175
4 Decadal ENSO Variability
As a result of the increased attention devoted to ENSO (El Nino/Southern
Oscillation) in the last decade or so, we know that ENSO is a Pacificwide phenomenon involving major eastward expansion of warm SSTs (of
the normally warm western Pacific) into the central and eastern Pacific.
This expansion occurs on interannual time scales and is accompanied by
westerly wind anomalies in the equatorial Pacific (to the west ofthe warm
SST anomalies) and anomalous rainfall, mostly over the expanded warm
water. We also know that the warm and cold phases of ENSO have global
teleconnections, especially during winter in each hemisphere and affect
Northwest North America. Northeast Brazil, Southern Africa, etc. (e.g.
Ropelewski and Halpert, 1987)
Although the mechanism for ENSO is believed to be interannual (e.g.
the so-called "retarded oscillator" mechanism, Battisti and Hirst, 1989)
there are known to be decadal variations in ENSO (Fig. 6). These decadal
variations have been associated with decadal changes in the circulation
over the northern Pacific (e.g. Trenberth and Hurrell, 1994) and it has
been noted that the shift in northern Pacific circulation in 1976 (an intensification of the PNA pattern) has been associated with an increase of the
intensity and regularity of ENSO in the tropical Pacific. Hurrell (1995)
has also noted that the winter warmth over Europe and Asia over the last
decade or so coincides with an abnormally strong North Atlantic Oscillation which guides the mid- latitude jet into a more southerly position over
Europe and continuing on to the Asia landmass.
Wallace, Zhang, and Renwick (1995) have put these circumstances together and shown that there is a global winter pattern (Fig. 7) with warmth
over the northern landmasses and cooling over the northern Atlantic and
Pacific that is strongly correlated with the ENSO signal. Although this
pattern normally has a month-to-month variability, it has been "stuck"
in the polarity that warms the land and cools the ocean and therefore
contributes to the mean land hemispheric winter warming that has been
apparent since 1976. The extra warmth in winter over that in summer is
primarily due to this pattern. This pattern correlated strongly with the
ENSO. A fuller account is given in the lectures by Wallace in this volume.
We must conclude that a substantial part of the warming is due to
4 Decadal ENSO Variability
As a result of the increased attention devoted to ENSO (El Nino/Southern
Oscillation) in the last decade or so, we know that ENSO is a Pacificwide phenomenon involving major eastward expansion of warm SSTs (of
the normally warm western Pacific) into the central and eastern Pacific.
This expansion occurs on interannual time scales and is accompanied by
westerly wind anomalies in the equatorial Pacific (to the west ofthe warm
SST anomalies) and anomalous rainfall, mostly over the expanded warm
water. We also know that the warm and cold phases of ENSO have global
teleconnections, especially during winter in each hemisphere and affect
Northwest North America. Northeast Brazil, Southern Africa, etc. (e.g.
Ropelewski and Halpert, 1987)
Although the mechanism for ENSO is believed to be interannual (e.g.
the so-called "retarded oscillator" mechanism, Battisti and Hirst, 1989)
there are known to be decadal variations in ENSO (Fig. 6). These decadal
variations have been associated with decadal changes in the circulation
over the northern Pacific (e.g. Trenberth and Hurrell, 1994) and it has
been noted that the shift in northern Pacific circulation in 1976 (an intensification of the PNA pattern) has been associated with an increase of the
intensity and regularity of ENSO in the tropical Pacific. Hurrell (1995)
has also noted that the winter warmth over Europe and Asia over the last
decade or so coincides with an abnormally strong North Atlantic Oscillation which guides the mid- latitude jet into a more southerly position over
Europe and continuing on to the Asia landmass.
Wallace, Zhang, and Renwick (1995) have put these circumstances together and shown that there is a global winter pattern (Fig. 7) with warmth
over the northern landmasses and cooling over the northern Atlantic and
Pacific that is strongly correlated with the ENSO signal. Although this
pattern normally has a month-to-month variability, it has been "stuck"
in the polarity that warms the land and cools the ocean and therefore
contributes to the mean land hemispheric winter warming that has been
apparent since 1976. The extra warmth in winter over that in summer is
primarily due to this pattern. This pattern correlated strongly with the
ENSO. A fuller account is given in the lectures by Wallace in this volume.
We must conclude that a substantial part of the warming is due to