Is a global warming signature emerging in the tropical Pacific?

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 39,, doi: /2011gl050232, 2012 Is a global warming signature emerging in the tropical Pacific? K. Ashok, 1 T. P. Sabin, 1 P. Swapna, 1 and R. G. Murtugudde 2 Received 3 November 2011; revised 6 December 2011; accepted 8 December 2011; published 18 January [1] The tropical pacific experienced a hitherto-unseen anomalous basinwide warming from May 2009 through April 2010 with the maximum warming to the east of the dateline, but for a weak anomalous cooling west of 140 E after early boreal fall. Our observed analysis and model experiments isolate the potential teleconnections from TP during the summer of Further, we show through an empirical orthogonal function analysis of the tropical Pacific SSTA that the anomalous conditions in TP during this period could have manifested as a canonical El Niño, but for a slowly intensifying background west east gradient. This zonal SST gradient is subject to an increasing trend associated with global warming. A possible implication is that any further increase in global warming may result in more basinwide warm events in place of canonical El Niños, along with the occurrence of more intense La Niñas and El Niño Modokis. Citation: Ashok, K., T. P. Sabin, P. Swapna, and R. G. Murtugudde (2012), Is a global warming signature emerging in the tropical Pacific?, Geophys. Res. Lett., 39,, doi: /2011gl Introduction [2] The tropical Pacific (henceforth TP) experienced, an anomalous basinwide warming ( com/en/news/idm/2009/nov-2009-if-it-s-not-el-nino-then-itmust-be-his-brother/index.html) [Ratnam et al., 2012] from May 2009 through April 2010 with the maximum warming to the east of the dateline (Figures 1a and 2a), except for a weak anomalous cooling west of 140 E after September In conjunction with the unique basinwide TP anomalous warming seen during June-September 2009, the east west gradient of the sea level pressure (SLP), a common measure of ENSO activity [Power and Kociuba, 2010], was significantly weaker in magnitude (16% of its standard deviation) than all other El Niño and La Niña events of the last 6 decades (Figure 3a). This SLP gradient outlier condition, and the associated anomalous upward motion everywhere in the TP except to the west of 125 E [Ratnam et al., 2010], are in accordance with an anomalously weak Walker circulation, which is also observed in recent years and attributed to global warming [Vecchi et al., 2006; Zhang et al., 2010]. The negative SLP anomaly in the tropical western Pacific (not shown) is apparently associated with unusually high seasonal mean SST of 29 C and above (not shown) in the western TP, well above the threshold of 27 C necessary to trigger convection. The unique SSTA (Figure 2a) and the weakened east west SLP gradient (Figure 3a) in the Pacific during the boreal 1 Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune, India. 2 Earth Sciences Interdisciplinary Center, University of Maryland, College Park, Maryland, USA. Copyright 2012 by the American Geophysical Union /12/2011GL summer and early fall of 2009 were associated with above normal rainfall in the TP (Figure 4a). On the other hand, most of the western Pacific region from Indonesia through central Japan, except the Philippines and some portions of the East Asian coast, experienced anomalously deficit rainfall. The India Meteorological Department reported a severe drought of 22% below the normal seasonal rainfall, with a massive deficit of 48% in June itself. Mexico experienced its worst drought in 70 years. The US Drought Monitor declared exceptional drought in 17 southern states. While regions in the tropical Africa suffered anomalous dry conditions, the southern region and northern portions received surplus rains. Furthermore, an intense heat wave during June (>40 C) across India resulted in nearly 100 fatalities; Britain experienced its warmest heat wave in three years at the beginning of July ( In addition, according to the Bureau of Meteorology, August 2009 was Australia s warmest in long-term records, with 2.47 C above average. The distinct rainfall and temperature anomalies around the globe during the 2009 summer may have some association with the concurrent basinwide warming in the TP as suggested by results from our atmospheric general circulation model experiments (Figure 4b; see sections 2 and 3 for details). [3] In this letter, we explore the potential mechanisms behind the unprecedented basinwide warming during June September The next section describes the data sets, atmospheric general circulation model experiments carried out, and other methodology used. The sections 3 and 4 describe the model results, and the evolution of the 2009 TP conditions and the role of background changes, respectively. We present the conclusions and discuss the implications in section Data, Model, and Methodology [4] Gridded monthly rainfall data from the Global Precipitation Climatology Project (GPCP) Version 2 Combination data [Adler et al., 2003], monthly Hadley Centre Global Sea Surface Temperature (HadISST) data [Rayner et al., 2003], and the daily National Center for Environmental Prediction and National Center for Atmospheric Research (NCEP/ NCAR) reanalysis [Kalnay et al., 1996] circulation fields, all principally for the post-satellite vintage are used in this study. Further, the weekly Optimally Interpolated Sea Surface Temperature (OISST) data [Reynolds et al., 2002] from January 1982 to 2010 were used to diagnose the submonthly evolution. We also use the long-term ( ) monthly HadISST data to examine the long-term trend in the eastern TP SST. In addition, we use the ocean heat content anomalies, thermocline depth (D20) and vertical temperature gradient (dt/dz) are computed from Global Ocean Data Assimilation System (GODAS) reanalysis data for the period of 1980 to 2010, and the monthly NOAA Climate Prediction 1of5

2 Figure 1. (a) SSTA ( C, contours) and heat content anomalies (10 9 Jm 2, shaded) and (b) anomalous winds (ms 1 )at 850 hpa averaged over 5 S to 5 N thermocline depth (m) anomalies averaged over 3 N to 6 N. (c) Similar to Figure 1b but averaged over 120 E 130 E. Centre Southern Oscillation Index (SOI; ncep.noaa.gov/data/indices/soi) for the period of 1951 to Any summer for which the 5-month running mean of monthly NINO3 Index ( indices/wksst.for) exceeds (falls below) 0.5 C throughout the June September, is cataloged as an El Niño (La Niña). The monthly NINO3 index is as an area average of the monthly SSTA over (5 S 5 N, 150 W 90 W). Global surface air temperature anomalies [Hansen et al., 2006] are obtained from for the period of 1880 to [5] In this study, we have used the LMDZ4 atmospheric general circulation model. This model, with standard physics packages such as the moist convection scheme [Emanuel 1993], reproduces the mean climatology of tropical summer precipitation satisfactorily (not shown). The model has a horizontal resolution of with 39 vertical levels. Further detailed description is provided by Hourdin et al. [2006, and references therein]. We have carried out two sets of experiments (following Ashok et al. [2004, 2009]), each with five ensembles with perturbed initial conditions, starting from January 1 and lasting for 9 months. In the control experiment, the monthly climatological HadISST ( ) are used as the lower boundary condition. In the second experiment, we impose the monthly June September SSTA (Figure 2a) on the climatological SSTA only in the tropical Pacific region (110E 90W, 15S 15N ) to obtain the lower boundary SST forcing. The seasonal ensemble mean differences in climate variables between these two experiments can be interpreted, from linear theory, as the responses induced by the 2009 boreal summer TP SSTA. Figure 2. (a) Observed SST anomalies ( C) for summer (JJAS) (b) Reconstructed summer 2009 SST anomaly from EOF1 and EOF3 of TP for the period (c) EOF1 of TP SST anomalies and (d) same as Figure 2c but for the EOF3. 2of5

3 Figure 3. (a) June September (JJAS) SOI index during various El Niño and La Niña, ranked from the lowest to the highest (from left to right) during the period of 1951 to (b) Time series of the principal components of TP SSTA variability from an EOF analysis for the period (PC2 in bar, PC1 in green to be multiplied by 1, and PC3 in red). (c) PC2 of TP from another EOF analysis of the TP SSTA for the period (green; scale on left axis), and time series of annual mean global air temperature (red; scale on right axis) and for the same period. We repeat the time series of PC3 of TP SSTA from Figure 3b (blue; scale on left axis), for the purpose of comparison. [6] We have carried out an Empirical Orthogonal Function (EOF) analysis [Bretherton et al., 1992] to decompose the dominant modes of the TP SST anomaly over a region of 110 E to 290 E and 30 S to 30 N. In addition, correlation and trend analyses have also been carried out. 3. The Tropical Pacific SST Teleconnections During June September 2009 [7] Figure 4b depicts the simulated summer monsoon rainfall anomaly distribution. Most features of the observed rainfall features in summer JJAS 2009 (Figure 4a), such as the anomalous precipitation over the entire equatorial Pacific Ocean, are well captured. The observed anomalous negative rainfall (Figure 4a) over the southern and north central India, southern Mexico, central and south central Australian continent and eastern African continent are reproduced (Figure 4b). The simulations also reasonably capture the observed excess rainfall over northern Japan, Tasmania Island and southern New Zealand, notwithstanding a few unrealistic signals in mid-latitudes pockets such as the southern Australian continent. The Model is able to reproduce most of the observed surface temperature anomalies (Figures S3a and S3b in the auxiliary material). 1 This experiment indicates that the anomalous rainfall and temperature signatures during the June September 2009, may be due to the concurrent anomalous basinwide warming of the TP. 4. Anomalous Evolution of the Tropical Pacific During 2009 and Potential Role of Global Warming [8] The zonal distribution of the anomalous SSTA shows (Figure 1a) that the equatorial western Pacific was anomalously warm by about 2 C during the first four months of 2009, and was flanked by an anomalous cooling in the equatorial eastern Pacific. In addition, anomalous easterlies were overlaid almost everywhere in the TP (Figure 1a) and the 20 C isotherm in the equatorial western Pacific was anomalously deep by m (not shown). All the features indicate the typical atmosphere-ocean coupling at interannual timescale. Subsequently, at the end of April/beginning of May an anomalous intraseasonal westerly wind (not shown) burst in the western Pacific apparently triggered a 1 Auxiliary materials are available in the HTML. doi: / 2011GL of5

4 Figure 4. (a) Anomalies of observed JJAS rainfall (mm/ day). (b) Same as Figure 4a, but from model simulations. Shaded regions in Figure 4b represent rainfall values significant at the 80% confidence level from a two tailed t test. downwelling Kelvin wave (Figure 1a) that deepened the thermocline in the equatorial central and eastern Pacific and increased the heat content to induce anomalously warm conditions (Figures 1a and 2a), causing positive SSTA there. However, these anomalous winds did not introduce anomalously cool negative SSTA in the western pacific nor did the thermocline therein change significantly from its climatological position in the west (Figure 1a), as it happens in typical El Niños (e.g., 1997) or during strong El Niño Modokis such as in 2004, (Figures S1a and S1b). Further, the vertical gradient of ocean temperature in the equatorial Pacific (Figure S2a) also indicates near-normal stratification in the west ruling out significant turbulent kinetic energy or vertical turbulent mixing [McPhaden, 1999]. The observed anomalous basinwide warming during the boreal summer of 2009 is apparently associated with a marginal flattened thermocline in the equatorial Pacific (Figure S2a). In fact, despite several intraseasonal westerly wind bursts that occurred over the western equatorial Pacific in the following few months (Figure 1a), we see basinwide positive SSTA in the equatorial Pacific with maximum in the eastern Pacific through November (see Figure 1a). Only by the late winter of 2009 do we see an El Niño Modoki signature in SSTA [Lee and McPhaden, 2010; Ratnam et al., 2010, 2012], with the maximum warming in the central TP and a marginal anomalous shoaling of the thermocline in the west (Figure 1a). In addition, a preliminary examination of the anomalous airsea fluxes for the June September period of 2009 shows a predominant role of atmospheric processes (see section S1 of Text S1). [9] In summary, these distinct SSTAs during the boreal summer and fall of 2009 were neither El Niño-like nor resemble a typical El Niño Modoki; also, despite the warm SSTA, we do not find the typical equatorial coupled dynamics associated with the basinwide TP SST variability. The atmospheric processes apparently played a dominant role, instead [e.g., Clement et al., 2011; Dommenget, 2010]. Theresultsare in line with recent studies [Ashok et al. 2007; Ashok and Yamagata, 2009; Kug et al., 2009; Yu et al., 2010],which demonstrate the increasing importance of processes such as of slow background changes, thermodynamics and subtropical processes, etc., in the recent evolution of the TP events. [10] The gravest two modes from the EOF analysis on the TP SSTA for the (Figure 2c for EOF1; figure not presented for EOF2) are found to be associated with canonical ENSO and ENSO Modoki, respectively [Ashok et al., 2007, Figures 2a and 2b], and explain 44% and 12% of the tropical SSTA variance respectively, in agreement with the recent findings. The EOF3 pattern (Figure 2d), explaining 7%, represents a warming of the entire TP except in the east, and has maximum loadings in the central TP with a positive trend in its principal component PC3 (Figure 3b). The trend is significant at 95% from a 2-tailed Student s t-test, after accounting for a decorrelation scale of 109, which gives 3 degrees of freedom. The EOF3 pattern, including the potential for a relatively small warming or even a cooling in the east, is consistent with the global warming signal in the tropics [Karnauskas et al., 2009; Zhang et al., 2010], as expected from the ocean dynamic thermostat [Cane et al., 1997; Clement et al., 1996; Seager and Murtugudde, 1997] hypothesis. The PC3 shows a very strong correlation of 0.85 with the time series of globally averaged temperature [Hansen et al., 2006] for the period (Figure 3c), which is statistically significant at 99% confidence level from a Student s t-test. This confirms that the EOF3 type of SSTA signal represents global warming in the TP (see auxiliary material for further details). However, the increasing trend associated with the global temperature since mid-1970s is largely associated with the anthropogenic activities [Meehl et al., 2007]. Hence, due to the strong correlation of the PC3 with the global temperature, it can be concluded that the TP SSTA trend is largely due to global warming. The increasing trend in the PC3 (Figure 3b) particularly reflects the changes associated with the climate regime shifts in the mid-1970s and later in the late 1990s; since 1998, it is at its peak phase. [11] The TP SSTA during the boreal summer of 2009 (Figure 2a) can be reasonably reconstructed (Figure 2b) through contributions from the EOF1 and EOF3 (Figures 2c and 2d). The reconstruction indicates that, but for this trend associated with the global warming in the TP, which acquired substantial amplitude by 2009, the TP would most likely have witnessed a canonical El Niño such as those during 1982 and 1997, with anomalous warming (cooling) east (west) of dateline. A similar reconstruction demonstrated that the trend associated with the EOF3 significantly contributed to strength of the 2004 El Niño Modoki [Ashok et al., 2007]. We further verified that, but for the contributions from the increasing trend of PC3, the TP SSTA in the boreal winters of 2009 and 2002 could have been like a conventional El Niño (not shown), with the maximum warming in the east. Interestingly, the TP SSTA in boreal summer of 2006 was also positive almost everywhere except west of 140 E. However, the SSTA magnitudes were substantially weak. 5. Concluding Remarks [12] Using observational and reanalysis datasets, we demonstrate that the unprecedented basinwide warming in the TP 4of5

5 during the 2009 was associated with increasing global warming trend in the TP SSTs. There is a debate [Bunge and Clarke, 2009; Deser et al., 2010; DiNezio et al., 2010] about the phase of the SST trend in eastern TP due to differences in some datasets when longer period data from late 19 century are concerned. However, for the study period that involves satellite data assimilation, we found that the pattern (Figure 1a) is qualitatively robust to the choice of the SST datasets (not shown). [13] Based on our EOF analysis of the TP SSTA, we can hypothesize that any further increase in global warming in the TP may result in more basinwide warm events in place of canonical El Niños, along with the occurrence of more intense La Niñas and El Niño Modokis. This speculation is subject to, among other issues, the linear methodology used. Interestingly, global warming scenarios from several IPCC models indicate a basinwide warming in the TP [e.g. Turner et al., 2007; DiNezio et al., 2009, Kug et al., 2010]. However, while Kug et al. [2010] indicate deepening (shoaling) of the thermocline in the eastern (western) TP associated with such basinwide warming, DiNezio et al. [2009, 2010] associate it with shoaling and flattening of the equatorial thermocline. Further research is necessary to confirm this hypothesis. [14] Acknowledgments. We thank two anonymous reviewers for the constructive comments, Dr. Jean-Louis Dufresne of LMD Paris for providing the LMDZ model. Drs. R. Krishnan, Pascal Terray, and Vinu Valsala for useful discussions, and Prof B.N Goswami, Director IITM for his support and encouragement. IITM is fully funded by the MoES, Govt. of India. RM acknowledges NASA PO grants and support by the Divecha Center for Climate Change, IISc, Bangalore. [15] The Editor thanks the anonymous reviewer for assistance in reviewing this paper. References Adler, R. F., et al. (2003), The version-2 Global Precipitation Climatology Project (GPCP) monthly precipitation analysis (1979 present), J. Hydrometeorol., 4, , doi: / (2003)004<1147: TVGPCP>2.0.CO;2. Ashok, K., and T. Yamagata (2009), Climate change: The El Niño with a difference, Nature, 461, , doi: /461481a. Ashok, K., Z. Guan, N. H. Saji, and T. Yamagata (2004), Individual and combined influences of the ENSO and Indian Ocean dipole on the Indian summer monsoon, J. Clim., 17, , doi: / (2004)017<3141:IACIOE>2.0.CO;2. Ashok, K., S. K. Behera, S. A. Rao, H. Weng, and T. Yamagata (2007), El Niño Modoki and its possible teleconnection, J. Geophys. Res., 112, C11007, doi: /2006jc Ashok, K., S. Iizuka, S. A. Rao, N. H. Saji, and W. J. Lee (2009), Processes and boreal summer impacts of the 2004 El Niño Modoki: An AGCM study, Geophys. Res. Lett., 36, L04703, doi: /2008gl Bretherton, C. S., C. Smith, and J. M. Wallace (1992), An intercomparison of methods for finding coupled patterns in climate data, J. Clim., 5, , doi: / (1992)005<0541:aiomff>2.0.co;2. Bunge, L., and A. J. Clarke (2009), Verified estimation of the El Niño index Niño-3.4 since 1877, J. Clim., 22, , doi: / 2009JCLI Cane, M. A., A. C. Clement, A. Kalpan, Y. Kushnir, D. Pozdnyakov, R. Seager, S. E. Zebiak, and R. Murtugudde (1997), Twentieth-century sea surface temperature trends, Science, 275, , doi: / science Clement, A. C., R. Seager, M. A. Cane, and S. E. Zebiak (1996), An ocean dynamical thermostat, J. Clim., 9, , doi: / (1996)009<2190:AODT>2.0.CO;2. Clement, A. C., P. DiNezio, and C. Deser (2011), Rethinking the ocean s role in the Southern Oscillation, J. Clim., 24, , doi: / 2011JCLI Deser, C., A. S. Phillips, and M. A. Alexander (2010), Twentieth century tropical sea surface temperature trends revisited, Geophys. Res. Lett., 37, L10701, doi: /2010gl DiNezio, P., A. C. Clement, G. A. Vecchi, B. J. Soden, B. Kirtman, and S. K. Lee (2009), Climate response of the equatorial Pacific to global warming, J. Clim., 22, , doi: /2009jcli DiNezio, P., A. C. Clement, and G. A. Vecchi (2010), Reconciling differing views of tropical Pacific climate change, Eos Trans. AGU, 91(16), 141, doi: /2010eo Dommenget, D. (2010), The slab ocean El Niño, Geophys. Res. Lett., 37, L20701, doi: /2010gl Emanuel, K. A. (1993), A cumulus representation based on the episodic mixing model: The importance of mixing and microphysics in predicting humidity, Am. Meteorol. Soc. Meteorol. Monogr., 24, Hansen, J., et al. (2006), Global temperature change, Proc. Natl. Acad. Sci. U. S. A., 103, 14,288 14,293, doi: /pnas Hourdin, F., et al. (2006), The LMDZ4 general circulation model: Climate performance and sensitivity to parameterized physics with emphasis on tropical convection, Clim. Dyn., 27, , doi: /s Kalnay, E., et al. (1996), The NCEP/NCAR 40-year reanalysis project, Bull. Am. Meteorol. Soc., 77, , doi: / (1996) 077<0437:TNYRP>2.0.CO;2. Karnauskas, K. B., R. Seager, A. Kaplan, Y. Kushnir, and M. A. Cane (2009), Observed strengthening of the zonal sea surface temperature gradient across the equatorial Pacific Ocean, J. Clim., 22, , doi: /2009jcli Kug, J. S., F. F. Jin, and S. I. An (2009), Two types of El Niño events: Cold tongue El Niño and warm pool El Niño, J. Clim., 22, , doi: /2008jcli Kug, J.-S., et al. (2010), A possible mechanism for El Nino like warming in response to the future greenhouse warming, Int. J. Climatol., 31, , doi: /joc.2163 Lee, T., and M. J. McPhaden (2010), Increasing intensity of El Niño in the central-equatorial Pacific, Geophys. Res. Lett., 37, L14603, doi: / 2010GL McPhaden, M. J. (1999), Genesis and evolution of the El Niño, Science, 283, , doi: /science Meehl, G. A., et al. (2007), Global climate projections, in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, edited by S. Solomon et al., pp , Cambridge Univ. Press, Cambridge, U. K. Power, S. B., and D. Kociuba (2010), Impact of global warming on the SOI, Clim. Dyn., 37, , doi: /s Ratnam, J. V., S. K. Behera, Y. Masumoto, K. Takahashi, and T. Yamagata (2010), Pacific Ocean origin for the 2009 Indian summer monsoon failure, Geophys. Res. Lett., 37, L07807, doi: /2010gl Ratnam, J. V., S. K. Behera, Y. Masumoto, K. Takahashi, and T. Yamagata (2012), Anomalous climatic conditions associated with the El Niño Modoki during boreal winter of 2009, Clim. Dyn., doi: /s z, in press. Rayner, N. A., D. E. Parker, E. B. Horton, C. K. Folland, L. V. Alexander, D. P. Rowell, E. C. Kent, and A. Kaplan (2003), Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century, J. Geophys. Res., 108(D14), 4407, doi: / 2002JD Reynolds, R. W., et al. (2002), An improved in situ and satellite SST analysis for climate, J. Clim., 15, , doi: / (2002) 015<1609:AIISAS>2.0.CO;2. Seager, R., and R. Murtugudde (1997), Ocean dynamics, thermocline adjustment and regulation of tropical SST, J. Clim., 10, , doi: / (1997)010<0521:odtaar>2.0.co;2. Turner, A. G., P. M. Inness, and J. M. Slingo (2007), The effect of doubled CO 2 and model basic state biases on the monsoon-enso system. I: Mean response and interannual variability, Q. J. R. Meteorol. Soc., 133, , doi: /qj.82. Vecchi, G. V., B. J. Soden, A. T. Wittenberg, I. M. Held, A. Leetmaa, and M. J. Harrison (2006), Weakening of tropical Pacific atmospheric circulation due to anthropogenic forcing, Nature, 441, 73 76, doi: / nature Yu, J.-Y., H.-Y. Kao, and T. Lee (2010), Subtropics-related interannual sea surface temperature variability in the central equatorial Pacific, J. Clim., 23, , doi: /2010jcli Zhang, W., J. Li, and X. Zhao (2010), Sea surface temperature cooling mode in the Pacific cold tongue, J. Geophys. Res., 115, C12042, doi: /2010jc K. Ashok, T. Sabin, and P. Swapna, Centre for Climate Change Research, Indian Institute of Tropical Meteorology, Pune , India. R. Murtugudde, Earth Sciences Interdisciplinary Center, University of Maryland, College Park, MD 20742, USA. (ashok@tropmet.res.in) 5of5