scholarly journals One plausible reason for the change in ENSO characteristics in the 2000s

2013 ◽  
Vol 10 (4) ◽  
pp. 951-984
Author(s):  
V. N. Stepanov
Keyword(s):  
Southern Ocean ◽  
Equatorial Pacific ◽  
Warm Water ◽  
Water Volume ◽  
Atmospheric Conditions ◽  
Enso Events ◽  
Wind Variability ◽  
Western Equatorial Pacific ◽  
Warm Water Volume ◽  
The Tropics

Abstract. It is well known that El Niño Southern Oscillation (ENSO) causes floods, droughts and the collapse of fisheries, therefore forecasting of ENSO is an important task in climate researches. Variations in the equatorial warm water volume of the tropical Pacific and wind variability in the western equatorial Pacific has been considered to be a good ENSO predictor. However, in the 2000s, the interrelationship between these two characteristics and ENSO onsets became weak. This article attempts to find some plausible explanation for this. The results presented here demonstrate a possible link between the variability of atmospheric conditions over the Southern Ocean and their impact on the ocean circulation leading to the amplifying/triggering of ENSO events. It is shown that the variability of the atmospheric conditions upstream of Drake Passage can strongly influence ENSO events. The interrelationship between ENSO and variability in the equatorial warm water volume of the equatorial Pacific, together with wind variability in the western equatorial Pacific has recently weakened. It can be explained by the fact that the process occurred in the Southern Ocean recently became a major contributor amplifying ENSO events (in comparison with the processes of interaction between the atmosphere and the ocean in the tropics of the Pacific). Likely it is due to a warmer ocean state observed from the end of the 1990s that led to smaller atmospheric variability in the tropics and insignificant their changes in the Southern Ocean.

scholarly journals One plausible reason for the change in ENSO characteristics in the 2000s

2014 ◽  
Vol 11 (2) ◽  
pp. 943-978
Author(s):  
V. N. Stepanov
Keyword(s):  
Southern Ocean ◽  
Equatorial Pacific ◽  
Water Volume ◽  
Atmospheric Conditions ◽  
Enso Events ◽  
Wind Variability ◽  
Western Equatorial Pacific ◽  
Warm Water Volume ◽  
The Tropics ◽  
The Tropical Pacific

Abstract. It is well known that El Niño Southern Oscillation (ENSO) causes floods, droughts in different regions of the Earth and the collapse of fisheries in the tropical Pacific, therefore forecasting of ENSO is an important task in climate researches. Variations in the equatorial warm water volume of the tropical Pacific and wind variability in the western equatorial Pacific has been considered to be a good ENSO predictor. However, in the 2000s, the interrelationship between these two characteristics and ENSO onsets became weak. This article attempts to find some plausible explanation for this. The results presented here demonstrate a possible link between the variability of atmospheric conditions over the Southern Ocean and their impact on the ocean circulation leading to the amplifying of ENSO events. It is shown that the variability of the atmospheric conditions upstream of Drake Passage can strongly influence ENSO events. The interrelationship between ENSO and variability in the equatorial warm water volume of the equatorial Pacific, together with wind variability in the western equatorial Pacific has recently weakened. It can be explained by the fact that the process occurred in the Southern Ocean recently became a major contributor amplifying ENSO events (in comparison with the processes of interaction between the atmosphere and the ocean in the tropics of the Pacific). Likely it is due to a warmer ocean state observed from the end of the 1990s that led to smaller atmospheric variability in the tropics and insignificant their changes in the Southern Ocean.

scholarly journals On the Warm Water Volume and Its Changing Relationship with ENSO

2014 ◽  
Vol 44 (5) ◽  
pp. 1372-1385 ◽  
Author(s):  
Lucia Bunge ◽  
Allan J. Clarke
Keyword(s):  
Southern Oscillation ◽  
Equatorial Pacific ◽  
Wind Forcing ◽  
Warm Water ◽  
Water Volume ◽  
Recent Theory ◽  
Second Mode ◽  
Western Equatorial Pacific ◽  
Warm Water Volume ◽  
Tilt Mode

Abstract The interannual, equatorial Pacific, 20°C isotherm depth variability since 1980 is dominated by two empirical orthogonal function (EOF) modes: the “tilt” mode, having opposite signs in the eastern and western equatorial Pacific and in phase with zonal wind forcing and El Niño–Southern Oscillation (ENSO) indices, and a second EOF mode of one sign across the Pacific. Because the tilt mode is of opposite sign in the eastern and western equatorial Pacific while the second EOF mode is of one sign, the second mode has been associated with the warm water volume (WWV), defined as the volume of water above the 20°C isotherm from 5°S to 5°N, 120°E to 80°W. Past work suggested that the WWV led the tilt mode by about 2–3 seasons, making it an ENSO predictor. But after 1998 the lead has decreased and WWV-based predictions of ENSO have failed. The authors constructed a sea level–based WWV proxy back to 1955, and before 1973 it also exhibited a smaller lead. Analysis of data since 1980 showed that the decreased WWV lead is related to a marked increase in the tilt mode contribution to the WWV and a marked decrease in second-mode EOF amplitude and its contribution. Both pre-1973 and post-1998 periods of reduced lead were characterized by “mean” La Niña–like conditions, including a westward displacement of the anomalous wind forcing. According to recent theory, and consistent with observations, such westward displacement increases the tilt mode contribution to the WWV and decreases the second-mode amplitude and its WWV contribution.

Ocean Response to Wind Variations, Warm Water Volume, and Simple Models of ENSO in the Low-Frequency Approximation

2010 ◽  
Vol 23 (14) ◽  
pp. 3855-3873 ◽  
Author(s):  
Alexey V. Fedorov
Keyword(s):  
Low Frequency ◽  
Equatorial Pacific ◽  
Warm Water ◽  
Water Volume ◽  
Thermocline Depth ◽  
Phase Lag ◽  
Eastern Equatorial Pacific ◽  
The Pacific ◽  
Warm Water Volume ◽  
Frequency Approximation

Abstract Physical processes that control ENSO are relatively fast. For instance, it takes only several months for a Kelvin wave to cross the Pacific basin (Tk ≈ 2 months), while Rossby waves travel the same distance in about half a year. Compared to such short time scales, the typical periodicity of El Niño is much longer (T ≈ 2–7 yr). Thus, ENSO is fundamentally a low-frequency phenomenon in the context of these faster processes. Here, the author takes advantage of this fact and uses the smallness of the ratio ɛk = Tk/T to expand solutions of the ocean shallow-water equations into power series (the actual parameter of expansion also includes the oceanic damping rate). Using such an expansion, referred to here as the low-frequency approximation, the author relates thermocline depth anomalies to temperature variations in the eastern equatorial Pacific via an explicit integral operator. This allows a simplified formulation of ENSO dynamics based on an integro-differential equation. Within this formulation, the author shows how the interplay between wind stress curl and oceanic damping rates affects 1) the amplitude and periodicity of El Niño and 2) the phase lag between variations in the equatorial warm water volume and SST in the eastern Pacific. A simple analytical expression is derived for the phase lag. Further, applying the low-frequency approximation to the observed variations in SST, the author computes thermocline depth anomalies in the western and eastern equatorial Pacific to show a good agreement with the observed variations in warm water volume. Ultimately, this approach provides a rigorous framework for deriving other simple models of ENSO (the delayed and recharge oscillators), highlights the limitations of such models, and can be easily used for decadal climate variability in the Pacific.

scholarly journals On the Physics of the Warm Water Volume and El Niño/La Niña Predictability

2019 ◽  
Vol 49 (6) ◽  
pp. 1541-1560 ◽  
Author(s):  
Allan J. Clarke ◽  
Xiaolin Zhang
Keyword(s):  
El Niño ◽  
El Nino ◽  
Warm Pool ◽  
Equatorial Pacific ◽  
La Niña ◽  
Warm Water ◽  
Water Volume ◽  
La Nina ◽  
Eastern Equatorial Pacific ◽  
Warm Water Volume

AbstractPrevious work has shown that warm water volume (WWV), usually defined as the volume of equatorial Pacific warm water above the 20°C isotherm between 5°S and 5°N, leads El Niño. In contrast to previous discharge–recharge oscillator theory, here it is shown that anomalous zonal flow acceleration right at the equator and the movement of the equatorial warm pool are crucial to understanding WWV–El Niño dynamics and the ability of WWV to predict ENSO. Specifically, after westerly equatorial wind anomalies in a coupled ocean–atmosphere instability push the warm pool eastward during El Niño, the westerly anomalies follow the warmest water south of the equator in the Southern Hemisphere summer in December–February. With the wind forcing that causes El Niño in the eastern Pacific removed, the eastern equatorial Pacific sea level and thermocline anomalies decrease. Through long Rossby wave dynamics this decrease results in an anomalous westward equatorial flow that tends to push the warm pool westward and often results in the generation of a La Niña during March–June. The anomalously negative eastern equatorial Pacific sea level typically does not change as much during La Niña, the negative feedback is not as strong, and El Niños tend to not follow La Niñas the next year. This El Niño/La Niña asymmetry is seen in the WWV/El Niño phase diagram and decreased predictability during “La Niña–like” decades.

scholarly journals Mechanisms controlling warm water volume interannual variations in the equatorial Pacific: diabatic versus adiabatic processes

2011 ◽  
Vol 38 (5-6) ◽  
pp. 1031-1046 ◽  
Author(s):  
M. Lengaigne ◽  
U. Hausmann ◽  
G. Madec ◽  
C. Menkes ◽  
J. Vialard ◽  
...  
Keyword(s):  
Equatorial Pacific ◽  
Warm Water ◽  
Water Volume ◽  
Interannual Variations ◽  
Warm Water Volume ◽  
Adiabatic Processes

scholarly journals Analytical Theory for the Quasi-Steady and Low-Frequency Equatorial Ocean Response to Wind Forcing: The “Tilt” and “Warm Water Volume” Modes

2010 ◽  
Vol 40 (1) ◽  
pp. 121-137 ◽  
Author(s):  
Allan J. Clarke
Keyword(s):  
Wind Stress ◽  
Large Scale ◽  
Low Frequency ◽  
Equatorial Pacific ◽  
Warm Water ◽  
Water Volume ◽  
Kelvin Wave ◽  
Eastern Equatorial Pacific ◽  
Warm Water Volume ◽  
Ocean Boundary

Abstract Analytical theory is used to examine the linear response of a meridionally unbounded stratified ocean to large-scale, low-frequency wind forcing. The following results, applied mainly to the equatorial Pacific, were obtained. (i) Provided that the wind stress curl vanishes at large distance from the equator, a general Sverdrup solution is valid in the quasi-steady (frequency ω → 0) limit. The meridionally averaged zonal flow toward the western boundary layer is zero so that there is no net mass flow into the boundary layer and the large-scale boundary condition is therefore satisfied. This solution predicts a zero pycnocline response in the eastern equatorial Pacific. It therefore predicts that, for the eastern equatorial Pacific, a slow weakening of the equatorial trade winds will not lead to long-term El Niño conditions there. (ii) Consistent with observations and other previous work, for finite but small frequencies there are two modes of equatorial motion. One is a “tilt” mode in which the equatorial sea level and thermocline are tilted by the in-phase zonal wind stress and the other is an equatorial warm water volume (WWV) mode in which the discharge of equatorial warm water (negative WWV anomaly) lags the wind stress forcing by a quarter of a period. (iii) The amplitude of the WWV mode approaches zero like ω1/2. Therefore, as ω → 0, the equatorial solution reduces to the tilt mode. (iv) The WWV mode is not due to a dominant meridional divergence driven by the wind, as suggested by some previous work. Meridional and zonal divergence approximately cancel. Reflection of energy at both ocean boundaries together with the strong dependence of long Rossby wave speed on latitude is crucial to the existence of the disequilibrium WWV mode. Because higher-latitude Rossby waves travel so much more slowly, the Rossby waves reflecting from the western ocean boundary are not in phase. This gives rise to a reflected equatorial Kelvin wave and a WWV that is not in phase with the wind stress forcing. (v) Observations from past work have shown that much low-frequency wave energy, particularly westward propagating Rossby wave energy poleward of about 5°N and 5°S, is damped out before it reaches the western ocean boundary. In this way dissipation likely has a strong influence on the equatorial Kelvin wave reflection and hence the disequilibrium WWV.

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