Eddy induced trapping and homogenization of freshwater in the Bay of Bengal

Freshwater from rivers influences Indian summer monsoon rainfall and regional tropical cyclones by shallowing the upper layer and warming the subsurface ocean in the Bay of Bengal. Here, we use in situ and satellite data with reanalysis products to showcase how river water can experience a significant increase in salinity on subseasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Specifically, in October 2015, river water is shown to enter a particularly long-lived eddy along with its attracting manifolds within a period of two weeks. The eddy itself is quite unique in that it lasted for 16 months in the Bay where average lifespans are of the order of 2-3 months. This low salinity water results in the formation of a highly stratified surface layer. In fact, when freshest, the eddy has the highest sea-level anomalies, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal progressive homogenization of salinity and relaxation of sea-level anomalies and salinity gradients within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity on the order of timescale of a month. This pathway is distinct from vertical mixing and likely to be important in the eddy-rich Bay of Bengal.Comment: 11 pages, 11 Figure

The potential of using remote sensing data to estimate air-sea CO2 exchange in the Baltic Sea

In this article, we present the first climatological map of air-sea CO2 flux over the Baltic Sea based on remote sensing data: estimates of pCO(2) derived from satellite imaging using self-organizing map classifications along with class-specific linear regressions (SOMLO methodology) and remotely sensed wind estimates. The estimates have a spatial resolution of 4 km both in latitude and longitude and a monthly temporal resolution from 1998 to 2011. The CO2 fluxes are estimated using two types of wind products, i.e. reanalysis winds and satellite wind products, the higher-resolution wind product generally leading to higher-amplitude flux estimations. Furthermore, the CO2 fluxes were also estimated using two methods: the method of Wanninkhof et al. (2013) and the method of Rutgersson and Smedman (2009). The seasonal variation in fluxes reflects the seasonal variation in pCO(2) unvaryingly over the whole Baltic Sea, with high winter CO2 emissions and high pCO(2) uptakes. All basins act as a source for the atmosphere, with a higher degree of emission in the southern regions (mean source of 1.6 mmol m(-2) d(-1) for the South Basin and 0.9 for the Central Basin) than in the northern regions (mean source of 0.1 mmol m(-2) d(-1)) and the coastal areas act as a larger sink (annual uptake of -4.2 mmol m(-2) d(-1)) than does the open sea (4 mmol m(-2) d(-1)). In its entirety, the Baltic Sea acts as a small source of 1.2 mmol m(-2) d(-1) on average and this annual uptake has increased from 1998 to 2012

Warm pool thermodynamics from the Arabian Sea Monsoon Experiment (ARMEX)

Before the onset of the south Asian summer monsoon, sea surface temperature (SST) of the north Indian Ocean warms to 30-32&#176; C. Climatological mean mixed layer depth in spring (March-May) is 10-20 m, and net surface heat flux (Q <SUB>net</SUB> ) is 80-100 W m<SUP>-2</SUP> into the ocean. Previous work suggests that observed spring SST warming is small mainly because of (1) penetrative flux of solar radiation through the base of the mixed layer (Q <SUB>pen</SUB> ) and (2) advective cooling by upper ocean currents. We estimate the role of these two processes in SST evolution from a two-week Arabian Sea Monsoon Experiment process experiment in April-May 2005 in the southeastern Arabian Sea. The upper ocean is stratified by salinity and temperature, and mixed layer depth is shallow (6 to 12 m). Current speed at 2 m depth is high even under light winds. Currents within the mixed layer are quite distinct from those at 25 m. On subseasonal scales, SST warming is followed by rapid cooling, although the ocean gains heat at the surface: Q<SUB> net</SUB> is about 105 W m<SUP>-2</SUP> in the warming phase and 25 W m<SUP>-2</SUP> in the cooling phase; penetrative loss Q <SUB>pen</SUB> is 80 W m<SUP>-2</SUP> and 70 W m<SUP>-2</SUP>. In the warming phase, SST rises mainly because of heat absorbed within the mixed layer, i.e., Q <SUB>net</SUB> minus Q <SUB>pen</SUB> ; Q<SUB> pen</SUB> reduces the rate of SST warming by a factor of 3. In the second phase, SST cools rapidly because (1) Q <SUB>pen</SUB> is larger than Q<SUB> net</SUB> and (2) advective cooling is ~85 W m<SUP>-2</SUP>. A calculation using time-averaged heat fluxes and mixed layer depth suggests that diurnal variability of fluxes and upper ocean stratification tends to warm SST on subseasonal timescale. Buoy and satellite data suggest that a typical premonsoon intraseasonal cooling event occurs under clear skies when the ocean is gaining heat through the surface. In this respect, premonsoon SST cooling in the north Indian Ocean is different from that due to the Madden-Julian oscillation or monsoon intraseasonal oscillation