shows very similar patterns. We believe that this data set is superior to an oGCM simulation with 20N 50 15N observed surface forcing only, because of insertion 10N 30 of the oceanographic observations.