The transition from a cold to a warm state of the El Nino-Southern Oscillation (ENSO) cycle is studied using Comprehensive Ocean-Atmosphere Data Sets (COADS) for the period 1950-1992.
The onset of El Nino (November to December of the year preceding the El Nino) is characterized by an occurrence of minimum sea-level pressure anomalies in the subtropics around the "node" line of the Southern Oscillation. This pressure fall favors the formation of the anomalous cyclonic circulations over the western Pacific and leads to the establishment of anomalous westerlies in the western equatorial Pacific during the boreal spring of the El Nino year. The westerly anomalies then intensify and propagate into the central Pacific by the end of the El Nino year. This is an essential feature of the development of a basin-wide warming.
It is argued that the development of the equatorial westerly anomalies over the western Pacific may result from the thermodynamic coupling between the atmosphere and ocean. In boreal winter and spring the mean zonal winds change from westerly to easterly over the western equatorial Pacific. A moderate equatorial westerly anomaly initially imposed on such a mean state may create eastward SST gradients via changing rates of evaporational cooling and turbulent mixing. The equatorial SST gradients would, in turn, induce differential heating and zonal pressure gradients which reinforce the westerly anomalies. The feedback between the eastward SST gradients and westerly anomalies promotes the eastward propagation of the westerly anomalies.
The anomalous warming of the eastern-central equatorial Pacific Ocean and along the South American coast (termed as El Nino in this paper) occurs concurrent with the weakening of the southeast trades, which is in turn tied to the simultaneous pressure fall in the southeast Pacific and pressure rise over Indonesia (a low-index phase of the Southern Oscillation (SO)). This interannual variation of the coupled atmosphere/ocean system is now popularly referred to as El Nino-Southern Oscillation (ENSO).
A central question concerning the nature and prediction of ENSO is how the turnabout from a cold to a warm state (or vice versa) takes place. After Bjerknes' (1966, 1969) pioneering studies, a considerable number of investigations were carried out to search for the "initial" changes or precursors in the atmosphere or/and ocean that may be responsible for an El Nino. A complete review of this topic is not attended but works that are most relevant to the present study are briefly discussed here.
After examining the El Ninos of 1957-58, 1965, and 1972, Wyrtki (1975) found that the South American coastal warming is not due to a local weakening of the southeast trades off Peru. He noticed that during the two years preceding the coastal warming excessively strong southeast trades build up warm water in the western Pacific. He hypothesized that in the southern winter of the year preceding the El Nino (year -1 hereafter), the wide-spread decrease of the southeast trades causes a strong El Nino. This, however, does not explain the origin of an ENSO event because the weakening of the trades is a part of the atmospheric response to the anomalously high SST (Philander and Rasmusson, 1985). Barnett (1981) tested Bjerknes' and Wyrtki's ideas by studying statistical relations between ocean/atmospheric fluctuations. They concluded that measures of both Hadley and Walker cells can be hindcasted at a lead time of a few months but with low skill; some key features of trade winds are effective in hindcasting ocean variables at both short (0-3 months) and long (10-12 months) lead time.
Rasmusson and Carpenter (1982) made a comprehensive description of a composite ENSO scenario based on six events during 1950-1976. They found that equatorial easterly anomalies occur west of the dateline during the antecedent period from July to October of the year -1, and equatorial westerly anomalies occur over the western Pacific in the onset phase (around the end of year -1). They were puzzled by the appearance of SST anomalies near the dateline concurrent with the westerly anomalies in the onset. These westerly anomalies occurring in the equatorial western Pacific were suggested as a trigger of the South American coastal warming which may induce eastward-propagating Kelvin waves (Wyrtki 1975, Philander 1981, 1985; Busalacchi and O'Brien 1981).
What is responsible for the initial changes in the western Pacific wind field? A variety of speculations has been made, including the twin cyclones which developed in the western-central Pacific (Keen 1982), the persistent development of intraseasonal oscillations (Lau and Chan 1986), the impacts of the cold surges from the east Asian winter monsoon (Lau et al. 1983), and the enhancement of the Australian summer monsoon (Hackert and Hastenrath 1986).
In a series of studies, Barnett (1983, 1984, 1985) suggested that the surface wind and sea-level pressure (SLP) anomalies, which eventually cause most of the change in SST in the equatorial Pacific, originate in the equatorial Indian Ocean and propagate slowly eastward into the Pacific. The eastward propagation of the zonal wind anomalies above the boundary layer from the southeast Asian monsoon region to the western Pacific described by Yasunari (1985, 1990) and Gutzler and Harrison (1987) seem to support this notion. An interpretation was offered by Meehl (1987) in which the important role of the biennial variation of the monsoon circulation is emphasized. Trenberth and Shea's (1987) analysis of long record SLP data, on the other hand, revealed that the most dominant feature of the Southern Oscillation (SO) is a standing seesaw, and the eastward propagation of SLP anomalies is not very regular and is not supported by the long-term record. They showed that changes over the South Pacific pole of the SO lead opposite changes in the Indonesian pole by one to two seasons, with the largest lead of three seasons beginning near New Zealand. Possible South Pacific roles in ENSO onset were discussed by van Loon and Shea (1985, 1987), Trenberth and Shea (1987) and Kiladis and van Loon (1988).
It appears that the exact origin of the equatorial westerly anomalies in the western Pacific and their roles in ENSO development have not yet been firmly determined and well understood. The purpose of the present study is to search for common characteristics and possible causes of the transition from a cold to a warm state of the ENSO cycle by examining all the major warm events during 1950-1992.
The ENSO cycle is highly nonstationary. Each El Nino has its own characteristics. It is more meaningful to investigate the evolution of ENSO event by event. Case studies are also desirable for revealing the differences from case to case, which are valuable for understanding the nature and cause of the deviations from a composite scenario. Section 2 will show that the available Comprehensive Ocean-Atmosphere Data Set (COADS) is adequate for a case analysis of the six most significant ENSO events in the last four decades. These warm events include 1957, 1965, 1972, 1982, 1986-87, and 1991 El Ninos. Section 3 displays phase propagation diagrams for the six most significant warm events in an attempt to trace the origins of the ENSO anomalies. A detailed documentation of the common characteristics is followed in section 4. A hypothesis regarding the development of the warm (ENSO) episode is proposed for further theoretical and numerical investigations in section 5. The last section summarizes the results and discusses questions remaining open for future studies.
During the transition periods, both the atmosphere and ocean are in normal climatological (annual cycle) conditions. The local coupling between the tropical atmosphere and ocean is weak, because the annual cycle, to a large extent, is determined by insolational forcing, especially in the western Pacific. Even in the tropical eastern Pacific where air-sea interaction is involved, the insolational forcing still fundamentally regulates the annual cycle (Wang 1994a). As a direct consequence of this weak coupling, relatively high frequency (intraseasonal to annual time-scales) disturbances, perhaps primarily in the extratropical atmosphere, could interfere with the ENSO mode and cause irregularities in the ENSO cycle.
The occurrence of minimum pressure anomalies in the subtropics around the node line of the Southern Oscillation leads the central Pacific warming (mature phase of an ENSO event) by about one year. This is probably the most conspicuous precursor for the onset of a warm episode. Both the location and the season are critical for the pressure fall to be in effect. This pressure fall favors the formation of anomalous cyclones and the establishment of equatorial westerly anomalies over the western Pacific. It is not a passive atmospheric response to tropical Pacific SST anomalies, because the latter are small in the onset phase. This pressure drop is likely linked to abnormal annual cycle in the atmosphere outside the tropics and serves as a mechanism that causes irregularities in the ENSO evolution. It is unclear, however, exactly what is responsible for the subtropical pressure fall in boreal fall and winter of year -1, which may play an important role in the initiation of the coupled ocean/atmosphere instability in the western-central Pacific.
A common feature of the development of basin-wide warming for all significant El Ninos in the last four decades was the establishment of equatorial westerly anomalies in the western Pacific and subsequent slow eastward propagation from boreal spring to winter of the El Nino year. It is hypothesized that this common development process results from the effects of zonal variations of the mean winds along the equator and the oceanic thermodynamic processes, by which westerly anomalies create an east-west SST gradient and the SST gradient in turn feeds back to the surface winds.
For the proposed mechanism to work, the zonal structure of the mean winds at the equator is a critical factor. In this regard, both the geographic location and the season are important elements. Geographically, the most favorable location is the equatorial Pacific between 135oE and 165oE. This area is the core of the warm pool and has virtually no east-west SST gradient nearly all year round (See Sadler et al. 1987 for instance). Seasonally, the most favorable seasons are boreal winter and spring, in particular from January to April. During this period the climatological equatorial winds are light (monthly mean wind speed is less than 2 ms-1), yet have relatively large zonal variation (about 3 ms-1 per 20o longitude) (Fig. 8). In fact, the onset and development of EL Ninos are characterized by the occurrence of equatorial westerly anomalies in this area during the boreal winter and spring. The most evident slow eastward propagation of the equatorial westerly anomalies also takes place in this area (Fig. 5). These observations appear to support the hypothesis. The occurrence of equatorial westerly anomalies over the core of the warm pool, however, is probably only a necessary (or favorable) condition, but not sufficient. For instance, significant anomalous westerlies occurred over the western equatorial Pacific in early 1980 and early 1990 (Figs. 7c and 8), the western-central Pacific during these two years experienced abnormal warming, but the warming did not spread into the eastern Pacific. Apparently, there are other factors that may also affect the proposed interactive instability in the western Pacific. Further studies are required to find out these factors.
In addition to the common features, differences between the cases in the transition are notable. The characteristic evolution of SST anomalies has changed notably since the late 1970s (Fig. 2). The causes of the changes are discussed in an accompanying paper (Wang 1994b). Another secular change was noticed in the evolution of SLP anomalies. While the lowest pressure anomalies associated with El Nino in the southeast Pacific tend to be quasi-stationary, the highest pressure anomalies over the Maritime Continent exhibit a significant northeastward propagation before, during, and after the central Pacific peak warming (Fig. 4). The propagation path of the highest SLP anomalies changed in the late 1960s. The causes of these changes call for further investigations.
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