A global analysis of Latent Heat Flux (LHF) sensitivity to Sea Surface Temperature (SST) is performed, with focus on the tropics and the North Indian Ocean (NIO). Sensitivity of LHF state variables (wind speed (Ws) and vertical humidity gradients (οq)) to SST give rise to mutually interacting dynamical (Ws-driven) and thermodynamical (οq-driven) coupled feedbacks. Generally, LHF sensitivity to SST is pronounced over tropics where SST increase causes Ws (οq) changes, resulting in a maximum decrease (increase) of LHF by ~15W/m2/°C. But Bay of Bengal (BoB) and Northern Arabian Sea (NAS) remain an exception that is opposite to the global feedback relationship. This uniqueness is attributed to strong seasonality in monsoon Ws and οq variations which brings in warm (cold) continental airmass into BoB and NAS during summer (winter), producing large seasonal cycle in air-sea temperature difference (οT, and hence on οq). In other tropical oceans, surface air is mostly of marine origin and blows from colder to warmer waters, resulting in a constant οT~1°C throughout the year, and hence a constant οq. Thus unlike other basins, when BoB and NAS are warming, air temperature warms faster than SST. The resultant decrease in οT and οq contributes to decrease the LHF with increased SST, contrary to other basins. Our analysis suggests that in NIO, LHF variability is largely controlled by thermodynamic processes, which peak during the monsoon period. These observed LHF sensitivities are then used to speculate how the surface energetics and coupled feedbacks may change in a warmer world.
Paper No
5
Publication ID : 617 & Year : 2016
Title
Variability of near-surface circulation and sea surface salinity observed from Lagrangian drifters in the northern Bay of Bengal during the Waning 2015 Southwest Monsoon
Authors
Hormann, V., L.R. Centurioni, A. Mahadevan, S. Essink, E.A. D⿿Asaro, and
B. Praveen Kumar.
Source
Oceanography, DOI
http://dx.doi.org/10.5670/oceanog.2016.45
Abstract
A dedicated drifter experiment was conducted in the northern
Bay of Bengal during the 2015 waning southwest monsoon. To sample a variety of
spatiotemporal scales, a total of 36 salinity drifters and 10 standard drifters were deployed
in a tight array across a freshwater front. The salinity drifters carried for the first time a
revised sensor algorithm, and its performance during the 2015 field experiment is very
encouraging for future efforts. Most of the drifters were quickly entrained in a mesoscale
feature centered at about 16.5°N, 89°E and stayed close together during the first month
of observations. While the eddy was associated with rather homogeneous temperature
and salinity characteristics, much larger variability was found outside of it toward the
coastline, and some of the observed salinity patches had amplitudes in excess of 1.5 psu.
To particularly quantify the smaller spatiotemporal scales, an autocorrelation analysis
of the drifter salinities for the first two deployment days was performed, indicating
not only spatial scales of less than 5 km but also temporal variations of the order of a
few hours. The hydrographic measurements were complemented by first estimates of
kinematic properties from the drifter clusters, however, more work is needed to link the
different observed characteristics.
Paper No
6
Publication ID : 616 & Year : 2016
Title
Air-sea interaction in the
Bay of Bengal
Authors
Weller, R.A., J.T. Farrar, J. Buckley, S. Mathew, R. Venkatesan, J. Sree Lekha,
D. Chaudhuri, N. Suresh Kumar, and B. Praveen Kumar.
Source
Oceanography, DOI
http://dx.doi.org/10.5670/oceanog.2016.36
Abstract
Recent observations of surface meteorology and exchanges of heat,
freshwater, and momentum between the ocean and the atmosphere in the Bay of Bengal
are presented. These observations characterize air-sea interaction at 18°N, 89.5°E from
December 2014 to January 2016 and also at other locations in the northern Bay of
Bengal. Monsoonal variability dominated the records, with winds to the northeast
in summer and to the southwest in winter. This variability included a strong annual
cycle in the atmospheric forcing of the ocean in the Bay of Bengal, with the winter
monsoon marked by sustained ocean heat loss resulting in ocean cooling, and the
summer monsoon marked by strong storm events with dark skies and rain that also
resulted in ocean cooling. The spring intermonsoon was a period of clear skies and low
winds, when strong solar heating and weak wind-driven mixing led to ocean warming.
The fall intermonsoon was a transitional period, with some storm events but also with
enough clear skies and sunlight that ocean surface temperature rose again. Mooring
and shipboard observations are used to examine the ability of model-based surface
fluxes to represent air-sea interaction in the Bay of Bengal; the model-based fluxes have
significant errors. The surface forcing observed at 18°N is also used together with a
one-dimensional ocean model to illustrate the potential for local air-sea interaction to
drive upper-ocean variability in the Bay of Bengal
Paper No
7
Publication ID : 619 & Year : 2016
Title
Teleconnection between the North Indian Ocean high swell events and meteorological conditions over the Southern Indian Ocean
Authors
P. G. Remya, S. Vishnu, B. Praveen Kumar, TM. Balakrishnan Nair, B. Rohith
Source
JGR-Oceans; DOI: 10.1002/2016JC011723
Abstract
The link between North Indian Ocean (NIO) high swell events and the meteorological conditions over the Southern Indian Ocean (SIO) is explored in this article, using a combination of in-situ measurements and model simulations for the year 2005. High waves, without any sign in the local winds, sometimes cause severe flooding events along the south-west coast of India, locally known as the Kallakkadal events and cause major societal problems along the coasts. In-situ observations report ten high swell events in NIO during 2005. Our study confirm that these events are caused by the swells propagating from south of 30°S. In all cases, 3-5 days prior to the high swell events in NIO, we observed a severe low pressure system, called the Cut-Off Low (COL) in the Southern Ocean. These COLs are quasi-stationary in nature, providing strong (⿼25 ms⿿1) and long duration (⿼3 days) surface winds over a large fetch; essential conditions for the generation of long period swells. The intense equator ward winds associated with COLs in the SIO trigger the generation of high waves, which propagate to NIO as swells. Furthermore, these swells cause high wave activity and sometimes Kallakkadal events along the NIO coastal regions, depending on the local topography, angle of incidence and tidal conditions. Our study shows that such natural hazards along the NIO coasts can be forecasted at least 2 days in advance if the meteorological conditions of the SIO are properly monitored. This article is protected by copyright. All rights reserved.
Paper No
8
Publication ID : 618 & Year : 2016
Title
Ocean atmosphere thermal decoupling in the eastern equatorial Indian ocean
Authors
Sudheer Joseph, M. Ravichandran, B. Praveen Kumar, Raju V. Jampana, Weiqing Han
Source
Clim Dyn, DOI: 10.1007/s00382-016-3359-1
Abstract
Eastern equatorial Indian ocean (EEIO) is one of the most climatically sensitive regions in the global ocean, which plays a vital role in modulating Indian ocean dipole (IOD) and El Niño southern oscillation (ENSO). Here we present evidences for a paradoxical and perpetual lower co-variability between sea-surface temperature (SST) and air-temperature (Tair) indicating instantaneous thermal decoupling in the same region, where signals of the strongly coupled variability of SST anomalies and zonal winds associated with IOD originate at inter-annual time scale. The correlation minimum between anomalies of Tair and SST occurs in the eastern equatorial Indian ocean warm pool region (⿿70°E⿿100°E, 5°S⿿5°N), associated with lower wind speeds and lower sensible heat fluxes. At sub-monthly and Madden⿿Julian oscillation time scales, correlation of both variables becomes very low. In above frequencies, precipitation positively contributes to the low correlation by dropping Tair considerably while leaving SST without any substantial instant impact. Precipitation is led by positive build up of SST and post-facto drop in it. The strong semi-annual response of SST to mixed layer variability and equatorial waves, with the absence of the same in the Tair, contributes further to the weak correlation at the sub-annual scale. The limited correlation found in the EEIO is mainly related to the annual warming of the region and ENSO which is hard to segregate from the impacts of IOD.
Paper No
9
Publication ID : 615 & Year : 2014
Title
Processes of interannual mixed layer temperature variability in the thermocline ridge of the Indian Ocean
Authors
B. Praveen Kumar, J. Vialard, M. Lengaigne, V. S. N. Murty, G. R. Foltz, M. J. McPhaden, S. Pous, C. de Boyer Montégut
Source
Clim Dyn, DOI: 10.1007/s00382-014-2059-y
Abstract
Sea-surface temperature interannual anomalies (SSTAs) in the thermocline ridge of the southwestern tropical Indian Ocean (TRIO) have several well-documented climate impacts. In this paper, we explore the physical processes responsible for SSTA evolution in the TRIO region using a combination of observational estimates and model-derived surface layer heat budget analyses. Vertical oceanic processes contribute most to SSTA variance from December to June, while lateral advection dominates from July to November. Atmospheric fluxes generally damp SSTA generation in the TRIO region. As a result of the phase opposition between the seasonal cycle of vertical processes and lateral advection, there is no obvious peak in SSTA amplitude in boreal winter, as previously noted for heat content anomalies. Positive Indian Ocean Dipole (IOD) events and the remote influence of El Niño induce comparable warming over the TRIO region, though IOD signals peak earlier (November⿿December) than those associated with El Niño (around March⿿May). Mechanisms controlling the SSTA growth in the TRIO region induced by these two climate modes differ strongly. While SSTA growth for the IOD mostly results from southward advection of warmer water, increased surface shortwave flux dominates the El Niño SSTA growth. In both cases, vertical oceanic processes do not contribute strongly to the initial SSTA growth, but rather maintain the SSTA by opposing the effect of atmospheric negative feedbacks during the decaying phase.
Paper No
10
Publication ID : 613 & Year : 2012
Title
TropFlux: air-sea fluxes for the global tropical oceans⿿description and evaluation
Authors
B. Praveen Kumar, J. Vialard, M. Lengaigne, V. S. N. Murty, M. J. McPhaden
Source
Clim Dyn, DOI: 10.1007/s00382-011-1115-0
Abstract
In this paper, we evaluate several timely, daily air-sea heat flux products (NCEP, NCEP2, ERA-Interim and OAFlux/ISCCP) against observations and present the newly developed TropFlux product. This new product uses bias-corrected ERA-interim and ISCCP data as input parameters to compute air-sea fluxes from the COARE v3.0 algorithm. Wind speed is corrected for mesoscale gustiness. Surface net shortwave radiation is based on corrected ISCCP data. We extend the shortwave radiation time series by using ⿿near real-time SWR estimated from outgoing longwave radiation. All products reproduce consistent intraseasonal surface net heat flux variations associated with the Madden-Julian Oscillation in the Indian Ocean, but display more disparate interannual heat flux variations associated with El Niño in the eastern Pacific. They also exhibit marked differences in mean values and seasonal cycle. Comparison with global tropical moored buoy array data, I-COADS and fully independent mooring data sets shows that the two NCEP products display lowest correlation to mooring turbulent fluxes and significant biases. ERA-interim data captures well temporal variability, but with significant biases. OAFlux and TropFlux perform best. All products have issues in reproducing observed longwave radiation. Shortwave flux is much better captured by ISCCP data than by any of the re-analyses. Our ⿿near real-time shortwave radiation performs better than most re-analyses, but tends to underestimate variability over the cold tongues of the Atlantic and Pacific. Compared to independent mooring data, NCEP and NCEP2 net heat fluxes display ~0.78 correlation and >65 W m⿿2 rms-difference, ERA-I performs better (~0.86 correlation and ~48 W m⿿2) while OAFlux and TropFlux perform best (~0.9 correlation and ~43 W m⿿2). TropFlux hence provides a useful option for studying flux variability associated with ocean⿿atmosphere interactions, oceanic heat budgets and climate fluctuations in the tropics.
Paper No
11
Publication ID : 614 & Year : 2012
Title
TropFlux wind stresses over the tropical oceans: evaluation and comparison with other products
Authors
B. Praveen Kumar, J. Vialard, M. Lengaigne, V. S. N. Murty, M. J. McPhaden, M. F. Cronin, F. Pinsard, K. Gopala Reddy
Source
Clim Dyn, DOI: 10.1007/s00382-012-1455-4
Abstract
In this paper, we present TropFlux wind stresses and evaluate them against observations along with other widely used daily air-sea momentum flux products (NCEP, NCEP2, ERA-I and QuikSCAT). TropFlux wind stresses are computed from the COARE v3.0 algorithm, using bias and amplitude corrected ERA-I input data and an additional climatological gustiness correction. The wind stress products are evaluated against dependent data from the TAO/TRITON, PIRATA and RAMA arrays and independent data from the OceanSITES mooring networks. Wind stress products are more consistent amongst each other than surface heat fluxes, suggesting that 10 m-winds are better constrained than near-surface thermodynamical parameters (2 m-humidity and temperature) and surface downward radiative fluxes. QuikSCAT overestimates wind stresses away from the equator, while NCEP and NCEP2 underestimate wind stresses, especially in the equatorial Pacific. QuikSCAT wind stress quality is strongly affected by rain under the Inter Tropical Convergence Zones. ERA-I and TropFlux display the best agreement with in situ data, with correlations >0.93 and rms-differences <0.012 Nm⿿2. TropFlux wind stresses exhibit a small, but consistent improvement (at all timescales and most locations) over ERA-I, with an overall 17 % reduction in root mean square error. ERA-I and TropFlux agree best with long-term mean zonal wind stress observations at equatorial latitudes. All products tend to underestimate the zonal wind stress seasonal cycle by ~20 % in the western and central equatorial Pacific. TropFlux and ERA-I equatorial zonal wind stresses have clearly the best phase agreement with mooring data at intraseasonal and interannual timescales (correlation of ~0.9 versus ~0.8 at best for any other product), with TropFlux correcting the ~13 % underestimation of ERA-I variance at both timescales. For example, TropFlux was the best at reproducing westerly wind bursts that played a key role in the 1997⿿1998 El Niño onset. Hence, we recommend the use of TropFlux for studies of equatorial ocean dynamics.
Paper No
12
Publication ID : 611 & Year : 2010
Title
Seasonal Mixed Layer Heat Balance of the Southwestern Tropical Indian Ocean
Authors
Gregory R. Foltz, Jérôme Vialard, B. Praveen Kumar, Michael J. McPhaden
Sea surface temperature (SST) in the southwestern tropical Indian Ocean exerts a significant influence on global climate through its influence on the Indian summer monsoon and Northern Hemisphere atmospheric circulation. In this study, measurements from a long-term moored buoy are used in conjunction with satellite, in situ, and atmospheric reanalysis datasets to analyze the seasonal mixed layer heat balance in the thermocline ridge region of the southwestern tropical Indian Ocean. This region is characterized by a shallow mean thermocline (90 m, as measured by the 20°C isotherm) and pronounced seasonal cycles of Ekman pumping and SST (seasonal ranges of ⿿0.1 to 0.6 m day⿿1 and 26°⿿29.5°C, respectively). It is found that surface heat fluxes and horizontal heat advection contribute significantly to the seasonal cycle of mixed layer heat storage. The net surface heat flux tends to warm the mixed layer throughout the year and is strongest during boreal fall and winter, when surface shortwave radiation is highest and latent heat loss is weakest. Horizontal heat advection provides warming during boreal summer and fall, when southwestward surface currents and horizontal SST gradients are strongest, and is close to zero during the remainder of the year. Vertical turbulent mixing, estimated as a residual in the heat balance, also undergoes a significant seasonal cycle. Cooling from this term is strongest in boreal summer, when surface wind and buoyancy forcing are strongest, the thermocline ridge is shallow (<90 m), and the mixed layer is deepening. These empirical results provide a framework for addressing intraseasonal and interannual climate variations, which are dynamically linked to the seasonal cycle, in the southwestern tropical Indian Ocean. They also provide a quantitative basis for assessing the accuracy of numerical ocean model simulations in the region.
