Alvarinho J. Luis

Short-term variability of phytoplankton blooms associated with a cold eddy in the northwestern Arabian Sea

The northern Arabian Sea is a semienclosed sea with high primary productivity and a complicated flow pattern consisting of several eddies. This paper reports on phytoplankton blooms, which were associated with a cold eddy in the northern Arabian Sea during November 1996, inferred from Ocean Color and Temperature Scanner (OCTS) and Sea-view Wide Field-of-view Sensor (SeaWiFS)-derived chlorophyll a (Chl-a hereafter), AVHRR sea surface temperature (SST), and other available oceanography data. The blooms emerged at 100 km from both coasts in the Gulf of Oman (60.5°E, 24.5°N) where the depth is about 3000 m. The Chl-a concentrations patch first appeared on November 2 and decayed after about 4 weeks (December 3, 1996). The high Chl-a concentrations patch was about 100 km in diameter and it was located at 60.3–61.3°E, 23.5–24.5°N. The bloom, having a mean Chl-a concentration of 6.8 mg m−3 on November 6, was located in a cold SST eddy, which was accompanied by another feature, an anticyclone eddy (of 100 km in diameter) with high SST and low Chl-a concentrations to the southwest (61.5°E, 22.5°N). An SST drop occurred around November 14, which coincides with a peak of the vertical pumping velocity derived from NSCAT-derived wind stress. Two SeaWiFS-derived Chl-a images obtained in November 1998 and 1999 show good agreement in terms of the locations and features with those described above through the OCTS observations. The possible mechanism for this newly identified Chl-a patch is discussed.

Wintertime wind forcing and sea surface cooling near the South India tip observed using NSCAT and AVHRR

Abstract This report addresses a case of topographic air–sea interaction in the Gulf of Mannar, near the Indian tip, for the winter monsoon of 1996–1997. Using high spatial resolution NASA-Scatterometer (NSCAT) wind data, a 1°×1° region of strong wind is identified in the Gulf during the peak of the winter monsoon. The characteristic topography of South India and Sri Lanka and their orientation to the monsoon wind tend to channel this strong wind. Air–sea heat flux analyses, using the NSCAT wind and the European Center for Medium-Range Weather Forecast objective analyses surface data, reveal that the strong winter monsoon bursts cause large latent heat loss (180 W/m 2 ) from a wind-fetch region centered on 7.5°N, 77.5°E during January. Weak air–sea temperature gradients result in weak sensible heat loss ( 2 ) from the ocean. The ocean response to this forcing is examined using weekly and monthly mean satellite-derived sea surface temperature (SST) maps; these indicate a sea surface cooling of about 1°C along the axis of the wind flow. A by-product of this cooling is an emergence of a SST front along the periphery of the strong wind-stress region. Time-series analyses of the surface meteorology reveal that this forcing has a periodicity of about 15 days, with a peak during the last week of December. Wind stress curl derived from the NSCAT wind data exhibits high negative values (anticyclonic Ekman pumping) on a 2-week time scale, concomitant with strong wind bursts. The features observed in the Gulf of Mannar are similar to those reported from some other locations.

Improved land cover mapping using high resolution multiangle 8-band WorldView-2 satellite remote sensing data

Abstract Here, we discuss the improvements in urban classification that were made using the spatial-spectral-angular information from a WorldView-2 (WV-2) multiangle image sequence. In this study, we evaluate the use of multiangle high resolution WV-2 panchromatic (PAN) and multispectral image (MSI) data for extracting urban geospatial information. Current multiangular WV-2 data were classified into misclassification-prone surfaces, such as vegetation, water bodies, and man-made features, using a cluster of normalized difference spectral index ratios (SIR). A novel multifold methodology protocol was designed to estimate the consequences of multiangularity and germane PAN-sharpening algorithms on the spectral characteristics (distortions) of satellite data and on the resulting land use/land cover (LU/LC) mapping using an array of SIRs. Eight existing PAN-sharpening algorithms were used for data fusion, followed by estimation of multiple SIRs to mitigate spectral distortions arising from the multiangularity of the data. This research highlights the benefits of using traditional PAN-sharpening techniques with a specific set of SIRs on land cover mapping based on five available tiles of satellite data. The research provides a method to overcome the atmospherically triggered spectral distortions of multiangular acquisitions, which will facilitate better mapping and understanding of the earth’s surface.

Air-Sea Interaction, Coastal Circulation and Primary Production in the Eastern Arabian Sea: A Review

Air-sea interaction, coastal circulation and primary production exhibit an annual cycle in the eastern Arabian Sea (AS). During June to September, strong southwesterly winds (4∼9 m s−1) promote sea surface cooling through surface heat loss and vertical mixing in the central AS and force the West India Coastal Current equatorward. Positive wind stress curl induced by the Findlater jet facilitates Ekman pumping in the northern AS, and equatorward-directed alongshore wind stress induces upwelling which lowers sea surface temperature by about 2.5°C (compared to the offshore value) along the southwestern shelf of India and enhances phytoplankton concentration by more than 70% as compared to that in the central AS. During winter monsoon, from November to March, dry and weak northeasterly winds (2–6 m s−1) from the Indo-China continent enhance convective cooling of the upper ocean and deepen the mixed layer by more than 80 m, thereby increasing the vertical flux of nutrients in the photic layer which promotes wintertime phytoplankton blooms in the northern AS. The primary production rate integrated for photic layer and surface chlorophyll-a estimated from the Coastal Zone Color Scanner, both averaged for the entire western India shelf, increases from winter to summer monsoon from 24 to 70 g C m−2month and from 9 to 24 mg m−2, respectively. Remotely-forced coastal Kelvin waves from the Bay of Bengal propagate into the coastal AS, which modulate circulation pattern along the western India shelf; these Kelvin waves in turn radiate Rossby waves which reverse the circulation in the Lakshadweep Sea semiannually. This review leads us to the conclusion that seasonal monsoon forcing and remotely forced waves modulate the circulation and primary production in the eastern AS.

A Semiautomatic Extraction of Antarctic Lake Features Using Worldview-2 Imagery

We devised a semiautomatic approach for extracting lake features based on a customized set of normalized difference water index (NDWI) information which was obtained by incorporating high resolution, 8-band WorldView-2 data. An extensive accuracy assessment was carried out for three semiautomatic feature extraction approaches for extracting 36 lake features on Larsemann Hills, Antarctica. The method was tested on five existing PAN-sharpening algorithms, which suggest that the customized NDWI approach renders intermediate performance (root mean square error varies from ~227 to ~235 m 2 ) and highest stability when compared with existing feature extraction techniques. In general, the customized NDWI rendered a least misclassification ( ≈11 percent), followed by target detection (≈16 percent) and spectral processing (≈17 percent) methods for extraction of 36 lakes. We also found that customized NDWI caused consistently least misclassification ( ≈21 percent) than the target detection (≈23 percent) and spectral processing (≈30 percent) methods for extraction of partially snow or ice-covered 11 lakes. Our results indicate that the use of the customized NDWI approach and appropriate PANsharpening algorithm can greatly improve the semiautomatic extraction of lake features in cryospheric environment.

Very high-resolution satellite data for improved land cover extraction of Larsemann Hills, Eastern Antarctica

Abstract We compared four different image classification methods to improve the accuracy of cryospheric land cover mapping from very high-resolution WorldView-2 (WV-2) satellite images. We used four pixel-by-pixel classification methods and then integrated the classified images using a winner-takes-all (WTA) approach. The images on which we performed the classification techniques were made up of eight-band multispectral images and panchromatic WV-2 images fused using the hyperspherical color sharpening method. We used four distinctly different methods to classify the WV-2 PAN-sharpened data: a support vector machine (SVM), a maximum likelihood classifier (MXL), a neural network classifier (NNC), and a spectral angle mapper (SAM). Three classes of land cover—land mass/rocks, water/lakes, and snow/ice—were classified using identical training samples. The final thematic land cover map of Larsemann Hills, east Antarctica, was integrated using ensemble classification based on a majority voting–coupled WTA method. Results indicate that the WTA integration method and the SVM classification method were more accurate than the MXL, NNC, and SAM classification methods. The overall accuracy of the WTA method was 97.23% (96.47% with the SVM classifier) with a 0.96 kappa coefficient (0.95 with the SVM classifier). The accuracy of the other classifiers were 93.73 to 95.55% with kappa coefficients of 0.91 to 0.93. This work demonstrates the strengths of different classifiers to extract land cover information from multispectral data collected in cryospheric regions.

Characteristics of atmospheric forcing and SST cooling events in the Gulf of Mannar during winter monsoon

Abstract This work addresses the analysis of winter monsoon forcing and sea surface temperature (SST) cooling events in the Gulf of Mannar, which is situated between the southeast of the Indian tip and northwest of Sri Lanka, using a 7-year data set derived from satellite sensors. The surface forcing consists of wind stress and turbulent heat flux, which were estimated through the TOGA/COARE algorithm using Special Sensor Microwave Imager (SSM/I) wind, Advanced Very High Resolution Radiometer (AVHRR) SST, and surface atmospheric conditions derived from National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalyses data. Net heat flux was derived by combining the turbulent heat flux with net short- and long-wave radiation from the NCEP/NCAR reanalysis data set. SST cooling was monitored by using 9-km spatial resolution pathfinder SST that was derived from the AVHRR. The 7-year weekly mean maps of wind stress, net heat flux, and SST indicate that SST cooling is locally influenced by surface forcing in- and offshore of the Gulf of Mannar. A time series of these parameters near the Indian tip reveal that the strong wind stress and high surface heat loss, which are punctuated in time varying from 15 days to more than a month, occur every winter and lower SST by ∼1.5°C. The occurrence of such a phenomenon is referred to as an event in this work. The bell-shaped events, which have a periodicity of 15 days with a maximum wind stress around Day 8, occur every winter and are characterized by the SST cooling of less than a degree. The features of these events are studied by using the statistical correlation and composite technique. It is inferred that the SST cooling is strongly correlated with the surface forcing.

Topographic meandering of Antarctic Circumpolar Current and Antarctic Circumpolar Wave in the ice-ocean- atmosphere system

[1] Topographic meandering of Antarctic Circumpolar Current (ACC) is found to be an impediment in the propagation of Antarctic Circumpolar Wave (ACW) in the Indian Ocean sector of Antarctica. Reasons for this are attributed to the southward advection of the ACW anomalies associated with the topographic meandering of the ACC. The southward meandering of ACC facilitates warming up of the region east of 20°E by about 1°C during winter, thereby reducing the sea ice; these processes interfere with the eastward propagating positive sea‐ice anomalies, and reduce its strength. Warming of ocean induced by topographic meandering leads to upward vertical velocities between 40°–60°E, where the ocean surface velocities are weak and southward, and the vertical/meridional advection of temperature dominates the zonal advection in the atmosphere. This results in the decoupling of the ACW in the region east of 40°E. In regions out side the Indian Ocean sector, vertical advection is minimum and zonal velocity of ACC becomes positive, which facilitates the ACW propagation in the Central Pacific, Ross and Weddell Seas. Citation: Nuncio, M., A. J. Luis, and X. Yuan (2011), Topographic meandering of Antarctic Circumpolar Current and Antarctic Circumpolar Wave in the ice‐ ocean‐atmosphere system, Geophys. Res. Lett., 38, L13708,

Seasonal SST patterns along the West India shelf inferred from AVHRR

Abstract Sea surface temperature (SST) patterns along the west India shelf, extending from 8° to 24°N, are analyzed during 1993–1996 to characterize seasonal variability using the advanced very high-resolution radiometer (AVHRR) SST, momentum and heat fluxes derived from ERS-1 winds and NCEP/NCAR reanalysis data. During winter monsoon (December–March), a 4-year mean SST spatial pattern shows a strong cooling north of 15°N due to surface heat depletion, while warm SSTs evolve in the south due to the intrusion of warm equatorial water. Cold water occupies the entire shelf during summer monsoon, with high degree of SST cooling dominating the Kerala coast, where Ekman pumping and upwelling promoted by the dominant alongshore wind stress component overwhelms the surface heat loss. The spectral analysis reveals semiannual and annual peaks in SST and forcing functions, which highlight the influence of monsoon forcing on the SST variability along the west India shelf.

Relationship between surface atmospheric convergence over Indian Ocean and Indian rainfall

Atmospheric convergence regions over the tropical Indian Ocean have been mapped for the first time using 31 years of vector wind data. The convergence fields reveal that the energetic summer monsoon winds enhance convergence in the central Arabian Sea and in the eastern Bay of Bengal when compared with weak winter monsoon winds. The summer monsoon is also effective in spatial migration of convergence regions in both basins. Area-integrated convergence time series reveals an annual cycle with high amplitude during summer monsoon, which occurs in phase with Indian rainfall. The study explores the prospects of using the convergence as one of the predictors of Indian rainfall.