Intraseasonal Variability of Sea Surface Wind in the Tropical Pacific and Indian Ocean during 1997‐98 and 1991‐93 El Niño Events Yuhei Kan-no1), Kunio Kutsuwada2) and Iwao Ueki3) 1), 2) 3)
School of Marine Science and Technology, Tokai University
Japan Marine Science Technology Center Tel: +81-543-37-0196
Fax: +81-543-36-1434
E-Mail: 1)
[email protected]
2)
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3)
[email protected] Abstract Data sets of surface wind vectors constructed by satellite scatterometer data and the objectively analyzed data are used to investigate variabilities of the zonal wind component along the equator in the tropical Pacific-Indian Oceans in the 1997-98 and 1991-93 El Nino events. Frequency spectra at longitude in the Indian-western central Pacific Ocean exhibit high energies at the period of 30-60 days both El Nino events, which mean the evolution of the intraseasonal variation. Time-longitude section of band-pass filtered (20-100 day) suggests that there are some signals propagating in the zonal direction. To clarify the zonal structure of these signals, we apply the complex Empirical Orthogonal Function (C-EOF) analyses for the band-passed time series. Results for the time series during August 1996-December 1998 reveal that both the first and second C-EOF modes, having their contribution ratios of 41.6% and 19.6%, respectively, have relatively high spatial amplitudes over the central Indian and western-central Pacific regions and that these spatial phases gradually increase to the east and to the west, respectively, which mean the eastward propagation of about 12-19 m s-1 and westward one of about 4-8 m s-1. Similar spatial features are also found in the lowest two C-EOF modes for the onset of the 1991-93 El Nino event. These results suggest that intraseasonal wind variations of having different zonal structures are dominant in the tropical region. Keyword: intraseasonal variation (MJO), zonal wind, frequency spectra, bandpass filter, Complex-EOF, westward propagation 1. Introduction It is well known that the westerly wind burst (WWB) is dominant in the onset of El Niño over the tropical western Pacific (Luther et al., 1983), and is related to 30-60 day intraseasonal variation (ISV), called often the Madden Julian Oscillation (MJO) (Lau et al., 1989). It is pointed out that the ISV is primary dominant over the tropical Indian-Pacific Ocean and that its maximum propagates eastward (e.g. Madden and Julian, 1972.a,b;
Cadet., 1983; Murakami., 1984; Wang and Rui., 1990). The ISV is considered to be a trigger of El Niño event. Thus, it is necessary to describe the ISV signal during the onset of El Niño event to understand the mechanism of El Niño event. To investigate the temporal and spatial characters of the ISV signal in detail, we need data set of surface wind/wind-stress with high resolution. Satellite scatterometer data and objectively analyzed data allows us to do
such an analysis, because they supply time series of daily base to us. European Remote Sensing Satellite (ERS-1/2), have been operated by European Space Agency(ESA) since 1991, and their data cover the entire 1997-98 El Niño event, which was the strongest one in the 20th century. We have evidences that the ISV developed in the western tropical Pacific in the onset of this event (e.g. Kazama et al., 1999). We attempt to describe a zonal structure of the ISV in the wind field in the event using the complex Empirical Orthogonal Function (C-EOF) analysis. Emphases are plaid upon the ISV of the zonal wind field during the 1991-93 El Niño event as well as the 1997-98 El Niño event 2. Data We use two types of data set of surface wind vectors. One is a data set of surface wind-vectors constructed using the level II product of ERS-1/2 which have been supplied by Institut Francais de Recherche pour l’ Exploitation de la Mer (IFREMER). The other is the objectively analyzed product (Basic level III surface data set) supplied by the European Center for Mediumrange Weather Forecast (ECMWF). Both of the products cover the Pacific and Indian Oceans of 60°N-60°S and 30°E-70°W with the spatial resolution of 2.5° x 2.5° grids. The first gridded product is constructed by the weighted average method (See Kutsuwada, 1998; Kubota et al., 2002). 3. Results Figure 1(a) and (b) show time-longitude cross section of low-pass filtered (11-days running mean) time series of the zonal component of surface wind-vector along the equator. The periods are (a) from October 1996 to October 1998 and (b) from January 1991 to January 1992 covering the 1997-98 and 1991-93 El Niño events, respectively. In the 1997-98 El Niño event, we find two contrastive features: the WWB originated over the western Pacific Ocean in December 1996 propagates eastward to about 160ºW in December 1997,whereas the WWB propagated into the west of the dateline in December 1997 and then weakened next month (Fig.1(a)). We also find a similar feature for the zonal structure in the 1991-93 El Niño event even if the eastward propagating signal of the WWB appears to be
weaker, compared with that in the 1997-98 El Niño event. In order to clarify the periodicity for these dominant signals during both El Niño events, we calculate frequency spectra, using the Fast Fourier Transform (FFT), for the time series of the zonal wind over three typical longitudes: the eastern Indian Ocean (80ºE), western Pacific (130°E) and central pacific Ocean (180°). Calculations are made for two periods: November 1996 to January 1998 and Apr. 1991 to Jun. 1992. Results reveal that the energy in the intraseasonal period band of 30-60 days is higher in both the El Niño events than in its neighboring bands, meaning the predominance of the ISV. Figure 2 (a) and (b) show time-longitude crosssections of band-pass filtered (20-100 day) time series of the zonal wind along the equator in the 1997-98 and 1991-93 El Niño events, respectively. We notice that during the 1997-98 El Niño event, strong westerly wind anomaly propagates eastward across the Indian Ocean to the western-central Pacific Ocean in December 1996, February, May and July-August 1997, then westward from the dateline in December 1997 (Fig.2 (a)). A similar structure is found in the 1991-93 El Niño event (Fig.2 (b)). These suggest that the propagations of strong westerly anomalies are almost simultaneous with the occurrence of the WWB in both the events, and are related to the dominance of the ISV signal. In order to clarify these zonal structures of the signal, we apply the Complex Empirical Orthogonal Function (C-EOF) analysis to the band-pass filtered time series of the zonal wind. Figure 3 (a) and (b) show the first and second C-EOF modes during the 1997-98 El Niño event (August 1996 to December 1998). The first C-EOF mode has its spatial amplitude with high coefficient over the eastern Indian Ocean (70°E-90°E) and western central Pacific Ocean (near 130°E, 170°E) in (upper figure of Fig.3 (a)) and its spatial phase which gradually increases eastward, meaning the eastward propagation. The maximum in the temporal amplitude is consistent with the eastward propagation of westerly wind anomaly in May 1997 shown in Fig.2 (a). Its characteristic period and phase speed are about 50day and 14 m s-1, respectively. On the other hand, the second C-EOF mode has a contrastive zonal structure that the spatial phase gradually increases westward, meaning the
westward propagation. This is related to the westward propagation of the ISV in December 1997 shown in Fig.2 (b). Its period and phase speed are about 50 day and 4 m s-1, respectively. The two modes in the 1991-93 El Niño event have also the similar structures to those in the 1997-98 El Niño event. The first mode is characterized by the eastward propagation, while the second mode by the westward one. Its period and phase speed are about 55 day and 5 m s-1 for the first mode, while the second about 45 day and 4 m s-1, respectively.
Kutsuwada, 2002: Japanese Ocean Flux Data Sets with Use of Remote Sensing Observations (J-OFURO). J. Oceanogr. 58, 213-225. Kutsuwada, K., 1998: Impact of wind/wind stress field in the North Pacific constructed by ADEOS / NSCAT data. J Oceanogr. 54, 443-456. Lau, K. M., Li. Peng, C. H. Sui, and T. Nakazawa, 1989: Dynamics of super cloud cluster, westerly wind bursts, 30-60 day oscillation and ENSO: An Unified View, J. Meteor. Soc. Japan. 61, 205-219. Luther, D., D. Harrison, and R. Knox, 1983: Zonal wind in
4. Concluding remarks It has been well known that the ISV propagates eastward not only surface wind but also other meteorological parameters in boreal summer (e.g. Madden and Julian., 1972a, b; Cadet, 1983; Murakami, 1984; Wang and Rui., 1990). The phase speed of east propagation in our analysis is almost consistent with the one of Super Cloud Cluster (SCC) in the previous study (Nakazawa., 1988). We have also found infrequent westward propagation of the ISV over the western Pacific Ocean during boreal summer. On the other hand, it has not been ever found that the ISV propagates westward over the central Pacific near the dateline in boreal winter during the El Niño event. Furthermore, the features of the zonal propagations are found not only in the unusual 1997-98 El Niño event but also in the 1991-92 El Nino event. We can conclude that the ISV in the tropical wind field has at least two contrastive zonal structures with eastward and westward propagations in both the 1997-98 and 1991-93 El Niño events. We are interested in whether the intraseasonal signals have these zonal structures in periods other than the El Niño event. This will be made in future studies. Reference Cadet, D L., 1983: The monsoon over the Indian Ocean during summer 1975. PartⅡ: Breaks and active monsoons. Mon. Wea. Rev., 111, 95-108. Kazama, T., I. Ueki, M. Kondou, and K. Kutsuwada, 1999: Characteristics of Westerly Wind Events in the Tropical Pacific and Indian Oceans during 1997-98 El Niño. Journal of the School of Marine Science and Technology Tokai University in Japanese with English abstract., 47, 205-222. Kubota, M., N. Iwasaka, S. Kizu, M. Konda, and K.
the central equatorial Pacific and El Niño. Science., 202, 327-330. Madden, R. A., and P. R. Julian, 1972 a: Description of global-scale circulation cells in the tropics with a 40-50 day period. J. Atoms. Sci., 29, 1109-1123. Madden, R. A. and P. R. Julian, 1972 b: Further evidence of global-scale circulation cells in the tropics with a 40-50 day period. J. Atmos. Sci., 29, 1464-1469. Murakami, M., 1984: Analysis of deep convective activity over the western Pacific and Southeast Asia 2. Seasonal and intraseasonal variations during northern summer. J. Meteor. Soc. Japan., 62, 88-108. Nakazawa, T., 1988: Tropical Super Clusters within Intraseasonal Variations over the Western Pacific. J. meteor. Soc. Japan,. 66, 823-839.
Wang, B. and H. Rui, 1990: Synoptic climatology of transient tropical intraseasonal convection anomalies. Meteor. Atomos. Phys. 44.,43-61
Fig.1: Time-longitude cross-section of low-pass filtered (11days running mean) zonal wind along Equator (a): during October
1996 - October 1998 from ERS-2 and (b): during
January 1991- December 1992 from ECMWF.
Fig. 2: The first and second C-EOF modes of band-pass filtered (20-100 day) zonal wind along Equator, during October 1996-December 1998 (ERS-2). Upper group (a) and lower group (b) panels show the first and the second C-EOF modes, respectively. Each group panel is composed of temporal amplitude (upper left), spatial amplitude (lower left), temporal phase (upper right) and spatial phase (lower right). The contribution ratios are (a) 41.6% and (b) 19.6%
Fig. 3: The first and second C-EOF modes of band-pass filtered (20-100 day) zonal wind along Equator, during October 1996 - December 1998 (ERS-2). Upper group and lower group panels show the first and the second C-EOF modes, respectively. Each group panel is composed of spatial amplitude (left) and spatial phase (right). The contribution ratios are (a) 41.6% and (b)19.6%.
Fig.4: The same as fig.3 except during April 1991 - July 1992. The contribution ratios are (c) 39.8% and (d) 14.8%.
