Analysis of Eastern Indian Ocean Cold and Warm Events:
The air-sea interaction under the Indian monsoon background
Qin Zhang
RSIS, Climate Prediction Center, NCEP/NOAA
The eastern Indian Ocean (EIO) sea surface temperature (SST) has a tendency of cooling (warming) in summer and fall during the El Nino (La Nina) developing year and warming (cooling) in winter and spring after El Nino (La Nina) mature phase. This coordination is linked with convective change over the Maritime Continents and induces a Rossby waves over the EIO. When the monsoon ridge shifts across the equator with the seasonal migration the local air-sea interaction has different effects corresponding to the same wind anomalies near the equator.
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Surface winds (arrows) and SST (shading) anomalies
Fall events (cold-warm) Spring events (warm-cold)
Fig. 3 Composite seasonal averaged surface wind (arrows) and SST(shading) anomalies for the fall cold minus warm events (left column) and spring warm minus cold events (right column) in the eastern Indian Ocean. The red lines indicate the regions where the differences in SST anomaly between the cold and warm events composite are statistically significant at 90% confidence level by t-test. Insignificant composite surface wind anomalies are ignored.
Fig. 6 Two month averaged precipitation (contours), unit: mm/month, and net solar radiation (shading) anomalies composite for the fall cold minus warm events (left column) and spring warm minus cold events (right column) in the eastern Indian Ocean. -1 and 0 in blocker indicate the prior and current year of the events. Unit: W/m2.
F irst E E O F m ode of surface w ind, SS T and precipitation anom alies (1958-1999)
F ig. 11 T he first extended E O F m ode of seasonal averaged surface w ind (arrow s) and S ST (shading) and precipitation (contours) anom alies.
M o n s o o n r i d g e s ( u = 0 ) a n d c l i m a t o l o g i c a l z o n a l s u r f a c e w i n d ( b l a c k c o n t o u r s )
F i g . 1 2 T h e m o n s o o n r i d g e s ( u = 0 ) o v e r t h e e a s t e r n I n d i a n O c e a n a v e r a g e d i n J J A f o r t h e f a l l c o l d e v e n t s ( s o l i d l i n e s ) a n d f o r s o m e E l N i n o y e a r s w i t h o u t s t r o n g S S T c o o l i n g i n t h e e a s t e r n I n d i a n O c e a n . T h e c o n t o u r s a r e c l i m a t o l o g i c a l z o n a l s u r f a c e w i n d .
The Rossby wave interaction with summer monsoon during the fall cold events
Conv. Decrease
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Fig. 13 The schematic diagram illustrating the mechanism formation of the fall cold events by the air-sea interaction under the summer monsoon background.
Fig. 7 Same as Fig6 except for latent heat flux anomaly (shading) and wind speed anomalies (contours).
Fig. 2 EIO index, SST anom aly averaged over the eastern Indian O cean (90E-110E, 10S-0) for (a) four cases and com posite of the fall cold events and six cases and com posite of the fall w arm events, (b) four cases and com posite of the early spring cold events and seven cases and com posite of the early spring w arm events.
• Fall cold (w arm ) events start developing in M ay and decay to nearly norm al in D ecem ber as indicated by the vertical dash lines.
Betw een 8 m onths life span, the tw o- sam ple student t-test of the cold and w arm events is above 95% confidence statistical significance. The com posite lines (thicker black lines) show the cold events reach the m ature phase in O ctober but the w arm events are early three m onths peaking in July.
• Spring w arm (cold) events begin in N ovem ber of the prior year and end in M ay w ith 7 m onths life span and
reach the m axim um SST anom alies in February.
• These tw o categories cold and w arm events (21 cases), sum m ed up 180 m onths (equivalent 15 years), account to 70% varianceinterannualSST anom alies out of total 42 years in the eastern Indian O cean.
The relationship of the eastern Indian O cean SST anom alies w ith Pacific E N SO
G roup1 (solid line) : fall events (10 cases) from Jun (0) to Jan (+1).
G roup 2 (dashed line): spring events (11 cases) from N ov ( -1) to Jun (0).
G roup2
G roup1
Fig. 9 M onthly m ean lag correlation coefficient of EIO index ref erence N ino 3.4 index for fall events (solid line) and spring events (dashed lin e). T he dashed red lines are test of statistical significant at 95% confidence leve l.
Seasonal mean SST anomalies averaged over the eastern Indian Ocean (0-10S, 90-110E) and Nino3.4 index (red line)
F61 F91
F94
F97 S68
S71 S74 S76 S64 S66 S69
S73
S78
S83
S87
F62 F73 F92 F95
F96 F98
Fall cold: 1961, 1991, 1994, 1997
Fall warm: 1962, 1973, 1992, 1995, 1996, 1998
Spring warm: 1964, 1966, 1969, 1973, 1978, 1983, 1987 Spring cold: 1968, 1971, 1974, 1976
Fig. 1 Seasonal mean SST anomaly averaged over the eastern Indian Ocean (90E-110E, 10S-0) called EIO index (green bar) and Nino 3.4 SST index (red line) from January 1958 to December 1999. The two dash straight lines are the standard deviation of the EIO index. The data removed liner tendency and the interdecadal signal (longer than 7 years).
Fig. 2 EIO index, SST anomaly averaged over the eastern Indian Ocean (90˚E- 110˚E, 10˚S-0˚) for (a) four cases and composite of the fall cold events and six cases and composite of the fall warm events, (b) four cases and composite of the early spring cold events and seven cases and composite of the early spring warm events.
Fall events (cold-warm) Spring events (warm-cold)
(a)
(b)
(c)
(d)
(e)
(f)
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Fig. 8 Distribution of the (a) anomalous horizontal ocean current (arrows) and climatological temperature (shading), (b) climatological horizontal ocean current (arrows) and anomalous temperature (shading), (c) anomalous oceanic upwelling speed (contours) and climatological vertical temperature stratification (shading) and (d) climatological oceanic upwelling speed (contours) and anomalous vertical temperature stratification (shading) and of various components of temperature advection. All results are based on composite fall cold events for the JAS season.
There are two domain modes (fall and spring patterns) to regress the Indian Ocean SST interannual variability.
• The convection reducing (increasing) over the Maritime Continents is a main factor linked the easterly (westerly) wind anomalies along the equator, which excites an anomalous anticyclone (cyclone) in the south Indian Ocean.
• The cooling (warming) in tropical eastern Indian Ocean is mainly caused by more (less) latent heat flux releasing to atmosphere from ocean and strengthen (weaken) upwelling along the equator.
• The Indian Ocean SST anomalies are linked to ENSO by the Maritime
Continents convection changes. The SST anomalies are negative in the eastern Indian ocean in summer and fall preceding El Nino year but positive in winter and spring after the El Nino mature phase.
Conclusions and Discussion
First EEOF mode of surface wind, SST and precipitation anomalies (1958-1999)
• The fall cold (warm) events start cooling (warming) SST at the Timor Sea with the easterly (westerly) anomalies along the equator. The negative (positive) SST anomalies develop near Sumatra mainly through the Rossby wave air-sea interaction with the Indian summer monsoon wind near the equator, as well as contributed by increasing (reducing) upwelling of the mixing layer and equatorial (pole) ward advection in fall. By southward migration of the monsoon ridge, the easterly (westerly) wind anomalies turn to reduce (increase) the total wind speed. Therefore, SST in the EIO recovers rapidly due to less (more) latent heat flux released from sea surface with thinner (thinker) mixing layer in winter.
• The spring warm (cold) events have an almost basin wide uniform sign pattern with the easterly (westerly) anomalies along the equator in winter and spring. The decreasing (increasing) wind speed on the equator and more (less) solar radiation received due to the anticyclonical (cyclonical) Rossby wave are two main factors warming (cooling) the sea surface in the EIO.
Fig. 3 Composite seasonal averaged surface wind (arrows) and SST (shading) anomalies for the fall cold minus warm events (left column) and spring warm minus cold events (right column) in the eastern Indian Ocean. The red lines indicate the regions where the differences in SST anomaly between the cold and warm events composite are statistically significant at 90% confidence level by t-test. Insignificant composite surface wind anomalies are ignored.
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Fig. 4 Composite seasonal averaged 500 hPa pressure vertical velocity (shading) and 200 hPa wind anomalies (arrows) for fall cold minus warm events (left column) and spring warm minus cold events (right column) over the eastern Indian Ocean.
Fig. 7 Same as Fig. 6 except for latent heat flux anomaly (shading) and wind speed anomalies (contours).
Fig. 6 Two month averaged precipitation (contours), unit: mm/month, and net solar radiation (shading) anomalies composite for the fall cold minus warm events (left column) and spring warm minus cold events (right column) in the eastern Indian Ocean. -1 and 0 in block indicate the prior and current year of the events. Unit: W/m2.
• Monthly mean NCEP reanalysis data from 1958 to 1999 (Kalnay et al. 1996).
• UKMO GISST data from 1958 to 1999 (Parker 1992).
• Reynolds reanalysis SST data from 1958 to 1999 (Reynolds and Smith, 1994).
• Version 7 SOAD data (Carton et al. 1999a, b) from 1958 to 1999.
All data sets are removed linear tendency and interdecadal signal longer than 7 years by
harmonic analysis. Then a three month running mean is conducted.
Data and Method
Surface wind (arrows) and SST (shading) anomalies
500hPa vertical velocity (shading) and 200hPa wind anomalies
Fall events (cold-warm) Spring events (warm-cold)
Fig. 4 Composite seasonal averaged 500hPapressure vertical velocity (shading) and 200hPawind anomalies (arrows) for fall cold minus warm events (left column) and spring warm minus cold events (right column) over the eastern Indian Ocean .
Pa/s
Fig. 5 Composite solar radiation anomaly (green line), latent heat flux anomaly (light blue line), net downward heat flux anomaly (red line), SST tendency (black line), the ocean dynamic processes contribution to the SST change (deep blue line) averaged over 90-110E, 0-10S, for fall cold minus warm events (upper penal) and spring warm minus cold (lower penal).
Unit: W/m2.
Fall cold (w arm ) events:
• The SST tendency (black line) is in the phase w ith the latent heat flux anom aly (light blue line) during the developm ent (July to A ugust) and m utual (Septem ber to N ovem ber) period.
• The solar radiation anom aly is out of the phase w ith ocean advection anom alies and lag SST tendency 3 m onths.
Spring w arm (cold) events:
• Solar radiation anom aly is the m ain contributor of the w arm (cold) SST over the eastern Indian O cean.
• O ceanic process is not im portant in spring events.
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