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Orbital modulation of ENSO seasonal phase locking

Lu, Zhengyao LU and Liu, Zhengyu (2019) In Climate Dynamics 52(7-8). p.4329-4350
Abstract

Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. Here, we show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but to peak in boreal summer when perihelion lies in between autumnal equinox and winter solstice. The mechanism for the change of ENSO’s phase locking is proposed to be caused by the change of seasonality of the growth rate, or the intensity of ocean–atmosphere feedbacks, of ENSO. It is found that the... (More)

Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. Here, we show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but to peak in boreal summer when perihelion lies in between autumnal equinox and winter solstice. The mechanism for the change of ENSO’s phase locking is proposed to be caused by the change of seasonality of the growth rate, or the intensity of ocean–atmosphere feedbacks, of ENSO. It is found that the December peak of ‘winter ENSO’ is caused by the continuous growth of ENSO anomaly from June to November, while the May–June peak of ‘summer ENSO’ appears to be caused jointly by the seasonal shift of higher growth rate into spring and stronger stochastic noise towards the first half of the year. Furthermore, the change of the seasonal cycle of feedbacks is contributed predominantly by that of the thermodynamic damping. The summer peak of ENSO is proposed to be caused by the following mechanism. A perihelion in the late fall to early winter leads to a cooling of the surface eastern equatorial Pacific (EEP) due to reduced insolation in spring. This cooling, reinforced by an oceanic process, reduces the latent heat flux damping in spring, and therefore favors the growth of the eastern Pacific-like ENSO (as opposed to the central Pacific-like ENSO). This EEP cooling is also likely to generate more effective short wave-cloud-SST feedback and, in turn, increased instability. Ultimately, the weakened thermodynamic damping in spring, combined with relatively intensive stochastic forcing, benefits the subsequent summer peak of ENSO.

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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
ENSO, Ocean–atmosphere feedbacks, Orbital forcing, Phase locking
in
Climate Dynamics
volume
52
issue
7-8
pages
4329 - 4350
publisher
Springer
external identifiers
  • scopus:85051626638
ISSN
0930-7575
DOI
10.1007/s00382-018-4382-1
language
English
LU publication?
yes
id
e4d9a203-a711-4c3c-97e4-a95a4766f47a
date added to LUP
2018-09-13 11:40:34
date last changed
2019-09-17 04:38:29
@article{e4d9a203-a711-4c3c-97e4-a95a4766f47a,
  abstract     = {<p>Modern El Niño-Southern Oscillation (ENSO) events are characterized by their phase locking of variability to the seasonal cycle and tend to peak at the end of calendar year. Here, we show that in an idealized NCAR-CCSM3 simulation of the climate of the last 300,000 years, ENSO seasonal phase locking is shifted periodically following the precessional forcing: ENSO tends to peak in boreal winter when perihelion is near vernal equinox, but to peak in boreal summer when perihelion lies in between autumnal equinox and winter solstice. The mechanism for the change of ENSO’s phase locking is proposed to be caused by the change of seasonality of the growth rate, or the intensity of ocean–atmosphere feedbacks, of ENSO. It is found that the December peak of ‘winter ENSO’ is caused by the continuous growth of ENSO anomaly from June to November, while the May–June peak of ‘summer ENSO’ appears to be caused jointly by the seasonal shift of higher growth rate into spring and stronger stochastic noise towards the first half of the year. Furthermore, the change of the seasonal cycle of feedbacks is contributed predominantly by that of the thermodynamic damping. The summer peak of ENSO is proposed to be caused by the following mechanism. A perihelion in the late fall to early winter leads to a cooling of the surface eastern equatorial Pacific (EEP) due to reduced insolation in spring. This cooling, reinforced by an oceanic process, reduces the latent heat flux damping in spring, and therefore favors the growth of the eastern Pacific-like ENSO (as opposed to the central Pacific-like ENSO). This EEP cooling is also likely to generate more effective short wave-cloud-SST feedback and, in turn, increased instability. Ultimately, the weakened thermodynamic damping in spring, combined with relatively intensive stochastic forcing, benefits the subsequent summer peak of ENSO.</p>},
  author       = {Lu, Zhengyao and Liu, Zhengyu},
  issn         = {0930-7575},
  keyword      = {ENSO,Ocean–atmosphere feedbacks,Orbital forcing,Phase locking},
  language     = {eng},
  number       = {7-8},
  pages        = {4329--4350},
  publisher    = {Springer},
  series       = {Climate Dynamics},
  title        = {Orbital modulation of ENSO seasonal phase locking},
  url          = {http://dx.doi.org/10.1007/s00382-018-4382-1},
  volume       = {52},
  year         = {2019},
}