El Niño induces a basin-wide increase in tropical Indian Ocean (tropical IO, TIO) sea surface temperature (SST) with a lag of one season. The North IO (NIO), in particular, displays a peculiar double-peak warming with the second peak larger in magnitude and persisting well through the summer. Motivated by recent studies suggesting the importance of the TIO warming for the Northwest Pacific and East Asian summer monsoons, the present study investigates the mechanisms for the second peak of the NIO warming using observations and general circulation models. Our analysis reveals that internal air-sea interaction within the TIO is key to sustaining the TIO warming through summer. During El Niño, anti-cyclonic wind curl anomalies force a downwelling Rossby wave in the South TIO through Walker circulation adjustments, causing a sustained SST warming in the tropical Southwest IO (SWIO) where the mean thermocline is shallow. During the spring and early summer following El Niño, this SWIO warming sustains an anti-symmetric pattern of atmospheric anomalies with northeasterly (northwesterly) wind anomalies north (south) of the equator. Over the NIO as the mean winds turn into southwesterly in May, the northeasterly anomalies force the second SST peak that

A unique open-ocean upwelling exists in the tropical South Indian Ocean (SIO), a result of the negative wind curl between the southeasterly trades and equatorial westerlies, raising the thermocline in the west. Analysis of in-situ measurements and a model-assimilated dataset reveals a strong influence of subsurface thermocline variability on sea surface temperature (SST) in this upwelling zone. El Nino/Southern Oscillation (ENSO) is found to be the dominant forcing for the SIO thermocline variability, with SST variability off Sumatra also making a significant contribution. When either an El Nino or Sumatra cooling event takes place, anomalous easterlies appear in the equatorial Indian Ocean, forcing a westward-propagating downwelling Rossby wave in the SIO. In phase with this dynamic Rossby wave, there is a pronounced co-propagation of SST. Moreover, a positive precipitation anomaly is found over, or just to the south of, the Rossby wave-induced positive SST anomaly, resu...

Multiple satellite records of tropical-mean water vapor are compared with a general circulation model (GCM) simulation to assess the ability to monitor and to predict low-frequency changes in total precipitable water. Particular attention is focused on the drying between 1979 and 1995 recorded by a TOVS statistical retrieval that is calibrated to radiosondes. Both a GCM integrated with observed SSTs and microwave and TOVS physical retrievals that overlap the drying period show no sustained drying. This discrepancy is consistent with the suggestion by Ross and Gaffen that the TOVS statistical algorithm is vulnerable to radiosonde instrumentation changes over this period that introduce an artificial drying trend into the retrieval. 1.

In this study, a mechanism is demonstrated whereby a large reduction in the Atlantic thermohaline circulation (THC) can induce global-scale changes in the Tropics that are consistent with paleoevidence of the global synchronization of millennial-scale abrupt climate change. Using GFDL’s newly developed global coupled ocean–atmosphere model (CM2.0), the global response to a sustained addition of freshwater to the model’s North Atlantic is simulated. This freshwater forcing substantially weakens the Atlantic THC, resulting in a southward shift of the intertropical convergence zone over the Atlantic and Pacific, an El Niño–like pattern in the southeastern tropical Pacific, and weakened Indian and Asian summer monsoons through air–sea interactions. 1.

The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Niño–Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo–western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these globalscale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the “atmospheric bridge ” mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated

The interaction of the ocean and atmosphere plays an important role in shaping the climate and its variations. This chapter reviews the current state of knowledge of air–sea interaction and climate variations over the global ocean. The largest source of climate variability in the instrumental record is El Niño–Southern Oscillation (ENSO), which extends its reach globally through the ability of the atmosphere to bridge ocean basins. The growth of ENSO owes its existence to a positive ocean–atmosphere feedback mechanism (originally envisioned by J. Bjerknes) that involves the interaction of ocean dynamics, atmospheric convection, and winds in the equatorial Pacific. The Bjerknes feedback and the resultant equatorial zonal mode of climate variability are a common feature to all three tropical oceans despite differences in dimension, geometry and mean climate. In addition to this zonal mode, the tropics also support a meridional mode, whose growth is due to a thermodynamic feedback mechanism involving the interaction of the cross–equatorial gradient of properties such as sea surface temperature and displacements of the seasonal intertropical convergence zone. This meridional mode is observed in the tropical

The impacts of ENSO on the evolution of the East Asian monsoon have been studied using output from a general circulation model experiment. Observed monthly variations of the sea surface temperature (SST) field have been prescribed in the tropical eastern and central Pacific, whereas the atmosphere has been coupled to an oceanic mixed layer model beyond this forcing region. During the boreal summer of typical El Niño events, a low-level cyclonic anomaly is simulated over the North Pacific in response to enhanced condensational heating over the equatorial central Pacific. Advective processes associated with the cyclone anomaly lead to temperature tendencies that set the stage for the abrupt establishment of a strong Philippine Sea anticyclone (PSAC) anomaly in the autumn. The synoptic development during the onset of the PSAC anomaly is similar to that accompanying cold-air surges over East Asia. The air–sea interactions accompanying the intraseasonal variations (ISV) in the model atmosphere exhibit a strong seasonal dependence. During the summer, the climatological monsoon trough over the subtropical western Pacific facilitates positive feedbacks between the atmospheric and oceanic fluctuations. Conversely, the prevalent northeasterly monsoon over this region in the winter leads to negative feedbacks. The onset of the PSAC anomaly is seen to be coincident with a prominent episode of the leading ISV mode. The ENSO events could influence the amplitude of the ISV by modulating the large-scale flow environment in which the ISV are embedded. Amplification of the summer monsoon trough over the western Pacific during El Niño enhances air–sea feedbacks on intraseasonal time scales, thereby raising the amplitudes of the ISV. A weakening of the northeasterly monsoon in El Niño winters suppresses the frequency and strength of the cold-air surges associated with the leading ISV mode in that season. Many aspects of the model simulation of the relationships between ENSO and the East Asian monsoon are in agreement with observational findings. 1.

A gap in the Indo-Pacific warm pool over the South China Sea in boreal winter:

Prior to the 1976–77 climate shift (1950–76), sea surface temperature (SST) anomalies in the tropical Indian Ocean consisted of a basinwide warming during boreal fall of the developing phase of most El Niños, whereas after the shift (1977–99) they had an east–west asymmetry—a consequence of El Niño being associated with the Indian Ocean Dipole/Zonal mode. In this study, the possible impact of these contrasting SST patterns on the ongoing El Niño is investigated, using atmospheric reanalysis products and solutions to both an atmospheric general circulation model (AGCM) and a simple atmospheric model (LBM), with the latter used to identify basic processes. Specifically, analyses of reanalysis products during the El Niño onset indicate that after the climate shift a low-level anticyclone over the South China Sea was shifted into the Bay of Bengal and that equatorial westerly anomalies in the Pacific Ocean were considerably stronger. The present study focuses on determining influence of Indian Ocean SST on these changes. A suite of AGCM experiments, each consisting of a 10-member ensemble, is carried out to assess the relative importance of remote (Pacific) versus local (Indian Ocean) SST anomalies in determining precipitation anomalies over the equatorial Indian Ocean. Solutions indicate that both local and remote SST anomalies are necessary for realistic simulations, with convection in the tropical west Pacific and the

An atmospheric general circulation model (AGCM) is used to examine the role of Indian Ocean sea surface temperature (SST) anomalies in regional climate variability. In particular, the authors focus on the effect of the basinwide warming that occurs during December through May after the mature phase of El Niño. To elucidate the relative importance of local and remote forcing, model solutions were sought for experiments where SST anomalies are inserted in the (i) tropical Indo-Pacific Oceans, (ii) tropical Pacific Ocean, and (iii) tropical Indian Ocean. A 10-member ensemble simulation is carried out for each of the three forcing scenarios. The model solutions demonstrate that precipitation variations over the southwest Indian Ocean are tied to local SST anomalies and are highly reproducible. Changes in the Indian Ocean–Walker circulation suppress precipitation over the tropical west Pacific–Maritime Continent, contributing to the development of a low-level anticyclone over the Philippine and South China Seas. Our model results indicate that more than 50 % of the total precipitation anomalies over the tropical west Pacific–Maritime Continent is forced by remote Indian Ocean SST anomalies, offering an additional mechanism for the Philippine Sea anticyclone apart from Pacific SST. This anticyclone increases precipitation along the East Asian winter monsoon