Abstract

Linear regression analysis of tropical Pacific winds versus a Southern Oscillation Index (SOI) show departures from the seasonal mean wind associated with the phase of SOI. When the SOI is low the largest departures are westerlies on the equator and in the subtropics west of the dateline. The Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ) shift equatorward. Departures from mean wind are strongest from September through February and weakest from March to May. The usual pattern of cyclonic wind stress curl in the tropics and anticyclonic wind stress curl in the subtropics intensifies when the SOI is low. Effects of these changes on ocean circulation are discussed and compared to changes in ocean circulation data.

Abstract

The authors have quality controlled six global datasets of drifting buoy data, made comparisons of 15-m drogued and undrogued buoy observations, and developed a 2D linear regression model of the difference between drogued and undrogued drifter velocity as a function of wind. The data were acquired from 2334 Surface Velocity Program (SVP) drifters, including 1845 SVP drifters after they lost their drogues; 704 AN/WSQ-6 Navy drifter buoys; and 503 First Global GARP Experiment (FGGE) drifter buoys. Meridional and zonal surface wind velocity components from the global synoptic FNMOC model, the global synoptic ECMWF model, and the global synoptic NCEP model were interpolated to naval AN/WSQ-6, WOCE–TOGA buoy, or FGGE buoy positions and date/times in the datasets. Two-day mean buoy drift velocities and positions were computed: 122 101 SVP drifter mean velocities before they lost their drogues and 58 201 SVP drifter mean velocities after they lost their drogues, 21 799 Navy drifter mean velocities, and 42 338 FGGE drifter mean velocities. A regression analysis was made on selected data in selected 2° lat × 8° long bins: U _undrogued = A _undrogued + B _undrogued W _undrogued, U _drogued = A _drogued + B _drogued W _drogued, where U _undrogued, U _drogued was ensemble mean buoy velocity and W was wind velocity, and the real and imaginary parts of these quantities were the zonal and meridional components, respectively. The difference in these complex valued regression coefficients, B _difference = B _undrogued − B _drogued measured the linear response to the wind. Navy and SVPL buoy response to the wind was identical and FGGE buoy response was generally the same; the global weighted mean value of |B _difference| was 0.0088 ± 0.002. The difference in the complex valued y intercept, A _difference = A _undrogued − A _drogued, was nearly always zero within error. The buoy response to wind was also estimated by b = ( U _undrogued − U _drogued)/ W _undrogued, where the velocity difference U _undrogued − U _drogued was calculated from ensemble mean buoy velocities and W was ensemble mean wind velocity. The global-weighted mean value of |b| was 0.0097 ± 0.005. The analysis also found that the phase angle of either B _difference or b, which measures the velocity difference with respect to the wind, was zero within error and not a function of the surface wind or the Coriolis parameter.

Abstract

Anomalous 14°C isotherm depth and anomalous geostrophic volume transport above the 14°C isotherm depth are calculated in the tropical Pacific for each bimonth of the four year period 1979 to 1982, based on 15 000 temperature/depth observations made by volunteer observing ships. The first two eigenfunctions (EOFs) of the anomalous 14°C isotherm depth, the anomalous zonal geostrophic volume transport, and the anomalous meridional Ekman volume transport explain 44%, 32% and 47% of their variances, respectively. Most of the variability explained by the first EOF is associated with the 1982–83 ENSO event. The EOFs indicate that a loss of anomalous volume above the 14°C isotherm in the western tropical North Pacific is correlated with an increase in anomalous volume in the eastern equatorial Pacific. There is a simultaneous intensification of anomalous eastward geostrophic currents across the width of the equatorial Pacific; however, from 150°E to 140°W there is also a simultaneous intensification of anomalous meridional Ekman currents convergent upon the equator. Net anomalous geostrophic volume transport into a rectangular study region defined by corners at (4°N, 160°E) and (16°N, 160°W) correlates at −0.60 with anomalous rate of change of volume above the 14°C isotherm depth. Net anomalous Ekman volume transport into the study region correlates at 0.76 with the anomalous rate of change of volume above the 14°C isotherm depth. When the net anomalous Ekman volume transport into the study region is added to the net anomalous geostrophic volume transport, the correlation between total net anomalous volume transport and anomalous rate of change of volume is 0.62. The magnitude of the interannual variability of total net anomalous volume transport is 5.5 sverdrups (sv = 10⁶ m³ s⁻¹), essentially equal to the magnitude of the interannual variability of the anomalous rate of change of volume of 5.9 Sv. Meridional Ekman and meridional geostrophic geostrophic volume transports through the southern boundary of the study region and zonal geostrophic transports through the eastern and western boundaries dominate the change in volume within the study region.

There is an anomalous drain of about 10 Sv from the study region during 1982, which includes the early and mature stages of the 1982–83 ENSO event. Anomalous Ekman volume transport through the southern boundary out of the study region is greater than this volume drop, but it is opposed by meridional anomalous geostrophic volume transport into the study region. Although the North Equatorial Countercurrent is highly correlated with the loss of volume during 1982, most of the volume loss is not due to the divergence of anomalous zonal transports but is due to meridional Ekman transport out of the southern boundary of the study region.

Abstract

A realistic simulation of short-term climatic variability in the relative dynamic height of the interior western tropical North Pacific (4°–22°N, 127°E–180°) for 9 years (1966–74) was conducted using a two-layer, wind-driven, baroclinic long-wave model. Results of this model were compared with bimonthly maps of observed dynamic height (0/400 db) for the region, constructed by White and Hasunuma from all available temperature/depth observations for the same period. The interannual rms differences about the mean annual cycle of the model dynamic height had a spatial pattern over the region of interest that was nearly identical to that observed, with maximum values (i.e., 3.5 and 4.5 dyn cm, respectively) in the vicinity of the North Equatorial Ridge at 15°N and minimum values (i.e., 2.0 and 3.5 dyn cm, respectively) in the vicinity of the North Equatorial Ridge at 15°N and minimum values (i.e., 2.0 and 3.5 dyn cm, respectively) in the Countercurrent Trough at 7°N. Model frequency spectra were identical in shape to those observed (i.e., increasing linearly toward low frequency at the fourth power of frequency for periods of 6 months to 2 years), with the model spectral magnitude slightly less than observed. Empirical orthogonal function (EOF) analysis of 9 years of bimonthly maps of observed and model dynamic height finds the first three functions explaining approximately 75% of the total short-term climatic variance of each parameter, most of which was associated with two ENSO (El Niño/Southern Oscillation) events, occurring in 1968–69 and 1972–73. In both the observed, and to a lesser extent, the model data sets, dynamic height decreased radically during these ENSO events; the maximum decrease was at 15°N in the vicinity of the North Equatorial Ridge, less near the equator. Cross-correlation of the model EOFs highly correlated, with 48% of observed variance explained by the model.

The dynamics of short-term climatic variability in the western interior tropical North Pacific is dominated by steady equilibrium processes. Frequency cross-spectra between long-wave model dynamic height and that produced by an equilibrium model of Sverdrup reveals that at periods of 1 year and longer, the two models produce identical time sequences for the latitude range 4°–12°N, the transient effects became important, so that at 21°N only 64% of the long-wave model variability could be explained by the equilibrium model at a period of one year. An important transient effect is produced by baroclinic long waves propagating information from east to west at speeds ranging from 100 cm s⁻¹ at 4°N to 10 cm s⁻¹ at 22°N. In the latitude band 10–15°N, where maximum short-term climatic variance occurred in both observed and model data, 50% of the total model variance was associated with baroclinic long wave propagation of information from east of 180°. Of the percent of model variance due to local wind forcing in this latitude band, approximately 25% was due to resonant wind forcing, 75% due to off-resonant wind forcing.

Abstract

The redistribution of observed upper-ocean heat content in the western tropical North Pacific for the four-year period 1979–82 was shown by Pazan et al. to provide a qualitative hindcast capability for the 1982–83 ENSO event. A related study (Inoue et al.) demonstrated that nearly 50% of the observed heat content redistribution in the western tropical North Pacific for the four-year period 1979–82 could be simulated by a linear, upper-ocean, numerical model driven by observed wind stress estimates. In this study, the redistribution of numerical model heat content as evidenced in model dynamic height in the western Pacific during the 22-year period 1964–85 is examined for its ability to hindcast and forecast ENSO events in this period. Complex EOF analysis is applied to the onset phase of ENSO events occurring in 1968–69, 1972–73, 1976–77, and 1982–83; it is used to determine the characteristic redistribution of heat content (dynamic height) prior to the mature phase of ENSO events. The first complex EOF explained 53% of the interannual variance of the numerical model anomalous dynamic height in the 22-year model data records. This analysis finds model dynamic height in the Northern Hemisphere to be characterized by wind-driven westward propagating, baroclinic Rossby wave activity, having a relatively stable period of 3–4 years over the 22-year period. The complex time series associated with these first spatial eigenfunctions are used to construct observed and model hindcast indices that yield high values one year prior to the mature phase of ENSO events of the period. They do not peak when ENSO events do not occur. These indices achieve these high values due to the incidence upon the Philippine coast in fall/winter of a positive anomaly in dynamic height propagating from the east at nondispersive (Rossby) long-wave speeds. The model dynamic height data have the advantage over the observed dynamic height of being available in near-real time (i.e., within a month of the present), which makes it useful in providing a near-real time forecast of future ENSO events. Application of this model forecast index to the upcoming year is discussed.

Abstract

Analysis of anomalous vertically averaged temperatures (TAV) in the tropical Pacific Ocean for the eight-year period 1979-86, finds the development during the onset phase, one to two years before the mature phase of the two ENSO events of the period, having dynamically significant similarities both north and south of the equator.

During the onset phase of both ENSO events, each beginning in the off-equatorial western tropical Pacific (about 10° latitude), positive anomalies of TAV occurred during northern autumn/winter, one year prior to the mature phase of each ENSO event, in both Northern and Southern hemispheres. This corroborates model results of Pazan et al. and supports the hindcasting/forecasting capability of ENSO events obtained by White et al., though the latter considered only the Northern Hemisphere. It is also demonstrated that the development of positive TAV anomalies in northern autumn/winter along the tropical maritime western boundary of both hemispheres was associated with wind-driven baroclinic long (Rossby) wave activity which had been transmitting positive TAV anomalies from the eastern and central tropical Pacific into the western tropical Pacific over the previous one-to-two years (between 1O° and 15° latitude).

The analysis shows that there were positive anomalies on the maritime western boundary in both hemispheres during autumn/winter of the onset phase, not in one hemisphere or the other. During both ENSO events, the dynamical off-equatorial influence was found only to have initiated the ENSO event; off-equatorial influence dominated the first half of the ENSO year, diminishing during the peak phase of the event (i.e., northern summer of both ENSO year), and replaced during the second half of the ENSO year by strictly equatorial influence related to the development of anomalous westerly winds on the equator in the western equatorial Pacific.

Abstract

Short-term climatic variability in both sea surface temperature (SST) and vertically averaged temperature over the upper 400 m of ocean (T _av ) is mapped over the Pacific from 20°S to 50°N each bimonth for four years from 1979 to 1982, leading up to the 1982–83 ENSO (El Nino–Southern Oscillation) event. This mapping was made possible by the collection of approximately 85 000 temperature/depth observations in the Pacific Ocean by volunteer observing ships. Anomalies of SST and T _av were approximately the same magnitude at midlatitude as in the tropics, with the exception of large changes occurring in the tropics during the 1982–83 ENSO event. During the ENSO event, (T _av variability was largest in the western tropical North Pacific and SST variability was largest in the eastern equatorial Pacific. Both parameters had spatial patterns which were of opposite phase on either side of the ocean, indicating an eastward shift of warm waters during the ENSO event. Correlation studies determined that on average during the four years extraequatorial T _av anomalies propagated from east to west at baroclinic long-wave speeds, travelling faster near the equator (i.e., 25–75 cm s⁻¹ at 10°N and 10°S) and slower at higher latitudes (i.e., 2 cm s⁻¹ new 40°N). At the equator, T _av anomaly propagation was to the east at internal Kelvin wave speeds (i.e., 50–250 cm s⁻¹). On the other hand, SST anomalies propagated on average during the four years in the direction of mean surface currents, so that, for example, at 5°N anomalies propagated to the east with the North Equatorial Countercurrent. Beginning in 1981, anomalously high T _av propagated westward from the central North Pacific near 16°N and approached the maritime coast of Asia late in the year. During early 1982 it propagated rapidly down the coast toward the equator, then along the equator to the west coast of South America. This initial development was similar to the development of the 1968–69 and 1972–73 ENSO events in the western North Pacific observed by White and others. In mid-1982, this initial development was followed by a reduction in the Southeast Trade Winds over the western and central equatorial Pacific and a rapid intensification of the ENSO episode.