scholarly journals A Comparison of Ocean Model Results with Satellite Observations during the Development of the strong 1997–98 El Niño

2019 ◽  
Author(s):  
David J. Webb ◽  
Andrew C. Coward ◽  
Helen M. Snaith
Keyword(s):  
High Resolution ◽  
Rossby Wave ◽  
El Niño ◽  
El Nino ◽  
Ocean Model ◽  
Equatorial Pacific ◽  
Satellite Observations ◽  
Sea Surface ◽  
Instability Waves ◽  
Tropical Instability Waves

Abstract. Satellite data from the equatorial Pacific is compared with results from a high resolution ocean model during a period including the strong El Niño of 1997–98. The results show that the ocean model realistically captures the changes in sea surface temperature and the propagation of the annual Rossby wave although it may overestimate the reduced energy of tropical instability waves during development of an El Niño. The results provide additional confidence in the oceanic mechanisms which model analysis implicated as being responsible for the development of both the 1982–83 and the 1997–98 El Niños.

scholarly journals A comparison of ocean model data and satellite observations of features affecting the growth of the North Equatorial Counter Current during the strong 1997–1998 El Niño

Ocean Science ◽  
2020 ◽  
Vol 16 (3) ◽  
pp. 565-574
Author(s):  
David J. Webb ◽  
Andrew C. Coward ◽  
Helen M. Snaith
Keyword(s):  
Sea Level ◽  
El Niño ◽  
El Nino ◽  
Ocean Model ◽  
Satellite Observations ◽  
Sea Surface ◽  
Warm Water ◽  
The North ◽  
North Equatorial Counter Current ◽  
Counter Current

Abstract. Descriptions of the ocean's role in the El Niño usually focus on equatorial Kelvin waves and the ability of such waves to change the mean thermocline depth and the sea surface temperature (SST) in the central and eastern Pacific. In contrast, starting from a study of the transport of water with temperatures greater than 28 ∘C, sufficient to trigger deep atmospheric convection, Webb (2018) found that, during the strong El Niños of 1983–1984 and 1997–1998, advection by the North Equatorial Counter Current (NECC) had a much greater impact on sea surface temperatures than processes occurring near the Equator. Webb's analysis, which supports the scheme proposed by Wyrtki (1973, 1974), made use of archived data from a high-resolution ocean model. Previously the model had been checked in a preliminary comparison against SST observations in the equatorial Pacific, but, given the contentious nature of the new analysis, the model's behaviour in key areas needs to be checked further against observations. In this paper this is done for the 1987–1988 El Niño, making use of satellite observations of SST and sea level. SST is used to check the movement of warm water near the Equator and at the latitudes of the NECC. Sea level is used to check the model results at the Equator and at 6∘ N in the North Equatorial Trough. Sea level differences between these latitudes affect the transport of the NECC, the increased transport at the start of each strong El Niño being associated with a drop in sea level at 6∘ N in the western Pacific. Later rises in sea level at the Equator increase the transport of the NECC in mid-ocean. The variability of sea level at 6∘ N is also used to compare the strength of tropical instability waves in the model and in the observations. The model showed that in a normal year these act to dilute the temperature in the core of the NECC. However their strength declined during the development of the strong El Niños, allowing the NECC to carry warm water much further than normal across the Pacific. The results of this paper should not be taken as providing proof of the hypotheses of Wyrtki (1973, 1974) or Webb (2018) but instead as a failure of a targeted study, using satellite observations, to disprove the hypotheses.

scholarly journals Improved Simulation of Tropical Cyclone Responses to ENSO in the Western North Pacific in the High-Resolution GFDL HiFLOR Coupled Climate Model*

2016 ◽  
Vol 29 (4) ◽  
pp. 1391-1415 ◽  
Author(s):  
Wei Zhang ◽  
Gabriel A. Vecchi ◽  
Hiroyuki Murakami ◽  
Thomas Delworth ◽  
Andrew T. Wittenberg ◽  
...  
Keyword(s):  
High Resolution ◽  
Tropical Cyclone ◽  
El Niño ◽  
North Pacific ◽  
Western North Pacific ◽  
El Nino ◽  
Walker Circulation ◽  
La Niña ◽  
Sea Surface ◽  
La Nina

Abstract This study aims to assess whether, and the extent to which, an increase in atmospheric resolution of the Geophysical Fluid Dynamics Laboratory (GFDL) Forecast-Oriented Low Ocean Resolution version of CM2.5 (FLOR) with 50-km resolution and the High-Resolution FLOR (HiFLOR) with 25-km resolution improves the simulation of the El Niño–Southern Oscillation (ENSO)–tropical cyclone (TC) connections in the western North Pacific (WNP). HiFLOR simulates better ENSO–TC connections in the WNP including TC track density, genesis, and landfall than FLOR in both long-term control experiments and sea surface temperature (SST)- and sea surface salinity (SSS)-restoring historical runs (1971–2012). Restoring experiments are performed with SSS and SST restored to observational estimates of climatological SSS and interannually varying monthly SST. In the control experiments of HiFLOR, an improved simulation of the Walker circulation arising from more realistic SST and precipitation is largely responsible for its better performance in simulating ENSO–TC connections in the WNP. In the SST-restoring experiments of HiFLOR, more realistic Walker circulation and steering flow during El Niño and La Niña are responsible for the improved simulation of ENSO–TC connections in the WNP. The improved simulation of ENSO–TC connections with HiFLOR arises from a better representation of SST and better responses of environmental large-scale circulation to SST anomalies associated with El Niño or La Niña. A better representation of ENSO–TC connections in HiFLOR can benefit the seasonal forecasting of TC genesis, track, and landfall; improve understanding of the interannual variation of TC activity; and provide better projection of TC activity under climate change.

Observed El Niño SSTA Development and the Effects of Easterly and Westerly Wind Events in 2014/15

2017 ◽  
Vol 30 (4) ◽  
pp. 1505-1519 ◽  
Author(s):  
Andrew M. Chiodi ◽  
D. E. Harrison
Keyword(s):  
Wind Stress ◽  
El Niño ◽  
El Nino ◽  
Ocean Model ◽  
Equatorial Pacific ◽  
Westerly Wind ◽  
Easterly Wind ◽  
Model Studies ◽  
Wind Events ◽  
Westerly Wind Events

Abstract The unexpected halt of warm sea surface temperature anomaly (SSTA) growth in 2014 and development of a major El Niño in 2015 has drawn attention to our ability to understand and predict El Niño development. Wind stress–forced ocean model studies have satisfactorily reproduced observed equatorial Pacific SSTAs during periods when data return from the TAO/TRITON buoy network was high. Unfortunately, TAO/TRITON data return in 2014 was poor. To study 2014 SSTA development, the observed wind gaps must be filled. The hypothesis that subseasonal wind events provided the dominant driver of observed waveguide SSTA development in 2014 and 2015 is used along with the available buoy winds to construct an oceanic waveguide-wide surface stress field of westerly wind events (WWEs) and easterly wind surges (EWSs). It is found that the observed Niño-3.4 SSTA development in 2014 and 2015 can thereby be reproduced satisfactorily. Previous 2014 studies used other wind fields and reached differing conclusions about the importance of WWEs and EWSs. Experiment results herein help explain these inconsistencies, and clarify the relative importance of WWEs and EWSs. It is found that the springtime surplus of WWEs and summertime balance between WWEs and EWSs (yielding small net wind stress anomaly) accounts for the early development and midyear reversal of El Niño–like SSTA development in 2014. A strong abundance of WWEs in 2015 accounts for the rapid SSTA warming observed then. Accurately forecasting equatorial Pacific SSTA in years like 2014 and 2015 may require learning to predict WWE and EWS occurrence characteristics.

scholarly journals Effects of Salinity Variability on Recent El Niño Events

Atmosphere ◽  
2019 ◽  
Vol 10 (8) ◽  
pp. 475 ◽  
Author(s):  
Hai Zhi ◽  
Rong-Hua Zhang ◽  
Pengfei Lin ◽  
Shiwei Shi
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Surface Salinity ◽  
Central Pacific ◽  
Salinity Variability ◽  
Central Equatorial Pacific

Ocean salinity variability provides a new way to study the evolution of the the El Niño-Southern Oscillation (ENSO). Comparisons between the salinity variation and related processes responsible for sea surface temperature anomaly (SSTA) were extensively examined for the two strong El Niño (EN) events in 1997/1998 and 2015/2016, and a special EN event in 2014/2015. The results show that the development of EN is significantly correlated with a sea surface salinity anomaly (SSSA) in the tropical western-central Pacific. In the spring of 1997 and 2015 with strong EN events, the western-central equatorial Pacific exhibited significant negative SSSA that propagated eastward to the west of the dateline. The negative SSSA induced increased barrier layer thickness (BLT) which enhanced sea surface temperature (SST) warming in the tropical central Pacific. In contrast, although a negative SSSA occurred during April of the 2014/2015 weak EN event in the western-central equatorial Pacific, this SSSA was mainly confined to between 160° E and 180° E without significant eastward movement, resulting in a weakened BLT thickening process and a weak modulation effect on SST. We also confirm that the surface forcing associated with fresh water flux (FWF: evaporation (E) minus precipitation (P)) plays a prominent role in the surface salinity tendency in the tropical Pacific during EN events. Moreover, the negative FWF anomaly leads a strong negative SSSA by two months. Compared with the two strong ENs, the early negative FWF anomaly in the weak 2014/2015 EN did not present distinct development and eastward propagation and weakened rapidly in the summer of 2015. We demonstrate that change in salinity can modulate the ENSO, and the variation of SSSA and associated physical processes in the tropical western-central Pacific and could be used as an indicator for predicting the development of ENSO.

scholarly journals A Model Study of Oceanic Mechanisms Affecting Equatorial Pacific Sea Surface Temperature during the 1997–98 El Niño

2001 ◽  
Vol 31 (7) ◽  
pp. 1649-1675 ◽  
Author(s):  
Jérôme Vialard ◽  
Christophe Menkes ◽  
Jean-Philippe Boulanger ◽  
Pascale Delecluse ◽  
Eric Guilyardi ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Model Study

The Termination of the 1997–98 El Niño. Part I: Mechanisms of Oceanic Change*

2006 ◽  
Vol 19 (12) ◽  
pp. 2633-2646 ◽  
Author(s):  
Gabriel A. Vecchi ◽  
D. E. Harrison
Keyword(s):  
El Niño ◽  
El Nino ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Central Pacific ◽  
Eastern Equatorial Pacific ◽  
Oceanic Response ◽  
Southward Shift ◽  
Sea Surface Temperature Anomalies ◽  
Subsurface Temperatures

Abstract The 1997–98 El Niño was both unusually strong and terminated unusually. Warm eastern equatorial Pacific (EEqP) sea surface temperature anomalies (SSTAs) exceeded 4°C at the event peak and lasted well into boreal spring of 1998, even though subsurface temperatures began cooling in December 1997. The oceanic processes that controlled this unusual termination are explored here and can be characterized by three features: (i) eastward propagating equatorial Pacific thermocline (Ztc) shoaling beginning in the central Pacific in November 1997; (ii) persistent warm EEqP SSTA between December 1997 and May 1998, despite strong EEqP Ztc shoaling (and subsurface cooling); and (iii) an abrupt cooling of EEqP SSTA in early May 1998 that exceeded 4°C within two weeks. It is shown here that these changes can be understood in terms of the oceanic response to changes to the meridional structure of the near-equatorial zonal wind field. Equatorial near-date-line westerly wind anomalies greatly decreased in late 1997, associated with a southward shift of convective and wind anomalies. In the EEqP, equatorial easterlies disappeared (reappeared) in late January (early May) 1998, driving the springtime extension (abrupt termination) of this El Niño event. The authors suggest that the wind changes arise from fundamentally meridional processes and are tied to the annual cycle of insolation.

scholarly journals Indian Ocean Sea Surface Temperature and El Niño–Southern Oscillation: A New Perspective

2005 ◽  
Vol 18 (9) ◽  
pp. 1351-1368 ◽  
Author(s):  
Pascal Terray ◽  
Sébastien Dominiak
Keyword(s):  
Indian Ocean ◽  
Surface Temperature ◽  
Sea Surface Temperature ◽  
El Niño ◽  
Regime Shift ◽  
El Nino ◽  
Equatorial Pacific ◽  
Sea Surface ◽  
Southern Indian Ocean ◽  
Boreal Winter

Abstract Here the 1976–77 climate regime shift that was accompanied by a remarkable change in the lead–lag relationships between Indian Ocean sea surface temperature (SST) and El Niño evolution is shown. After the 1976–77 regime shift, a correlation analysis suggests that southern Indian Ocean SSTs observed during late boreal winter are a key precursor in predicting El Niño evolution as the traditional oceanic heat content anomalies in the equatorial Pacific or zonal wind anomalies over the equatorial western Pacific. The possible physical mechanisms underlying this highly significant statistical relationship are discussed. After the 1976–77 regime shift, southern Indian Ocean SST anomalies produced by Mascarene high pulses during boreal winter trigger coupled air–sea processes in the tropical eastern Indian Ocean during the following seasons. This produces a persistent remote forcing on the Pacific climate system, promoting wind anomalies over the western equatorial Pacific and modulating the regional Hadley cell in the southwest Pacific. These modulations, in turn, excite Rossby waves, which produce quasi-stationary circulation anomalies in the extratropical South Pacific, responsible for the development of the southern branch of the “horseshoe” El Niño pattern. The change of the background SST state that occurred in the late 1970s over the Indian Ocean may also explain why ENSO evolution is different before and after the 1976–77 regime shift. These results shed some light on the possible influence of global warming or decadal fluctuations on El Niño evolution through changes in teleconnection patterns between the Indian and Pacific Oceans.

scholarly journals Oceanic Rossby Waves over Eastern Tropical Pacific of Both Hemispheres Forced by Anomalous Surface Winds after Mature Phase of ENSO

2016 ◽  
Vol 46 (11) ◽  
pp. 3397-3414 ◽  
Author(s):  
Hiroto Abe ◽  
Youichi Tanimoto ◽  
Takuya Hasegawa ◽  
Naoto Ebuchi
Keyword(s):  
Rossby Wave ◽  
El Niño ◽  
Rossby Waves ◽  
El Nino ◽  
Tropical Pacific ◽  
Sea Surface ◽  
Eastern Coast ◽  
Eastern Tropical Pacific ◽  
Enso Events ◽  
Anomalous Surface

AbstractThe present study examined ENSO-related wind forcing contribution to off-equatorial Rossby wave formations in the eastern tropical regions of the North and South Pacific using satellite altimeter data and atmospheric reanalysis data during the period of 1993–2013. After mature phases of ENSO events, the sea surface height anomaly fields showed that off-equatorial Rossby waves propagated westward along 11°N and 8°S from the eastern Pacific. Starting longitudes of the westward propagation were distant from the eastern coast, especially for weak El Niño events in the 2000s, in contrast to the strong 1997/98 El Niño event in which the propagations started from the coast. Based on observational data, it was hypothesized that the Rossby waves could be formed by off-equatorial zonal belts of wind stress curl anomalies (WSCAs) in 135°–90°W rather than by wave emissions from the eastern coast. A numerical model forced only by WSCAs, that is, without wave emissions from the coast, successfully reproduced observed features of the Rossby waves in 180°–120°W, supporting the study’s hypothesis. During mature phases of El Niño events, equatorially symmetric negative sea level pressure anomalies (SLPAs) resulting from hydrostatic adjustment to the underlying warm sea surface temperature anomalies dominated over the eastern tropical Pacific. Anomalous surface easterlies blowing around the negative SLPA area as geostrophic winds were a major contributor in forming the anticyclonic WSCAs. The polarity of the anomalies is reversed during La Niña events. Therefore, spatial patterns of the SLPAs associated with the ENSO events are necessary to understand the Rossby wave formations.

Interdecadal Change in Properties of El Niño–Southern Oscillation in an Intermediate Coupled Model

2005 ◽  
Vol 18 (9) ◽  
pp. 1369-1380 ◽  
Author(s):  
Rong-Hua Zhang ◽  
Antonio J. Busalacchi
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
El Niño ◽  
El Nino ◽  
Southern Oscillation ◽  
Coupled Model ◽  
Ocean Model ◽  
Sea Surface ◽  
Subsurface Temperature ◽  
Intermediate Coupled Model

Abstract The role of subsurface temperature variability in modulating El Niño–Southern Oscillation (ENSO) properties is examined using an intermediate coupled model (ICM), consisting of an intermediate dynamic ocean model and a sea surface temperature (SST) anomaly model. An empirical procedure is used to parameterize the temperature of subsurface water entrained into the mixed layer (Te) from sea level (SL) anomalies via a singular value decomposition (SVD) analysis for use in simulating sea surface temperature anomalies (SSTAs). The ocean model is coupled to a statistical atmospheric model that estimates wind stress anomalies also from an SVD analysis. Using the empirical Te models constructed from two subperiods, 1963–79 (T63–79e) and 1980–96 (T80–96e), the coupled system exhibits strikingly different properties of interannual variability (the oscillation period, spatial structure, and temporal evolution). For the T63–79e model, the system features a 2-yr oscillation and westward propagation of SSTAs on the equator, while for the T80–96e model, it is characterized by a 5-yr oscillation and eastward propagation. These changes in ENSO properties are consistent with the behavior shift of El Niño observed in the late 1970s. Heat budget analyses further demonstrate a controlling role played by the vertical advection of subsurface temperature anomalies in determining the ENSO properties.

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