The ocean response to operational westerly wind bursts during the 1991–1992 El Niño

Abstract

Numerical simulations of the remotely forced ocean response to westerly wind bursts prior to and during the 1991–1992 El Niño are examined; the models are forced by wind stress from the U.S. Navy's atmospheric global operational analysis/forecast system. The study focuses on (1) the relative response of the first and second internal modes to a single episode of westerly bursts; (2) the role of westerly bursts in producing the eastern Pacific sea level variations from October 1990 to February 1992; and (3) the relative importance of the remotely forced sea level responses generated by central and western Pacific wind anomalies. The simulations use the Naval Research Laboratory global multilayer formulation; the suite of experiments includes hydrodynamic simulations that use both one‐ and three‐active‐layer reduced gravity models as well as an experiment that also includes thermodynamic effects. The models are executed from January 1, 1990, to March 1, 1992, a period that includes 10 significant westerly wind bursts or burst clusters and the 1991–1992 El Niño event. The numerical experiments reveal an ability to accurately simulate the eastern Pacific sea level variations during this period. In response to a single burst the three‐layer hydrodynamic simulation reveals that the second internal mode Kelvin wave yields a sea level change at the eastern boundary that is approximately one third that of the first mode and a surface velocity signature that is equivalent to the first mode. During the onset of the El Niño event the inclusion of higher modes also produces a more realistic representation of the observed eastern boundary sea level signal. Furthermore, by comparing the model response to particular wind bursts with the observed sea level at Baltra, Galapagos, a value of 2.5–2.6 m/s is suggested as the most appropriate mean speed for the first internal mode Kelvin wave during the onset phase. The models reveal the following scenario for the onset of the 1991–1992 El Niño. The eastern boundary sea level exhibited three, distinct pulselike events superimposed on a general rise; the observed sea level pulses occurred in October–November, December–January, and February. The numerical simulations indicate that the pulses in December–January and February were the result of three powerful, westerly wind bursts in the western Pacific preceded by westerly anomalies in the central Pacific. The Kelvin wave pulse generated in the western Pacific by the wind burst in late December to early January was reinforced by a strong central Pacific westerly wind anomaly in mid‐January. These westerly wind events were associated with the development of western Pacific tropical cyclones near the equator in one or both hemispheres.

California cooperative oceanic fisheries investigations. Reports volume 37, January 1--December 31, 1995

“…These shifts indxate eastward phase velocities of about 2.5 ms-', similar to those found in theoretical and modeling studies (e.g., Gill 1982;Geise and Harrison 1990;Kindle and Phoebus 1995). Correlations of the temperature time series in figure 2b with equatorial surface winds indicate that these Kelvin waves were initiated by eastward surface-wind anomalies, especially those associated with tropical cyclone winds, in the western Pacific.4 Figure 2b shows that the major Kelvin wave activity occurred in October 1991-April 1992 and October 1992-April 1993.…”

“…subsurface fluctuations was also a time of warm SSTs (28"-30°C) and positive SSTAs (1"-2"C; figures la, 2a -April 1993McPhaden 1993). The temporal and spatial patterns of these intraseasonal variations in upper-ocean temperature, along with current-velocity data (not shown), indicate internal equatorial ocean Kelvin waves (Kindle and Phoebus 1995;Murphree et aL3). Coriolis effects cause these waves to be confined within a few degrees of the equator and to propagate eastward along the equator (Gill 1982).…”

The effect of El Niño on intraseasonal Kelvin waves

“…In contrast, Kindle and Phoebus (1995) suggested that a sloping thermocline increases the amplitudes of the SLA eld, basing their argument on the results from a study with a 1.5-layer ocean model. The situation is more complicated in a General Circulation Model (GCM) with a continuous strati cation, since the vertical extent of the waves is not limited to the surface layer de ned by the thermocline.…”

“…To further investigate the in uence of the oceanic background state on the IKWs, we carried out a set of numerical experiments with a highly resolved ocean model. These experiments have some similarities to Kindle and Phoebus (1995), who applied realistic wind forcing to an ocean model and studied the effect of a sloping thermocline on Kelvin wave amplitudes. We also attempted to elaborate on the previous studies by identifying the damping mechanisms that may take place during the different ENSO phases.…”

“…Busalacchi and Cane (1988), Giese and Harrison (1990), and Kindle and Phoebus (1995) all suggested that a west-to-east sloping thermocline may affect the Kelvin wave amplitude as measured by sea-level anomaly (SLA). However, they all gave different accounts of how the Kelvin wave amplitudes change, basing their arguments on different assumptions.…”