Howard F. Seidel

Pacific meridional mode and El Niño—Southern Oscillation

(1) We present intriguing evidence that the majority of El Nino events over the past four decades are preceded by a distinctive sea-surface warming and southwesterly wind anomaly in the vicinity of the Intertropical Convergence Zone (ITCZ) during the boreal spring. This phenomenon, known as the Meridional Mode (MM), is shown to be intrinsic to the thermodynamic coupling between the atmosphere and ocean. The MM effectively acts as a conduit through which the extratropical atmosphere influences ENSO. Modeling results further suggest that the MM plays a vital role in the seasonal phase-locking behavior of ENSO. The findings provide a new perspective for understanding the important role of thermodynamic ocean-atmosphere feedback in ENSO and may have profound implications for ENSO prediction, particularly the unresolved issue of the spring predictability barrier. Citation: Chang, P., L. Zhang, R. Saravanan, D. J. Vimont, J. C. H. Chiang, L. Ji, H. Seidel, and M. K. Tippett (2007), Pacific meridional mode and El Nino—Southern Oscillation, Geophys. Res. Lett., 34, L16608, doi:10.1029/2007GL030302.

The cause of the fragile relationship between the Pacific El Niño and the Atlantic Niño.

El Niño, the most prominent climate fluctuation at seasonal-to-interannual timescales, has long been known to have a remote impact on climate variability in the tropical Atlantic Ocean, but a robust influence is found only in the northern tropical Atlantic region. Fluctuations in the equatorial Atlantic are dominated by the Atlantic Niño, a phenomenon analogous to El Niño, characterized by irregular episodes of anomalous warming during the boreal summer. The Atlantic Niño strongly affects seasonal climate prediction in African countries bordering the Gulf of Guinea. The relationship between El Niño and the Atlantic Niño is ambiguous and inconsistent. Here we combine observational and modelling analysis to show that the fragile relationship is a result of destructive interference between atmospheric and oceanic processes in response to El Niño. The net effect of El Niño on the Atlantic Niño depends not only on the atmospheric response that propagates the El Niño signal to the tropical Atlantic, but also on a dynamic ocean–atmosphere interaction in the equatorial Atlantic that works against the atmospheric response. These results emphasize the importance of having an improved ocean-observing system in the tropical Atlantic, because our ability to predict the Atlantic Niño will depend not only on our knowledge of conditions in the tropical Pacific, but also on an accurate estimate of the state of the upper ocean in the equatorial Atlantic.

Decadal change in the south tropical Pacific in a global assimilation analysis

Using an ocean general circulation model that incorporates data assimilation, we show that the northward flowing portion of the subtropical gyre in the South Pacific undergoes a well defined decadal variation. The maximum variability of ocean temperature is located at around 10 degrees S and is largest in the western side of the basin with a vertical extension of up to 1000 m. The anomalous temperature shows a rapid warm-to-cold transition in the late 1970s with a peak-to-peak value of more than 1 degrees C. Coupled with this change is a strengthened circulation in the northern portion of the subtropical gyre. Further analysis indicates that the decadal change in the south tropical Pacific is linked to increased Ekman suction induced by enhanced cyclonic wind-stress curl in the region.

Equatorial currents in the Pacific Ocean 1992–1997

A medium resolution ocean general circulation model of the tropical Pacific Ocean is used to explore current structure and variability on the equator for the period from 1992 to 1997. The model assimilates surface and subsurface temperature data from expendable bathythermographs and the Tropical Ocean Global Atmosphere-Tropical Atmosphere Ocean (TOGA-TAO) moorings and altimetry data from the TOPEX/Poseidon satellite. Currents from the model are compared with current observations from the TOGA-TAO moorings at 110°W, 140°W, and 165°E. Since current data are not assimilated, these data provide an independent verification of the model results. The comparison shows that the model produces accurate currents over a wide range of spatial and temporal scales. In particular, the model correctly resolves temporal variability from instability waves with periods of less than a month to interannual changes with periods of several years. The model results are used to explore zonal velocity changes associated with the onset of the 1997–1998 El Nino-Southern Oscillation (ENSO) event. The event began with a series of equatorial Kelvin waves which were excited by westerly wind bursts in the western Pacific and propagated to the eastern Pacific. The passage of these waves is associated with an initial strengthening of the Equatorial Undercurrent (EUC) followed by a rapid shutdown of the EUC. The weakening of the Undercurrent during ENSO is consistent with previous descriptions and results from the collapse of the easterly trade winds in the western Pacific.

Impact of Bathythermograph Temperature Bias Models on an Ocean Reanalysis

Abstract Historical bathythermograph datasets are known to be biased, and there have been several efforts to model this bias. Three different correction models of temperature bias in the historical bathythermograph dataset are compared here: the steady model of Hanawa et al. and the time-dependent models of Levitus et al. and Wijffels et al. The impact of these different models is examined in the context of global analysis experiments using the Simple Ocean Data Assimilation system. The results show that the two time-dependent bias models significantly reduce warm bias in global heat content, notably in the 10 years starting in the early 1970s and again in the early 1990s. Overall, the Levitus et al. model has its greatest impact near the surface and the Wijffels et al. model has its greatest impact at subtropical thermocline depths. Examination of the vertical structure of temperature error shows that at thermocline depths the Wijffels et al. model overcompensates, leading to a slight cool bias, while at...