arabian sea phytoplankton snow

THE ARABIAN SEA MONSOON - A GIANT SEA BREEZE

The Arabian Sea’s predictable, seasonally reversing monsoons (click HERE for animated picture) drive one of the most energetic current systems in the world and the greatest seasonal variability observed in any ocean basin. It is the only ocean that fully reverses its circulation on a semi-annual basis, a phenomenon in which the Indian Ocean, the Eurasian continent and the Pacific Ocean play significant roles.The strength of the monsoon winds is regulated by a thermal gradient that develops from differential heating of land and sea. In summer, (southwest monsoon, June-September), heating of the Eurasian land mass results in low pressure over Asia, while high pressure prevails over the Indian Ocean. The direction of the monsoon winds is then southwesterly. In winter, (northeast monsoon, November-February), cooling of the northern hemispheric land mass results in high pressure over land and low pressure over the Indian Ocean, causing a reversal in the direction of the monsoon winds from southwesterly to northeasterly. CLICK ON ANY IMAGE TO ENLARGE

Wind direction resulting from differential pressure over sea and land superimposed on a SeaWiFS chlorophyll image for the southwest monsoon. High chlorophyll concentrations in the western Arabian Sea are due to coastal upwelling

Schematic showing snow cover extent and wind direction superimposed on a SeaWiFS chlorophyll image for the northwest monsoon season. High chlorophyll concentrations are due to nutrients inputs from of winter convective mixing

MONSOONS AND ARABIAN SEA BLOOMS

The southwest monsoon winds are responsible for bringing rain from June to September to a third of the world's population. As the southwesterly winds blow off the coasts of Somalia, Yemen, and Oman, the elevated east African coastline intensifies the wind at the surface and directs it parallel to the coast. This strong flow, known as the Findlater Jet, can exceed 36 knots in July and results in intense upwelling of deeper, nutrient rich waters to the surface. The consequent high phytoplankton productivity and biomass along the coasts of Somalia, Yemen, and Oman make this region one of the richest fisheries grounds in the world.

In contrast, during winter, when the Eurasian continent cools, cold dry winds blow from the continent. These winds are not as strong as during summer, but, combined with strong surface cooling lead to deepening of the mixed layer. Significant nutrient inputs from this deep convective mixing produce large phytoplankton blooms in the northern and central Arabian Sea

FOR MORE DETAILS VISIT THE WEBPAGE OF OUR COLLBARORATOR PRASAD THOPPIL

Schematic depicting the relationship between winter-time snow cover and the strength of the southwest monsoon winds

Departures from monthly means of % snow cover extent over Southwest Asia (SWA) and Himalayas-Tibetan Plateau (HTP) between 1967 and 2003. Note especially the abrupt decline in snow cover extent after 1997

Interannnual trends showing increase in satellite derived chlorophyll a and zonal wind stress in the western Arabian Sea

Funded by NASA's Earth Science Enterprise

EURASIAN SNOW AND MONSOON WINDS

The extent of winter and spring-time snow cover over the Eurasian land mass and the latent heat released during spring have a major impact on the land-sea thermal gradient that drives the monsoons. Positive snow anomalies in winter and spring give rise to colder ground temperatures in the subsequent summer because a substantial fraction of the available solar energy during spring and early summer goes toward melting the snow and evaporating water from the wet soil rather than toward heating the ground. Excessive snowfall in the early part of winter also tends to reduce solar radiation in winter by increasing the surface albedo, resulting in persistently colder temperatures. Conversely, reduced snow cover over Eurasia strengthens the spring and summer land-sea thermal contrast and is considered to be responsible for the stronger southwest monsoonal winds and positive rainfall anomalies over the subcontinent .

GLOBAL WARMING, SNOW MELT AND PHYTOPLANKTON BLOOMS

Over the past 7 years, the western half of the Arabian Sea has witnessed record increases in phytoplankton blooms due to a year-by-year intensification of monsoonal winds. In a recent study that has appeared in the journal Science (22 April 2005, Vol. 308), we show that these changes are being triggered by the rapid decline and meltdown of winter-time snow over southwest Asia and the Himalayas. CLICK HERE TO SEE RETREAT OF THE GANGOTRI GLACIER, ONE OF THE LARGEST GLACIERS IN THE HIMALAYAS.

While large phytoplankton blooms can enhance fisheries, unusually high increases could be detrimental to the ecosystem by causing oxygen depletion at depth. If the present warming trend continues, the Arabian Sea could slowly become devoid of oxygen. In recent years, fishermen off the coast of Oman have encountered several instances of massive fish mortality, according to scientists at the Dept. of Marine Sciences and Fisheries at the Sultan Qaboos University in Oman. These fish kill incidences are generally preceded by unusually high fish landings in association with phytoplankton blooms.

Oxygen depleted waters also provide an ideal environment for the growth of a specialized group of bacteria called the denitrifying bacteria, which convert nitrate in seawater into less oxidized forms of nitrogen. One such form is nitrous oxide, also known as laughing gas. Nitrous oxide is a greenhouse gas whose global warming potential is over 300 times that of carbon dioxide. Increased phytoplankton in the oxygen limited deeper waters of the Arabian Sea could therefore exacerbate the greenhouse problem.

Comparision of chlorophyll fields in the Arabian Sea during the bloom season of 1997 (left) and 2003 (right)

“Nothing in the sea falls haphazard; if we cannot predict, it is because we do not know the cause, or how the cause works..." Henry Bigelow (1929)

For more press coverage click here

Comments or questions to Dr. Helga do Rosario Gomes

Please visit The Rise & Fall of El Niños - Their impact on Phytoplankton