OS21F-01
Secular Warming in the Indian Ocean: Natural Variability or Global Warming?
The impacts of anthropogenic greenhouse gas emissions are now widespread on each component of the Earth System. One ocean that shows a rather unique response as a monotonic, basin-scale, warming is the Indian Ocean. The secular warming of the tropical Indian Ocean over the last several decades is evident in the observations and a limited number of modeling studies exist that attribute this secular warming to anthropogenic forcing. It is not clear how model deficiencies are affecting the response of the Indian Ocean itself to anthropogenic forcings. We investigate the natural variability in the Indian Ocean to test the hypothesis that the Indian Ocean variability is unique due to the net downward annual mean heat flux with additional heat input from the Indonesian throughflow (ITF) and the meridional circulation with deep northward transport from the Southern Ocean with a natural flushing time-scale which is decadal to multi- decadal. We analyze the unforced coupled simulations of the GFDL and the CCSM models to understand the water-column heat balance. The trade-winds in the southern tropics do display a natural decadal time-scale very similar to the variability observed in the last six decades with associated water vapor feedbacks and thermocline responses. The MOC introduces the low-frequency variability from the Southern Ocean and the ITF tunnels in the Pacific low-frequency variability generating secular trend-like variability in SSTs. The details of this recharge-discharge processes are discussed in the context of the observed warming in the past few decades with implications for projections into the 21st century.
OS21F-02 INVITED
Regional Climate Responses To 20th Century Indian Ocean Warming
Sea surface temperatures over the warmest portions of the world ocean have risen +0.5°C in the past three decades, and the areal extent of 29°C waters over the Indian and western Pacific Oceans has grown considerably. This warm state does not appear to be transient, but is principally due to external radiative forcing resulting from increasing greenhouse gases. The presentation summarizes recent research results on the regional climate impacts of sea surface warming taking place over the Indian Ocean and the tropical warm-pool as a whole. We focus first on North Atlantic climate change, and summarize the understanding of Indian Ocean impact on trend components of the North Atlantic Oscillation. Indian Ocean warming has also been argued to impact North American surface temperature and precipitation, and we review evidence for its role in forcing Pacific-North American circulation patterns conducive for drought. Finally, the hypothesis that Indian Ocean warming may have caused 20th Century Sahel drought is examined. The presentation concludes with a discussion of challenges and opportunities to advance understanding of regional climate change trajectories related to the Indian Ocean's response to greenhouse gas forcing.
OS21F-03
Why the Indian Ocean is Important for North Atlantic Climate
Evidence is provided that the influence of the Indian ocean on the North Atlantic climate, in particular on the NAO, may be much larger than previously thought. This influence has generally been assessed by forcing atmospheric GCMs with prescribed anomalous SSTs in the Indian ocean. Such an approach assumes that the Indian ocean anomalies act as a forcing, when in fact they are often largely a response to changes in the Walker circulation associated with the ENSO phenomenon. In this study, the global impacts of such remotely forced Indian ocean changes are investigated in GCM simulations with and without anomalous air-sea coupling in the Indian ocean. A dramatically stronger NAO response is obtained in the coupled simulations. The weaker response in the prescribed-SST simulations is due to a previously unappreciated spurious transient-eddy feedback. Briefly, the erroneous surface heat fluxes in those simulations generate spuriously larger deep convective heating variability over the Indian ocean, stronger eddies in the circumpolar jet stream waveguide, and enhanced eddy-mean flow interactions over the Atlantic. This spurious enhancement is associated with stronger incursions of low potential vorticity subtropical air into the northern Atlantic middle latitudes, whose net effect is to force a negative-NAO error pattern reminiscent of the classic pattern of north Atlantic blocking. This may be the basic reason why numerous previous prescribed-SST simulations have severely underestimated the magnitude of the observed NAO trend over the past half century.
OS21F-04 INVITED
RAMA: Research Moored Array for African - Asian - Australian Monsoon Analysis and Prediction
The Indian Ocean is unique among the three tropical oceans in that it is blocked at 25N by the Asian land mass. Seasonal heating and cooling over this land mass sets the stage for dramatic monsoon wind reversals and intense summer rains over areas surrounding the basin. These climate variations have significant societal and economic impacts that affect half the world's population. Despite the importance of the Indian Ocean for both the regional and global climate though, it is the most poorly observed and least well understood of the three tropical oceans. This presentation describes the Research Moored Array for African-Asian-Australian Monsoon Analysis and Prediction (RAMA), which has been designed to provide sustained, basin scale time series data in the Indian Ocean for climate research and forecasting. RAMA is intended to complement other satellite and in situ components of the Indian Ocean Observing System and it is being implemented through a coordinated multi- national effort involving institutions in several countries. We will review the scientific rationale, design criteria, and implementation status of RAMA. We will also illustrate some of the important intraseasonal to interannual time scale phenomena in the region observed with new RAMA time series data. Potential applications of the data for forecasting purposes will also be discussed.
OS21F-05
Causes of decadal subsurface cooling in the tropical Indian Ocean during 1961- 2000
The vertical structure of temperature change in the Indian Ocean (IO) during 1961-2000 indicates that a region of tropical thermocline cooling accompanies the upper level warming. Results from data analysis and ocean general circulation model experiments suggest that the cooling signals exceed the cross-data and cross-model differences. Spatial patterns of the temperature trend above 200m resemble the negative IO dipole structure, with the strongest cooling occurring in the western-central basin south of the equator. The upper thermocline cooling is mainly caused by enhanced Ekman pumping velocity, which shoals the thermocline. The enhanced upwelling is consistent with the strengthened Southern subtropical cell. Enhanced equatorial westerly winds contribute to the negative dipole pattern. Remote forcing from the Pacific may contribute to the cooling below 200m and further south.
OS21F-06 INVITED
A Holistic View of the Coupled Monsoon System
The basic dynamical constraint on both the atmospheric and oceanic components of the monsoon is the strong cross-equatorial pressure gradient (CEPG). The CEPG is positive and strongest in the lower troposphere during the boreal summer and weakest and negative in the boreal winter. Counter gradients exist at higher elevations. The CEPG is a slowly varying field set up by land-sea differences, convective heating and the seasonal cycle of sea-surface temperature. The dynamic response to this evolving CEPG creates the seasonal structure of the ocean and the atmosphere and determines how the monsoon system will respond to forcing from outside the system. It determines the mode of interannual variability of the system. The CEPG drives a cross-equatorial flow that gains moisture through evaporation. Strong latent heat release occurs in littoral seas and land areas during the summer and to the south of the equator during winter creating net cross-equatorial heat fluxes from the winter to summer hemispheres. However, the cross- equatorial wind fields, so generated, cause an Ekman heat transport from the winter to the summer hemisphere. The net flux is large with a seasonal amplitude of about 2 PW. This almost matches the net atmospheric heat transport, but with reversed sign. For example, the oceanic heat flux is sufficient to reduce the north Indian Ocean upper temperature by 1-2C during summer and warm it by a comparable amount during winter. The net effect is to reduce the vigor of the atmospheric monsoon. To a large degree, the couple ocean-atmosphere system is self-regulated and closed system. Occasional outside influences (ENSO, anomalous springtime snow cover etc.) influence the monsoon. For example there is evidence that El Nino (La Nina) is associated with a weak (strong) monsoon. But a strong (weak) monsoon creates a stronger (weaker) cross-equatorial flow and an enhanced (reduced) oceanic heat flux to the winter hemisphere. In this manner, the system returns to its original annual cycle within a year. The Dipole or Zonal Mode is a component of this self-regulation. The self-regulation also explains why the interannual variability of the monsoon is relatively small and why it is rare to find a chain of years that possess the same sign anomaly. It also explains why there is a strong biennial variability of the monsoon Finally, the higher frequency intraseasonal variability is considered and shows a modulation of cross- equatorial heat on 20-60 day time scales in both the atmosphere and the ocean suggesting mechanisms for coupled intraseasonal variability.
OS21F-07
Indonesian Throughflow time series based on the profile of Pacific-Indian pressure differences
Interocean pressure differences as a function of depth between the Pacific and Indian Oceans are used to calculate a timeseries of Indonesian Throughflow (ITF) variability. The validity of this method at interannual frequencies is confirmed by comparison to in-situ data from the ARLINDO and INSTANT observational periods. The timeseries of pressure-gradient derived ITF variability is compared to timeseries of ITF variability as derived from Godfrey's Island Rule and reanalysis data, as well as their relationships to El Niņo- Southern Oscillation and the Indian Ocean Dipole Mode. The comparison of multiple estimates of ITF variability allows for a better understanding of the balance of forces that govern the ITF and its links to climate.
OS21F-08 INVITED
MJO-Related Variability over the Indian Ocean: Ocean Responses and AGCM/AOGCM Simulation Fidelity
Over the Indian Ocean, the Madden-Julian Oscillation (MJO) is the most prolific mode of atmospheric variability on subseasonal time scales. MJO events arise in conjunction with large-scale organized convective systems forming over the equatorial western Indian Ocean, intensifying and propagating eastward in the Pacific basin. These systems are associated with significant variations in the ocean surface fluxes of momentum, mass, and radiation/heat. Modeling and observation studies have shown that MJO-related flux variations induce strong responses in the surface and upper levels of the Indian Ocean, that include both physical as well as biological (i.e. Chl) properties. This presentation will highlight these physical and biological responses in order to motivate the necessity of accounting for MJO variability in considerations of Indian Ocean climate variability and in achieving accurate representations of MJO variability in our atmosphere (AGCM) and coupled atmosphere-ocean (AOGCM) climate models. Results will be shown of recent efforts by the US CLIVAR MJO Working Group to develop standardized diagnostics to assess MJO fidelity in GCMs and an associated assessment of the MJO variability in a set of present-day AGCMs and AOGCMs.
OS21F-09
Oceanic Precondition and Evolution of the Indian Ocean Dipole Events
Indian Ocean Dipole (IOD) is one of the interannual climate variability in the Indian Ocean, associated with the negative (positive) SST anomaly in the eastern (western) equatorial region developing during boreal summer/autumn seasons. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has been deploying TRITON buoys in the eastern equatorial Indian Ocean since October 2001. Details of subsurface ocean conditions associated with IOD events were observed by the mooring buoys in the eastern equatorial Indian Ocean in 2006, 2007, and 2008. In the 2006 IOD event, large-scale sea surface signals in the tropical Indian Ocean associated with the positive IOD started in August 2006, and the anomalous conditions continued until December 2006. Data from the mooring buoys, however, captured the first appearance of the negative temperature anomaly at the thermocline depth with strong westward current anomalies in May 2006, about three months earlier than the development of the surface signatures. Similar appearance of negative temperature anomalies in the subsurface were also observed in 2007 and 2008, while the amplitude, the timing, and the relation to the surface layer were different among the events. The implications of the subsurface conditions for the occurrences of these IOD events are discussed.