Southern high latitude climate anomalies associated with the Indian Ocean dipole mode

Using a 23-year database consisting of sea level pressure, surface air temperature and sea surface temperature, the authors studied southern high latitude climate anomalies associated with IOD (Indian Ocean Dipole). Correlation analysis of the spatial variability regarding monthly sea level pressure, surface air tempera- ture, and sea surface temperature anomalies with IOD index suggests that IOD signal exists in southern high latitudes. The correlation fields exhibit a wavenumber-3 pattern around the circumpolar Southern Ocean. Lead-lag correlation analysis on the strongest correlation areas with IOD index shows that IOD in the tropical Indian Ocean responses to the southern high latitude climate almost instantaneously. It is proposed in the present paper that this connection is realized through atmospheric propagation rather than through oceanic one.

RELATIONSHIPS BETWEEN AUTUMN INDIAN OCEAN DIPOLE MODE AND THE STRENGTH OF SCS SUMMER MONSOON

Based on 1948 - 2004 monthly Reynolds Sea Surface Temperature （SST） and NCEP/NCAR atmospheric reanalysis data, the relationships between autumn Indian Ocean Dipole Mode （IODM） and the strength of South China Sea （SCS） Summer Monsoon are investigated through the EOF and smooth correlation methods. The results are as the following. （1） There are two dominant modes of autumn SSTA over the tropical Indian Ocean. They are the uniformly signed basin-wide mode （USBM） and Indian Ocean dipole mode （IODM）, respectively. The SSTA associated with USBM are prevailing deeadal to interdecadal variability characterized by a unanimous pattern, while the IODM mainly represents interannual variability of SSTA. （2） When positive （negative） IODM exists over the tropical Indian Ocean during the preceding fall, the SCS summer monsoon will be weak （strong）. The negative correlation between the interannual variability of IODM and that of SCS summer monsoon is significant during the warm phase of long-term trend but insignificant during the cool phase. （3） When the SCS summer monsoon is strong （weak）, the IODM will be in its positive （negative） phase during the following fall season. The positive correlation between the interannual variability of SCS summer monsoon and that of IODM is significant during both the warm and cool phase of the long-term trend, but insignificant during the transition between the two phases.

North Indian Ocean tropical cyclone activities influenced by the Indian Ocean Dipole mode

Using Joint Typhoon Warning Center tropical cyclone(TC)track data over the North Indian Ocean(NIO),National Centers for Environmental Prediction monthly reanalysis wind and outgoing long-wave radiation data,and National Oceanic and Atmospheric Administration sea surface temperature data from 1981 to 2010,spatiotemporal distributions of NIO TC activity and relationships with local sea surface temperature(SST)were studied with statistical diagnosis methods.Results of empirical orthogonal function(EOF)analysis of NIO TC occurrence frequency show that the EOF1 mode,which accounts for 16%of total variance,consistently represents variations of TC occurrence frequency over the whole NIO basin.However,spatial dis- tributions of EOF1 mode are not uniform,mainly indicating variations of westward-moving TCs in the Bay of Bengal.The prevailing TC activity variation mode oscillates significantly on a quasi-5 year interannual time scale.NIO TC activity is notably influenced by the Indian Ocean dipole(IOD)mode.When the Indian Ocean is in a positive(negative)phase of the IOD, NIO SST anomalies are warm in the west(east)and cold in the east(west),which can weaken(strengthen)convection over the Bay of Bengal and eastern Arabian Sea,and cause anticyclonic(cyclonic)atmospheric circulation anomalies at low levels. This results in less(more)TC genesis and reduced(increased)opportunities for TC occurrence in the NIO.In addition,positive(negative)IOD events may strengthen(weaken)westerly steering flow over the Bay of Bengal,which further leads to fewer(more)westward-moving TCs which appear in regions west of 90°E in that bay.

Interannual Variability of Sea Surface Temperature in the Northern Indian Ocean Associated with ENSO and IOD

The Northern Indian Ocean （NIO） sea surface temperature （SST） warming, associated with the E1 Nifio/Southern Oscillations （ENSO） and the Indian Ocean Dipole （IOD） mode, is investigated using the International Comprehensive Ocean-Atmosphere Data Set （ICOADS） monthly data for the period 1979-2010. Statistical analy- ses are used to identify respective contribution from ENSO and IOD. The results indicate that the first NIO SST warming in September-November is associated with an IOD event, while the second NIO SST warming in spring-summer following the mature phase of ENSO is associated with an ENSO event. In the year that IOD co-occurred with ENSO, NIO SST warms twice, rising in the ENSO developing year and decay year. Both short- wave radiation and latent heat flux contribute to the NIO SST variation. The change in shortwave radiation is due to the change in cloudiness. A cloud-SST feedback plays an important role in NIO SST warming. The latent heat flux is related to the change in monsoonal wind. In the first NIO warming, the SST anomaly is mainly due to the change in the latent heat flux. In the second NIO warming, both factors are important.

Impacts of the Leading Modes of Tropical Indian Ocean Sea Surface Temperature Anomaly on Sub-Seasonal Evolution of the Circulation and Rainfall over East Asia during Boreal Spring and Summer

The two leading modes of the interannual variability of the tropical Indian Ocean （TIO） sea surface temperature （SST） anomaly are the Indian Ocean basin mode （IOBM） and the Indian Ocean dipole mode （IODM） from March to August. In this paper, the relationship between the TIO SST anomaly and the sub-seasonal evolution of the circula- tion and rainfall over East Asia during boreal spring and summer is investigated by using correlation analysis and composite analysis based on multi-source observation data from 1979 to 2013, together with numerical simulations from an atmospheric general circulation model. The results indicate that the impacts of the IOBM on the circulation and rainfall over East Asia vary remarkably from spring to summer. The anomalous anticyclone over the tropical Northwest Pacific induced by the warm IOBM is closely linked with the Pacific-Japan or East Asia-Pacific telecon- nection pattern, which persists from March to August. In the upper troposphere over East Asia, the warm phase of the IOBM generates a significant anticyclonic response from March to May. In June and July, however, the circulation response is characterized by enhanced subtropical westerly flow. A distinct anomalous cyclone is found in August. Overall, the IOBM can exert significant influence on the western North Pacific subtropical high, the South Asian high, and the East Asian jet, which collectively modulate the precipitation anomaly over East Asia. In contrast, the ef- fects of the IODM on the climate anomaly over East Asia are relatively weak in boreal spring and summer. There- fore, studying the impacts of the TIO SST anomaly on the climate anomaly in East Asia should take full account of the different sub-seasonal response during boreal spring and summer.

Southern Meridional Atmospheric Circulation Associated with IOD

Using the monthly wind and sea surface temperature （SST） data, southern meridional atmospheric circulation cells associated with the Indian Ocean Dipole Mode （IOD） events in the Indian Ocean are for the first time described and examineS. The divergent wind and pressure vertical velocity are employed for the identification of atmospheric circulation cells. During the four different phases of the positive IOD events, the anomalous meridional Hadley circulation over the western Indian Ocean shows that the air rises in the tropics, flows poleward in the upper troposphere, sinks in the subtropics, and returns back to the tropics in the lower troposphere. The anomalous Hadley circulation over the eastern Indian Ocean is opposite to that over the western Indian Ocean. During positive IOD events, the meridional Hadley circulation over the eastern Indian Ocean is weakened while it is strengthened over the western Indian Ocean. Correlation analysis between the IOD index and the indices of the Hadley cells also proves that, the atmospheric circulation patterns are evident in every IOD event over the period of record.