Evolution of the 2006-2007 El Niño: the role of intraseasonal to interannual time scale dynamics

International audienceWe describe development of the 2006-2007 El Niño, which started late, ended early and was below average strength. Emphasis is on the interplay between large scale, low frequency (i.e., seasonal-to-interannual time scale) deterministic dynamics and episodic intraseasonal wind forcing in the evolution of the event. Efforts to forecast the El Niño are reviewed, with discussion of factors affecting its predictability. Perspectives on the contemporaneous development of an Indian Ocean Dipole Zonal Mode event in 2006 and possible influences of global warming on the ENSO cycle, which exhibited unusual behavior in the first decade of the 21st century, will also be presented

Variability in the central equatorial Indian Ocean, Part II: Oceanic heat and turbulent energy balances

Heat and turbulent energy balances in the central equatorial Indian Ocean are examined using over two years (January 1973-May 1975) of simultaneous oceanic and atmospheric measurements from the island of Gan (00°41S, 73°10\u27E)…

Variability in the central equatorial Indian Ocean Part I: Ocean dynamics

Time series of simultaneous wind stress, ocean temperature and velocity from the Island of Gan (00°41\u27S, 73°10\u27E) in the equatorial Indian Ocean are examined for the period January 1973-May 1975. Means, trends, and variance at 1 and 2 cycles per year are removed by regression techniques and compared to existing equatorial ocean theories...

Moored rainfall measurements during COARE

This presentation discusses mini-ORG rainfall estimates collected from an array of six moornings in the western equatorial Pacific during the TOGA-COARE experiment. The moorings were clustered in the vicinity of the COARE intensive flux array (IFA) centered near 2 deg S, 156 deg E. The basic data set consisted of hourly means computed from 5-second samples

Symmetry of the Atlantic Nino mode

El Niño-Southern Oscillation (ENSO) in the Pacific is asymmetric for warm and cold events with respect to amplitude, spatial patterns and temporal evolution. Here the symmetry of the Atlantic Niño mode, which many previous studies have argued is governed by atmosphere–ocean dynamics similar to those of ENSO, is investigated using two different ocean reanalysis products. Calculation of Bjerknes feedback terms for the Pacific reveals a pronounced asymmetry between warm and cold events, though unlike most previous studies, the largest asymmetry is found in the relationship between eastern Pacific thermocline depth and SST anomalies. For the Atlantic, cold events are effectively mirror images of warm events with Bjerknes feedbacks of similar strength. The analysis supports not only the conclusion that Atlantic Niños are more symmetric than ENSO, but the hypothesis itself that the Bjerknes feedback is operative in the Atlantic given the strength of the relationship between the key variables involved

Assessing the Twenty-First-Century Shift in ENSO Variability in Terms of the Bjerknes Stability Index*

A decadal change in the character of ENSO was observed around year 2000 toward weaker-amplitude, higher-frequency events with an increased occurrence of central Pacific El Niños. Here these changes are assessed in terms of the Bjerknes stability index (BJ index), which is a measure of the growth rate of ENSO-related SST anomalies. The individual terms of the index are calculated from ocean reanalysis products separately for the time periods 1980–99 and 2000–10. The spread between the products is large, but they show a robust weakening of the thermocline feedback due to a reduced thermocline slope response to anomalous zonal wind stress as well as a weakened wind stress response to eastern equatorial Pacific SST anomalies. These changes are consistent with changes in the background state of the tropical Pacific: cooler mean SST in the eastern and central equatorial Pacific results in reduced convection there together with a westward shift in the ascending branch of the Walker circulation. This shift leads to a weakening in the relationship between eastern Pacific SST and longitudinally averaged equatorial zonal wind stress. Also, despite a steeper mean thermocline slope in the more recent period, the thermocline slope response to wind stress anomalies weakened due to a smaller zonal wind fetch that results from ENSO-related wind anomalies being more confined to the western basin. As a result, the total BJ index is more negative, corresponding to a more strongly damped system in the past decade compared to the 1980s and 1990s

A Comparative Stability Analysis of Atlantic and Pacific Nino Modes

El Niño–Southern Oscillation (ENSO) in the Pacific and the analogous Atlantic Niño mode are generated by processes involving coupled ocean–atmosphere interactions known as the Bjerknes feedback. It has been argued that the Atlantic Niño mode is more strongly damped than ENSO, which is presumed to be closer to neutrally stable. In this study the stability of ENSO and the Atlantic Niño mode is compared via an analysis of the Bjerknes stability index. This index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean–atmosphere variability. Using observational data, an ocean reanalysis product, and output from an ocean general circulation model, the individual terms of the Bjerknes index are calculated for the first time for the Atlantic and then compared to results for the Pacific. Positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins, but they are twice as large in the Pacific compared to the Atlantic. Thermocline feedback is related to the fetch of the zonal winds, which is much greater in the equatorial Pacific than in the equatorial Atlantic due to larger basin size. Negative feedbacks are dominated by thermal damping of sea surface temperature anomalies in both basins. Overall, it is found that both ENSO and the Atlantic Niño mode are damped oscillators, but the Atlantic is more strongly damped than the Pacific primarily because of the weaker thermocline feedback

Assessment of radiative heating errors in Tropical Atmosphere Ocean array marine air temperature measurements

We assess the radiative heating error affecting marine air temperature (MAT) measurements in the Tropical Atmosphere Ocean array. The error in historical observations is found to be ubiquitous across the array, spatially variable and approximately stationary in time. The error induces spurious warming during daytime hours, but does not affect night-time temperatures. The range encompassing the real, unknown daily- and monthly-mean values is determined using daytime and night-time mean temperatures as upper and lower limits. The uncertainty in MAT is less than or equal to 0.5 °C and 0.2 °C for 95% of daily and monthly estimates, respectively. Uncertainties impact surface turbulent heat flux estimates, with potentially significant influences on the quantification of coupled ocean-atmosphere processes

Variability in the South Atlantic Anticyclone and the Atlantic Niño mode

Sea surface temperature (SST) anomalies in the eastern equatorial Atlantic are connected to modulations in the strength of the South Atlantic subtropical high-pressure system, referred to as the South Atlantic Anticyclone (SAA). Using ocean and atmosphere reanalysis products we show here that the strength of the SAA from February to May impacts the timing of the cold tongue onset and the intensity of its development in the eastern equatorial Atlantic (EEA) via anomalous tropical wind power. This modulation of the timing and amplitude of the seasonal cold tongue development manifests as anomalous SST events peaking between June and August. The timing and impact of this connection is not completely symmetric for warm and cold events. For cold events, an anomalously strong SAA in February and March leads to positive wind power anomalies from February to June resulting in an early cold tongue onset and subsequent cold SST anomalies in June and July. For warm events the anomalously weak SAA persists until May, generating negative wind power anomalies that lead to a late cold tongue onset as well as a suppression of the cold tongue development and associated warm SST anomalies. Mechanisms by which SAA induced wind power variations south of the equator influence EEA SST are discussed, including ocean adjustment via Rossby and Kelvin wave propagation, meridional advection, and local intraseasonal wind variation

Evaluation of NSCAT scatterometer winds using equatorial Pacific buoy observations

As part of the calibration/validation effort for NASA's Scatterometer (NSCAT) we compare the satellite data to winds measured at the sea surface with an array of buoys moored in the equatorial Pacific Ocean. The NSCAT data record runs from September, 1996 through the end of June, 1997. The raw NSCAT data, radar backscatter, is converted to wind vectors at 10 meters above the surface assuming a neutrally stratified atmosphere, using the NSCAT-1 and NSCAT-2 model functions. The surface winds were measured directly by the TAO (Tropical Atmosphere Ocean) buoy array which spans the width of the equatorial Pacific within about 8° of the equator. The buoy program and data archive are maintained by the Pacific Marine Environmental Laboratory, at the National Oceanic and Atmospheric Administration, in collaboration with institutions in Japan, France and Taiwan. We also use data from two buoys maintained by the Woods Hole Oceanographic Institution located along 125°W. Since the buoy winds are measured at various heights above the surface, they are adjusted for both height and atmospheric surface layer stratification before comparisons are made to the NSCAT data. Co-location requirements include measurements within 100 km and 60 minutes of each other. There was a total of 5580 comparisons for the NSCAT-1 model function and 6364 comparisons for the NSCAT-2 model function. The NSCAT wind speeds, using the NSCAT-1 model function, are lower than the buoy wind speeds by about 0.54 ms-1 and have a 9.8° directional bias. The NSCAT-2 winds speeds were lower than the TAO buoy winds by only 0.08 ms-1, but still have the same 9.8° directional bias. The wind retrieval algorithm selects the vector closest to the buoy approximately 88% of the time. However, in the relatively low wind speed regime of the TAO array, approximately 4% of the wind vectors are more than 120° from the buoy wind.Funding was provided by the National Aeronautics and Space Administration under Contract No. 957652