On the Interannual Patterns and Dynamic Factors

Affecting Trends of Western North Pacific Subtropical High

Climatology Final Report, Winter 2016-2017

Dung Bai YenDepartment of Bioenvironmental Systems Engineering, National Taiwan University

Review

The tropical and subtropical pacific region is full of thermal and dynamic driven factors. From an energetic point of view, the net radiation gain in this region is strong, therefore making it the main “heat pump” of this planet. The eastern part of Pacific usually has a cooler sea surface temperature and thus smaller outgoing longwave radiation, leading to a net energy transport from east to west via ocean current. The inter-tropical convergence zone, the south Pacific convergence zone and the monsoon regions form deep convective clouds, inhibiting outgoing longwave radiation and thus form an area of net moist static energy convergence.

From a dynamic point of view, this region exhibits many important oscillations that affect heavily on the climate. In an intra-seasonal period, the Madden–Julian oscillation is of most fame and shapes the monsoon onset for each year. Longer period of oscillations, such as the troposphere biennial oscillation and ENSO, shape the region’s climate in a longer term. In this article we shall focus on one of such examples: how these factors affect the amplitude of north Pacific subtropical high.

Since Hadley cell circulation itself is insufficient to explain the NPSH, many other theories have been developed. Among the first was the global monsoon desert mechanism. Rodwell and Hoskins proposed that the subtropical anticyclones can be seen as a Kelvin wave response to the monsoon heating over the continent to the west, and the Rossby wave response to the west of subtropical monsoon heating strengthens the descending via diabatic enhancement1. This picture of vorticity balancing among the continents and the ocean in between is challenged by some who believe that direct thermal driven circulations is the main factor of shaping the subtropical anticyclones. The type of theory they propose is the local sea-land contrast mechanism, in which the deep convections of monsoons are not the necessary factor that form the

subtropical highs; rather, it is the shallow cooling-heating couplet caused by local difference on thermal properties between land and sea2.

Besides the ongoing debate of how the subtropical highs are formed, many researches focus their attention on the long term patterns and trends of the north pacific subtropical anticyclone, and analyze how long term oscillations and trends may alter them. The study of Wang et al. in 2013 suggested two modes of western north pacific subtropical high can be found empirically. Empirical orthogonal function one is coupled by warming in Indian Ocean and cooling in western Pacific. Empirical orthogonal function two is coupled by central Pacific La Nina 3.

Investigations on how long term oscillations may affect the modes of western north Pacific subtropical high are also conducted. In 2007, Sui et al. showed that the WNPSH index consists two modes of oscillation. A 2-3 year periodic mode is related to the change of strength of the Hadley cell in western pacific, and thus is suspected to be related to the tropospheric biennial oscillation. A 3-5 year periodic mode is related to ENSO events, as sea surface temperature drops in western Pacific. In the two decades previous to the research, the 2-3 year periodic mode had been growing with respect to its magnitude 4.

Such phenomenon can be connected to two other long term trends happening in the tropical Pacific region. The first is the weakening of the Walker cell and strengthening of the Hadley cell in this region. This is perhaps a trend due to the warming of Indian Ocean 5 and global warming 6. The other is a shift of the type of ENSO, from eastern Pacific to central Pacific 7.

The last long term trend regarding the north Pacific subtropical high which will be related to this article is a westward span of the western north Pacific subtropical high since the 1970s. The trend may also be attributed to a warming in the India Ocean 8.

Data Analysis

To examine the above mentioned trends, in this article an empirical orthogonalization is performed on the anomalies of sea level pressure, surface temperature, 850 hPa wind field and precipitation. The time span of the data is from 1977 to 2012. The spatial resolution is 1x1 degree. The temporal resolution is per month. Area weighting is used.

After the five features are orthogonally decomposed, a multi-variant principle component analysis is then performed, giving five dominating empirical orthogonal modes. From the spectrum domain and the time series chart, it is obvious that mode one two, and five reveals long term oscillation and trends, while mode three and four contain high frequency variation (Appendix 1 and Appendix 2).

For mode one, an obvious difference in pressure and temperature between eastern and western Pacific and a dominating 4 year periodicity spectrum suggests that this is a mode representing the regular oscillation of ENSO. The cooling anomaly in western Pacific aligns with Wang et al. in 2013 and Sui et al. in 2007 (Appendix 3 and Appendix 4). For mode two, a dominating 2.5 year periodicity and an obvious warming in Indian Ocean and tropical monsoon region suggests this to be representing the Hadley circulation in west Pacific and the tropospheric biennial oscillation. A steady increase of the time series of this mode further confirms this suggestion. It should also be noted that a cooling anomaly in central Pacific can also be seen in this mode, suggesting that central Pacific La Nina may also play a part of role in this growing mode, as suggested by Wang et al. in 2013 (Appendix 3 and Appendix 4).

The correlations of original anomaly data and the scores of the modes are also revealing (Appendix 5). The correlation of surface temperature and score one suggests that in summer, the tropical region of western Pacific is negatively correlated while the eastern Pacific is positive correlated to score one. This reaffirms that mode one might be representing the ENSO, with positive value indicating El Nino events and negative value indicating La Nina events.

The correlation of sea level pressure and score two suggests that in summer, sea level pressure in the Indian Ocean and tropical monsoon region is negatively correlated to this score while the subtropical areas of western Pacific and the eastern Pacific are positively correlated to this score. The correlation for surface temperature and score two also suggests positive correlation in western tropical Pacific and negative in subtropical region. This further suggests the relationship between the heating of Indian Ocean and the convection in the monsoon region with the subtropical anticyclone. In winter, the score is strongly positively correlated with sea level pressure in central Pacific; its correlation with surface temperature is strongly negative in the central Pacific area. These patterns reaffirm the growing trend of

stronger Hadley circulation and central Pacific ENSO events.

Since the signal of mode two is expected to continue to increase in the near future, the correlation patterns can also give a rough prediction on how the climate characteristics are apt to change. Western north Pacific subtropical high is likely to grow stronger in the future, so does the summer monsoon in Indian Ocean and tropical western Pacific.

Reference

1. M. J. RODWELL and B. J. HOSKINS, Journal of Climate, 2001, 14, 20.2. T. Miyasaka and H. Nakamura, Journal of Climate, 2005, 18, 20.3. B. Wanga, B. Xiang and J.-Y. Leeb, PNAS, 2013, 110, 5.4. C.-H. Sui, P.-H. Chung and T. Li, Geophysical Research Letters, 2007, 34, 6.5. H. Tokinaga, S.-P. Xie, C. Deser, Y. Kosaka and Y. M. Okumura, Nature, 2012,

491, 5.6. J. MA, S.-P. XIE and Y. KOSAKA, Journal of Climate, 2012, 25, 15.7. J.-Y. Yu, Pacific Subtropical High: An Overview).8. T. Zhou, R. Yu, J. Zhang, H. Drange, C. Cassou, C. Deser, D. L. R. Hodson, E.

Sanchez-Gomez, J. Li, N. Keenlyside, X. Xin and Y. Okumura, Journal of Climate, 2009, 22, 16.

Appendix 1: Time Series of the Five Principle Components

Appendix 2: Power Spectrum of the Five Principle Components

The red line is the theoretical AR-1 line red dashed line is the 95% significance level.

Appendix 3: Spatial Variation of Five Principle Components I

This is the sea level pressure, surface temperature and 850 hPa wind field variations under different principle components. The colored image indicates surface temperature, the contour indicates sea level pressure, and the vector indicates the 850 hPa wind field.

Appendix 4: Spatial Variation of Five Principle Components II

This is the precipitation, surface temperature and 850 hPa wind field variations under different principle components. The colored image indicates precipitation, the contour indicates sea level pressure, and the vector indicates the 850 hPa wind field.

Appendix 5: Correlations of Scores and Variables

Below we show the correlation charts that are mentioned in this article. To view all the plots, readers can visit the following web site:

https://drive.google.com/open?id=0ByumDBjHbLo5eDJIZ1JvNE96REk Surface Temperature

Sea Level Pressure

Precipitation