3. Atmospheric and oceanic circulation

a. Southern Oscillation

1) Overview

The Southern Oscillation refers to large-scale pressure anomalies, that develop at irregular intervals between the Indian Ocean-western tropical Pacific and the east-central tropical Pacific (Berlage 1957; Bjerknes 1969). Extreme phases of the SO are accompanied by large-scale patterns of anomalous sea level pressure (SLP), wind, tropical convection, SST, and subsurface ocean temperature, that generally span the entire equatorial Pacific. Collectively, these anomaly patterns represent a strongly coupled ocean-atmosphere interaction and comprise the dominant mode of interannual variability in the global Tropics.

From SON 1995 through SON 1996, the positive phase of the SO (Fig. 20a) and abnormally cold sea surface temperatures were observed across the central and east-central equatorial Pacific (Figs. 20b, 21a ). These conditions are in marked contrast to the negative phase of the SO and abnormally warm ocean temperatures that dominated these regions from 1991 to early 1995 (Halpert et al. 1996). During this time, three periods of mature-phase ENSO conditions were evident: November 1991-May 1992, December 1992-May 1993, and November 1994-February 1995.

These periods of mature ENSO conditions were associated with enhanced atmospheric convection [negative anomalies of outgoing longwave radiation (OLR)] over the central and east-central equatorial Pacific, and with generally suppressed convective activity (positive OLR anomalies) over the western equatorial Pacific and Indonesia (Fig. 21b ). This distribution of anomalous convection reflects several features typical of ENSO (Rasmusson and Carpenter 1982; Janowiak and Arkin 1991), including an eastward extension of the climatological mean region of deep tropospheric heating well east of the date line, a strengthening and equatorward shift of the intertropical convergence zone (ITCZ) over the central and eastern Pacific, and a northeastward shift of the South Pacific convergence zone (SPCZ). Other ENSO-related features during these periods included 1) above-normal SLP over the western equatorial Pacific and below-normal SLP over the eastern equatorial Pacific (Fig. 21c), 2) westerly wind anomalies at 850 hPa across the equatorial Pacific (Fig. 21d), and 3) abnormally warm (cold) subsurface ocean temperatures east (west) of the date line down to thermocline depth, that is indicated by the depth of the 20°C isotherm (Fig. 22a). These subsurface anomalies reflect a reduced (increased) depth of the oceanic thermocline over the western (eastern) equatorial Pacific and a reduction in the overall slope of the thermocline.

The development of cold episode conditions during 1995 was accompanied in the Tropics by enhanced convection over Indonesia and suppressed convection over the central equatorial Pacific in the vicinity of the date line (Fig. 21b) and by enhanced low-level easterly winds across the equatorial Pacific (Fig. 21d). It was also accompanied by a transition to an increased slope of the oceanic thermocline across the equatorial Pacific, resulting in abnormally warm (cold) subsurface ocean temperatures down to thermocline depth in the western (eastern) Pacific (Fig. 22b).

These cold­episode conditions contributed to pronounced large-scale changes in the anomaly patterns from those that had prevailed during much of the early 1990s. For example, above-normal rainfall returned to Indonesia, northern Australia [see section 4d(1)], and southern Africa [see section 4c(2)], following a prolonged period of below-normal rainfall and periodic drought. Additionally, a poleward shift of the jet stream and storm track position over the eastern half of the North Pacific was evident during DJF 1995/96 [see section 4b(1) ], following several winter seasons (three in the last four) characterized by a marked strengthening, equatorward shift, and eastward extension of these features toward the southwestern United States. In the southwestern United States, there was a transition to significantly below-normal precipitation and severe drought [see section 4b(2)], following above-normal precipitation in this region during the previous several winter seasons. Over the North Atlantic, there was a dramatic return to very active hurricane seasons during 1995 and 1996, following four consecutive years (1991-94) of significantly below-normal hurricane activity [see section 4b(4)]. This strong relationship between the phase of the SO and intense hurricane activity over the North Atlantic has been previously documented by Gray et al. (1992).

2) Mature cold-episode conditions during December 1995-May 1996

The period December 1995-May 1996 featured the development of mature-phase cold-episode conditions for the first time since the 1988-89 cold episode (Fig. 20b). During DJF 1995/96, below-normal SSTs cov ered the equatorial Pacific east of the date line (Fig. 23a), with temperatures more than 1.0oC below normal observed between 170o and 145oW. Farther west, abnormally warm ocean temperatures averaging 0.5°-1.0oC above normal dominated the western equatorial Pacific during the season. Accompanying this anomalous SST pattern, enhanced convection was observed throughout Indonesia and northern Australia, while suppressed convection covered the central equatorial Pacific (Fig. 24a ). This pattern reflected three important changes in the normal distribution of tropical convection: 1) a westward retraction of the mean region of deep tropical convection toward Indonesia and a nearly complete dissipation of the normal pattern of tropical convection over the central equatorial Pacific, 2) an overall weakening of the Northern Hemisphere ITCZ over the central and eastern Pacific, and 3) a strengthening of the western flank of the SPCZ.

The 200-hPa wind field during the season featured a large-scale anticyclonic circulation anomaly centered over the western subtropical North Pacific (Fig. 24a), which represents an amplification of the mean subtropical ridge along the equatorward (or anticyclonic) flank of the East Asian jet entrance region. [The mean axis of the East Asian jet crosses central China and southern Japan near 32oN.] This anticyclonic anomaly comprises several important changes in the atmospheric circulation over the western Pacific and is also a well known feature linking enhanced convection over Indonesia with changes in extratropical circulation farther north (Mo and Kousky 1993). For example, the enhanced westerly flow along its northern flank is coupled to an amplification of the East Asian jet stream, while the enhanced poleward geostrophic flow along its western flank is linked to enhanced confluence (not shown) along the southern half of the East Asian jet entrance region.

We suggest that additional aspects of the link between the enhanced convection over Indonesia and the intensified East Asian jet are revealed by the ageostrophic (both rotational and divergent contributions) component of the flow. For example, a coherent pattern of southerly ageostrophic flow at upper levels normally extends from the deep subtropics to the cyclonic-shear (poleward) side of the East Asian jet entrance region (Fig. 25a ). Accompanying this flow is ascending motion at low latitudes and descending motion at higher latitudes, primarily on the cyclonic-shear side of the East Asian jet. This overall
circulation represents a strong coupling of two prominent features: 1) the subtropical Hadley cell, with its associated outflow from deep tropical convection over the western tropical Pacific; and 2) the thermodynamically direct, transverse ageostrophic circulation that characterizes the entrance region of the East Asian jet. This transverse ageostrophic circulation is a required feature of any jet stream of finite length (Hoskins et al. 1978) and, among its many properties, produces the westerly momentum and kinetic energy increases that air parcels experience as they approach the jet (Fig. 25a), via cross-contour flow toward lower geopotential height (Keyser and Shapiro 1986; Palmén and Newton 1969, sections 1.5 and 8.3; and Holton 1979, section 10.7)

The anomalous ageostrophic flow over the western North Pacific during DJF 1995/96, along with the accompanying patterns of anomalous Lagrangian kinetic energy tendency, are shown in Figs. 25b-d. Enhanced southerly ageostrophic flow was evident at upper levels during the season, extending poleward from the area of enhanced convection well into the axis of the amplified East Asian jet (Fig. 25b). This flow was associated with an enhanced subtropical Hadley circulation. Additionally, it was directed toward lower geopotential height throughout the anticyclonic flank of the jet entrance region, resulting in anomalously strong Lagrangian increases in kinetic energy in that region. The rotational and divergent components of the ageostrophic flow (Figs. 25c and 25d, respectively) each accounted for approximately one-half of these total ageostrophic flow and kinetic energy tendency anomalies.

These results suggest that the ageostrophic component (both rotational and divergent contributions) of the flow along the western flank of the subtropical anticyclonic circulation (Fig. 24a ) reflected a coupling of both an enhanced subtropical Hadley circulation and an amplified transverse ageostrophic circulation in the East Asian jet entrance region. Collectively, these conditions are consistent with both the increase in deep tropical convection and enhanced deep tropospheric heating over Indonesia during the season, and with the stronger westerly momentum and kinetic energy increases experienced by air parcels migrating through the amplified East Asian jet.

By MAM 1996, the region of largest negative SST anomalies had shifted westward to near 175oW and had diminished considerably (Fig. 23b). Despite this evolution, the pattern of tropical convection remained suppressed (Fig. 24b). Additionally, well-developed cyclonic circulation anomalies were observed in both hemispheres at upper levels flanking the region of suppressed tropical convection, while farther west the anomalous anticyclonic circulation over the western subtropical North Pacific persisted but had shifted 30° to the east (Fig. 24b ).

By the end of May, mature-phase cold-episode conditions had ended, with only near-normal to slightly below-normal SSTs observed during the next two seasons across the tropical Pacific east of the date line (Figs. 23c,d). Nonetheless, abnormally warm SSTs (averaging 0.5°-1.0oC) persisted over much of the tropical and subtropical western Pacific during June-August and September-November, along with a continuation of enhanced (suppressed) convection over Indonesia (near the date line) (Figs. 24c,d). This large-scale pattern of anomalous tropical convection is consistent with a continued positive phase of the Southern Oscillation during these seasons.

3) Equatorial Walker circulation

The equatorial Walker circulation is an important component of the atmospheric general circulation over the tropical Pacific and is also an integral part of the still larger Southern Oscillation (Bjerknes 1969). During DJF, the mean Walker circulation consists of a maximum in ascending motion over Indonesia, a maximum in descending motion over the east-central equatorial Pacific, and upper-level westerly (lower-level easterly) flow completing the direct circulation cell.

Mature phase warm (cold) episode conditions are accompanied by a reduced (enhanced) strength of the equatorial Walker circulation. These circulation anomalies can be illustrated in equatorial height-longitude plots of vector anomalies (Fig. 26), where the horizontal component is the divergent zonal wind (referred to as u*) and the vertical component is the anomalous vertical velocity. The accompanying relative humidity anomalies are also shown shaded in Fig. 26.

During DJF 1994/95 the strength of the Walker circulation was weaker than normal with the u* and vertical motion anomalies confined to the eastern portion of the Walker circulation. During this period the region of enhanced ascending motion and above-normal relative humidity covered the area between 175oE and 150oW (Fig. 26a). These conditions then disappeared during March-May 1995 (Fig. 26b) as SSTs returned to near normal throughout the region (Fig. 21a).

By June-August 1995 enhanced descending motion and dryness developed throughout the troposphere across the central equatorial Pacific, and enhanced ascending motion and increased relative humidity became established over the western equatorial Pacific (Fig. 26c). Additionally, a well-defined pattern of anomalous easterly u* in the lower and middle troposphere (up to near the 500­hPa level) and anomalous westerly u* at upper levels was evident. This enhanced Walker circulation became increasingly well defined during September-November 1995 (Fig. 26d) and reached peak amplitude during DJF 1995/96 (Fig. 26e) as mature cold-episode conditions became established.

The strength of the equatorial Walker circulation weakened during MAM 1996, with the area of anomalous descending motion restricted to the vicinity of the date line (Fig. 26f). This evolution is consistent with the decrease in magnitude of both the negative SST anomalies (Fig. 23b) and suppressed convection over the central equatorial Pacific during the season (Fig. 24b).

An enhanced Walker circulation persisted during the next two seasons (Figs. 26g,h), consistent with the continued positive phase of the Southern Oscillation and with the continuation of abnormally warm SSTs throughout the western equatorial Pacific (Figs. 23c,d). Thus, by late 1996 the tropical atmosphere still retained its positive phase SO characteristics, despite the return to near-normal SSTs over the central and east-central equatorial Pacific.

Back to Table of Contents