Wind stress curl and ENSO discharge/recharge in the equatorial Pacific

被引:76
作者
Clarke, Allan J. [1 ]
Van Gorder, Stephen [1 ]
Colantuono, Giuseppe [1 ]
机构
[1] Florida State Univ, Dept Oceanog, Tallahassee, FL 32306 USA
关键词
D O I
10.1175/JPO3035.1
中图分类号
P7 [海洋学];
学科分类号
0707 ;
摘要
Discharge and recharge of the warm water volume (WWV) above the 20 degrees C isotherm in an equatorial Pacific Ocean box extending across the Pacific from 156 degrees E to the eastern ocean boundary between latitudes 5 degrees S and 5 degrees N are key variables in ENSO dynamics. A formula linking WWV anomalies, zonally integrated wind stress curl anomalies along the northern and southern edges of the box, and flow into the western end of the box is derived and tested using monthly data since 1993. Consistent with previous work, a WWV balance can only be achieved if the 20 degrees C isotherm Surface is not a material surface; that is, warm water can pass through it. For example, during El Nino, part of the WWV anomaly entering the box is cooled so that it is less than 20 degrees C and therefore passes out of the bottom of the box, the 20 degrees C isotherm surface. The observations suggest that the anomalous volume passing through the 20 degrees C isotherm is approximately the same as T'(W), the anomalous WWV entering the western end of the box. Therefore the observed WWV anomaly can be regarded as being driven by the anomalous wind stress curl along the northern and southern edges of the box. The curl anomaly changes the WWV both by divergent meridional flow at the edges of the box and vortex stretching; that is, the Sverdrup balance does not hold in the upper ocean. A typical amplitude for the rate of change of WWV for the 5 degrees S-5 degrees N box is 9.6 Sv (Sv = 10(6) m(3) s(-1)). The wind stress curl anomaly and the transport anomaly into the western end of the box are highly correlated with the El Nino index Nino-3.4 [the average sea surface temperature anomaly (SSTA) ever the region 5 degrees S-5 degrees N, 170 degrees-120 degrees W] and Nino-3.4 leads minus the WWV anomaly by one-quarter of a cycle. Based on the preceding results, a simple discharge/recharge coupled ENSO model is derived. Only water warmer than about 27.5 degrees-28 degrees C can give rise to deep atmospheric convection, so, unlike past discharge/recharge oscillator models, the west-central rather than eastern equatorial SSTAs are emphasized. The model consists of two variables: T', the SSTA averaged over the region of strong ENSO air-sea interaction in the west-central Pacific equatorial strip 5 degrees S-5 degrees N, 156 degrees E-140 degrees W and D', the 20 degrees C isotherm depth anomaly averaged over the same region. As in the observations, T' lags D' by one-quarter of a cycle; that is, delta T'/delta t vD' for some positive constant v. Physically, when D' > 0, the thermocline is deeper and warmer water is entrained through the base of the mixed layer, the anomalous heat flux causing delta T'/delta t > 0. Also, when D' > 0, the eastward current anomaly is greater than zero and warm water is advected into the region, again causing delta T'/delta t > 0. Opposite effects occur for D' < 0. A second relationship between T' and D' results because the water is warm enough that T' causes deep atmospheric convection anomalies that drive the wind stress curl anomalies that change the heat storage delta D'/delta t. The atmosphere responds essentially instantly to the T' forcing and the curl causes a discharge of WWV during El Nino (T' > 0) and recharge during La Nina (T' < 0), so delta D'/delta t = -mu T' for some positive constant mu. The two relationships between T' and D' result in a harmonic oscillator with period 2 pi/root nu mu approximate to 51 months.
引用
收藏
页码:1077 / 1091
页数:15
相关论文
共 38 条
[1]   Tropical Pacific basin-wide adjustment and oceanic waves [J].
An, SI ;
Kang, IS .
GEOPHYSICAL RESEARCH LETTERS, 2001, 28 (20) :3975-3978
[2]  
CLARKE AJ, 1994, J CLIMATE, V7, P1623, DOI 10.1175/1520-0442(1994)007<1623:WASEEW>2.0.CO
[3]  
2
[4]  
Ebisuzaki W, 1997, J CLIMATE, V10, P2147, DOI 10.1175/1520-0442(1997)010<2147:AMTETS>2.0.CO
[5]  
2
[6]   OCEAN ATMOSPHERE COUPLING OVER MONSOON REGIONS [J].
GADGIL, S ;
JOSEPH, PV ;
JOSHI, NV .
NATURE, 1984, 312 (5990) :141-143
[7]   SEA-SURFACE TEMPERATURE, SURFACE WIND DIVERGENCE, AND CONVECTION OVER TROPICAL OCEANS [J].
GRAHAM, NE ;
BARNETT, TP .
SCIENCE, 1987, 238 (4827) :657-659
[8]   Interannual volume variability in the tropical Pacific [J].
Holland, CL ;
Mitchum, GT .
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS, 2003, 108 (C11)
[9]   Thermocline and zonal advective feedbacks within the equatorial ocean recharge oscillator model for ENSO [J].
Jin, FF ;
An, SI .
GEOPHYSICAL RESEARCH LETTERS, 1999, 26 (19) :2989-2992
[10]  
Jin FF, 1997, J ATMOS SCI, V54, P811, DOI 10.1175/1520-0469(1997)054<0811:AEORPF>2.0.CO