Delayed Response of the Extratropical Northern Atmosphere to ENSO: A Revisit *

Transcription

1 Delayed Response of the Extratropical Northern Atmosphere to ENSO: A Revisit * Ruping Mo Pacific Storm Prediction Centre, Environment Canada, Vancouver, BC, Canada Corresponding author s address: Ruping Mo Pacific Storm Prediction Centre Environment Canada Burrard Street Vancouver, BC V6C 3S5 Canada ruping.mo@ec.gc.ca Technical Report Pacific Storm Prediction Centre 31 May 2006 * This report is based on a poster, presented at the 40 th CMOS Congress, 29 May 1 June 2006, Toronto, Ontario, Canada; see Fig. 7 at the end of the report.

2 1. Introduction El Niño/Southern Oscillation (ENSO) is known as the most prominent source of interannual variability in weather and climate around the world. In an earlier study, Mo et al. (1998) showed that the atmospheric anomalies in the Pacific/North American (PNA) sector from December to March are correlated most significantly with the ENSO signal in October. These delayed responses, together with their dynamical implications, are revisited in the current study. The goal is not just to confirm the delayed responses identified earlier, but also to search for other unknown delays that may be useful in longrange operational forecast. As it turns out, the most remarkable atmospheric response occurs in February, with a robust teleconnection (similar to the PNA pattern) related to the ENSO signal appearing as early as in June of the previous year. The existence of such a long delay cannot be explained by the atmospheric bridge theory alone. The possibility for an oceanic bridge to carry the early ENSO signal to the Northeast Pacific in the following winter is investigated in this study. As a straightforward application, the summer ENSO signal is used as a predictor to construct linear regression models for the winter climate of the Inner South Coast of British Columbia. Such models could provide a valuable service to the 2010 Olympic and Paralympic Winter Games, to be held in Vancouver from February 12 to March Data and methods The following datasets are used in this study: UK Met Office monthly mean SSTs (HadISST, 1º 1º, ) NCEP/NCAR Reanalysis monthly mean 500 hpa heights (2.5º 2.5º, ) 1

3 Daily maximum and minimum temperatures, precipitation, and snowfall amount from 5 airports in the BC South Coast (See Fig. 1), The ENSO signal is represented by the NINO-3 index, given as the SST anomaly averaged over the eastern equatorial Pacific (5ºS-5ºN, 150ºW-90ºW; see Fig. 2). Figure 2: The NINO-3 region. 2

4 Correlation is the main statistical technique used to reveal the linear relations between the ENSO signal and the atmospheric response. Regression technique is used for the forecast application. 3. Evidences of delayed responses of atmosphere to ENSO Table 1 shows the explained variance of the NH 500 hpa heights by the NINO-3 index, with the atmosphere lagging from 0 to 11 months. Lag correlations with the PNA index (not shown) reveal the same features: Strongest responses occur in the winter months (December February) The ENSO signal in October or November achieves the highest scores For the February atmosphere, strong ENSO signal appears as early as in June of the previous year (8-month lead time) Table 1: The proportion of total variance of the NH 500 hpa geopotential height (0 N- 90 N) explained by the NINO-3 index. The first column indicates the lead time (months) of the ocean ahead of the atmosphere. Values greater than 0.5 are boldfaced. Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

5 Correlation maps for the above-mentioned ENSO impact are shown in Fig. 3 and Fig. 4. Figure 3: Correlation maps. The PNA patterns (right panel) are derived from the Empirical orthogonal teleconnections (EOT, van den Dool et al. 2000). 4

6 Figure 4: Correlation maps. Significant correlations over the NH extratropical area include: December: Significant responses to ENSO occur as a dipole pattern in the western Pacific and East Asia. The PNA pattern is barely affected by ENSO. Possible delay: 0 to 3 months January: The ENSO-induced pattern bears more resemblance to the PNA pattern. But responses over western Canada are very weak. Possible delay: 3 months February: The ENSO-induced pattern is best in-phase with the PNA pattern Responses over western Canada are stronger than in January. Possible delay: 4 to 8 months March: Significant responses to ENSO are confined to North America. Possible delay: 9 months? 4. Possible causalities Well-known theories for ENSO-induced extratropical teleconnections include: Hadley-circulation anomaly (Bjerknes 1966) 5

7 ENSO-induced Rossby waves (Hoskins and Karoly 1981; Horel and Wallace 1981) Atmospheric bridge plus mid-latitude ocean feedback (Mo et al. 1998; Alexander et al. 2002) Does an oceanic bridge exist? Figure 5 shows the correlations between June ENSO signal and SSTs from June to March. Implications: ENSO-induced equatorial oceanic Kelvin waves travel eastward along the equator. Their energy splits upon reaching the eastern boundary of the Pacific: a portion reflects as equatorial Rossby waves propagating westward and the remaining energy deflects as coastal Kelvin waves traveling poleward. SST anomalies associated with the slowly moving equatorial Rossby waves could be more effective in forcing the tropical atmosphere, as they are closer to the climatological position of the Intertropical Convergence Zone (ITCZ). The coastal Kelvin waves generate extratropical Rossby waves, which act to widen the along-shore SST anomalies and then enhance their feedback to the atmosphere; note that along-shore anomaly band is at least 10 times wider than the typical width of the coastal Kelvin waves (~30 km). In January (February) over W. Canada, atmospheric response to the along-shore SST anomalies partly cancels (reinforces) the response associated with the atmospheric Rossby waves, according to its dependence on background flow (Peng et al. 1997). 6

8 Figure 5: Correlations between June ENSO signal and SSTs from June to March. 7

9 5. Applications Previous studies have indicated that the winter climate in southern BC is influenced by ENSO (e.g., Taylor 1998). To test the ENSO-induced June-February teleconnection, linear regression models are constructed to predict temperature, precipitation, and snowfall amount for the BC South Coast. As an example, Figure 6 shows the regression prediction for the monthly mean daily maximum temperature at the Vancouver International Airport. The mean-squared-error skill scores for all predictions are given in Table 2. It is shown that these statistical predictions are skilful for the temperature and snowfall amount, but not for the precipitation. Skill from the warm water case is slightly higher for the temperature, and much higher for the snowfall amount, than that from the cold water case. Nonlinearity, therefore, should be taken into account. Table 2: Mean-squares-error skill scores. CYVR CYXX CYXX CYQQ CYZT All Tmean SSTA> SSTA< All Snow SSTA> SSTA< All Precip SSTA> SSTA<

10 Figure 6: Linear regression predictions for the monthly mean daily maximum temperature in February at CYVR. 6. Concluding remarks It is demonstrated that delayed response to the ENSO signal exists in the wintertime extratropical NH atmosphere. Evidences suggest that the equatorial/coastal Kelvin wave, through their interaction with the oceanic Rossby waves, are capable of carrying the summer ENSO signal from the tropical Pacific to the Northeast Pacific in the following 9

11 winter. The existence and robustness of such an oceanic bridge remain to be confirmed through further theoretical and modeling studies. The summer ENSO signal can be used as a statistical predictor for the winter (February, in particular) climate in the BC South Coast. Skilful forecasts can be achieved through linear regression. Including nonlinear effect may help to improve the forecast skill. This report is based on a poster (see Fig. 7), presented at the 40 th CMOS Congress, 29 May 1 June 2006, Toronto, Ontario, Canada. References Alexander, M. A., I. Bladé, M. Newman, J. R. Lanzante, N.-C. Lau, and J. D. Scott, 2002: The atmospheric bridge: The influence of ENSO teleconnections on air sea interaction over the global oceans. J. Climate, 15, Bjerknes, J., 1966: A possible response of the atmospheric Hadley circulation to equatorial anomalies of ocean temperature. Tellus, 18, Horel, J. D., and J. M. Wallace, 1981: Planetary-scale atmospheric phenomena associated with the Southern Oscillation. Mon. Wea. Rev., 109, Hoskins, B. J., and D. J. Karoly, 1981: The steady linear response of a spherical atmosphere to thermal and orographic forcing. J. Atmos. Sci., 38, Mo, R., J. Fyfe, and J. Derome, 1998: Phase-locked and asymmetric correlations of the wintertime atmospheric patterns with the ENSO. Atmosphere-Ocean, 36,

12 Figure 7: Poster presented at the 40 th CMOS Congress, 29 May 1 June 2006, Toronto, Ontario, Canada. 11

13 Peng, S., W. A. Robinson, and M. P. Hoerling, 1997: The modeled atmospheric response to midlatitude SST anomalies and its dependence on background circulation states. J. Climate, 10, Taylor, B., 1998: Effect of El Niño/Southern Oscillation (ENSO) on British Columbia and Yukon winter weather. Report 98-02, Aquatic and Atmos. Sci. Division, Pacific and Yukon Region, Environment Canada, 10 pp. van den Dool, H. M., S. Saha, and Å. Johansson, 2000: Empirical orthogonal teleconnections. J. Climate, 13,