Zonal Displacement Research Articles

Equatorial wave sequence associated with warm pool displacements during the 1986–1989 El Niño‐La Niña

In the Pacific equatorial band the zonal displacement of the eastern edge of the warm pool, subject to insignificant seasonal variations, is dominated by strong interannual variations almost in phase with the Southern Oscillation Index. Over the 1971–1973 El Niño‐La Niña period, Gill (1983) suggested that such displacement was caused solely by horizontal advection by zonal current anomaly. Basin‐wide Geosat‐derived zonal surface current anomalies are available during the 1986–1989 period, and they agree quite well with observed equatorial currents in the western and central Pacific. Within the 4°N–4°S equatorial band the cumulative contribution of first‐baroclinic Kelvin and first‐symmetric (m = 1) Rossby modes to surface zonal equatorial current anomalies account for most of the Geosat‐derived current variability. From these Geosat‐derived currents it is demonstrated that the eastward (westward) displacement of the eastern edge of the warm pool was primarily due to horizontal advection by zonal current anomalies during the 1986–1987 El Niño (1988–1989 La Niña). As a corollary, the El Niño (La Niña) warm (cold) sea surface temperature (SST) anomaly in the central western Pacific was the result of anomalous zonal advection. The 1986–1989 displacement of the warm pool appeared as a low‐frequency movement resulting from high‐frequency forcing, i.e., a succession of local wind forcing and its remote Kelvin and m = 1 Rossby wave responses in the equatorial waveguide. In the central western Pacific a m = 1 downwelling Rossby wave, issued from the eastern Pacific, shifted the displacement from eastward to westward and therefore seems to have been the cause for El Niño to turn to La Niña in boreal fall 1987. In the eastern Pacific the simultaneous occurrence of free‐propagating downwelling (upwelling) Kelvin waves with the seasonal SST warming (cooling) appeared necessary for the development of local El Niño (La Niña) SST anomaly. Over most of the equatorial Pacific, slow migration of SST anomalies clearly showed up, distinct during El Niño and La Niña. These zonal migrations appeared to result from the delayed association of two phenomena: the zonal advection in the central western Pacific and the arrival of downwelling (upwelling) Kelvin waves into the eastern Pacific, synchronous with the seasonal SST warming (cooling). Albeit different in terms of equatorial wave action, as compared to the delayed action oscillator theory, our results suggest that zonal advection and wave dynamics were both important for the generation and migration of El Niño‐ Southern Oscillation SST anomalies in 1986–1989. Finally, our observational results about warm pool displacements, equatorial wave sequence, the shift from El Niño to La Niña, and the slow SST migrations are presented in a schematic form involving sequential ocean‐atmosphere coupling which appears continuously over the November 1986 to February 1989 period.

An Exploratory Analysis of the Relationship between Tropical Storm Formation in the Western North Pacific and ENSO

Abstract El Nino-Southern Oscillation (ENSO)-related variations in the observed character (e.g., annual storm totals, preferred genesis region, etc.) of the tropical cyclone activity in the western North Pacific are sought. With respect to annual storm totals, no ENSO signal is found; with respect to genesis region, a strong relationship is found between ENSO indices and the zonal displacement of the annual mean genesis locations. ENSO indices during the first half of the calendar year were found to be weakly predictive of the number and genesis locations of tropical cyclones occurring from July through December.

Indices for mid-latitude North Pacific winter wind systems; an exploratory investigation

Biological distributions and fish migrations during the spring and summer in the mid-latitude N Pacific are believed to be affected by the large-scale wind systems during the preceding winter. These wind systems are related to the field of atmospheric pressure which, on a monthly or quarterly time scale, show pronounced interannual variations. This exploratory investigation is concerned with measures of the large-scale wind systems that are indicative of their intensity, location, and size, and that can be derived from monthly or quarterly sea-level pressure distributions. Of interest are the NW, W, and SW wind systems which are related to the pressure gradients between the Siberian High and Aleutian Low, the Aleutian Low and the N Pacific Subtropical High, and the Aleutian Low and the N American High, respectively. Interannual changes in the winter sea-level pressure distribution pattern are primarily the result of changes in the pressure and zonal displacements of the Aleutian Low near 50°N. This property permitted simple derivation of wind indices from grid values of N hemisphere mean winter pressures distributions for the years 1947 to 1985. Indices, presented as normalized anomalies, are for the meridional component of the wind system at 50°N over the ocean W and E of the Aleutian Low, for the westerlies S of the Aleutian Low and for the longitude of this Low. The paper concludes with a discussion of the limitations of the indices in terms of reflecting characteristic atmospheric pressure distribution patterns, an examination of the relationship between wind anomalies and El Nino events, and a discussion of the results in terms of oceanographic, climate, and biological implications.

A Statistical-Climatological Tropical Cyclone Track Prediction Technique Using an EOF Representation of the Synoptic Forcing

Abstract An empirical orthogonal function (EOF) analysis is performed on a series of 504 normalized D-value fields at 500 mb. Each field is defined on a 120-point grid that is centered on the geographical position of a tropical cyclone. The EOF representation (eigenvectors) of the environmental flow field requires less than 10% of the storage needed for the original grid points. Each eigenvector represents a distinctly different synoptic-scale forcing pattern for the tropical cyclone motion. Statistical regression equations are developed to predict the zonal and meridional displacements of the tropical cyclone at 12 h intervals to 84 h. The EOF coefficients are used to represent the synoptic-scale forcing in the equations. The track forecast errors are competitive with other statistical schemes. The average displacement errors in an independent sample were ∼15% smaller than the official Joint Typhoon Warning Center forecasts. Thus, the EOF-based regression approach provides a simple and low cost technique...

Statistical Post-Processing of Dynamical Tropical Cyclone Model Track Forecasts

Abstract The Fleet Numerical Oceanography Center Tropical Cyclone Model (TCM) exhibits a systematic bias in track forecasts. Based on a 1975–78 sample of western North Pacific tropical cyclones, the mean zonal and meridional 72 h errors are about 280 and 350 km, respectively. A series of statistical regression equations is derived to adjust the TCM forecasts. Predictors include zonal and meridional displacements and speeds over various forecast intervals to 72 h. The dynamical model is also integrated backward in time for 36 h to derive additional predictors. Inclusion of predictors derived from comparing the backward-integrated positions with the known warning positions at −12, −24 and −36 h resulted in smaller forecast errors than the case which involved only forward integration predictors. Use of the forward-backward statistical post-processing with an independent set of TCM forecasts resulted in an error reduction of ∼200 km at 72 h.

Large-Scale Dispersion of Clusters of Particles in the Atmosphere

The characteristics of the relative particle displacement tensor, the correlation functions, and spectra of the relative particle velocities at 200-, 500- and 850-mb levels are investigated. It is found that similarity and stability exist for the autocorrelation functions as well as the power spectra at various levels, which indicate that a quasi-stationary process exists in the large-scale relative diffusion in the atmosphere. The high frequency portion of the power spectra of both the zonal and meridional components of the relative velocities is found to be more or less proportional to k−8; the relative meridional velocity shows a maximum at the low frequency end. The low frequency portion of the cross spectra of the zonal and meridional relative velocities at 850 mb shows an opposite transfer to that at 200 mb. The mean square of the relative zonal displacement at a level is found to be about twice that of the relative meridional displacement at that level. It is also found that anisotropy exists in the field of the large-scale turbulent dispersion and that the major axis of the dispersion is generally oriented in the ESE to WNW direction.

A new method for the prediction of typhoon movement using the 700 mb chart

AbstractStorm tracks which crossed the area between the Equator and 35°N, 105°E and 160°E, during the period 1957 to 1960 have been analysed. For each storm position values of the 700 mb contour height (geopotential metres) He, Hw, Hs and Hn, were extracted at four selected grid points labelled as E, W, S and N respectively. E and W were at distances of 10° longitude east and west of the storm centre, S and N at distances of 10° latitude south and north of the storm centre. The differences (He − Hw) = ΔϕH and (Hs − Hn) = ΔϕH were computed for 516 storm positions and used as predictors. The subsequent 24‐hr meridional and zonal displacements of the storm in degrees latitude and longitude, Δϕ and Δλ, were chosen as predictands. The over‐all synoptic pattern at 700 mb was then introduced as an additional criterion. Regression lines corresponding to a few easily recognized synoptic patterns were derived from scatter diagrams of ΔϕH versus Δϕ and ΔλH versus Δλ. A nomogram was constructed to give Δϕ and Δλ for values of ΔϕH and ΔλH respectively.The method, tested in 44 forecasts on 8 storms during May to September 1961, gave an average error of 87 nautical miles; this compares favourably with other objective methods.