The first attempt to establish a relation between the Loop Current extension and deep flows in Yucatan Channel was made by Maul et al. [1985]; it was unsuccessful, probably because of the low spatial resolution of their observations. From September 8, 1999, to June 17, 2000, eight moorings with acoustic Doppler current profilers, current meters, and thermometers were deployed across the Yucatan Channel. The data from these arrays were used to compute time series of the transports below the level of the deepest isotherm observed in the Florida Straits, as required by a simple box model that restricts deep exchanges with the Gulf of Mexico to the Yucatan Channel. The surface extension of the Loop Current was inferred from 3 day advanced very high resolution radiometer imagery from October to May, when temperature gradients were sufficient to map the warm water unambiguously. The deep transports appear at first unrelated to the rate of change of the Loop Current extension, but filtering the series with a 20 day running mean increases the correlation between the low‐pass series to 0.62, and up to 0.83 with a lag of 8.5 days, with Loop Current changes leading the deep flows. The cumulative deep transport, a quantity that favors lower frequencies, is very well related (correlations >0.9) to the surface extension of the Loop Current, also with a lag of about a week. These lags are not statistically significant but suggest a timescale for internal adjustment processes in the Gulf of Mexico. The empirical orthogonal function of the current best related to the area extension of the Loop Current represents a unidirectional flow across the entire deep section, flowing either toward or from the Gulf of Mexico, and includes a strong expression of the Yucatan Undercurrent.

Decadal and longer time-scale variabilities of the best known El Niñ o-Southern Oscillation (ENSO) indexes are poorly correlated before 1950, and so knowledge of interdecadal variability and trend in ENSO indexes is dubious, especially before 1950. To address this problem, the authors constructed and compared physically related monthly ENSO indexes. The base index was El Niñ o index Niñ o-3.4, the sea surface temperature (SST) anomaly averaged over the equatorial box bounded by 58N, 58S, 1708W, and 1208W; the authors also constructed indexes based on the nighttime marine air temperature over the Niñ o-3.4 region (NMAT3.4) and an equatorial Southern Oscillation index (ESOI). The Niñ o-3.4 index used the ''uninterpolated'' sea surface temperature data from the Second Hadley Centre Sea Surface Temperature dataset (HadSST2), a dataset with smaller uncertainty and better geographical coverage than others. In constructing the index, data at each point for a given month were weighted to take into account the typical considerable spatial variation of the SST anomaly over the Niñ o-3.4 box as well as the number of observations at that point for that month. Missing monthly data were interpolated and ''noise'' was reduced by using the result that Niñ o-3.4 has essentially the same calendar month amplitude structure every year. This 12-point calendar month structure from April to March was obtained by an EOF analysis over the last 58 yr and then was fitted to the entire monthly time series using a least squares approach. Equivalent procedures were followed for NMAT3.4 and ESOI. The new ESOI uses Darwin atmospheric pressure in the west and is based on theory that allows for variations of the atmospheric boundary layer depth across the Pacific.The new Niñ o-3.4 index was compared with NMAT3.4, the new ESOI, and with a record of d 18 O from a coral at Palmyra, an atoll inside the region Niñ o-3.4 (Cobb et al.). Correlation coefficients between Niñ o-3.4 and the three monthly indexes mentioned above before 1950 are 0.84, 0.87, 0.73 and 0.93, 0.86, 0.73 for decadal time scales. These relatively high correlation coefficients between physically related but independent monthly time series suggest that this study has improved knowledge of low-frequency variability. All four indexes are consistent with a rise in Niñ o-3.4 SST and the weakening of the equatorial Pacific winds since about 1970.

The interannual, equatorial Pacific, 20°C isotherm depth variability since 1980 is dominated by two empirical orthogonal function (EOF) modes: the “tilt” mode, having opposite signs in the eastern and western equatorial Pacific and in phase with zonal wind forcing and El Niño–Southern Oscillation (ENSO) indices, and a second EOF mode of one sign across the Pacific. Because the tilt mode is of opposite sign in the eastern and western equatorial Pacific while the second EOF mode is of one sign, the second mode has been associated with the warm water volume (WWV), defined as the volume of water above the 20°C isotherm from 5°S to 5°N, 120°E to 80°W. Past work suggested that the WWV led the tilt mode by about 2–3 seasons, making it an ENSO predictor. But after 1998 the lead has decreased and WWV-based predictions of ENSO have failed. The authors constructed a sea level–based WWV proxy back to 1955, and before 1973 it also exhibited a smaller lead. Analysis of data since 1980 showed that the decreased WWV lead is related to a marked increase in the tilt mode contribution to the WWV and a marked decrease in second-mode EOF amplitude and its contribution. Both pre-1973 and post-1998 periods of reduced lead were characterized by “mean” La Niña–like conditions, including a westward displacement of the anomalous wind forcing. According to recent theory, and consistent with observations, such westward displacement increases the tilt mode contribution to the WWV and decreases the second-mode amplitude and its WWV contribution.

Time series of high vertical resolution current meter measurements between 600-m and 1800-m depths on the equator in the Atlantic were obtained at two locations, 10°and 23°W. The measurements have a time span of almost 7 years (2000-06) and provide insights into the temporal scales and vertical structure of variability at intermediate depths. Variability in the zonal velocity component records is dominated by semiannual, annual, and interannual fluctuations. At semiannual and annual periodicities, vertical scales are large, on the order of 2000 stretched meters (sm), and show upward phase propagation. In contrast, interannual variability is associated with small vertical scale flows, called equatorial deep jets (EDJs), presenting downward phase propagation most of the time. Fitting a plane wave to these small vertical-scale flows leads to velocity amplitude, vertical scale, and temporal scale estimates of 8 (normalized) cm s Ϫ1 , 440 sm, and 4.4 yr. However, this plane wave cannot explain all the variability presenting small vertical scales. Indeed, the data suggest that, along with a seasonal cycle of much larger vertical scale, different features with EDJ vertical scale coexist, with the possibility of a semipermanent eastward jet at around 1500 sm. Variability in the meridional velocity component is dominated by intraseasonal fluctuations. In addition, at 23°W, the meridional component shows low-frequency flows that may be due to the interaction of zonal fluctuations with the Mid-Atlantic Ridge.