Spatial-temporal Features of Multi-scale Vorticity Disturbances over the Tropical North Pacific and Its Roles in the Turnabout of 1997-98 El Niño

Zuqiang Zhang1,3 Renhe Zhang2 Song Yang3 1) NCC/CMA, China 2) CAMS/CMA, China 3) CPC/NCEP/NWS/ NOAA, USA

INTRODUCTIONINTRODUCTION

Low-level wind anomalies over the western tropical Pacific play a crucial role in triggering El Niño-Southern Oscillation (ENSO) phase transitions through the excited equatorial oceanic Kelvin waves of which accumulative effects are comparable with the western boundary wave reflection qualifying as a negative feedback to maintain ENSO cycle. The wind anomalies not only are associated with the onset of cyclonic or anti-cyclonic circulation over the western North Pacific (WNP), but also are attributed to shorter timescale disturbances, e.g. synoptic and intraseasonal oscillations (SO and ISO). Combined roles of the involved multi-scale disturbances, as well as their space-time characteristics, in ENSO phase transition are demonstrated with respect to the turnabout of 1997-98 El Nino in terms of NCEP reanalysis data and TAO data, and by use of Morlet Wavelet analysis and Extended EOF(EEOF).

99% significant level

NWP Vorticity leading months Nino 3.4 leading

ConclusionSO, ISO and LFO timescale fluctuations over the western North Pacific Ocean are intimately involved in the phase transition of 1997-98 ENSO by effectively producing near-equatorial zonal wind anomalies.

Those three components distinguish from each other not only by the dominant periodicity but also by the time evolution, spatial structure and migration orientation.

It is worthy to highlight the significant roles of the predominance of SO and ISO flucuations over the entire longitudes before the end of 1997 and its ensuing sudden decrease and withdrawal to the central Pacific in the decay of warm episode and the onset of cold one, respectively.

ImplicationRegime shift of SO and ISO activities from the active to the depressed may provide some pre-signals for the phase change of El Niño.

Warm SSTA-vertical shear of U-SO+ISO activity-Easterly anomaly may serve as a self-provided negative feedback necessary for the decay and reversal of El Niño event.

1997.9 1998. 1

1997.11 1998.3

TAO 2S-2N Sea Temp. Anom. Profile

5N-20N SO+ISO Vor. & U anom. 5N-20N Interannual Vor. 5N-20N U850-U200 Anom.

Easterly is shaded, Contour: SO+ISO Vorticity Climatology easterly shear is shaded

P3: SO+ISO+LFO & U & ENSO Phase Shift

P2: spatial-temporal features of SO, ISO and LFO

P1: Motivation

Westward-traveling vorticity disturbances exhibit significantly decrease in the reversal of El Niño

Three timescale disturbances are determined significantly at 95% confidence level, with SO and ISO depressed at the end of 1997.

SO, ISO and LFO differ each other in the structure and propagation orientation.

Decay of 1997 El Niño is mainly due to the upwelling Kelvin wave produced in Nov. 1997

Easterlies over WNP before and after the end of 1997 are dominated by SO+ISO and LFO, respectively.

SO+ISO is modulated by anomalous U shear