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HOME > Climate System Monitoring > Atmosphere Circulation Regression and Correlation map > Explanation

Explanation

Features of Atmospheric Circulation Associated with SST in Monitoring Regions ~ Based on Regression Maps Produced from the JRA-25 and JCDAS ~

1. Introduction

An investigation on how the atmospheric circulation is related to tropical sea surface temperatures (SST) in monitoring regions was conducted using distribution maps of regression coefficients between the regional mean SST anomalies (El Niño Monitoring Indices) in each monitoring region and atmospheric circulation anomalies such as Outgoing Long-wave Radiation (OLR), and 850-hPa and 200-hPa stream function anomalies. Response to the heat source in the tropics has the baroclinic patterns in which the circulations in the lower and upper troposphere are reversed each other. Therefore, to investigate the response to the heat source in the tropics, the circulation patterns in 850 hPa and 200 hPa were mainly used. The features highlighted below correspond to the atmospheric circulation anomalies that tend to appear when the index is one standard deviation (+/-1.0).

It has been suggested that the atmospheric response to the tropical SST anomalies during El Niño events is not always symmetric to the response to La Niña events (Hoerling et al., 2001). In spite of this suggestion, it is assumed that the circulation patterns when the monitoring index is +/-1.0 could be generally considered symmetric with each other because of small sample number.



2. Data and methods

For the SST data, COBE SST analysis dataset (http://ds.data.jma.go.jp/tcc/tcc/library/MRCS_SV12/index_e.htm) produced by the Marine Division of the Japan Meteorological Agency (JMA) were used.

For the atmospheric circulation data, the JRA-25 reanalysis dataset (http://www.jreap.org/indexe.html) produced by the collaboration between JMA and the Central Research Institute of Electric Power Industry (CRIEPI) and the JCDAS which is consistent in quality with the JRA-25 were used. OLR data derived from observations by NOAA's polar-orbiting satellite and provided by the Earth System Research Laboratory (ESRL) in the National Oceanic and Atmospheric Administration (NOAA) were used as estimates of convective activity.

Anomalies are defined as deviations from the average from 1979/80 to 2008/09 for winter and from 1979 to 2008 for other seasons.

The details of El Niño Monitoring Indices are shown in Table 1.


Table 1 El Niño Monitoring Indices
Name of Index Area Latitude & Longitude
NINO.1+2 West of Peru 90ºW - 80ºW 10ºS - EQ, 90ºW - 80ºW
NINO.3 Eastern Pacific 5ºS - 5ºN, 150ºW - 90ºW
NINO.4 Central Pacific 5ºS - 5ºN, 160ºE - 150ºW
NINO.WEST Western Pacific EQ - 15ºN, 130ºE - 150ºE
IOBW Indian Ocean Basin 20ºS - 20ºN, 40ºE - 100ºE


3. NINO.3 and Atmospheric Circulation

(1) Winter (December - February)

Fig. 1 shows regression coefficients between NINO.3 and winter mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 1b), a west-east contrast of suppressed convections over the South China Sea and from Indonesia to the western Pacific and enhanced convections from the central to the eastern Pacific is clearly seen. Suppressed convections are also seen over the South Pacific Convergence Zone (SPCZ) and the equatorward-shifted Intertropical Convergence Zone (ITCZ). Convective activities are enhanced over the Gulf of Mexico and suppressed over the northern part of South America.

In the 850-hPa stream function field (Fig. 1c), corresponding to the convection anomalies, equatorially symmetric anti-cyclonic and cyclonic circulation anomalies are clearly seen from the Indian Ocean to Indonesia and over the Pacific, respectively. These patterns correspond to "Matsuno - Gill" response (Matsuno 1966 and Gill 1980) to the heat source (convection) anomalies. Cyclonic circulation anomalies clearly seen over the Gulf of Alaska mean eastward shift of the Aleutian Low.

In the 200-hPa stream function field (Fig. 1d), corresponding to the convection anomalies, equatorially symmetric cyclonic and anti-cyclonic circulation anomalies are clearly seen over East Asia and the South Indian Ocean, and from the equatorial central to eastern Pacific, respectively. Equatorially symmetric cyclonic circulation anomalies are also seen over the equatorial Atlantic. Corresponding to the cyclonic circulation anomalies over East Asia, the subtropical jet streams shift southward from China to Japan (Fig. 2). In the regression coefficients between NINO.3 and 500-hPa height anomalies (Fig. 3), wave trains are distributed from the North Pacific to North America. This is similar to the Tropical Northern Hemisphere (TNH) pattern (Mo and Livezey 1986), which appears in winter during El Niño events.

(2) Spring (March - May)

Fig. 4 shows regression coefficients between NINO.3 and spring mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 4b), convective activities are enhanced over the equatorial Pacific as well as winter. Suppressed convections shift northward from the Arabian Sea to the east off the Philippines compared with winter. Convections are suppressed from the northern part of Brazil to the Gulf of Guinea.

In the 850-hPa stream function field (Fig. 4c), equatorially symmetric anti-cyclonic circulation anomalies are seen from the Indian Ocean to near Indonesia, though they are obscure compared with winter. Over the equatorial Pacific, equatorially symmetric cyclonic circulation anomalies are seen like winter. Cyclonic circulation anomalies are also seen over eastern Siberia and near New Zealand.

In the 200-hPa stream function field (Fig. 4d), equatorially symmetric cyclonic and anti-cyclonic circulation anomalies are seen over Asia and the South Indian Ocean, and over the equatorial Pacific, respectively. Equatorially symmetric cyclonic circulation anomalies are seen over the equatorial Atlantic.

(3) Summer (June - August)

Fig. 5 shows regression coefficients between NINO.3 and summer mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 5b), convective activities are suppressed over Indonesia and enhanced over the equatorial Pacific. On the other hand, statistical significant anomalies are not seen over the Philippines, which are clearly seen in other seasons.

In the 850-hPa stream function field (Fig. 5c), equatorially symmetric anti-cyclonic circulation anomalies from the Indian Ocean to Indonesia seen in other seasons are obscure. Equatorially symmetric cyclonic circulation anomalies over the equatorial Pacific shift westward compared with other seasons. Cyclonic circulation anomalies are seen around Japan.

In the 200-hPa stream function field (Fig. 5d), equatorially symmetric cyclonic circulation anomalies over the equatorial Pacific are obscure compared with other seasons. In the extra-tropics, cyclonic circulation anomalies are broadly distributed over the Northern Hemisphere. The Tibetan High is weaker than normal in its northern side.

(4) Autumn (September - November)

Fig. 6 shows regression coefficients between NINO.3 and autumn mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 6b), suppressed convections over Indonesia extend to the west off the Java Island. They are associated with the enhanced convection over the western Indian Ocean, indicating the relationship between ENSO and Indian Ocean Dipole (Saji et al., 1999).

In the 850-hPa stream function field (Fig. 6c), equatorially symmetric anti-cyclonic and cyclonic circulation anomalies are clearly seen from the Indian Ocean to Indonesia and over the Pacific, respectively.

In the 200-hPa stream function field (Fig. 6d), equatorially cyclonic and anti-cyclonic circulation anomalies are seen from the northern Africa to Asia and the Indian Ocean, and over the equatorial Pacific, respectively. Corresponding to cyclonic circulation anomalies over Asia, the Asian jet shifts southward from its normal position (Fig. 7).



4. NINO.WEST and Atmospheric Circulation

(1) Winter (December - February)

Fig. 8 shows regression coefficients between NINO.WEST and winter mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig.8b), convective activities are enhanced from around Indonesia to the SPCZ and suppressed from the western to the central Pacific. These patterns have opposite sign to those of NINO.3. Enhanced and suppressed convections are also seen from the Caribbean Sea to the northern part of South America and near the Gulf of Mexico, respectively.

In the 850-hPa stream function field (Fig. 8c), similar patterns to the reversed ones of NINO.3 (Fig. 1c) are clearly seen around the western Pacific.

In the 200-hPa stream function field (Fig. 8d), corresponding to the convection anomalies, equatorially symmetric anti-cyclonic and cyclonic circulation anomalies are clearly seen around Indonesia and over the equatorial central Pacific, respectively. Corresponding to the anti-cyclonic circulation anomalies over East Asia, the subtropical jet shifts northward from its normal position (Fig. 9).

(2) Spring (March - May)

Fig. 10 shows regression coefficients between NINO.WEST and spring mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 9b), convective activities are enhanced from the Arabian Sea to the Bay of Bengal and off the east of the Philippines and suppressed over the equatorial Pacific.

In the 850-hPa stream function field (Fig. 10c), similar patterns to those of winter are seen.

In the 200-hPa stream function field (Fig. 10d), equatorially symmetric anti-cyclonic and cyclonic circulation anomalies are seen over Asia and the South Indian Ocean, and over the central Pacific, respectively.

(3) Summer (June - August)

Fig. 11 shows regression coefficients between NINO.WEST and summer mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 10b), convection anomalies from the Bay of Bengal to near the Philippines are obscure while enhanced convections are seen over Indonesia. It is noteworthy that there are no clear statistical relationships between NINO.WEST and the convective activities over the NINO.WEST region in summer.

In the 850-hPa stream function field (Fig. 11c), anti-cyclonic circulation anomalies are distributed around and to the east of the Philippines, which means shallower-than-normal monsoon trough. The monsoon circulation is weak over the Indian Ocean. Anti-cyclonic circulation anomalies are also distributed near the Aleutian Islands, which indicates the stronger-than-normal North Pacific High over the region.

In the 200-hPa stream function field (Fig. 11d), equatorially symmetric cyclonic circulation anomalies are seen from South America to the Atlantic.

(4) Autumn (September - November)

Fig. 12 shows regression coefficients between NINO.WEST and autumn mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 12b), convective activities are enhanced over from Indonesia to the Philippines. The suppressed convections over the equatorial Pacific shift westward compared with winter.

In the 850-hPa stream function field (Fig. 12c), equatorially symmetric cyclonic and anti-cyclonic circulation anomalies are clearly seen over the Indian Ocean and from the western to the central Pacific, respectively. These patterns are clear and shift westward compared with winter.

In the 200-hPa stream function field (Fig. 12d), equatorially symmetric cyclonic circulation anomalies are clearly seen over the Pacific. In the regression coefficients between NINO.WEST and sea level pressure anomalies (Fig. 13), low pressure anomalies are distributed around Japan.



5. IOBW and Atmospheric Circulation

(1) Winter (December - February)

Fig. 14 shows regression coefficients between IOBW and winter mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 14b), convective activities are enhanced over the equatorial Pacific, and suppressed around the Philippines and from the northern part of South America to the equatorial Atlantic. These patterns are similar to those of NINO.3 (Fig. 1b) and indicate that positive IOBW and positive NINO.3 have a tendency to weaken the Walker Circulation.

In the 850-hPa stream function field (Fig. 14c), corresponding to the convection anomalies, equatorially symmetric anti-cyclonic and anti-cyclonic circulation anomalies are clearly seen from the Indian Ocean to Indonesia and over the Pacific, respectively, which are similar to the patterns of NINO.3 (Fig. 1c). Cyclonic circulation anomalies seen over the Aleutian Islands indicate eastward-shifted Aleutian Low. In the regression coefficients between IOBW and SLP anomalies (Fig. 15), high pressure anomalies covering Eastern and Northern Japan mean weaker-than-normal winter monsoon over Japan.

In the 200-hPa stream function field (Fig. 14d), corresponding to the convection anomalies, equatorially symmetric cyclonic and anti-cyclonic circulation anomalies are clearly seen around Indonesia and from the equatorial central to eastern Pacific, respectively.

(2) Spring (March - May)

Fig. 16 shows regression coefficients between IOBW and spring mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR distributions (Fig. 16b), convective activities are enhanced over the equatorial Pacific. Suppressed convections are seen from the Bay of Bengal to near the Philippines; they shift northward compared with winter. Suppressed convections from the northern part of South America to the equatorial Atlantic and enhanced ones over the northeast off Madagascar are similar to those of NINO.3 (Fig. 4b).

In the 850-hPa stream function field (Fig. 16c), cyclonic circulation anomalies are seen over the Bering Sea; they shift westward compared with winter. Monsoon circulations in the lower troposphere weakened over the equatorial Indian Ocean.

In the 200-hPa stream function field (Fig. 16d), corresponding to the convection anomalies, equatorially symmetric cyclonic and anti-cyclonic circulation anomalies are seen around Indonesia and over the central Pacific, respectively.

(3) Summer (June - August)

Fig. 17 shows regression coefficients between IOBW and summer mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR (Fig. 17b), though enhanced convections are seen over the equatorial central Pacific, their statistical significance is small. Suppressed convections are seen from the Philippines to the Date Line along 15ºN, which are different from the patterns of winter.

In the 850-hPa stream function field (Fig. 17c), anti-cyclonic circulation anomalies over the Philippines and cyclonic ones near Japan are seen, though their statistical significance is small. In the regression coefficients between IOBW and 500-hPa height anomalies (Fig. 18), the tripole pattern, which have positive anomalies over eastern Siberia and around the Philippines and negative anomalies over Japan, are clearly seen. The positive anomalies over eastern Siberia indicate the development of a blocking high over the region in summer. In association with the blocking high, the Okhotsk high is seen near the surface over the region (Fig. 19). In Fig. 19, low pressure anomalies are seen over Japan, which indicates the North Pacific High is weaker than normal around Japan.

In the 200-hPa stream function field (Fig. 17d), cyclonic circulation anomalies are distributed from Central Asia to Japan and around Alaska. This means that the Tibetan High is weaker than normal over its northern side, being consistent with the southward-shifted Asian jet (Fig. 20).

(4) Autumn (September - November)

Fig. 21 shows regression coefficients between IOBW and autumn mean SST, OLR, and 850- and 200-hPa stream function anomalies. In the OLR (Fig. 21b), convective activities are enhanced over the equatorial Pacific. The suppressed convections over Indonesia extend to the west off the Java Island. This feature is similar to the pattern of NINO.3 (Fig. 6b).

In the 850-hPa stream function field (Fig. 21c), though the similar patterns to winter are seen from the Indian Ocean to the equatorial Pacific, their statistical significance is small. Cyclonic circulation anomalies seen over the Gulf of Alaska indicate the eastward-shifted Aleutian Low.

In the 200-hPa stream function field (Fig. 21d), though the wave trains are seen from the north Atlantic to the Middle East, near Japan and from the equatorial North Pacific to the Gulf of Alaska to North America, their statistical significance is small.



6. Summary

An investigation on the statistical relationships between El Niño Monitoring Indices and the atmospheric circulations were conducted. Positive NINO.3 indicates El Niño-like patterns which have suppressed convections from Indonesia to the western Pacific and weaker-than-normal Walker Circulation. Negative NINO.WEST and positive IOBW also indicate these patterns, being consistent with ENSO cycle. While NINO.3 and IOBW have some tendencies in common (ex. about Aleutian Low in winter, Tibetan High in summer, Indian Ocean Dipole in autumn), the similar relationships are not seen in some cases (e.g. winter and summer Asian monsoon, north Pacific High and Okhotsk High in summer). These are brought through the interaction between ocean and atmosphere, and the teleconnections in association with the stationary Rossby wave. Because these have significant influence on the weather over the extra-tropics, it is important to monitor the variations of not only the Pacific but also the Indian Ocean.



References

  • Gill, A. E., 1980: Some simple solutions for heat-induced tropical circulation. Q. J. Roy. Met. Soc., 106, 447-462.
  • Hoerling, M. P. and A. Kumar, T. Xu, 2001: Robustness of the nonlinear climate response to ENSO's extreme phases. J. Climate, 14, 1277-1293.
  • Matsuno, T., 1966: Quasi-geostrophic motions in the equatorial area. J. Meteor. Soc. Japan, 44, 25-43.
  • Mo, K. and R. E. Livezey, 1986: Tropical-extratropical geopotential height teleconnections during the Northern Hemisphere winter. Mon. Wea. Rev., 114, 2488-2515.
  • Saji, N. H., B. N. Goswami, P. N. Vinayachandran, and T. Yamagata, 1999 : A Dipole Mode in the Tropical Indian Ocean. Nature, 401, 360-363.