Low-frequency Sea Surface Temperature Research Articles

Characteristics of Low-Frequency Sea Surface Temperature Fluctuations in the Tropical Atlantic

Abstract Sea surface temperature anomalies in the tropical Atlantic Ocean are reexamined to investigate an apparent low-frequency oscillation that has been described as a fluctuating dipole structure with poles north and south of the equator and a node near the ITCZ. Using principal components rotated by the varimax method and simple correlations of area-averaged temperatures, we show that during the 1964–88 interval SST anomalies north and south of the ITCZ are not significantly correlated. Therefore, the low-frequency variation, with an apparent decadal period observed in the SST gradient across the ITCZ during 1964–88, does not arise from temporally coherent and out-of-phase fluctuations in each hemisphere and cannot be characterized as a dipole.

Simulation of ENSO with a Global Atmospheric GCM Coupled to a High-Resolution, Tropical Pacific Ocean GCM

A global atmospheric general circulation model (GCM) coupled to an oceanic GCM that is dynamically active only in the tropical Pacific simulates variability over a broad spectrum of frequencies even though the forcing, the annual mean incoming solar radiation, is steady. Of special interest is the simulation of a realistically irregular Southern Oscillation between warm El Niño and cold La Niña states. Its time scale is on the order of 5 years. The spatial structure is strikingly different in the eastern and western halves of the ocean basin. Sea surface temperature changes have their largest amplitude in the central and eastern tropical Pacific, but the low-frequency zonal wind fluctuations are displaced westward and are large over the western half of the basin. These zonal wind anomalies are essentially confined to the band of latitudes 10° to 10°S so that they form a jet and have considerable latitudinal shear. During El Niño the associated curl contributes to a pair of pronounced minima in thermocline depth, symmetrically about the equator in the west, near 8°N and 8°S. In the east, where the low-frequency wind forcing is at a minimum, the deepening of the thermocline in response to the winds in the west has a very different shape—an approximate Gaussian shape centered on the equator. The low-frequency sea surface temperature and zonal wind anomalies wax and wane practically in place and in phase without significant zonal phase propagation. Thermocline depth variations have phase propagation; it is eastward at a speed near 15 cm s−1 along the equator in the western half of the basin and is westward off the equator. This phase propagation, a property of the oceanic response to the quasi-periodic winds that force currents and excite a host of waves with periods near 5 years, indicates that the ocean is not in equilibrium with the forcing. In other words, the ocean-atmosphere interactions that cause El Niño to develop at a certain time are countered and, in due course, reversed by the delayed response of the ocean to earlier winds. This “delayed oscillator” mechanism that sustains interannual oscillations in the model differ in its details from that prevoiusly discussed by Schopf and Suarez and others. The latter investigators invoke an explicit role for Kelvin and Rossby waves. These waves cannot be identified in the low-frequency fluctuations of this model, but they are energetic at relatively short periods and are of vital importance to a quasi-resonant oceanic mode with a period near 7 months that is excited in the model. The similarities and differences between the results of this simulation and those with other models, especially the one described in a companion paper, are discussed.

Equatorial Sea Surface Temperature Sensitivity to Net Surface Heat Flux: Some Ocean Circulation Model Results

Abstract Several primitive-equation ocean general circulation model experiments have been carried out in order to explore the sensitivity of equatorial sea surface temperature (SST) results to uncertainty in the net surface heal flux (Q) imposed at the surface. Both climatological seasonal cycle experiments and hindcasts of the 1982/33 ENSO event are considered. It is found that regions of light winds, which typically reach values of SST in excess of 31°C using this ocean model and past Q parameterizations, attain more realistic SST values of 29°–30°C when Q is reduced by as little as 10 W m−2. Sensitivity in this regime is about 0.1–0.2°C (W m−2)−1 for low-frequency SST changes. In regions of easterly winds with their associated upwelling, horizontal advection, and stronger mixing, changes of Q in excess of 50 W m−2 produce SST changes typically of 0.7°C, for a sensitivity of about 0.02°C (W m−2)−1. These results apply equally well to the ENSO hindcasts and the seasonal cycle studies. The reasons for the...

The Annual Cycle of Satellite-derived Sea Surface Temperature in the Southwestern Atlantic Ocean

The annual cycle of sea surface temperature (SST) in the southwestern Atlantic Ocean was estimated using four years (July 1984-July 1988) of NOAA Advanced Very High Resolution Radiometer observations. High resolution satellite observations at 1-km space and daily time resolution were grided at 100-km space and 5-day time intervals to develop an analysis dataset for determination of low frequency SST variability. The integral time scale, a measure of serial correlation, was found to vary from 40 to 60 days in the domain of interest. The existence of superannual trends in the SST data was investigated, but conclusive results could not be obtained. The annual cycle (and, in particular, the annual harmonic) explains a large proportion of the SST variability. The estimated amplitude of the cycle ranges between 5 deg and 13 deg C throughout the study area, with minima in August-September and maxima in February. The resultant climatology is compared with an arbitrary 5-day satellite SST field, and with the COADS/ICE SST climatology. It was found that the higher resolution satellite-based SST climatology resolves boundary current structure and has significantly better structural agreement with the observed field.

Associations between North Atlantic Sea Surface Temperature Anomalies, Latitude of the Polar Front Zone, and Precipitation over Northwest Europe

Based on available data, a 100-year series of annual sea surface temperature (SST) is established for a given area in the North Atlantic. The oscillations of the annual SST anomalies are characterized as small oscillations about a stable state of equilibrium obviously determined by the heat balance. The behavior of such a system can be described by linear integro-differential equations with constant coefficients. It is shown that the power spectrum of a second-order autoregressive linear stochastic process reveals the main features of the spectrum of the given SST time series. The observed low frequency significant peak of the annual SST anomaly spectrum is assumed to be the oceanic integral response to the white noise atmospheric input on shorter time scales. The low frequency location of the peak, corresponding to a period of 25–30 years, is physically explained by the large thermal inertia caused by the large heat capacity of the ocean. A thermal feedback of low frequency SST oscillations to the atmosphere explains the low frequency latitudinal oscillations of the annual mean tropospheric thermal field, including the polar frontal zone from the North Atlantic toward Europe.

A Statistical Study of Large-Scale, Long-Period Variability in North Pacific Sea Surface Temperature Anomalies

Abstract Frequency domain principal components analysis, a technique which involves extracting the eigenvalues and eigenvectors of the cross spectrum of a multivariate time series, is used to analyze the development and movement of sea surface temperature (SST) anomalies in the North Pacific. A prominent feature of the low-frequency SST field is an oscillatory pattern with a return period of about four to seven years. It involves the slow migration of an anomaly northward into the western North Pacific where the anomaly undergoes rapid expansion and intensification and then begins to deteriorate as it moves eastward. It is suggested that the rapid expansion in midlatitudes is caused by a positive feedback between the SST anomalies, the atmospheric long waves, and cyclonic short waves.