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  • Air–Sea Interactions from the Diurnal to the Intraseasonal during the PISTON, MISOBOB, and CAMP2Ex Observational Campaigns in the Tropics x
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C. A. Luecke
,
H. W. Wijesekera
,
E. Jarosz
,
D. W. Wang
,
T. G. Jensen
,
S. U. P. Jinadasa
,
H. J. S. Fernando
, and
W. J. Teague

Abstract

The formation of a sharp oceanic front located south-southeast of Sri Lanka during the southwest monsoon is examined through in situ and remote observations and high-resolution model output. Remote sensing and model output reveal that the front extends approximately 200 km eastward from the southeast coast of Sri Lanka toward the southern Bay of Bengal (BoB). This annually occurring front is associated with the boundary between the southwest monsoon current with high-salinity water to the south, and a weak flow field comprised of relatively fresh BoB water to the north. The front contains a line of high chlorophyll extending from the coastal upwelling zone, often for several hundred kilometers. Elevated turbulent diffusivities ∼10−2 m2 s−1 along with large diapycnal fluxes of heat and salt were found within the front. The formation of the front and vertical transports are linked to local wind stress curl. Large vertical velocities (∼50 m day−1) indicate the importance of ageostrophic, submesoscale processes. To examine these processes, the Ertel potential vorticity (PV) was computed using the observations and numerical model output. The model output shows a ribbon of negative PV along the front between the coastal upwelling zone and two eddies (Sri Lanka Dome and an anticyclonic eddy) typically found in the southern BoB. PV estimates support the view that the flow is susceptible to submesoscale instabilities, which in turn generate high vertical velocities within the front. Frontal upwelling and heightened mixing show that the seasonal front is regionally important to linking the fresh surface water of the BoB with the Arabian Sea.

Significance Statement

Within the ocean, motions span extraordinarily wide ranges of sizes and time scales. In this study we focus on a narrow, intensified feature called a front. This front occurs in the southern Bay of Bengal during the summer monsoon and forms a boundary between fresher water to the north and saltier water to the south. Features such as this are difficult to study, however, by combining observations made from ships and satellites with output from numerical models of the ocean, we are able to better understand the front. This is important because fronts like the one studied here play a role in determining the pathways of heat within the ocean, which, in turn, may feedback into the atmosphere and weather patterns.

Open access
Jaynise M. Pérez Valentín
,
Harindra J. S. Fernando
,
G. S. Bhat
,
Hemantha W. Wijesekera
,
Jayesh Phadtare
, and
Edgar Gonzalez

Abstract

The relationship between eastward-propagating convective equatorial signals (CES) along the equatorial Indian Ocean (EIO) and the northward-propagating monsoon intraseasonal oscillations (MISOs) in the Bay of Bengal (BOB) was studied using observational datasets acquired during the 2018 and 2019 MISO-BOB field campaigns. Convective envelopes of MISOs originating from just south of the BOB were associated with both strong and weak eastward CES (average speed ∼6.4 m s−1). Strong CES contributed to ∼20% of the precipitation budget of BOB, and they spurred northward-propagating convective signals that matched the canonical speed of MISOs (1–2 m s−1). In contrast, weak CES contributed to ∼14% of the BOB precipitation budget, and they dissipated without significant northward propagation. Eastward-propagating intraseasonal oscillations (ISOs; period 30–60 days) and convectively coupled Kelvin waves (CCKWs; period 4–15 days) accounted for most precipitation variability across the EIO during the 2019 boreal summer as compared with that of 2018. An agreement could be noted between high moisture content in the midtroposphere and the active phases of CCKWs and ISOs for two observational locations in the BOB. Basin-scale thermodynamic conditions prior to the arrival of strong or weak CES revealed warmer or cooler sea surface temperatures, respectively. Flux measurements aboard a research vessel suggest that the evolution of MISOs associated with strong CES are signified by local enhanced air–sea interactions, in particular the supply of local moisture and sensible heat, which could enhance deep convection and further moisten the upper troposphere.

Significance Statement

Eastward-propagating convective signals along the equatorial Indian Ocean and their relationship to the northward-propagating spells of rainfall that lead to moisture variability in the Bay of Bengal are studied for the 2018 and 2019 southwest monsoon seasons using observational datasets acquired during field campaigns. Strong convective equatorial signals spurred northward-propagating convection, as compared with weak signals that dissipated without significant northward propagation. Wave spectral analysis showed CCKWs (period 4–15 days), and eastward ISOs (period 30–60 days) accounted for most of the precipitation variability, with the former dominating during the 2018 boreal summer. High moisture periods observed from radiosonde measurements show agreement with the active phases of CCKWs and ISOs.

Free access
Adam V. Rydbeck
,
Jonathan A. Christophersen
,
Maria K. Flatau
,
Matthew A. Janiga
,
Tommy G. Jensen
,
Carolyn A. Reynolds
,
James A. Ridout
,
Travis A. Smith
, and
Hemantha Wijesekera

Abstract

Moist static energy (MSE) and ocean heat content (OHC) in the tropics are inextricably linked. The processes by which sources and sinks of OHC modulate column integrated MSE in the Indian Ocean (IO) are explored through a reformulation of the MSE budget using atmosphere and ocean reanalysis data. In the reframed MSE budget, interfacial air–sea turbulent and radiative fluxes are replaced for information on upper ocean dynamics, thus “mooring” the MSE tendency to the subsurface ocean. On subseasonal time scales, ocean forcing is largely responsible for the amplification of MSE anomalies across the IO, with basin average growth rates of 10% day−1. Local OHC depletion is the leading contributor to anomalous MSE amplification with average rates of 12% day−1. Along the equator, MSE is amplified by OHC vertical advection. Ocean forcing only weakly reduces the propagation tendency of MSE anomalies (−2% day−1), with propagation predominantly resulting from atmosphere forcing (10% day−1). OHC in the IO acts as an MSE reservoir that is expended during periods of enhanced intraseasonal atmosphere convection and recharged during periods of suppressed convection. Because OHC is an MSE source during enhanced intraseasonal convection periods, it largely offsets the negative MSE tendency produced by horizontal advection in the atmosphere. The opposite effect occurs during suppressed convection periods, where OHC is a sink of MSE and counters the positive MSE tendency produced by horizontal advection in the atmosphere.

Open access
Yolande L. Serra
,
Steven A. Rutledge
,
Kyle Chudler
, and
Chidong Zhang

Abstract

This study evaluates rainfall, cloudiness, and related fields in the European Centre for Medium-Range Weather Forecasts fifth-generation climate reanalysis (ERA5) and the National Aeronautics and Space Administration’s Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), gridded global reanalysis products against observations from the Office of Naval Research’s Propagation of Intraseasonal Tropical Oscillations (PISTON) field campaign. We focus on the first PISTON cruise, which took place from August to October 2018 in the northern equatorial western Pacific Ocean. We find biases in the mean surface heat and radiative fluxes consistent with observed biases in high and low cloud fraction and convective activity in the reanalyses. Biases in the high, middle, and low cloud fraction are also consistent with the biases in the thermodynamic profiles, with positive biases in upper-level humidity associated with excessive high cloud in both products, whereas negative biases in humidity above the boundary layer are associated with too few low and middle clouds and increased static stability. ERA5 exhibits a profile that is more top-heavy than that of MERRA-2 during periods dominated by MCSs and stronger upward motion during rainy periods, consistent with higher total rainfall in this product during PISTON. The coarser grid size in MERRA-2 relative to ERA5 and the fact that MERRA-2 did not assimilate PISTON data likely both contribute to the overall larger biases seen in MERRA-2. The observed biases in the reanalyses during PISTON have also been seen in comparisons of these products with satellite data, suggesting that the results of this study are more broadly applicable.

Free access
Michael B. Natoli
and
Eric D. Maloney

Abstract

The impact of the environmental background wind on the diurnal cycle near tropical islands is examined in observations and an idealized model. Luzon Island in the northern Philippines is used as an observational test case. Composite diurnal cycles of CMORPH precipitation are constructed based on an index derived from the first empirical orthogonal function (EOF) of ERA5 zonal wind profiles. A strong precipitation diurnal cycle and pronounced offshore propagation in the leeward direction tends to occur on days with a weak, offshore prevailing wind. Strong background winds, particularly in the onshore direction, are associated with a suppressed diurnal cycle. Idealized high-resolution 2D Cloud Model 1 (CM1) simulations test the dependence of the diurnal cycle on environmental wind speed and direction by nudging the model base state toward composite profiles derived from the reanalysis zonal wind index. These simulations can qualitatively replicate the observed development, strength, and offshore propagation of diurnally generated convection under varying wind regimes. Under strong background winds, the land–sea contrast is reduced, which leads to a substantial reduction in the strength of the sea-breeze circulation and precipitation diurnal cycle. Weak offshore prevailing winds favor a strong diurnal cycle and offshore leeward propagation, with the direction of propagation highly sensitive to the background wind in the lower free troposphere. Offshore propagation speed appears consistent with density current theory rather than a direct coupling to a single gravity wave mode, though gravity waves may contribute to a destabilization of the offshore environment.

Free access
Iury T. Simoes-Sousa
,
Amit Tandon
,
Jared Buckley
,
Debasis Sengupta
,
Sree Lekha J. Sree Lekha J.
,
Emily Shroyer
, and
Simon P. de Szoeke

Abstract

Atmospheric cold pools, generated by evaporative downdrafts from precipitating clouds, are ubiquitous in the Bay of Bengal. We use data from three moorings near 18°N to characterize a total of 465 cold pools. The cold pools are all dry, with a typical temperature drop of 2°C (maximum 5°C) and specific humidity drop of 1 g kg−1 (maximum = 6 g kg−1). Most cold pools last 1.5–3.5 h (maximum = 14 h). Cold pools occur almost every day in the north bay from April to November, principally in the late morning, associated with intense precipitation that accounts for 80% of total rain. They increase the latent heat flux to the atmosphere by about 32 W m−2 (median), although the instantaneous enhancement of latent heat flux for individual cold pools reaches 150 W m−2. During the rainiest month (July), the cold pools occur 21% of the time and contribute nearly 14% to the mean evaporation. A composite analysis of all cold pools shows that the temperature and specific humidity anomalies are responsible for ∼90% of the enhancement of sensible and latent heat flux, while variations in wind speed are responsible for the remainder. Depending on their gust-front speed, the estimated height of the cold pools primarily ranges from 850 to 3200 m, with taller fronts more likely to occur during the summer monsoon season (June–September). Our results indicate that the realistic representation of cold pools in climate models is likely to be important for improved simulation of air–sea fluxes and monsoon rainfall.

Significance Statement

Atmospheric cold pools form over the ocean when falling rain evaporates, leading to a dense cold air mass spreading over the surface. They impact air–sea heat exchanges over tropical regions and give rise to new rainstorms. We analyze data from three fixed, closely spaced buoys to describe cold pools and investigate their role in rainfall and air–sea interactions in the northern Bay of Bengal (Indian Ocean). We find that cold pools are associated with about 80% of all rain and are important for ocean–atmosphere heat and moisture exchange, especially from April to November. We estimate the speed of cold pools and derive their heights (850–3200 m) using theory.

Free access
A. Anutaliya
,
U. Send
,
J. L. McClean
,
J. Sprintall
,
M. Lankhorst
,
C. M. Lee
,
L. Rainville
,
W. N. C. Priyadarshani
, and
S. U. P. Jinadasa

Abstract

Boundary currents along the Sri Lankan eastern and southern coasts serve as a pathway for salt exchange between the Bay of Bengal and the Arabian Sea basins in the northern Indian Ocean, which are characterized by their contrasting salinities. Measurements from two pairs of pressure-sensing inverted echo sounders (PIES) deployed along the Sri Lankan eastern and southern coasts as well as satellite measurements are used to understand the variability of these boundary currents and the associated salt transport. The volume transport in the surface (0–200-m depth) layer exhibits a seasonal cycle associated with the monsoonal wind reversal and interannual variability associated with the Indian Ocean dipole (IOD). In this layer, the boundary currents transport low-salinity water out of the Bay of Bengal during the northeast monsoon and transport high-salinity water into the Bay of Bengal during the fall monsoon transition of some years (e.g., 2015 and 2018). The Bay of Bengal salt input increases during the 2016 negative IOD as the eastward flow of high-salinity water during the fall monsoon transition intensifies, whereas the effect of the 2015/16 El Niño on the Bay of Bengal salt input is still unclear. The time-mean eddy salt flux over the upper 200 m estimated for the April 2015–March 2019 period along the eastern coast accounts for 9% of the salt budget required to balance an estimated 0.13 Sv (1 Sv ≡ 106 m3 s−1) of annual freshwater input into the Bay of Bengal. The time-mean eddy salt flux over the upper 200 m estimated for the December 2015–November 2019 period along the southern coast accounts for 27% of that same salt budget.

Significance Statement

In the northern Indian Ocean, the highly saline Arabian Sea undergoes extreme evaporation while the Bay of Bengal (BoB) receives excess freshwater input. The focus of this study is the role of the observed time-variable circulation around Sri Lanka that permits the exchange between these basins to maintain their salinity distributions. The circulation fluctuates seasonally following the monsoon wind reversal and interannually in response to large-scale climate modes. The BoB freshwater export around Sri Lanka occurs during the northeast monsoon, whereas saline water import occurs during the fall monsoon transition of some years. However, rapid changes in both water volume transport and salt exchange can occur. The circulation over 0–200-m depth transports ∼9%–27% of the BoB salt budget.

Free access
Kyle Chudler
,
Steven A. Rutledge
, and
Brenda Dolan

Abstract

Isolated warm-rain cells are an important feature over the tropical oceans. Although warm rain is typically associated with relatively small raindrops, large raindrops (>4.5 mm in diameter) have been observed in some cases. Previous studies have examined warm rain cells with large drops on a case-study basis, but they have yet to be investigated in a broader, statistical sense. During the recent Propagation of Intraseasonal Oscillations (PISTON) field campaign, a C-band polarimetric radar routinely measured extreme values of differential reflectivity in small, isolated convection, indicating the presence of large drops. Using an objective feature identification and tracking algorithm, this study offers new insights to the structure and frequency of cells containing large drops. Cells with high differential reflectivity (>3.5 dB) were present in 24% of all radar scans. The cells were typically small (8-km2 mean area), short lived (usually <10 min), and shallow (3.7-km mean height). High differential reflectivity was more often found on the upwind side of the cells, suggesting a size sorting mechanism was operating establishing a low concentration of large drops on the upwind side. Differential reflectivity also tended to increase at lower altitudes, which is hypothesized to be due to continued drop growth and increasing temperature (increasing the dielectric constant of water). Rapid vertical cross-section radar scans, as well as transects made by a Learjet aircraft with onboard particle probes, are also used to analyze these cells, and support the conclusions drawn from statistical analysis.

Free access
Luc Rainville
,
Craig M. Lee
,
K. Arulananthan
,
S. U. P. Jinadasa
,
Harindra J. S. Fernando
,
W. N. C. Priyadarshani
, and
Hemantha Wijesekera

Abstract

We present high-resolution sustained, persistent observations of the ocean around Sri Lanka from autonomous gliders collected over several years, a region with complex, variable circulation patterns connecting the Bay of Bengal and the Arabian Sea to each other and the rest of the Indian Ocean. The Seaglider surveys resolve seasonal to interannual variability in vertical and horizontal structure, allowing quantification of volume, heat, and freshwater fluxes, as well as the transformations and transports of key water mass classes across sections normal to the east (2014–15) and south (2016–19) coasts of Sri Lanka. The resulting transports point to the importance of both surface and subsurface flows and show that the direct pathway along the Sri Lankan coast plays a significant role in the exchanges of waters between the Arabian Sea and the Bay of Bengal. Significant section-to-section variability highlights the need for sustained, long-term observations to quantify the circulation pathways and dynamics associated with exchange between the Bay of Bengal and Arabian Sea and provides context for interpreting observations collected as “snapshots” of more limited duration.

Significance Statement

The strong seasonal variations of the wind in the Indian Ocean create large and rapid changes in the ocean’s properties near Sri Lanka. This variable and poorly observed circulation is very important for how temperature and salinity are distributed across the northern Indian Ocean, both at the surface and at depths. Long-term and repeated surveys from autonomous Seagliders allow us to understand how freshwater inflow, atmospheric forcing, and underlying ocean variability act to produce observed contrasts (spatial and seasonal) in upper-ocean structure of the Bay of Bengal and Arabian Sea.

Open access
Sebastian Essink
,
Verena Hormann
,
Luca R. Centurioni
, and
Amala Mahadevan

Abstract

Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales O ( 1 10 ) km , kinematic properties become as large as planetary vorticity f, but challenging to observe because they evolve on short time scales O ( hours to days ) . By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions estimated empirically and theoretically. The most robust method—a two-dimensional, linear least squares fit—is applied to the first few days of a drifter dataset from the Bay of Bengal. Application of the length scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the total number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.

Significance Statement

The purpose of this study is to provide insights and guidance for computing horizontal velocity gradients from clusters (i.e., three or more) of Lagrangian surface ocean drifters. The uncertainty in velocity gradient estimates depends strongly on the shape deformation of drifter clusters by the ocean currents. We propose criteria for drifter cluster length scales and aspect ratios to reduce uncertainties and develop ways of estimating the magnitude of the resulting errors. The findings are applied to a real ocean dataset from the Bay of Bengal.

Full access