Distinct influence of air–sea interactions mediated by mesoscale sea surface temperature and surface current in the Arabian Sea

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 8061-8080, doi:10.1175/JCLI-D-16-0834.1.During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.This work is supported by ONR (N00014-15-1-2588 and N00014-17-1-2398), NSF (OCE- 1419235), and NOAA (NA15OAR4310176).2018-03-0

Distinct influence of air–sea interactions mediated by mesoscale sea surface temperature and surface current in the Arabian Sea

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 30 (2017): 8061-8080, doi:10.1175/JCLI-D-16-0834.1.During the southwest monsoons, the Arabian Sea (AS) develops highly energetic mesoscale variability associated with the Somali Current (SC), Great Whirl (GW), and cold filaments (CF). The resultant high-amplitude anomalies and gradients of sea surface temperature (SST) and surface currents modify the wind stress, triggering the so-called mesoscale coupled feedbacks. This study uses a high-resolution regional coupled model with a novel coupling procedure that separates spatial scales of the air–sea coupling to show that SST and surface currents are coupled to the atmosphere at distinct spatial scales, exerting distinct dynamic influences. The effect of mesoscale SST–wind interaction is manifested most strongly in wind work and Ekman pumping over the GW, primarily affecting the position of GW and the separation latitude of the SC. If this effect is suppressed, enhanced wind work and a weakened Ekman pumping dipole cause the GW to extend northeastward, delaying the SC separation by 1°. Current–wind interaction, in contrast, is related to the amount of wind energy input. When it is suppressed, especially as a result of background-scale currents, depth-integrated kinetic energy, both the mean and eddy, is significantly enhanced. Ekman pumping velocity over the GW is overly negative because of a lack of vorticity that offsets the wind stress curl, further invigorating the GW. Moreover, significant changes in time-mean SST and evaporation are generated in response to the current–wind interaction, accompanied by a noticeable southward shift in the Findlater Jet. The significant increase in moisture transport in the central AS implies that air–sea interaction mediated by the surface current is a potentially important process for simulation and prediction of the monsoon rainfall.This work is supported by ONR (N00014-15-1-2588 and N00014-17-1-2398), NSF (OCE- 1419235), and NOAA (NA15OAR4310176).2018-03-0

Dynamical response of the Arctic atmospheric boundary layer process to uncertainties in sea-ice concentration

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Atmospheres 118 (2013): 12,383–12,402, doi:10.1002/2013JD020312.Impact of sea-ice concentration (SIC) on the Arctic atmospheric boundary layer (ABL) is investigated using a polar-optimized version of the Weather Research and Forecasting (Polar WRF) model forced with SIC conditions during three different years. We present a detailed comparison of the simulations with historical ship and ice station based data focusing on September. Our analysis shows that Polar WRF provides a reasonable representation of the observed ABL evolution provided that SIC uncertainties are small. Lower skill is obtained, however, with elevated SIC uncertainties associated with incorrect seasonal evolution of sea ice and misrepresentation of ice thickness near the marginal ice zone (MIZ). The result underscores the importance of accurate representation of ice conditions for skillful simulation of the Arctic ABL. Further, two dynamically distinctive effects of sea ice on the surface wind were found, which act on different spatial scales. Reduced SIC lowers ABL stability, thereby increasing surface-wind (W10) speeds. The spatial scale of this response is comparable to the basin scale of the SIC difference. In contrast, near-surface geostrophic wind (Wg) shows a strong response in the MIZ, where a good spatial correspondence exists among the Laplacian of the sea level pressure (SLP), the surface-wind convergence, and the vertical motion within the ABL. This indicates that SIC affects Wg through variation in SLP but on a much narrower scale. Larger-amplitude and broader-scale response in W10 implies that surface-wind stress derived from Wg to drive ice-ocean models may not fully reflect the effect of SIC changes.The authors acknowledge the support from WHOI Arctic Research Initiative and National Science Foundation’s Office of Polar Program. H.S. thanks Andrey Proshutinsky (WHOI), Sang- Hun Park (NCAR), Keith Hines (BPRC/OSU), and Jun Inoue (JAMSTEC) for insightful comments.2014-05-2

A new framework for near‐surface wind convergence over the Kuroshio Extension and Gulf Stream in wintertime : the role of atmospheric fronts

Author Posting. © American Geophysical Union, 2018. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 45 (2018): 9909-9918, doi:10.1029/2018GL080135.It is well known that the wintertime time‐mean surface wind convergence patterns over the Kuroshio Extension and Gulf Stream show significant imprints of the underlying oceanic fronts. Previous studies have suggested that this collocation results from a time‐mean response to sea level pressure forcing from sea surface temperature gradients. However, more recent work has illustrated this phenomenon is heavily influenced by extratropical cyclones, although exact mechanisms are still debated. The purpose of this study is to introduce a new framework that explicitly distinguishes between two separate components in their contribution to the time‐mean surface wind convergence, that associated with and without atmospheric fronts. It is then argued that this distinction can help better explain the mechanisms driving the Kuroshio Extension and Gulf Stream influence on the atmosphere.National Science Foundation (NSF); NSF Physical Oceanography program Grant Numbers: AGS‐1355339, OCE‐14192352019-03-2

Impact of ocean warm layer thickness on the intensity of hurricane Katrina in a regional coupled model

Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Meteorology and Atmospheric Physics 122 (2013): 19-32, doi:10.1007/s00703-013-0275-3.The effect of pre-storm subsurface thermal structure on the intensity of hurricane Katrina (2005) is examined using a regional coupled model. The Estimating Circulation and Climate of Ocean (ECCO) ocean state estimate is used to initialize the ocean component of the coupled model, and the source of deficiencies in the simulation of Katrina intensity is investigated in relation to the initial depth of 26°C isotherm (D26). The model underestimates the intensity of Katrina partly due to shallow D26 in ECCO. Sensitivity tests with various ECCO initial fields indicate that the correct relationship between intensity and D26 cannot be derived because D26 variability is underestimated in ECCO. A series of idealized experiments is carried out by modifying initial ECCO D26 to match the observed range. A more reasonable relationship between Katrina’s intensity and pre-storm D26 emerges: the intensity is much more sensitive to D26 than to sea surface temperature (SST). Ocean mixed layer process plays a critical role in modulating inner-core SSTs when D26 is deep, reducing mixed layer cooling and lowering the center pressure of the Katrina. Our result lends strong support to the notion that accurate initialization of pre-storm subsurface thermal structure in prediction models is critical for a skillful forecast of intensity of Katrina and likely other intense storms.HS and SPX thank the support from NSF, NOAA, NASA and Japan Agency for Marine-Earth Science and Technology. HS acknowledges support from the Penzance Endowed Fund in Support of Assistant Scientists at WHOI.2014-10-0

Response and impact of equatorial ocean dynamics and tropical instability waves in the tropical Atlantic under global warming : a regional coupled downscaling study

Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 116 (2011): C03026, doi:10.1029/2010JC006670.A regional coupled model is used for a dynamic downscaling over the tropical Atlantic based on a global warming simulation carried out with the Geophysical Fluid Dynamics Laboratory CM2.1. The regional coupled model features a realistic representation of equatorial ocean dynamical processes such as the tropical instability waves (TIWs) that are not adequately simulated in many global coupled climate models. The coupled downscaling hence provides a unique opportunity to assess their response and impact in a changing climate. Under global warming, both global and regional models exhibit an increased (decreased) rainfall in the tropical northeast (South) Atlantic. Given this asymmetric change in mean state, the regional model produces the intensified near-surface cross-equatorial southerly wind and zonal currents. The equatorial cold tongue exhibits a reduced surface warming due to the enhanced upwelling. It is mainly associated with the increased vertical velocities driven by cross-equatorial wind, in contrast to the equatorial Pacific, where thermal stratification is suggested to be more important under global warming. The strengthened upwelling and zonal currents in turn amplify the dynamic instability of the equatorial ocean, thereby intensifying TIWs. The increased eddy heat flux significantly warms the equator and counters the effect of enhanced upwelling. Zonal eddy heat flux makes the largest contribution, suggesting a need for sustained monitoring of TIWs with spatially denser observational arrays in the equatorial oceans. Overall, results suggest that eddy heat flux is an important factor that may impact the mean state warming of equatorial oceans, as it does in the current climate.H.S. acknowledges the support from the NOAA Climate and Global Change Postdoctoral Fellowship Program and the Penzance Endowed Fund in Support of Assistant Scientists at WHOI. H.S. and S.‐P.X. are thankful for support from NOAA, NSF, and the Japan Agency for Marine‐Earth Science and Technology

Continental shelf baroclinic instability. Part II : oscillating wind forcing

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 569-582, doi:10.1175/JPO-D-15-0048.1.Continental shelf baroclinic instability energized by fluctuating alongshore winds is treated using idealized primitive equation numerical model experiments. A spatially uniform alongshore wind, sinusoidal in time, alternately drives upwelling and downwelling and so creates highly variable, but slowly increasing, available potential energy. For all of the 30 model runs, conducted with a wide range of parameters (varying Coriolis parameter, initial stratification, bottom friction, forcing period, wind strength, and bottom slope), a baroclinic instability and subsequent eddy field develop. Model results and scalings show that the eddy kinetic energy increases with wind amplitude, forcing period, stratification, and bottom slope. The dominant alongshore length scale of the eddy field is essentially an internal Rossby radius of deformation. The resulting depth-averaged alongshore flow field is dominated by the large-scale, periodic wind forcing, while the cross-shelf flow field is dominated by the eddy variability. The result is that correlation length scales for alongshore flow are far greater than those for cross-shelf velocity. This scale discrepancy is qualitatively consistent with midshelf observations by Kundu and Allen, among others.This work was funded by the Woods Hole Oceanographic Institution and by the National Science Foundation, Physical Oceanography section through Grant OCE-1433953.2016-06-0

Intraseasonal rainfall variability in the Bay of Bengal during the Summer Monsoon: coupling with the ocean and modulation by the Indian Ocean Dipole

The Indian Summer Monsoon rainfall exhibits pronounced intraseasonal variability in the Bay of Bengal (BoB). This study examines the intraseasonal rainfall variability with foci on the coupling with sea surface temperatures (SST) and its interannual modulation. The lagged composite analysis reveals that, in the northern BoB, SST warming leads the onset of intraseasonal rainfall by 5 days. Latent heat flux is reduced before the rain event but is greatly amplified during the rainfall maxima. Further analysis reveals that this intraseasonal rainfall-SST relationship through latent heating is strengthened in negative Indian Ocean Dipole (IOD) years when the bay-wide local SST is anomalously warm. Latent heat flux is further increased during the intraseasonal rainfall maxima leading to strengthened rainfall variability. The moisture budget analysis shows this is primarily due to stronger low-level moisture convergence in negative IOD years. The results provide important predictive information on the monsoon rainfall and its active/break cycles

Effect of eddy-wind interaction on Ekman pumping and eddy kinetic energy : a regional coupled modeling study for the California Current System

The definitive version was published in CalCOFI Reports 56 (2015): 83The California Current system (CCS) is characterized by the energetic summertime mesoscale and filamentary eddies with typical anomalies in sea surface temperature (SST) and surface current exceeding 2˚C and 0.5 cms–1, respectively. Recent satellite observations show that both SST and surface current at oceanic mesoscales significantly influence the Ekman pumping velocity, suggestive of a subsequent dynamical feedback effect on the eddy energetics. The extent to which this mesoscale coupling is important for the Ekman pumping and the eddy kinetic energy (EKE) budget in the CCS is the focus of this study

Eddy–wind interaction in the California Current System : dynamics and impacts

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 439-459, doi:10.1175/JPO-D-15-0086.1.The summertime California Current System (CCS) is characterized by energetic mesoscale eddies, whose sea surface temperature (SST) and surface current can significantly modify the wind stress and Ekman pumping. Relative importance of the eddy–wind interactions via SST and surface current in the CCS is examined using a high-resolution (7 km) regional coupled model with a novel coupling approach to isolate the small-scale air–sea coupling by SST and surface current. Results show that when the eddy-induced surface current is allowed to modify the wind stress, the spatially averaged surface eddy kinetic energy (EKE) is reduced by 42%, and this is primarily due to enhanced surface eddy drag and reduced wind energy transfer. In contrast, the eddy-induced SST–wind coupling has no significant impact on the EKE. Furthermore, eddy-induced SST and surface current modify the Ekman pumping via their crosswind SST gradient and surface vorticity gradient, respectively. The resultant magnitudes of the Ekman pumping velocity are comparable, but the implied feedback effects on the eddy statistics are different. The surface current-induced Ekman pumping mainly attenuates the amplitude of cyclonic and anticyclonic eddies, acting to reduce the eddy activity, while the SST-induced Ekman pumping primarily affects the propagation. Time mean–rectified change in SST is determined by the altered offshore temperature advection by the mean and eddy currents, but the magnitude of the mean SST change is greater with the eddy-induced current effect. The demonstrated remarkably strong dynamical response in the CCS system to the eddy-induced current–wind coupling indicates that eddy-induced current should play an important role in the regional coupled ocean–atmosphere system.We thank NSF for support under GrantsOCE-0960770,OCE-1419235, andOCE-1419306. HS is grateful for the WHOI internal support from the Andrew W. Mellon Foundation Awards for Innovative Research and the additional support from the ONR We thank NSF for support under GrantsOCE-0960770,OCE-1419235, andOCE-1419306. HS is grateful for the WHOI internal support from the Andrew W. Mellon Foundation Awards for Innovative Research and the additional support from the ONR2016-05-3