Analysis of Eastern Indian Ocean Cold and Warm Events:

1. Analysis of Eastern Indian Ocean Cold and Warm Events: The air-sea interaction under the Indian monsoon backgroundQin ZhangRSIS, Climate Prediction Center, NCEP/NOAA Data and Method • 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. 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. First EEOF mode of surface wind, SST and precipitation anomalies (1958-1999) 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. 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. Pa/S 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. Surface wind (arrows) and SST (shading) anomalies Fall events (cold-warm) Spring events (warm-cold) Fig. 7 Same as Fig. 6 except for latent heat flux anomaly (shading) and wind speed anomalies (contours). 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. Conclusions and Discussion • 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. 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. 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.