C. Gnanaseelan

Variability in the Indian Ocean circulation and salinity and its impact on SST anomalies during dipole events

The GFDL Modular Ocean Model (MOM4) has been used to understand the variability of the Indian Ocean circulation and salinity during Indian Ocean Dipole events. The model simulations are compared with HadISST, SODA and ECCO data sets. During the positive dipole years, the climatological cyclonic circulation in the Bay of Bengal weakens or is replaced by an anticyclonic circulation. The interannual variability in the Wyrtki Jet and Bay of Bengal circulation has significant influence on fresh water transport between the equatorial Indian Ocean and Bay of Bengal. The salinity anomalies in the equatorial Indian Ocean are significant during the positive dipole years. The salinity anomalies are positive in the southeastern equatorial Indian Ocean and negative in the central equatorial Indian Ocean. The advection of low salinity water from the eastern equatorial Indian Ocean and Bay of Bengal is attributed to the salinity anomalies in the central equatorial Indian Ocean. The salinity variability in the equatorial Indian Ocean influences the surface and subsurface temperatures by forming or eroding the barrier layer.

Hydrography and water masses in the southeastern Arabian Sea during March–June 2003

This paper describes the hydrographic observations in the southeastern Arabian Sea (SEAS) during two cruises carried out in March–June 2003 as part of the Arabian Sea Monsoon Experiment. The surface hydrography during March–April was dominated by the intrusion of low-salinity waters from the south; during May–June, the low-salinity waters were beginning to be replaced by the highsalinity waters from the north. There was considerable mixing at the bottom of the surface mixed layer, leading to interleaving of low-salinity and high-salinity layers. The flow paths constructed following the spatial patterns of salinity along the sections mimic those inferred from numerical models. Time-series measurements showed the presence of Persian Gulf and Red Sea Waters in the SEAS to be intermittent during both cruises: they appeared and disappeared during both the fortnight-long time series.

Spring asymmetric mode in the tropical Indian Ocean: role of El Niño and IOD

The spring asymmetric mode over the Tropical Indian Ocean (TIO) is characterized by contrasting patterns of rainfall and surface wind anomalies north and south of Equator. The asymmetric pattern in rainfall has evolved as a leading mode of variability in the TIO and is strongly correlated with El Niño-Southern Oscillation (ENSO) and positive Indian Ocean Dipole (IOD). The evolution of the asymmetric pattern in rainfall and surface wind during pure El Niño/IOD and co-occurrence years are examined in the twentieth century reanalysis for the period of 1871–2008 and atmospheric general circulation model (AGCM) simulations. The study revealed that spring asymmetric mode is well developed when El Niño co-occurred with IOD (positive) and is driven by the associated meridional gradients in sea surface temperature (SST) and sea level pressure (SLP). The pure El Niño composites are characterized by homogeneous (spatially) SST anomalies (positive) and weaker SLP gradients and convection, leading to weak asymmetric mode. The asymmetric mode is absent in the pure IOD (positive) composites due to the persistence of east west SST gradient for a longer duration than the co-occurrence years. The meridional gradient in SST anomalies over the TIO associated with the ENSO-IOD forcing is therefore crucial in developing/strengthening the spring asymmetric mode. The northwest Pacific anticyclonic circulation further strengthen the asymmetric mode in surface winds by inducing northeasterlies in the north Indian Ocean during pure El Niño and co-occurrence years. The simulations based on AGCM, forced by observed SSTs during the period of 1871–2000 supported the findings. The analysis of available station and ship track data further strengthens our results.

Tropical Indian Ocean subsurface temperature variability and the forcing mechanisms

Abstract The first two leading modes of interannual variability of sea surface temperature in the Tropical Indian Ocean (TIO) are governed by El Niño Southern Oscillation and Indian Ocean Dipole (IOD) respectively. TIO subsurface however does not co-vary with the surface. The patterns of the first mode of TIO subsurface temperature variability and their vertical structure are found to closely resemble the patterns of IOD and El Niño co-occurrence years. These co-occurrence years are characterized by a north–south subsurface dipole rather than a conventional IOD forced east–west dipole. This subsurface dipole is forced by wind stress curl anomalies, driven mainly by meridional shear in the zonal wind anomalies. A new subsurface dipole index (SDI) has been defined in this study to quantify the intensity of the north–south dipole mode. The SDI peaks during December to February (DJF), a season after the dipole mode index peaks. It is found that this subsurface north–south dipole is a manifestation of the internal mode of variability of the Indian Ocean forced by IOD but modulated by Pacific forcing. The seasonal evolution of thermocline, subsurface temperature and the corresponding leading modes of variability further support this hypothesis. Positive wind stress curl anomalies in the south and negative wind stress curl anomalies in the north of 5°S force (or intensify) downwelling and upwelling waves respectively during DJF. These waves induce strong subsurface warming in the south and cooling in the north (especially during DJF) and assist the formation and/or maintenance of the north–south subsurface dipole. A thick barrier layer forms in the southern TIO, supporting the long persistence of anomalous subsurface warming. To the best of our knowledge the existence of such north–south subsurface dipole in TIO is being reported for the first time.

Impact of Biannual Rossby Waves on the Indian Ocean Dipole

TOPEX/Poseidon sea surface height anomalies during 1993-2002 are decomposed using 2-D finite impulse response filters which showed biannual Rossby waves (BRWs) in the equatorial Indian Ocean (peak at 1.5degS) and in the southern tropical Indian Ocean (peak at 10.5degS) during Indian Ocean dipole (IOD) years. Anomalous downwelling BRWs in the equatorial Indian Ocean triggered by the wind stress curl-induced Ekman pumping near the eastern boundary started propagating westward from the eastern boundary in July/August 1993 and 1996, i.e., more than one year prior to the formation of the IOD events of 1994 and 1997 respectively. These strong downwelling signals reach the western equatorial Indian Ocean during the peak dipole time.

Relative role of El Niño and IOD forcing on the southern tropical Indian Ocean Rossby waves

The role of local air-sea interactions over the tropical Indian Ocean (TIO) and remote forcing from the tropical Pacific Ocean in the formation and maintenance of southern TIO Rossby waves during El Nino and positive Indian Ocean Dipole (IOD) years is investigated. These Rossby waves are significantly intensified during the El Nino and IOD cooccurrence years, as compared to those during pure El Nino or IOD years. Coupled ocean-atmosphere model sensitivity experiments reveal that air-sea coupled processes in the TIO are responsible for the Rossby wave formation and its maintenance from boreal summer to fall, while remote forcing from the Pacific intensifies and maintains these waves up to the following spring. During the cooccurrence years, the Rossby waves are generated by both the persistent equatorial easterlies and off-equatorial wind stress curl. During pure El Nino years, however, only off-equatorial wind stress curl exists to drive weak Rossby wave. Asymmetric heating associated with IOD and the mean background easterly vertical wind shear (in the northern hemisphere) during summer and fall excite two symmetric anticyclones in both sides of the equator as atmospheric Rossby wave response, which are responsible for the anomalous equatorial surface easterlies. In contrast, symmetric heat sink over the Maritime Continent in winter associated with El Nino-induced subsidence and mean easterly vertical shear (in southern hemisphere) are responsible for strong anticyclone in the southern TIO, which supports off-equatorial wind stress curl.

Impact of Northwest Pacific anticyclone on the Indian summer monsoon region

Influence of northwest (NW) Pacific anticyclone on the Indian summer monsoon (ISM), particularly over the head Bay of Bengal and monsoon trough region, is investigated. Strong NW Pacific anticyclone during summer induces negative precipitation anomalies over the head Bay of Bengal and Gangetic Plain region. Westward extension of moisture divergence and dry moisture transport from NW Pacific associated with anticyclone (ridge) and local Hadley cell-induced subsidence are responsible for these negative precipitation anomalies. The impact is maximum when the anticyclone and Indian Ocean basin warming co-occur. This contributes significantly to year-to-year variability of ISM.

Prediction of the Diurnal Change Using a Multimodel Superensemble. Part I: Precipitation

Modeling the geographical distribution of the phase and amplitude of the diurnal change is a challenging problem. This paper addresses the issues of modeling the diurnal mode of precipitation over the Tropics. Largely an early morning precipitation maximum over the oceans and an afternoon rainfall maximum over land areas describe the first-order diurnal variability. However, large variability in phase and amplitude prevails even within the land and oceanic areas. This paper addresses the importance of a multimodel superensemble for much improved prediction of the diurnal mode as compared to what is possible from individual models. To begin this exercise, the skills of the member models, the ensemble mean of the member models, a unified cloud model, and the superensemble for the prediction of total rain as well as its day versus night distribution were examined. Here it is shown that the distributions of total rain over the earth (tropical belt) and over certain geographical regions are predicted reasonably well (RMSE less than 18%) from the construction of a multimodel superensemble. This dataset is well suited for addressing the diurnal change. The large errors in phase of the diurnal modes in individual models usually stem from numerous physical processes such as the cloud radiation, shallow and deep cumulus convection, and the physics of the planetary boundary layer. The multimodel superensemble is designed to reduce such systematic errors and provide meaningful forecasts. That application for the diurnal mode appears very promising. This paper examines some of the regions such as the Tibetan Plateau, the eastern foothills of the Himalayas, and the Amazon region of South America that are traditionally difficult for modeling the diurnal change. In nearly all of these regions, errors in phase and amplitude of the diurnal mode of precipitation increase with the increased length of forecasts. Model forecast errors on the order of 6–12 h for phase and 50% for the amplitude are often seen from the member models. The multimodel superensemble reduces these errors and provides a close match (RMSE 6 h) to the observed phase. The percent of daily rain and their phases obtained from the multimodel superensemble at 3-hourly intervals for different regions of the Tropics showed a closer match (pattern correlation about 0.4) with the satellite estimates. This is another area where the individual member models conveyed a much lower skill.

Prediction of the Diurnal Cycle Using a Multimodel Superensemble. Part II: Clouds

Abstract This study addresses the issue of cloud parameterization in general circulation models utilizing a twofold approach. Four versions of the Florida State University (FSU) global spectral model (GSM) were used, including four different cloud parameterization schemes in order to construct ensemble forecasts of cloud covers. Next, a superensemble approach was used to combine these model forecasts based on their past performance. It was shown that it is possible to substantially reduce the 1–5-day forecast errors of phase and amplitude of the diurnal cycle of clouds from the use of a multimodel superensemble. Further, the statistical information generated in the construction of a superensemble was used to develop a unified cloud parameterization scheme for a single model. This new cloud scheme, when implemented in the FSU GSM, carried a higher forecast accuracy compared to those of the individual cloud schemes and their ensemble mean for the diurnal cycle of cloud cover up to day 5 of the forecasts. Th...

Epochal changes in the seasonal evolution of tropical Indian Ocean warming associated with El Niño

The epochal changes in the seasonal evolution of El Niño induced tropical Indian Ocean (TIO) warming in the context of mid-1970s regime shift is investigated in this study. El Niño induced warming is delayed by one season in the northern TIO during epoch-2 (post mid-1970) and southern TIO during epoch-1 (pre mid-1970). Significant spatiotemporal changes in TIO (especially in the north) warming are apparent during the developing phase of El Niño. The ocean dynamics is the major driver in the basin wide warming during epoch-2 whereas heat fluxes are the dominant processes during epoch-1. Strong coupling between thermocline and sea surface temperature (SST) in epoch-2 indicates that El Niño induced oceanic changes are very significant in the seasonal evolution of basin-wide warming. The thermocline-SST coupling is strengthened by the upward propagating subsurface warming in epoch-2. The westward propagating barrier layer over southern TIO supports persistence of warm SST (over southwest TIO in epoch-2), which in turn induce spring asymmetric mode in winds and precipitation. The asymmetric wind pattern and persistent subsidence over maritime continent are primarily responsible for stronger spring warming in epoch-2. The strong east equatorial Indian Ocean cooling in epoch-2 is mainly driven by coastal upwelling over Java–Sumatra coast, whereas in epoch-1 the weak cooling is controlled by the latent heat flux. The spatiotemporal changes in TIO SST warming and their evolution have strong impact on atmospheric circulation and rainfall distribution over the Indian Oceanic rim through local air–sea interaction.