Abstract. The Indian Ocean presents two distinct climate regimes. The north Indian Ocean is dominated by the monsoons, whereas the seasonal reversal is less pronounced in the south. The prevailing wind pattern produces upwelling along different parts of the coast in both hemispheres during different times of the year. Additionally, dynamical processes and eddies either cause or enhance upwelling. This paper reviews the phenomena of upwelling along the coast of the Indian Ocean extending from the tip of South Africa to the southern tip of the west coast of Australia. Observed features, underlying mechanisms, and the impact of upwelling on the ecosystem are presented. In the Agulhas Current region, cyclonic eddies associated with Natal pulses drive slope upwelling and enhance chlorophyll concentrations along the continental margin. The Durban break-away eddy spun up by the Agulhas upwells cold nutrient-rich water. Additionally, topographically induced upwelling occurs along the inshore edges of the Agulhas Current. Wind-driven coastal upwelling occurs along the south coast of Africa and augments the dynamical upwelling in the Agulhas Current. Upwelling hotspots along the Mozambique coast are present in the northern and southern sectors of the channel and are ascribed to dynamical effects of ocean circulation in addition to wind forcing. Interaction of mesoscale eddies with the western boundary, dipole eddy pair interactions, and passage of cyclonic eddies cause upwelling. Upwelling along the southern coast of Madagascar is caused by the Ekman wind-driven mechanism and by eddy generation and is inhibited by the Southwest Madagascar Coastal Current. Seasonal upwelling along the East African coast is primarily driven by the northeast monsoon winds and enhanced by topographically induced shelf breaking and shear instability between the East African Coastal Current and the island chains. The Somali coast presents a strong case for the classical Ekman type of upwelling; such upwelling can be inhibited by the arrival of deeper thermocline signals generated in the offshore region by wind stress curl. Upwelling is nearly uniform along the coast of Arabia, caused by the alongshore component of the summer monsoon winds and modulated by the arrival of Rossby waves generated in the offshore region by cyclonic wind stress curl. Along the west coast of India, upwelling is driven by coastally trapped waves together with the alongshore component of the monsoon winds. Along the southern tip of India and Sri Lanka, the strong Ekman transport drives upwelling. Upwelling along the east coast of India is weak and occurs during summer, caused by alongshore winds. In addition, mesoscale eddies lead to upwelling, but the arrival of river water plumes inhibits upwelling along this coast. Southeasterly winds drive upwelling along the coast of Sumatra and Java during summer, with Kelvin wave propagation originating from the equatorial Indian Ocean affecting the magnitude and extent of the upwelling. Both El Niño–Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events cause large variability in upwelling here. Along the west coast of Australia, which is characterized by the anomalous Leeuwin Current, southerly winds can cause sporadic upwelling, which is prominent along the southwest, central, and Gascoyne coasts during summer. Open-ocean upwelling in the southern tropical Indian Ocean and within the Sri Lanka Dome is driven primarily by the wind stress curl but is also impacted by Rossby wave propagations. Upwelling is a key driver enhancing biological productivity in all sectors of the coast, as indicated by enhanced sea surface chlorophyll concentrations. Additional knowledge at varying levels has been gained through in situ observations and model simulations. In the Mozambique Channel, upwelling simulates new production and circulation redistributes the production generated by upwelling and mesoscale eddies, leading to observations of higher ecosystem impacts along the edges of eddies. Similarly, along the southern Madagascar coast, biological connectivity is influenced by the transport of phytoplankton from upwelling zones. Along the coast of Kenya, both productivity rates and zooplankton biomass are higher during the upwelling season. Along the Somali coast, accumulation of upwelled nutrients in the northern part of the coast leads to spatial heterogeneity in productivity. In contrast, productivity is more uniform along the coasts of Yemen and Oman. Upwelling along the west coast of India has several biogeochemical implications, including oxygen depletion, denitrification, and high production of CH4 and dimethyl sulfide. Although weak, wind-driven upwelling leads to significant enhancement of phytoplankton in the northwest Bay of Bengal during the summer monsoon. Along the Sumatra and Java coasts, upwelling affects the phytoplankton composition and assemblages. Dissimilarities in copepod assemblages occur during the upwelling periods along the west coast of Australia. Phytoplankton abundance characterizes inshore edges of the slope during upwelling season, and upwelling eddies are associated with krill abundance. The review identifies the northern coast of the Arabian Sea and eastern coasts of the Bay of Bengal as the least observed sectors. Additionally, sustained long-term observations with high temporal and spatial resolutions along with high-resolution modelling efforts are recommended for a deeper understanding of upwelling, its variability, and its impact on the ecosystem.
more »
« less
Evaporative Implications of Dry‐Air Outbreaks Over the Northern Red Sea
We investigate the impacts of westward wind events on the Red Sea evaporation using the 35‐year second Modern‐Era Retrospective analysis for Research and Applications reanalysis and a 2‐year‐long record of in situ observations from a heavily instrumented air‐sea interaction mooring. These events are common during boreal winter, and their effects are similar to cold‐air outbreaks that occur in midpolar and subpolar latitudes. They cause extreme heat loss from the sea, which is dominated by latent heat fluxes. Different from cold‐air outbreaks, the intensified heat loss is due to the low relative humidity as we show through latent heat flux decomposition. Rainfall is negligible during these events, and we refer to them as dry‐air outbreaks. We also investigate the general atmospheric circulation pattern that favors their occurrence, which is associated with an intensified Arabian High at the north‐central portion of the Arabian Peninsula—a feature that seems to be an extension of the Siberian High. The analyses reveal that the westward winds over the northern Red Sea and the winter Shamal winds in the Persian Gulf are very likely to be part of the same subsynoptic‐scale feature. The second Modern‐Era Retrospective analysis for Research and Applications reanalysis indicates that the occurrence of westward wind events over the northern Red Sea has grown from 1980 to 2015, especially the frequency of large‐scale events, the cause of which is to be investigated. We hypothesize that dry‐air outbreaks may induce surface water mass transformation in the surface Red Sea Eastern Boundary Current and could represent a significant process for the oceanic thermohaline‐driven overturning circulation.
more » « less- NSF-PAR ID:
- 10375265
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 124
- Issue:
- 9
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- p. 4829-4861
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract The oceanographic response and atmospheric forcing associated with downwelling along the Alaskan Beaufort Sea shelf/slope is described using mooring data collected from August 2002 to September 2004, along with meteorological time series, satellite data, and reanalysis fields. In total, 55 downwelling events are identified with peak occurrence in July and August. Downwelling is initiated by cyclonic low‐pressure systems displacing the Beaufort High and driving westerly winds over the region. The shelfbreak jet responds by accelerating to the east, followed by a depression of isopycnals along the outer shelf and slope. The storms last 3.25 ± 1.80 days, at which point conditions relax toward their mean state. To determine the effect of sea ice on the oceanographic response, the storms are classified into four ice seasons: open water, partial ice, full ice, and fast ice (immobile). For a given wind strength, the largest response occurs during partial ice cover, while the most subdued response occurs in the fast ice season. Over the two‐year study period, the winds were strongest during the open water season; thus, the shelfbreak jet intensified the most during this period and the cross‐stream Ekman flow was largest. During downwelling, the cold water fluxed off the shelf ventilates the upper halocline of the Canada Basin. The storms approach the Beaufort Sea along three distinct pathways: a northerly route from the high Arctic, a westerly route from northern Siberia, and a southerly route from south of Bering Strait. Differences in the vertical structure of the storms are presented as well.
-
more » « less
Wintertime surface ocean heat loss is the key process driving the formation of Subantarctic Mode Water (SAMW), but there are few direct observations of heat fluxes, particularly during winter. The Ocean Observatories Initiative (OOI) Southern Ocean mooring in the southeast Pacific Ocean and the Southern Ocean Flux Station (SOFS) in the southeast Indian Ocean provide the first concurrent, multiyear time series of air–sea fluxes in the Southern Ocean from two key SAMW formation regions. In this work we compare drivers of wintertime heat loss and SAMW formation by comparing air–sea fluxes and mixed layers at these two mooring locations. A gridded Argo product and the ERA5 reanalysis product provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is on average 1.5 times larger in the southeast Indian (SOFS) than in the southeast Pacific (OOI), with stronger extreme heat flux events in the southeast Indian leading to larger cumulative winter ocean heat loss. Turbulent heat loss events in the southeast Indian (SOFS) occur in two atmospheric regimes (cold air from the south or dry air circulating via the north), while heat loss events in the southeast Pacific (OOI) occur in a single atmospheric regime (cold air from the south). On interannual time scales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. This relationship is part of a larger basin-scale zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific basins, associated with anomalous meridional atmospheric circulation.
-
more » « less
Abstract Using 17 years of Modern‐Era Retrospective analysis for Research and Applications, Version 2 (MERRA‐2) data, significant responses of gravity wave (GW) variances, zonal winds and parameterized GW drag to the Madden‐Julian Oscillation (MJO) are identified globally during boreal winter, and their relations are examined. The relative anomalies of GW variances range from −4% (phase 7) to 8% (phase 4) in tropics, and −20% (phase 1) to 20% (phase 5) in the northern polar region (NPR). The anomalies of zonal winds are from −3–3 m/s and −4–8 m/s in tropics and NPR, respectively. The vertical and latitudinal structures of MJO signals in GW, wind and GW drag show coherent patterns. Further analysis implies that in the NPR, the eastward wind leads to westward momentum flux carried by the GWs. This flux leads to westward drag, which drives that of zonal winds and imprint the MJO signal in GWs to the wind.
-
more » « less
Abstract Traveling planetary waves surrounding sudden stratospheric warming events can result from direct propagation from below or in situ generation. They can have significant impacts on the circulation in the mesosphere and lower thermosphere. Our study runs a series of ensembles initialized from the Whole Atmosphere Community Climate Model, version 4, nudged up to 50 km by 6-hourly Modern-Era Retrospective Analysis for Research and Application, version 2, reanalysis to compile a library of sudden stratospheric warming events. To our knowledge, we present the first composite or ensemble study that attempts to link direct propagation and in situ generation by evaluating the wave geometries associated with the overreflection perspective, a framework used to describe how planetary waves interact with critical and turning levels. The present study looks at the evolution of these interactions through the onset of sudden stratospheric warmings with an elevated stratopause (ES-SSWs). Robust and unique features of ES-SSWs are determined by employing an ensemble study that compares ES-SSWs with normal winters. Our study evaluates the production and impacts of westward-propagating, quasi-stationary, and eastward-propagating planetary waves surrounding ES-SSWs. Our results show that eastward-propagating planetary waves are generated within the westward stratospheric wind layer after ES-SSW onset which aids in restoring the eastward stratospheric wind. The interaction of quasi-stationary and westward-propagating waves with the westward stratospheric wind is explored from an overreflection perspective and reaffirms that westward-propagating planetary waves are produced from instabilities at the top of the westward stratospheric wind reversal.
