A major accomplishment of the recently completed Tropical Ocean-Global Atmosphere (TOGA) Program was the development of an ocean observing system to support seasonal-to-interannual climate studies. This paper reviews the scientific motivations for the development of that observing system, the technological advances that made it possible, and the scientific advances that resulted from the availability of a significantly expanded observational database. A primary phenomenological focus of TOGA was interannual variability of the coupled ocean-atmosphere system associated with El Nino and the Southern Oscillation (ENSO).Prior to the start of TOGA, our understanding of the physical processes responsible for the ENSO cycle was limited, our ability to monitor variability in the tropical oceans was primitive, and the capability to predict ENSO was nonexistent. TOGA therefore initiated and/or supported efforts to provide real-time measurements of the following key oceanographic variables: surface winds, sea surface temperature, subsurface temperature, sea level and ocean velocity. Specific in situ observational programs developed to provide these data sets included the Tropical Atmosphere-Ocean (TAO) array of moored buoys in the Pacific, a surface drifting buoy program, an island and coastal tide gauge network, and a volunteer observing ship network of expendable bathythermograph measurements. Complementing these in situ efforts were satellite missions which provided near-global coverage of surface winds, sea surface temperature, and sea level. These new TOGA data sets led to fundamental progress in our understanding of the physical processes responsible for ENSO and to the development of coupled ocean-atmosphere models for ENSO prediction.

/pdf/the-tropical-ocean-global-atmosphere-observing-system-a-c3sujtrlum.pdf

The Tropical Ocean‐Global Atmosphere observing system: A decade of progress

Rossby waves play a critical role in the transient adjustment of ocean circulation to changes in large-scale atmospheric forcing. The TOPEX/POSEIDON satellite altimeter has detected Rossby waves throughout much of the world ocean from sea level signals with ≲10-centimeter amplitude and ≳500-kilometer wavelength. Outside of the tropics, Rossby waves are abruptly amplified by major topographic features. Analysis of 3 years of data reveals discrepancies between observed and theoretical Rossby wave phase speeds that indicate that the standard theory for free, linear Rossby waves is an incomplete description of the observed waves.

https://science.sciencemag.org/content/sci/272/5259/234.full.pdf

Global Observations of Oceanic Rossby Waves

Recent findings about zonal displacements of the Pacific warm pool required a notable modification of the delayed action oscillator theory, the current leading theory for the El Nino-Southern Oscillation (ENSO) Simulations with a linearized coupled ocean-atmosphere model resulted in 3- to 6-year ENSO-like oscillations, with many of the variable model parameters found to be very close to their observed values This simple model suggests that ocean processes that are ignored or underestimated in the delayed action oscillator theory, such as zonal current convergence, zonal advection of sea surface temperature, and equatorial wave reflection from the eastern ocean boundary, are fundamental to the development of the ENSO, in particular to its manifestations in the central equatorial Pacific

/pdf/an-advective-reflective-conceptual-model-for-the-oscillatory-14c3vyqono.pdf

An Advective-Reflective Conceptual Model for the Oscillatory Nature of the ENSO

El Nino and La Nina are not a simple mirror image, but exhibit significant differences in their spatial structure and seasonal evolution. In particular, sea surface temperature (SST) anomalies over the equatorial Pacific cold tongue are larger in magnitude during El Nino compared to La Nina, resulting in positive skewness of interannual SST variations. The associated atmospheric deep convection anomalies are displaced eastward during El Nino compared to La Nina because of the nonlinear atmospheric response to SST. In addition to these well-known features, an analysis of observational data for the past century shows that there is a robust asymmetry in the duration of El Nino and La Nina. Most El Ninos and La Ninas develop in late boreal spring/summer, when the climatological cold tongue is intensifying, and they peak near the end of the calendar year. After the mature phase, El Ninos tend to decay rapidly by next summer, but many La Ninas persist through the following year and often reintensify in...

/pdf/asymmetry-in-the-duration-of-el-nino-and-la-nina-51vprjz0ya.pdf

Asymmetry in the Duration of El Niño and La Niña

Observations of the development of recent El Nino events suggest a pivotal role for the Madden–Julian oscillation (MJO). Previous attempts to uncover a systematic relationship between MJO activity and the El Nino–Southern Oscillation (ENSO), however, have yielded conflicting results. In this study the MJO–ENSO relationship is stratified by season, and the focus is on MJO activity in the equatorial western Pacific. The results demonstrate that MJO activity in late boreal spring leads El Nino in the subsequent autumn–winter for the period 1979–2005. Spring is the season when MJO activity is least asymmetric with respect to the equator and displays the most sensitivity to SST variations at the eastern edge of the warm pool. Enhanced MJO activity in the western Pacific in spring is associated with an eastward-expanded warm pool and low-frequency westerly surface zonal wind anomalies. These sustained westerly anomalies in the western Pacific are hypothesized to project favorably onto a developing El N...

Seasonal Dependence of the MJO-ENSO Relationship

In the equatorial Pacific, zonal displacements of the eastern edge of the warm pool represent an intrinsic manifestation of El Nino Southern Oscillation (ENSO) events, with numerous dynamical and biogeochemical consequences. Following a previous work dedicated to the 1986–1989 Geosat period, we focus on the 1992–1998 zonal displacements of the warm pool using mainly TOPEX/Poseidon data. We also used a simple linear model forced by monthly ERS winds to help in the interpretation of the results. We found that the 1992–1998 zonal displacements of the warm pool resulted mainly from horizontal advection by zonal current anomalies, through a combination of interannual equatorial Kelvin and first meridional mode Rossby waves. The interannual equatorial Kelvin waves were essentially wind forced in the western and central equatorial Pacific, with some minor contribution from reflected Rossby waves on the western Pacific boundary. In particular, westerly wind anomalies and the resulting downwelling Kelvin waves (entailing eastward surface current anomalies and thermocline deepening) contributed strongly to the onset of the 1993, 1994–1995 and 1997–1998 El Nino events. In contrast, easterly wind anomalies and the resulting upwelling Kelvin waves (with westward surface current anomalies and thermocline shoaling) played a role in stopping the 1993 El Nino and in shifting the 1994–1995 and 1997–1998 El Nino into La Nina events. Consistently with the 1987–1988 El Nino-La Nina scenario, two main downwelling Rossby wave packets, originating from eastern boundary reflections and wind forcing, crossed the entire basin in 1993 and 1994–1995. These waves favored the decay of the corresponding El Nino events, in the sense that their associated current anomalies contributed to shifting the displacements of the eastern edge of the warm pool from eastward to westward. Unlike what happened for the termination of the 1993 and 1994–1995 El Nino events, downwelling Rossby wave packets, mostly reflected from impinging Kelvin waves, did not propagate all the way to the western Pacific during the 1997–1998 El Nino. They stopped propagating in the central basin where they met unfavorable eastward migrating westerly wind anomalies which generated upwelling Rossby waves. Hence reflected downwelling and wind-forced upwelling Rossby waves opposed each other for shifting the eastern edge of the warm pool. The rapid demise of the 1997–1998 El Nino and its shift into La Nina in mid-1998 are interpreted as resulting mainly from the effect of upwelling Kelvin waves forced by easterly wind anomalies occurring in the west from the end of 1997. The associated thermocline shoaling was further enhanced by the wind-forced upwelling Rossby waves in the central basin in mid-1998, strongly influencing the fast sea surface temperature (SST) cooling at times when the thermocline was very close to the surface at the end of the mature phase of the 1997–1998 El Nino.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JC900113

Equatorial waves and warm pool displacements during the 1992–1998 El Niño Southern Oscillation events: Observation and modeling

Equatorial waves and warm pool displacements during the 1992–1998 ENSO events. Observations and modeling

Profiling current meter (PCM) measurements under adrifting buoy are compared with concurrent shipboard acoustic Doppler current profiler (ADCP) measurements camed out in the western equatorial Pacific in March 1991, from 10"s to 7ON along the 165"E meridian. The mean (ADCP minus PCM) zk rms differences between zonal and meridional velocity components are 5.7 t 11.2 cm s-' and 0.0 t 8.8 cm s-'. respectively, when PCM measurements are relative to 600 m. The mean f rms differences decrease to 2.3 t 7.8 cm s-' and 0.0 k 6.3 cm s-' when the PCM and ADCP data are both referenced to the same layer (on a mean, 16-240 m). As compared with ADCP, it is found that PCM underestimates velocities of less than 20 cm s-' by about 25%.

/pdf/comparison-of-profiling-current-meter-and-shipboard-adcp-1yqr0l81hn.pdf

Comparison of Profiling Current Meter and Shipboard ADCP Measurements in the Western Equatorial Pacific

/pdf/rapport-de-la-campagne-surtropac-14-a-bord-du-n-o-le-noroit-1oxx0l5m3c.pdf

Rapport de la campagne SURTROPAC 14 à bord du N.O. LE NOROIT : 11 mars au 8 avril 1991, de 20°S à 8°N le long du méridien 165°E

/pdf/atlas-des-variations-de-temperature-et-de-salinite-de-41k1cvh850.pdf

Atlas des variations de température et de salinité de surface du Pacifique tropical (1969-1988)

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