The dynamics of wind-driven intraseasonal variability in the equatorial Indian Ocean

This commentary provides a discussion of the concept of `bounded rationality' as it applies to the theses advanced by Lopes (1991) and Evans (1991). Lopes's (1991) assessment of the irrationalist consequences of Tversky and Kahneman's (1974) work on heuristics and biases is premature because bounded rationality implies that people could not employ optimal strategies. Considerations of bounded rationality also provide additional criteria by which to judge the theories of deductive reasoning discussed by Evans (1991). Judged by this criterion, theories whose goal is to explain logically competent performance are inadequate (Oaksford & Chater, 1991). Thus Evans's assessment of the state of current theories of reasoning requires revision

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

Closing the sea surface mixed layer temperature budget from in situ observations alone: Operation Advection during BoBBLE

Sea surface temperature (SST) is a fundamental driver of tropical weather systems such as monsoon rainfall and tropical cyclones. However, understanding of the factors that control SST variability is lacking, especially during the monsoons when in situ observations are sparse. Here we use a ground-breaking observational approach to determine the controls on the SST variability in the southern Bay of Bengal. We achieve this through the first full closure of the ocean mixed layer energy budget derived entirely from in situ observations during the Bay of Bengal Boundary Layer Experiment (BoBBLE). Locally measured horizontal advection and entrainment contribute more significantly than expected to SST evolution and thus oceanic variability during the observation period. These processes are poorly resolved by state-of-the-art climate models, which may contribute to poor representation of monsoon rainfall variability. The novel techniques presented here provide a blueprint for future observational experiments to quantify the mixed layer heat budget on longer time scales and to evaluate these processes in models

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...

Modes of ocean variability in the tropical Pacific as derived from Geosat altimetry

Satellite-derived (Geosat altimetry) sea surface height anomalies for the period November 1986 to September 1989 were investigated in order to extract the dominant modes of climate variability in the tropical Pacific. We applied the technique of principal oscillation patterns and computed associated wind stress patterns for each mode. Four modes were identified. The first mode has a time scale of about 3 months and can be identified with the first baroclinic equatorial Kelvin wave mode, which is excited by intraseasonal wind variations over the western equatorial Pacific. The second mode has a time scale of about 6 months and describes the semiannual cycle in the tropical Pacific sea level. Equatorial wave dynamics appears to be crucial for this mode also. The third mode is the annual cycle which shows evidence of off-equatorial Rossby wave propagation. The fourth mode is associated with the El Niño/Southern Oscillation (ENSO) phenomenon. The ENSO mode is found to be consistent with the “delayed action oscillator” scenario

On the sensitivity of Sverdrup transport estimates to the specification of wind stress forcing in the Tropical Pacific

We use Sverdrup dynamics to estimate geostrophic transports between 20°N and 20°S in the Tropical Pacific Ocean averaged over the period 1979-1981. Three wind stress products are used to force the model. Results are compared to geostrophic transports computed along expendable bathythermograph transects in the Western, Central, and Eastern Pacific for the same period. Depending on the choice of wind stress, modeled transports may differ from the observations by a factor of 2 and, in some cases, flow is opposite to that observed. Possible limitations of the Sverdrup theory are discussed; however, we conclude that detailed and accurate simulation of the general circulation in the Tropical Pacific is limited more by the uncertainties in presently available estimates of the surface wind stresses than by deviations from Sverdrup balance. (Résumé d'auteur

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