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Oceanography
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Sea surface temperature variability in the seas surrounding The philippines in relation to ENSO events
EOF Analysis
The EOF decomposition of SST data, for the three ENSO phases, yields three dominant modes. On the average, the three modes accounted 71%, 7% and 5% of the total variations, respectively.
The first mode, shows typical spatial pattern associated with the monsoon. It is dominated by the intrusion of cold water from the northeast that reaches southward off the southern coast of Vietnam: (~9°N) during non-ENSO, north of 14°N during weak ENSO and north of 23°N during ENSO. The Philippine Sea is characterized by weak latitudinal temperature gradient and homogeneous warm water at the southern portion up to 18°N. The amplitudes of the first mode, which is identical for the three data sets, are characterizes by almost uniform
positive values throughout the year - weakening during southwest monsoon months (i.e. from June to September).
Figure 1. Spatial distributions (a, b and c) and seasonal amplitudes (d) of the 1st EOF mode.
For the three periods considered, the first mode, shows typical spatial pattern associated with the monsoon. With or without ENSO, for both the SCS and the Philippine Sea, there exists a north to south temperature gradient - with a decreasing temperature from the north. The weaker signal given during the southwest monsoon suggests that the north to south gradient pattern (which is very prominent during the northeast monsoon) diminish with the onset of this season. Reduced inflow of cold water in the northern extremities, during strong ENSO, can be associated to the anomalous southerlies or southwesterlies in the SCS during ENSO, which opposes the mean wintertime northeasterly flow that advects cold and dry air off the Asian continent (Klein et al., 1999e). This eventually lead to the reduction in the wind speed and occurrence of cold-air outbreaks, for that reason, the latent and sensible heat flux in the northern portion of the SCS as well as in regions northeast of the philippines were also reduced.
The second mode exhibits strong interannual variations. The spatial variability of temperature in the Philippine Sea is smaller during period of non-ENSO compared with ENSO years. The basin-wide circulation in the SCS during strong ENSO, on the other hand, seems to weaken as the temperature difference between the northern and southern portion of the basin is very distinct. The cold upwelled water off Vietnam during normal year did not also developed as ENSO occurs.
Its monthly amplitudes during non-ENSO, are symmetrical with the seasonal variations. However, periods with weak and strong ENSO indicate a reversal of the amplitude values as compared with non-ENSO. Period with weak ENSO also shows the same temporal variability as that of strong ENSO, however the negative amplitude only extended up to October. The negative amplitude and the negative anomalies in the northern part of the SCS and Philippine Sea, during strong ENSO, can be associated to the weak transition of northeast monsoon (Wu et al., 1998f).
Figure 2. Spatial distributions (a, b and c) and seasonal amplitudes (d) of the 2nd EOF mode.
The third mode highlights the formation of a warm/ cool pool in the northwestern part of Luzon (centered at 16.4°N, 118.3°E) during strong ENSO. This spatial pattern may already be affected by noise due to some undefined patterns, particularly during weak ENSO. The feasible reason behind the formation of warm water on this area, during strong ENSO, is quite hard to define. Whether this feature was caused by the occurrence of ENSO or by processes of a large-scale origin, requires further investigation. In terms of its amplitude, this mode only shows one significant feature during non-ENSO, which is the appearance of the extreme negative amplitude in July. This features may be an indication of a northeastward warm current off the southern coast of China (Chao et al., 1995g) during winter.
The results of the EOF analysis within the Sulu Sea and Celebes Sea are not that distinct compared with the SCS and the Philippine Sea. Since the two basins are partly bounded by land, land masking highly influences the result of the analysis. Though the spatial and temporal patterns in these two regions are quite hard to define, the characteristics of their surface temperature can still be compared based on the satellite images. Surface temperature in the Sulu Sea is much warmer compared to the Celebes Sea since the primary source of the water in the former basin comes from the SCS while the latter is primarily influence by the Mindanao current. And in general, since the water in the SCS is distinctly colder than the Pacific (Shaw, 1991h), it is expected that water in the Sulu Sea is colder than the Celebes Sea.
Summary and Conclusion
The seasonal, temporal and internannual variations of sea surface temperature, highlighting its association to the ENSO event, in the water surrounding the philippines was examined by performing an EOF analysis. Satellite-derived monthly SST images of the area were used as the principal basis. The images were classified into three periods, representing specific year with strong, weak and non-ENSO. Results from the first three EOF modes reflect various significant characteristics linking the SST variations to the ENSO event, such as reduced inflow of cold water in the northern extremities, reduced upwelling and weak seasonal variation. It was also observed that the dominant spatial pattern in the South China Sea can be associated with the northeast monsoon. Distinct latitudinal difference exists in the Philippine Sea with or without ENSO. Lastly, sudden change in the prevailing local wind system in the SCS during ENSO primarily affects the circulation in the basin and results in the warming of the upper ocean.
References
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Chao, S. Y., P. T. Shaw, and J. Wang, 1995g. Wind Relaxation as a Possible Cause of the South China Sea Warm Current. J. Oceanography, 51, pp. 111-132.
- Chao, S. Y., P.T. Shaw, and S.Y. Wu, 1996d. El Niño Modulation of the South China Sea Circulation. Prog. Oceanog., 38, pp.51-93.
- Emery, W. J. and R. E. Thomson, 1997b. Data Analysis Methods in Physical Oceanography. Gray Publishing, Tunbridge Wells, Kent, pp.319-340.
- Klein, S.A., B.J. Soden. and N.C. Lau, 1999e. Remote Sea Surface Temperature Variations during ENSO: Evidence for a Tropical Atmospheric Bridge. J. Climate, 12, pp. 917-932.
- Masumoto, Y. and T. Yamagata, 1991c. Response of the Western Tropical Pacific to the Asian Winter Monsoon: The Generation of Mindanao Dome. J. Phys. Oceanogr., 21, pp.1386-1398.
- Shaw, P.T., 1991h. The seasonal variation of the intrusion of the Philippine Sea water into the South China Sea. J. Geophys. Res., 96, pp. 821-827.
- Thurman, H. V., 1998a: Introductory Oceanography, 7th. Ed. Macmillan Publishing Company, New York, pp.185-212.
- Wu, C.R., P.T. Shaw and S.Y. Chao, 1998f. Seasonal and interannual variations in the velocity field of the South China Sea. J. Oceanogr., 54, pp. 361-372.
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