Analysis of sea surface temperature (SST) observations of the last 150 years reveals strong changes in the tropical Pacific climate system during the most recent decades. These changes can be best described as a slow variation of the mean state of the tropical Pacific. The superimposed interannual variability associated with the El Niño/Southern Oscillation (ENSO) phenomenon

does not exhibit any significant changes. However, the change in the mean state is “El

Niño-like”, with many aspects observed during present-day El Niño events. Thus, the change

in the mean state biasses the SSTs in the tropical Pacific towards the warm side, which explains the stronger and more frequent El Niños observed during the recent decades

Latif, Mojib

English

Latif, M.

MPG.PuRe

On the Interpretation of Climate Change in the Tropical Pacific

 1 On the Interpretation of Climate Change in the Tropical Pacific 1 Mojib LatifMax-Planck-Institut für MeteorologieBundesstr. 55, D-20146 Hamburg, Germany Abstract Analysis of sea surface temperature (SST) observations of the last 150 years reveals strongchanges in the tropical Pacific climate system during the most recent decades. These changescan be best described as a slow variation of the mean state of the tropical Pacific. The superim-posed interannual variability associated with the El Niño/Southern Oscillation (ENSO) phe-nomenon does not exhibit any significant changes. However, the change in the mean state is “ElNiño-like”, with many aspects observed during present-day El Niño events. Thus, the changein the mean state biasses the SSTs in the tropical Pacific towards the warm side, which explainsthe stronger and more frequent El Niños observed during the recent decades. *** The El Niño/Southern Oscillation (ENSO) phenomenon (Philander 1990) is the strongest natu-ral interannual climate fluctuation. ENSO can be understood as an irregular oscillation betweena warm state (El Niño) and a cold state (La Niña). Although ENSO originates in the tropicalPacific, it influences the global climate system (Glantz et al. 1991). ENSO is therefore not onlyof scientific but also of large public interest. The strong El Niños of 1982/1983 and 1997/1998,along with the more frequent occurrences of El Niños during the past few decades (Fig. 1), hasled to a discussion of whether human-induced (anthropogenic) greenhouse warming already hasor will affect ENSO (Trenberth and Hoar 1996, Goddard and Graham 1997, Rajagopolan et al.1997, Latif et al. 1997, Timmermann et al. 1999). From a scientific point of view it is importantto distinguish between changes in the mean state and changes in the nature of the interannualvariability. The observed changes in the tropical Pacific can be understood in two differentways, if interactions between the mean state and the variability are excluded. First, there is achange in the mean state and the superimposed interannual variability remains unchanged. Sec-ond, the mean state is stable and the statistics of the interannual variability change. The intentionbehind this paper is to shed light on this problem.In order to investigate this question, sea surface temperature (SST) observations were analysed.The dataset used is the Kaplan dataset (Kaplan et al. 1997) which provides monthly mean SSTsfor the period 1856-1998 on a 5 o  x 5 o  grid. This dataset has been used, for instance, by Cane etal. 1997 to study the sea surface temperature trends in the tropical Pacific and by Kumar et al.1999 to investigate the interdecadal variability in the ENSO/Indian Monsoon relationship. Al-though the focus of this paper is the tropical Pacific, the SSTs in all three ocean basins have beenretained in the analysis, in order to investigate potential teleconnections from the tropical Pacif-ic. The analysis is restricted to the region 30 o S-60 o N, and annual mean values were calculatedin order to smooth the data in time. The investigation is based on the Principal Oscillation Pat-tern (POP) analysis (Hasselmann 1998, von Storch et al. 1988), which is designed to extract thedominant modes of variability from a multi-variate dataset. 1.submitted as a Note to J. Climate, 5 June 2000. 2 Figure 1: Observed eastern equatorial SST anomaly (K) averaged over the Niño-3 region (5 o N-5 o S, 150 o W-90 o W). Note the two strong El Niños 1982/1983 and 1997/1998 and the more fre-quent occurrences of El Niños during the last few decades. The POP analysis yields two modes that account for a relatively large amount of variance in theSSTs. These two POP modes together describe the variability in the Niño-3 region very well.The correlation between the reconstructed Niño-3 time series using the two leading POP modeswith the original time series (Fig. 1) amounts to 0.96. The leading POP mode accounting forabout 27% of the total variance is the ENSO mode. This mode accounts between 60 and 80%of the variability in the eastern and central equatorial Pacific. It is oscillatory, has a rotation pe-riod of about 5 years and a damping time of about 1 year. Please note that the relatively smalldamping time results partly from the large domain considered. The two POP coefficient timeseries (Fig. 2a) are clearly dominated by interannual variations and show all known majorENSO extremes, including the 1982/1983 and 1997/1998 record El Niños. The real part patternof the this leading POP mode (Fig. 2b) shows the familiar spatial structure of ENSO extremes.There is a strong anomalous warming in the eastern and central equatorial Pacific and anoma-lous cooling in the western equatorial Pacific, in the North and South Pacific. Some weak tele-connections to the tropical Indian and Atlantic Oceans are also found, consistent with earlierstudies (Klein et al. 1999, Enfield and Mayer 1997). The imaginary part pattern (Fig. 2c) is gen-erally much weaker, supporting the theoretical picture that ENSO is basically a standing oscil-lation if SST is considered. Propagating anomalies are theoretically expected and observed inthe upper ocean heat content of the equatorial Pacific (Neelin et al. 1994).1850 1900 1950 2000time (a)-2-1012SSTA (K)Nino-3 SSTAKaplan dataset 3 Figure 2: The leading POP mode. a) Time series, b) real part pattern, c) imaginary part pattern.This mode is the ENSO mode which is an interannual oscillatory mode. There is no majorchange during the last few decades. The time series are in units of (K), while the patterns aredimensionless.1850 1900 1950 2000time (a)-0.4-0.20.00.20.40.6POP amplitudeleading POP mode (27%)annual data, Kaplan datasetreal partimag. part 4 The most important feature of the leading (ENSO) POP mode concerning the problem underconsideration is that there does not exist any systematic change of the interannual variability intime. Although the time series exhibit some pronounced interdecadal variations, with active andless active decades, no strong change in the character of the oscillation during the last few dec-ades is discernible (Fig. 2a). It is thus concluded that the statistics of the ENSO variability, asdescribed by the leading POP mode, have not changed during the most recent decades and thatthere is no apparent impact of greenhouse warming on the nature of the ENSO-related interan-nual variability. The next energetic POP mode accounting for about 14% of the total variance has a rotation pe-riod of about 110 years and a damping time of about 5 years. The very small damping time to-gether with the weak amplitude of the imaginary part time series (Fig. 3a) indicate that this POPmode does not reflect an oscillatory but rather a growing mode which is represented by its realpart. Most outstanding is the increase of the real part time series towards the end of the last cen-tury from 1950 onwards (Fig. 3a). Whether this behaviour is due to anthropogenic greenhousewarming or an expression of natural interdecadal variability is not the subject of this paper. It isonly concluded here that a strong and almost monotonous change in the SST has occurred dur-ing the last 50 years. This slowly evolving change can be regarded as a change in the mean staterelative to ENSO and its short period. Please note that in contrast to earlier studies (e. g. Knutsonet al. 1999) no a priori information about the nature of the slow mode has been assumed. ThePOP analysis is able to distinguish between different modes, which have similar spatial struc-tures (as shown below) but very different time evolutions. EOF analysis failed to distinguishbetween the two modes.The real part pattern of the second energetic POP mode (Fig. 3b) shows many features that havebeen discussed in the context of anthropogenic climate change (e. g. IPCC 1996, Meehl andWashington 1996, Hegerl et al. 1997, Cane et al. 1997, Knutson et al. 1999), with a strong ElNiño-like change in the tropical Pacific. This change is referred to as “El Niño-like” to distin-guish it from the El Niño itself. While the latter is the warm phase of the leading interannualoscillatory mode, as decribed above, the “El Niño-like” SST change associated with the secondenergetic POP mode evolves much more slowly in time and does not seem to be oscillatory innature (Fig. 3a). It is this slowly growing “El Niño-like” pattern which biassed the SSTs in theeastern tropical Pacific towards the warm side during the most recent decades, so that El Niñosappear to have become stronger and more frequent. This bias amounts to typically 0.3 o C in theNiño-3 SST anomaly index during the last 25 years. It is this warm bias that made the two ElNiños 1982/1983 and 1997/1998 to record El Niños. This can be shown by reconstructing thepast El Niños by using only the leading POP mode (not shown). Such a reconstruction yieldsrecord El Niños in 1877/1888 and 1941/1942. If the second POP mode is added to the recon-struction, the El Niños 1982/1983 and 1997/1998 become the record El Niños.The SST anomaly pattern associated with the real part of the second mode (Fig. 3b), however,shows some interesting differences to the classical El Niño pattern (Fig. 2b). The most impor-tant difference is the lack of strong warming in the equatorial east Pacific, a region where warm-ing during present-day El Niños is strongest. Instead, strongest warming is found off the equatorin the eastern Pacific and at the equator in the west (Fig. 3b), features which have not been fullyexplaind yet and may be related to specific equatorial ocean dynamics, as hypothesised by e. g.Cane et al. 1997. Teleconnections to the North and South Pacific and to the Indian Ocean arepresent in the SST anomaly pattern of the real part of the second energetic POP mode, featuresobserved also during present-day El Niños (Fig. 2b). Other features to be noted are the strongwarming of the subtropical South Indian Ocean, the warming of the subtropical South Atlantic,and the cooling of the North Atlantic.  5 Figure 3: The second energetic POP mode. a) Time series, b) real part pattern, c) imaginarypart pattern. This mode is a growing mode that is mainly represented by its real part. Note thestrong increase of the real part time series in a) during the recent decades. The time series arein units of (K), while the patterns are dimensionless.1850 1900 1950 2000time (a)-1.0-0.50.00.51.0POP amplitude2nd most energetic POP mode (14 %)annual data, Kaplan datasetreal partimag. part 6 The results of the POP analysis indicate that the recent tropical Pacific climate change can bedescribed best by a superposition of a slowly changing mean state (or ultra low-frequency var-iability) and an unchanged superimposed interannual variability. It is important from a scientificpoint of view to distinguish between the slowly growing “El Niño-like” change in the tropicalPacific and the oscillatory ENSO mode, since they may involve completely different physics,which is indicated by the differences in the spatial patterns of the two modes. While the formermay perhaps represent a forced mode resulting from enhanced greenhouse warming, the latteris an internal mode of interannual variabilty of the coupled ocean-atmosphere system in thetropical Pacific. The observational results presented here are consistent with a greenhousewarming simulation described by Timmermann et al. 1999, in which the ENSO variability doesnot change significantly prior to the middle of this century and changes before are mostly dueto a change in the mean state. However, the study by Timmermann et al. 1999 shows also thatthe ENSO statistics may change in response to the changes in the mean state, if the latter becomesufficiently strong. Thus, it may well be possible that the nature of the ENSO-related interan-nual variability will change, if the trend observed in the tropical Pacific continues in the future. Acknowledgements Thanks to A. Kaplan for providing the SST dataset. This work was supported by the EuropeanUnion’s SINTEX and PREDICATE programmes. References  Cane, M. A., A. C. Clement, A. Kaplan, Y. Kushnir, D. Pozdnyakov, R. Seager, S. E. Zebiak,and R. Murtugudde, 1997: Twentieth-century sea surface temperature trends. Science, 275,957-960.Enfield, D. 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