Upper ocean response to a hurricane

Also published as: Journal of Physical Oceanography 11 (1981): 153-175The upper ocean response to a moving hurricane is studied using historical air-sea data and a three-dimensional numerical ocean model. Sea surface temperature (SST) response is emphasized. The model has a surface mixed-layer (ML) that entrains according to a velocity dependent parameterization, and two lower layers that simulate the response in the thermocline. The passage of Hurricane Eloise (1975) over buoy EB-10 is simulated in detail. SST decreased 2°C as Eloise passed directly over EB-10 at 8.5 m s-1. Model results indicate that entrainment caused 85% of the irreversible heat flux into the ML; air-sea heat exchange accounted for the remainder. The maximum SST response was predicted to be -3°C and to occur 60 km to the right of the hurricane track. This is consistent with the well-documented rightward bias in the SST response to rapidly moving hurricanes. The rightward bias occurs in the model solution because the hurricane wind-stress vector turns clockwise with time on the right side of the track and is roughly resonant with the ML velocity. High ML velocities cause strong entrainment and thus a strong SST response. Model comparisons with EB-10 data suggest that a wind-speed-dependent drag coefficient similar to Garratt's (1977) is appropriate for hurricane conditions. A constant drag coefficient 1.5 x w-s underpredicts the amplitude of upwelling and the SST response by -40%. Numerical experiments show that the response has a lively dependence on a number of air-sea parameters. Intense, slowly moving hurricanes cause the largest response. The SST response is largest where cold water is near the sea surface, i.e., where the initial ML is thin and the upper thermocline temperature gradient is sharp. Nonlocal processes are important to some aspects of the upper ocean response. Upwelling significantly enhances entrainment under slowly moving hurricanes (≤4 m s-1) and reduces the rightward bias of the SST response. Horizontal advection dominates the pointwise ML heat balance during the several-day period following a hurricane passage. Pressure gradients set up by the upwelling do not play an important role in the entrainment process, but are an effective mechanism for dispersing energy from the ML over a 5-10 day time scale.Prepared for the Office of Naval Research under Contract N00014-76-C-0226

Metrics of hurricane-ocean interaction : vertically-integrated or vertically-averaged ocean temperature?

© 2009 The Author. This is an open-access article distributed under the terms of the Creative Commons Attribution 3.0 License. The definitive version was published in Ocean Science 5 (2009): 351-368, doi:10.5194/os-5-351-2009The ocean thermal field is often represented in hurricane-ocean interaction by a metric termed upper Ocean Heat Content (OHC), the vertical integral of ocean temperature in excess of 26°C. High values of OHC have proven useful for identifying ocean regions that are especially favorable for hurricane intensification. Nevertheless, it is argued here that a more direct and robust metric of the ocean thermal field may be afforded by a vertical average of temperature. In the simplest version, dubbed T100, the averaging is from the surface to 100 m, a typical depth of vertical mixing by a category 3 hurricane. OHC and T100 are well correlated over the deep open ocean in the high range of OHC, ≥75 kJ cm−2. They are poorly correlated in the low range of OHC, ≤50 kJ cm−2, in part because OHC is degenerate when evaluated on cool ocean regions, ≤26°C. OHC and T100 can be qualitatively different also over shallow continental shelves: OHC will generally indicate comparatively low values regardless of the ocean temperature, while T100 will take on high values over a shelf that is warm and upwelling neutral or negative. In so far as the ocean thermal field alone is concerned, these warm, shallow continental shelves would appear to be as favorable for hurricane intensification as are warm, deep ocean regions.This research was supported by the US Office of Naval Research through the project Impact of Typhoons on the Western North Pacific (ITOP)

Water-mass formation and potential vorticity balance in an abyssal ocean circulation

Our goal is to develop some understanding and intuition regarding abyssal ocean circulations. To do this we investigate highly idealized, source/sink-driven flows computed by a single layer, numerical ocean model forced by a prescribed source or sink. The interior circulation is always found to be very slow and Stommel-Arons like. On the other hand, the intense boundary currents may vary considerably from case to case, depending largely upon the potential vorticity (PV) associated with the source or sink. If the source is imposed by downwelling along a northern boundary, then the associated PV flux is zero, and the resulting steady circulation can induce no net frictional torque. The result is a rather complex pattern of boundary layer flow that includes a strong recirculation along the northern boundary. If the same mass flux is injected as a laminar, horizontal inflow, then the associated PV flux is significant, and must be balanced in steady state by frictional torque. The result is a unidirectional boundary layer flow away from the source. Other experiments elucidate the effect of vortex stretching on topography. For example, a horizontal outflow over shallowing topography induces a net cyclonic frictional torque in the boundary layer circulation of the basin. An understanding of the steady state PV balance thus appears to confer some insight into the form of boundary layer flows in these abyssal circulations

Observations of a barotropic planetary wave in the western North Atlantic

SOFAR float observations from 1300 m depth are used to describe a major feature of the large-scale, subthermocline velocity field observed in the western North Atlantic (31 N, 70W), during the 1978 POLYMODE Local Dynamics Experiment (LDE). The two-month-long intensive phase of the LDE was dominated by a highly polarized, oscillatory flow which had many of the characteristics of a barotropic planetary wave...

Marginal sea overflows and the upper ocean interaction

Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 387-403, doi:10.1175/2008JPO3934.1.Marginal sea overflows and the overlying upper ocean are coupled in the vertical by two distinct mechanisms—by an interfacial mass flux from the upper ocean to the overflow layer that accompanies entrainment and by a divergent eddy flux associated with baroclinic instability. Because both mechanisms tend to be localized in space, the resulting upper ocean circulation can be characterized as a β plume for which the relevant background potential vorticity is set by the slope of the topography, that is, a topographic β plume. The entrainment-driven topographic β plume consists of a single gyre that is aligned along isobaths. The circulation is cyclonic within the upper ocean (water columns are stretched). The transport within one branch of the topographic β plume may exceed the entrainment flux by a factor of 2 or more. Overflows are likely to be baroclinically unstable, especially near the strait. This creates eddy variability in both the upper ocean and overflow layers and a flux of momentum and energy in the vertical. In the time mean, the eddies accompanying baroclinic instability set up a double-gyre circulation in the upper ocean, an eddy-driven topographic β plume. In regions where baroclinic instability is growing, the momentum flux from the overflow into the upper ocean acts as a drag on the overflow and causes the overflow to descend the slope at a steeper angle than what would arise from bottom friction alone. Numerical model experiments suggest that the Faroe Bank Channel overflow should be the most prominent example of an eddy-driven topographic β plume and that the resulting upper-layer transport should be comparable to that of the overflow. The overflow-layer eddies that accompany baroclinic instability are analogous to those observed in moored array data. In contrast, the upper layer of the Mediterranean overflow is likely to be dominated more by an entrainment-driven topographic β plume. The difference arises because entrainment occurs at a much shallower location for the Mediterranean case and the background potential vorticity gradient of the upper ocean is much larger.SK’s support during the time of his Ph.D. research in the MIT/WHOI Joint Program was provided by the National Science Foundation through Grant OCE04-24741. JP and JY have also received support from the Climate Process Team on Gravity Current Entrainment, NSF Grant OCE-0611530. JY has also been supported by NSF Grant OCE-0351055

Highly resolved observations and simulations of the ocean response to a hurricane

Author Posting. © American Geophysical Union, 2007. 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 34 (2007): L13604, doi:10.1029/2007GL029679.An autonomous, profiling float called EM-APEX was developed to provide a quantitative and comprehensive description of the ocean side of hurricane-ocean interaction. EM-APEX measures temperature, salinity and pressure to CTD quality and relative horizontal velocity with an electric field sensor. Three prototype floats were air-deployed into the upper ocean ahead of Hurricane Frances (2004). All worked properly and returned a highly resolved description of the upper ocean response to a category 4 hurricane. At a float launched 55 km to the right of the track, the hurricane generated large amplitude, inertially rotating velocity in the upper 120 m of the water column. Coincident with the hurricane passage there was intense vertical mixing that cooled the near surface layer by about 2.2°C. We find consistent model simulations of this event provided the wind stress is computed from the observed winds using a high wind-speed saturated drag coefficient.The development of the EM-APEX float system was supported by the Office of Naval Research through SBIR contract N00014-03-C-0242 to Webb Research Corporation and with a subcontract to APL-UW

Satellite-derived ocean thermal structure for the North Atlantic hurricane season

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 Monthly Weather Review 144 (2016): 877-896, doi:10.1175/MWR-D-15-0275.1.This paper describes a new model (method) called Satellite-derived North Atlantic Profiles (SNAP) that seeks to provide a high-resolution, near-real-time ocean thermal field to aid tropical cyclone (TC) forecasting. Using about 139 000 observed temperature profiles, a spatially dependent regression model is developed for the North Atlantic Ocean during hurricane season. A new step introduced in this work is that the daily mixed layer depth is derived from the output of a one-dimensional Price–Weller–Pinkel ocean mixed layer model with time-dependent surface forcing. The accuracy of SNAP is assessed by comparison to 19 076 independent Argo profiles from the hurricane seasons of 2011 and 2013. The rms differences of the SNAP-estimated isotherm depths are found to be 10–25 m for upper thermocline isotherms (29°–19°C), 35–55 m for middle isotherms (18°–7°C), and 60–100 m for lower isotherms (6°–4°C). The primary error sources include uncertainty of sea surface height anomaly (SSHA), high-frequency fluctuations of isotherm depths, salinity effects, and the barotropic component of SSHA. These account for roughly 29%, 25%, 19%, and 10% of the estimation error, respectively. The rms differences of TC-related ocean parameters, upper-ocean heat content, and averaged temperature of the upper 100 m, are ~10 kJ cm−2 and ~0.8°C, respectively, over the North Atlantic basin. These errors are typical also of the open ocean underlying the majority of TC tracks. Errors are somewhat larger over regions of greatest mesoscale variability (i.e., the Gulf Stream and the Loop Current within the Gulf of Mexico).IFP is supported by Grants NSC 101-2628-M-002-001-MY4 and MOST 103-2111-M-002 -002 -MY3. JFP and SRJ were supported by the U.S. Office of Naval Research under the project “Impact of Typhoons on the North Pacific, ITOP.”2016-06-0

SOFAR float Mediterranean outflow experiment : summary and data from 1986-88

In October, 1984, the Woods Hole Oceanographic Institution SOFAR float group began a three and a half year field program to measure the velocity field of the Mediterranean water in the eastern North Atlantic. The principal scientific goal was to learn how the Mediterranean salt tongue is produced by the general circulation and the eddy diffusion of the Canary Basin. Thirty-two floats were launched at depths near 1100 m: 14 in a cluster centered on 32°N, 24°W, with nearest neighbors at 20 km spacing, 10 at much wider spacing to explore regional variations of first order flow statistics, and 8 in three different Meddies (Mediterranean water eddies) in collaboration with investigators from Scripps Institution of Oceanography and the University of Rhode Island. The floats were launched in 1984 and 1985, and tracked with U.S. and French ALSs (moored listening stations) from October 1984 to June 1988. This report includes a summary of the whole three and a half year experiment, the final year and a half of data processed from the third ALS setting (October 1986-June 1988), and the first deep sea test of Bobber EB014 in the eastern subtropical North Atlantic (May 1986-May 1988). Approximately 60 years of float trajectories were produced during the three and a half years of the experiment.Funding was provided by the National Science Foundation through Grant Nos. OCE 82-14066, OCE 85-17375, OCE 86-00055, OCE 88-22826

Ocean response to a hurricane, part II : data tabulations and numerical modeling

Field observations of the ocean's forced stage response to three hurricanes, Norbert (1984), Josephine (1984) and Gloria (1985), are analyzed and presented in a storm-centered coordinate system. All three hurricanes had a non-dimensional speed of O(1) and produced a strongly rightward biased response of the ocean surface mixed layer (SML) transport and current. The maximum layer-averaged SML currents varried from 0.8 m S-1 in response to Josephine, which was a fairly weak hurricane, to 1.7 m S.l in response to Gloria, which was much stronger. In these two cases the current amplitude is set primarly by the strength of the wind stress and its efficiency of coupling with the SML current, and the depth of vertical mixing of the SML. The Norbert case (SML Burger number ≈ 1/2) was also affected by significant pressure-coupling with the thermocline that caused appreciable upwellng by inertial pumping and strong thermocline-depth currents, up to 0.3 m S-l, under the trailing edge of Norbert. The observed SML current has a vertical shear in the direction of the local wind of up to 0.01 S-l. This vertical shear causes the surface current to be larger than the layer-averaged SML current described above by typically 0.2 m S.l.Funding was provided by the Office of Naval Research under grant No. N00014-89-J-I053

SOFAR float Mediterranean outflow experiment data from the second year, 1985-86

In October, 1984, the Woods Hole Oceanographic Institution SOFAR float group began a three-year-long field program to observe the low frequency currents in the Canary Basin. The principal scientific goal was to learn how advection and diffusion by these currents determine the shape and amplitude of the Mediterranean salt tongue. Fourteen floats were launched at a depth of 1100 min a cluster centered on 32°N, 24°W, and seven other floats were launched incoherently along a north/south line from 24°N to 37°N. At the same time investigators from Scripps Institution of Oceanography and the University of Rhode Island used four other SOFAR floats to tag a Meddy, a submesoscale lens of Mediterranean water. In October, 1985, seven additional floats were launched, four in three different Meddies, one of which was tracked during year 1. This report describes the second year of the floats launched in 1984 and the first year of the ones launched in 1985. Approximately 41 years of float trajectories were produced during the first two years of the experiment. One of the striking accomplishments is the successful tracking of one Meddy over two full years plus the tracking of two other Meddies during the second year.Funding was provided by the National Science Foundation under grant Numbers OCE 82-14066 and OCE 86-00055