lecture 11 El Niño/Southern Oscillation (ENSO) Part II

Transcription

1 lecture 11 El Niño/Southern Oscillation (ENSO) Part II

2 SYSTEM MEMORY: OCEANIC WAVE PROPAGATION

3 ASYMMETRY BETWEEN THE ATMOSPHERE AND OCEAN The atmosphere and ocean are not symmetrical in their responses to changes in boundary conditions. The atmosphere responds very quickly to changes in SST, usually within days, while the ocean can take months to equilibrate to changes in wind forcing. This phase lag is the fundamental reason the tropical Pacific exhibits oscillatory behavior. Moreover, the ocean s memory of earlier wind patterns can propagate along the equator in the form of waves. These waves are expressed as undulations of the thermocline. These have a much larger effect on SST when the thermocline is close to the surface.

4 Oceanic Equatorial Waves Horizontal Structures Rossby waves propagate westwards, and have amplitude maxima on either side of the equator. Kelvin waves propagate eastwards, and have maximum amplitude at the equator. They can also be trapped along coastlines, propagating with the coast on the right in the northern hemisphere

5 Kelvin Wave propagation

6 Idealized Experiment Illustrating Tropical Wave Propagation The shallow-water equations in equatorial beta-plane are solved numerically in a 1.5 layer model. The basin is idealized with a similar dimension as the Pacific. The ocean is forced with westerly winds in the equatorial region, Gaussian and symmetrical about the equator. The winds are kept constant for 30 days and then are switched off. The meridional structure of the wind forcing (dynes/cm2) along 175 E is also shown. (

7 Propagation of Kelvin and Rossby Waves Excited by Wind Forcing (Vectors are ocean currents, colors are sea surface height.) Which are Kelvin and which are Rossby waves? Which type of wave propagates faster?

8 Propagation of Kelvin and Rossby Waves Excited by Wind Forcing (Vectors are ocean currents, colors are sea surface height.) What is happening at the eastern boundary? What is happening at the western boundary?

9 The propagation of equatorial Kelvin waves is clearly visible is this Hovmuller diagram of the depth of the 20 deg C isotherm averaged from 2N to 2S. Approximately how long does it take for one of these waves to cross the Pacific? courtesy of NOAA PMEL (from the TAO array)

10 ENSO as a Stable Mode Triggered by Random Forcing This hypothesis proposes that disturbances, unrelated to internal ENSO dynamics, are the source of random (stochastic) forcing that drives ENSO from one phase to another (e.g., Penland & Sardeshmukh 1995; Moore & Kleeman 1999; Thompson & Battisti 2001; Kessler 2002; Philander & Fedorov 2003).

11 ENSO IMPACTS

12 Schematic of temperature and precipitation anomalies generally associated with the warm phase of ENSO during the northern winter and summer seasons. To a good approximation, relationships with the cold phase of ENSO are simply reversed in sign. [After Halpert and Ropelewski (1992, J. Climate, 5, ) and supplemented by Aceituno (1988, Mon. Wea. Rev., 116, )] (courtesy of NOAA/PMEL).

13 ENSO has a strong impact on the position of the jet stream over the Pacific. During La Niña, the jet stream is pushed far to the north of California. During El Niño, the jet stream tends to be located at about the same latitude as Southern California. Thus the storm activity associated with the jet stream is also located at the same latitude as Southern California.

14 The jet stream position shifts because SST anomalies in the Pacific excite a natural mode of pressure variability called the PNA. It is characterized by an wave train that arcs from the equatorial Pacific to the Southeastern U.S.

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17 MODULATION AND PREDICTION OF ENSO

18 Niño Indices: Recent Evolution

19 ENSO prediction

20 ENSO AND CLIMATE CHANGE ENSO underwent some kind of regime shift in the 1970s toward a more El- Niño-like state. Could this shift have been due to anthropogenic forcing? How will climate change affect ENSO in the future? Figure 2.29 from the 2001 IPCC report on climate change: El Niño-La Niña variations from 1876 to 2000 measured by sea surface temperature in the region 5 N to 5 S, 150 to 90 W. Reconstructions using pattern analysis methods from (a) red: UK Met Office (UKMO) Hadley Centre sea ice and sea surface temperature data set version 1 (Rayner et al., 2000); (b) black: from the Lamont-Doherty Earth Observatory (LDEO) (Kaplan et al., 1998); (c) blue: the National Centers for Environmental Prediction (NCEP) analysis (Smith et al., 1998) is close to the earliest date for which reasonably reliable reconstructions can be made. Unfortunately climate change simulations do not provide much guidance on this question.

21 Low-Frequency Variability of ENSO and the link with mid-latitudes Reconstructed Nino4 SSTA using two leading modes. Regression patterns of SSTA using above two time-series. From Wang & Picaut (2004)

22 PACIFIC DECADAL OSCILLATION Is there a role for the mid-latitudes in modulating ENSO? The PDO is the leading mode of North Pacific SST variability. Clearly it projects strongly on the interdecadal component of ENSO. The causal relationship between these two modes is not known at the present time. TOP: SST (colors), surface wind stress (arrows), and sea level pressure (contours) during opposite phases of the PDO. BOTTOM: PDO time series. Courtesy Nate Mantua/Steven Hare