Ocean Feedback Research Articles

Diagnosing Ocean Feedbacks to the BSISO: SST‐Modulated Surface Fluxes and the Moist Static Energy Budget

AbstractThe oceanic feedback to the atmospheric boreal summer intraseasonal oscillation (BSISO) is examined by diagnosing the sea surface temperature (SST) modification of surface fluxes and moist static energy on intraseasonal scales. SST variability affects intraseasonal surface latent heat (LH) and sensible heat (SH) fluxes, through its influence on air‐sea moisture and temperature gradients (∆q and ∆T, respectively). According to bulk formula decomposition, LH is mainly determined by wind‐driven flux perturbations, while SH is more sensitive to thermodynamic flux perturbations. SST fluctuations tend to increase the thermodynamic flux perturbations over active BSISO regions, but this is largely offset by the wind‐driven flux perturbations. Enhanced surface fluxes induced by intraseasonal SST anomalies are located ahead (north) of the convective center over both the Indian Ocean and the western Pacific, favoring BSISO northward propagation. Analysis of budgets of column‐integrated moist static energy (⟨m⟩) and its time rate of change (∂⟨m⟩/∂t) shows that SST‐modulated surface fluxes can influence the development and propagation of the BSISO, respectively. LH and SH variability induced by intraseasonal SSTs maintain 1–2% of ⟨m⟩ day−1 over the equatorial western Indian Ocean, Arabian Sea, and Bay of Bengal but damp about 1% of ⟨m⟩ day−1 over the western North Pacific. The contribution of intraseasonal SST variability to ∂⟨m⟩/∂t can reach 12–20% over active BSISO regions. These results suggest that SST variability is conducive, but perhaps not essential, for the propagation of convection during the BSISO life cycle.

Thermal Effects of Magma Intrusion on Ocean Temperatures Through Ice Ages, with Particular Reference to Interglacial Events

Recent observations are used to crudely quantify and scale the direct heating of the global ocean surface by submarine volcanism. These suggest a current submarine magma budget that is substantially greater than the prevailing consensus. The current ocean heating effect is estimated at c. 0.7W/m2. Interglacial events are geologically sudden and predictably cyclical, overtly caused by the combined effect of Milankovitch cycles. Several possible causes for interglacial warmings arising from these cycles have been proposed, and rejected as inadequate. These are summarily reviewed. It is concluded, in the absence of other capable causes, and in particular given the evidence of continued ocean warming throughout the Dryas cooling events, that ice age interglacial events are probably caused by a sustained increase in the rate of submarine volcanic heating of the oceans. The primary cause of increasing emissions is suggested as the rising levels of solid gravitational tides caused by the combined orbital forcing of all three main Milankovitch cycles. These tides are at their largest during interglacial warmings. How the dominant climate control system of oceanic feedback responds to this internal perturbation change is also discussed. In particular how the upper equilibrium temperature of an interglacial period is imposed by this control.

The Influence of Oceanic Stochastic Forcing on the Atmospheric Response to Midlatitude North Pacific SST Anomalies

AbstractThe stochastic sea surface temperature (SST) anomalies are always present because of the internal oceanic variability due to small‐scale SST variability and based on a set of ensemble experiments forced by Pacific Decadal Oscillation‐related midlatitude North Pacific SST anomalies with and without oceanic stochastic forcing, their influences on atmospheric variability in coarse‐resolution models are investigated in this study. Comparisons of experiment results show that oceanic stochastic forcing can improve the simulated atmospheric response by generating more organized atmospheric transient eddies through the increases of both thermal activity and atmospheric baroclinicity, similar to the high‐resolution observations and simulations. Dynamic diagnostics further reveal the dominant contributions of corresponding transient eddy vorticity forcing to the significantly enhanced westerly wind with equivalent barotropic structure. It suggests that the oceanic stochastic forcing is important and nonnegligible for the understanding and theory of oceanic feedback to the atmosphere in the midlatitudes.

Difference between the North Atlantic and Pacific meridional overturning circulation in response to the uplift of the Tibetan Plateau

Abstract. The role of the Tibetan Plateau (TP) in maintaining the large-scale overturning circulation in the Atlantic and Pacific is investigated using a coupled atmosphere–ocean model. For the present day with a realistic topography, model simulation shows a strong Atlantic meridional overturning circulation (AMOC) but a near absence of the Pacific meridional overturning circulation (PMOC), which are in good agreement with the present observations. In contrast, the simulation without the TP depicts a collapsed AMOC and a strong PMOC that dominates deep-water formation. The switch in deep-water formation between the two basins results from changes in the large-scale atmospheric circulation and atmosphere–ocean feedback over the Atlantic and Pacific. The intensified westerly winds and increased freshwater flux over the North Atlantic cause an initial slowdown of the AMOC, while the weakened East Asian monsoon circulation and associated decreased freshwater flux over the North Pacific give rise to the initial intensification of the PMOC. The further decreased heat flux and the associated increase in sea-ice fraction promote the final AMOC collapse over the Atlantic, while the further increased heat flux leads to the final PMOC establishment over the Pacific. Although the simulations were performed in a cold world, it still importantly implicates that the uplift of the TP alone could have been a potential driver for the reorganization of PMOC–AMOC between the late Eocene and early Oligocene.

On the Use of Ocean Dynamic Temperature for Hurricane Intensity Forecasting

Abstract Sea surface temperature (SST) and tropical cyclone heat potential (TCHP) are metrics used to incorporate the ocean’s influence on hurricane intensification into the National Hurricane Center’s Statistical Hurricane Intensity Prediction Scheme (SHIPS). While both SST and TCHP serve as useful measures of the upper-ocean heat content, they do not accurately represent ocean stratification effects. Here, it is shown that replacing SST within the SHIPS framework with a dynamic temperature Tdy, which accounts for the oceanic negative feedback to the hurricane’s intensity arising from storm-induced vertical mixing and sea surface cooling, improves the model performance. While the model with SST and TCHP explains about 41% of the variance in 36-h intensity changes, replacing SST with Tdy increases the variance explained to nearly 44%. These results suggest that representation of the oceanic feedback, even through relatively simple formulations such as Tdy, may improve the performance of statistical hurricane intensity prediction models such as SHIPS.

Changing response of the North Atlantic/European winter climate to the 11 year solar cycle

Recent studies have presented conflicting results regarding the 11 year solar cycle (SC) influences on winter climate over the North Atlantic/European region. Analyses of only the most recent decades suggest a synchronized North Atlantic Oscillation (NAO)-like response pattern to the SC. Analyses of long-term climate data sets dating back to the late 19th century, however, suggest a mean sea level pressure (mslp) response that lags the SC by 2–4 years in the southern node of the NAO (i.e. Azores region). To understand the conflicting nature and cause of these time dependencies in the SC surface response, the present study employs a lead/lag multi-linear regression technique with a sliding window of 44 years over the period 1751–2016. Results confirm previous analyses, in which the average response for the whole time period features a statistically significant 2–4 year lagged mslp response centered over the Azores region. Overall, the lagged nature of Azores mslp response is generally consistent in time. Stronger and statistically significant SC signals tend to appear in the periods when the SC forcing amplitudes are relatively larger. Individual month analysis indicates the consistent lagged response in December–January–February average arises primarily from early winter months (i.e. December and January), which has been associated with ocean feedback processes that involve reinforcement by anomalies from the previous winter. Additional analysis suggests that the synchronous NAO-like response in recent decades arises primarily from late winter (February), possibly reflecting a result of strong internal noise.

The 30–50-Day Intraseasonal Oscillation of SST and Precipitation in the South Tropical Indian Ocean

The Sea Surface Temperature (SST) in the South Tropical Indian Ocean (STIO) displays significant intraseasonal oscillation (ISO) in two regions. A striking 30–50-day ISO found over the east of thermocline ridge (Region A, 80–90° E, 6–12° S), as identified by the Empirical Mode Decomposition (EMD) method, is distinguished from the SST signature over the thermocline ridge (Region B, 52.5–65° E, 6–13° S). The 30–50-day ISO of SST in the Region A is active in March–May (MAM) and suppressed in September–November (SON). Meanwhile, a 30–50-day ISO of precipitation correlates with the SST over the Region A. SST leads precipitation by 10 days, implying a pronounced ocean–atmosphere interaction at the intraseasonal timescale, especially the oceanic feedback to the atmosphere during Madden–Julian Oscillation (MJO) events. Analysis on mechanism of the ISO manifests heat fluxes are critical to the development of the intraseasonal SST variability. The local thermocline in Region A, as the shallowest in MAM and the thickest in SON, is likely to modulate the strength of ISO and contribute to its sustainability. It suggests that thermocline plays a more important role in Region A than in Region B, leading to the difference between the two regions.

Slow ocean response and the 1.5 and 2°C warming targets

Recent studies reveal that to achieve the 1.5 or 2°C warming target proposed in the 2015 Paris Agreement, greenhouse gas (GHGs) concentration is required to decrease after reaching a peak around the middle of this century, distinct from most scenarios without configuring GHGs concentration decrease. Climate response to external radiative forcing under two different scenarios is then investigated in this study by simulations from 8 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5). Based on an idealized two-box (ocean mixed layer and the deeper layer) model, global-mean surface temperature (GMST) change is further divided into components due to the fast response of the ocean mixed-layer (fast contribution) and the deeper ocean slow evolution (slow contribution), respectively, to explain the GMST trajectory in CMIP5 simulations. The response timescale is 3–5 a for the fast contribution and decades to centuries for the slow contribution. Under the scenario with radiative forcing (RF) firstly increases and then levels off towards a constant after 2070 (RCP4.5), GMST rises rapidly at the first stage and slowly after RF levels off. In comparison, under the scenario with RF firstly increases and then decreases after 2045 (RCP2.6), GMST rises rapidly to a peak below 2°C and then declines at a very slow rate, following a nearly flat trajectory between 2050 and 2100. Corresponding to the GMST change, the deeper ocean warms faster than the upper ocean after the RF levels off in RCP4.5 while decreasing in RCP2.6. In fact, the GMST trajectory depends on the ratio between the fast and slow contributions from the ocean at different stages under different RF pathways. During the RF increase period, GMST mainly follows the RF pathway because the fast contribution dominates. After RF holds constant or decreases, slow contribution due to the deeper ocean warming increases persistently, causing the GMST change to deviate from the RF pathway when RF holds constant or decreases. As a result, contribution from the deeper ocean slow warming to the magnitude and trajectory of GMST change cannot be neglected in scenarios for the 1.5 and 2°C low warming target. For RCP2.6, GMST increase is 1.83°C in 2100, the time when RF is in decrease. In contrast, for RCP4.5, GMST reaches the same increase in 2033, the time when RF is still increasing. Despite the same GMST increase, climate responses in 2033 under RCP4.5 are distinct from that in 2100 under RCP2.6. For example, ocean stratification weakens in 50–300 m in 2100 under RCP2.6 but strengthens at all depths in 2033 under RCP4.5 due to the persistent heat accumulation in the deeper ocean. Global-mean thermosteric sea-level rise due to thermal expansion of seawater warming is much higher in RCP2.6 (16.39 cm) than that in RCP4.5 (9.01 cm). Moreover, surface warming pattern displays substantial structural differences between RCP2.6 and RCP4.5, especially over regions with strong ocean dynamics and hence large deeper ocean feedback to surface warming. Specifically, surface warming is notably larger over the North Pacific Ocean and Southern Ocean in 2100 under RCP2.6 than that in 2033 under RCP4.5, while the opposite is true over the subtropical and mid-latitude land regions in the Northern Hemisphere. Under most CMIP5 scenarios without a RF decrease, the 1.5 or 2°C GMST warming target is projected to reach much earlier than 2100, which are used in some studies to represent the climate responses for the Paris targets at the end of this century. These results underestimate the effect of the deeper ocean slow contribution and are biased in projecting climate response for the Paris targets.

Role of Air–Sea Interaction in the 30–60-Day Boreal Summer Intraseasonal Oscillation over the Western North Pacific

AbstractThis study investigates the role of air–sea interaction in the 30–60-day boreal summer intraseasonal oscillation (BSISO) over the western North Pacific with daily outgoing longwave radiation (OLR), CFSR, and OAFlux datasets for 1985–2009. The BSISO events are identified with the first principal component of 30–60-day bandpass filtered OLR anomalies. Composite analysis of these events reveals that during the northward migration of BSISO, the convection can interact with underlying sea surface temperature (SST). A near-quadrature phase relationship exists between the convection and SST anomalies. An active (a suppressed) convection tends to induce a cold (warm) underlying SST anomaly by reducing (increasing) downward solar radiation but a warm SST anomaly in its northern (southern) portion by reducing near-surface wind and upward latent and sensible heat fluxes, resulting in a 10-day delayed maximized warm SST anomaly ahead of the active convection. In turn, this warm SST anomaly tends to increase upward surface sensible and latent heat fluxes via amplifying sea–air temperature and humidity differences. This oceanic feedback acts to heat, moisten, and destabilize the low-level atmosphere, favoring the trigger of shallow convection, which can further develop into deep convection. The maximum warm SST anomaly lies in the southern (northern) portion of the convectively suppressed (enhanced) area, which weakens the anomalous descending motion in the southern portion of convectively suppressed area and preconditions the boundary layer to promote convection development in the northern portion of convectively enhanced area. Such a spatial and temporal phase relationship between the convection and SST anomalies suggest that air–sea interaction can play a delayed negative feedback role in the BSISO cycle and provide an alternative mechanism responsible for its northward propagation.

Evaluating AMIP Skill in Simulating Interannual Variability over the Indo–Western Pacific

AbstractLocal correlation between sea surface temperature (SST) and rainfall is weak or even negative in summer over the Indo–western Pacific warm pool, a fact often taken as indicative of weak ocean feedback on the atmosphere. An Atmospheric Model Intercomparison Project (AMIP) simulation forced by monthly varying SSTs derived from a parallel coupled general circulation model (CGCM) run is used to evaluate AMIP skills in simulating interannual variability of rainfall. Local correlation of rainfall variability between AMIP and CGCM simulations is used as a direct metric of AMIP skill. This “perfect model” approach sidesteps the issue of model biases that complicates the traditional skill metric based on the correlation between AMIP and observations. Despite weak local SST–rainfall correlation, the AMIP–CGCM rainfall correlation exceeds a 95% significance level over most of the Indo–western Pacific warm pool, indicating the importance of remote (e.g., El Niño in the equatorial Pacific) rather than local SST forcing. Indeed, the AMIP successfully reproduces large-scale modes of rainfall variability over the Indo–western Pacific warm pool. Compared to the northwest Pacific east of the Philippines, the AMIP–CGCM rainfall correlation is low from the Bay of Bengal through the South China Sea, limited by internal variability of the atmosphere that is damped in CGCM by negative feedback from the ocean. Implications for evaluating AMIP skill in simulating observations are discussed.

Pan-Antarctic analysis aggregating spatial estimates of Ad\xe9lie penguin abundance reveals robust dynamics despite stochastic noise

Colonially-breeding seabirds have long served as indicator species for the health of the oceans on which they depend. Abundance and breeding data are repeatedly collected at fixed study sites in the hopes that changes in abundance and productivity may be useful for adaptive management of marine resources, but their suitability for this purpose is often unknown. To address this, we fit a Bayesian population dynamics model that includes process and observation error to all known Adélie penguin abundance data (1982–2015) in the Antarctic, covering >95% of their population globally. We find that process error exceeds observation error in this system, and that continent-wide “year effects” strongly influence population growth rates. Our findings have important implications for the use of Adélie penguins in Southern Ocean feedback management, and suggest that aggregating abundance across space provides the fastest reliable signal of true population change for species whose dynamics are driven by stochastic processes.

Improved simulation of Antarctic sea ice due to the radiative effects of falling snow

Southern Ocean sea-ice cover exerts critical control on local albedo and Antarctic precipitation, but simulated Antarctic sea-ice concentration commonly disagrees with observations. Here we show that the radiative effects of precipitating ice (falling snow) contribute substantially to this discrepancy. Many models exclude these radiative effects, so they underestimate both shortwave albedo and downward longwave radiation. Using two simulations with the climate model CESM1, we show that including falling-snow radiative effects improves the simulations relative to cloud properties from CloudSat-CALIPSO, radiation from CERES-EBAF and sea-ice concentration from passive microwave sensors. From 50–70°S, the simulated sea-ice-area bias is reduced by 2.12 × 106 km2 (55%) in winter and by 1.17 × 106 km2 (39%) in summer, mainly because increased wintertime longwave heating restricts sea-ice growth and so reduces summer albedo. Improved Antarctic sea-ice simulations will increase confidence in projected Antarctic sea level contributions and changes in global warming driven by long-term changes in Southern Ocean feedbacks.

An analytical model of maximum potential intensity for tropical cyclones incorporating the effect of ocean mixing

AbstractAn analytical model of maximum potential intensity (PI) for tropical cyclones (TCs) incorporating wind‐induced ocean cooling is developed on the basis of Emanuel's PI theory. The model consists of a one‐dimensional ocean and an axisymmetric TC vortex that is translating at a constant speed. The model advances upon previous approaches by accounting for TC size and shape. The PI is determined at the radius of maximum winds by radially integrating the effect of ocean mixing using a specified wind profile. The resulting analytic ocean cooling agrees well with numerical solutions, and the new PI model better captures maximum intensities of observed TCs compared with Emanuel's original PI. It is also shown that the degree of cooling (i.e., ocean's feedback to atmosphere) can be quantified by a dimensionless parameter, representing the ratio of atmospheric forcing to ocean stability.

Seasonal Modulations of El Niño–Related Atmospheric Variability: Indo–Western Pacific Ocean Feedback

The spatial structure of atmospheric anomalies associated with El Niño–Southern Oscillation varies with season because of the seasonal variations in sea surface temperature (SST) anomaly pattern and in the climatological basic state. The latter effect is demonstrated using an atmospheric model forced with a time-invariant pattern of El Niño warming over the equatorial Pacific. The seasonal modulation is most pronounced over the north Indian Ocean to northwest Pacific where the monsoonal winds vary from northeasterly in winter to southwesterly in summer. Specifically, the constant El Niño run captures the abrupt transition from a summer cyclonic to winter anticyclonic anomalous circulation over the northwest Pacific, in support of the combination mode idea that emphasizes nonlinear interactions of equatorial Pacific SST forcing and the climatological seasonal cycle. In post–El Niño summers when equatorial Pacific warming has dissipated, SST anomalies over the Indo–northwest Pacific Oceans dominate and anchor the coherent persisting anomalous anticyclonic circulation. A conceptual model is presented that incorporates the combination mode in the existing framework of regional Indo–western Pacific Ocean coupling.

Tropical Cyclone Interaction with the Ocean: The Role of High-Frequency (Subdaily) Coupled Processes

Through tropical cyclone (TC) activity the ocean and the atmosphere exchange a large amount of energy. In this work possible improvements introduced by a higher coupling frequency are tested between the two components of a climate model in the representation of TC intensity and TC–ocean feedbacks. The analysis is based on the new Centro Euro-Mediterraneo per I Cambiamenti Climatici Climate Model (CMCC-CM2-VHR), capable of representing realistic TCs up to category-5 storms. A significant role of the negative sea surface temperature (SST) feedback, leading to a weakening of the cyclone intensity, is made apparent by the improved representation of high-frequency coupled processes. The first part of this study demonstrates that a more realistic representation of strong TC count is obtained by coupling atmosphere and ocean components at hourly instead of daily frequency. Coherently, the positive bias of the annually averaged power dissipation index associated with TCs is reduced by one order of magnitude when coupling at the hourly frequency, compared to both forced mode and daily coupling frequency results. The second part of this work shows a case study (a modeled category-5 typhoon) analysis to verify the impact of a more realistic representation of the high-frequency coupling in representing the TC effect on the ocean; the theoretical subsurface warming induced by TCs is well represented when coupling the two components at the higher frequency. This work demonstrates that an increased horizontal resolution of model components is not sufficient to ensure a realistic representation of intense and fast-moving systems, such as tropical and extratropical cyclones, but a concurrent increase in coupling frequency is required.

Ocean feedback to pulses of the Madden-Julian Oscillation in the equatorial Indian Ocean.

Dynamical understanding of the Madden–Julian Oscillation (MJO) has been elusive, and predictive capabilities therefore limited. New measurements of the ocean's response to the intense surface winds and cooling by two successive MJO pulses, separated by several weeks, show persistent ocean currents and subsurface mixing after pulse passage, thereby reducing ocean heat energy available for later pulses by an amount significantly greater than via atmospheric surface cooling alone. This suggests that thermal mixing in the upper ocean from a particular pulse might affect the amplitude of the following pulse. Here we test this hypothesis by comparing 18 pulse pairs, each separated by <55 days, measured over a 33-year period. We find a significant tendency for weak (strong) pulses, associated with low (high) cooling rates, to be followed by stronger (weaker) pulses. We therefore propose that the ocean introduces a memory effect into the MJO, whereby each event is governed in part by the previous event.

High resolution projections for the western Iberian coastal low level jet in a changing climate

The Iberian coastal low-level jet (CLLJ) is one of the less studied boundary layer wind jet features in the Eastern Boundary Currents Systems (EBCS). These regions are amongst the most productive ocean ecosystems, where the atmosphere–land–ocean feedbacks, which include marine boundary layer clouds, coastal jets, upwelling and inland soil temperature and moisture, play an important role in defining the regional climate along the sub-tropical mid-latitude western coastal areas. Recently, the present climate western Iberian CLLJ properties were extensively described using a high resolution regional climate hindcast simulation. A summer maximum frequency of occurrence above 30 % was found, with mean maximum wind speeds around 15 ms−1, between 300 and 400 m heights (at the jet core). Since the 1990s the climate change impact on the EBCS is being studied, nevertheless some lack of consensus still persists regarding the evolution of upwelling and other components of the climate system in these areas. However, recently some authors have shown that changes are to be expected concerning the timing, intensity and spatial homogeneity of coastal upwelling, in response to future warming, especially at higher latitudes, namely in Iberia and Canaries. In this study, the first climate change assessment study regarding the Western Iberian CLLJ, using a high resolution (9 km) regional climate simulation, is presented. The properties of this CLLJ are studied and compared using two 30 years simulations: one historical simulation for the 1971–2000 period, and another simulation for future climate, in agreement with the RCP8.5 scenario, for the 2071–2100 period. Robust and consistent changes are found: (1) the hourly frequency of occurrence of the CLLJ is expected to increase in summer along the western Iberian coast, from mean maximum values of around 35 % to approximately 50 %; (2) the relative increase of the CLLJ frequency of occurrence is higher in the north off western Iberia; (3) the occurrence of the CLLJ covers larger areas both latitudinal and longitudinal; (4) the CLLJ season is lengthened extending to May and September; and, (5) there are shifts for higher occurrences of higher wind speeds and for the jet core to occur at higher heights.

How can we understand the global distribution of the solar cycle signal on the Earth's surface?

Abstract. To understand solar cycle signals on the Earth's surface and identify the physical mechanisms responsible, surface temperature variations from observations as well as climate model data are analysed to characterize their spatial structure. The solar signal in the annual mean surface temperature is characterized by (i) mid-latitude warming and (ii) no overall tropical warming. The mid-latitude warming during solar maxima in both hemispheres is associated with a downward penetration of zonal mean zonal wind anomalies from the upper stratosphere during late winter. During the Northern Hemisphere winter this is manifested by a modulation of the polar-night jet, whereas in the Southern Hemisphere, the upper stratospheric subtropical jet plays the major role. Warming signals are particularly apparent over the Eurasian continent and ocean frontal zones, including a previously reported lagged response over the North Atlantic. In the tropics, local warming occurs over the Indian and central Pacific oceans during high solar activity. However, this warming is counterbalanced by cooling over the cold tongue sectors in the southeastern Pacific and the South Atlantic, and results in a very weak zonally averaged tropical mean signal. The cooling in the ocean basins is associated with stronger cross-equatorial winds resulting from a northward shift of the ascending branch of the Hadley circulation during solar maxima. To understand the complex processes involved in the solar signal transfer, results of an idealized middle atmosphere–ocean coupled model experiment on the impact of stratospheric zonal wind changes are compared with solar signals in observations. Model integration of 100 years of strong or weak stratospheric westerly jet condition in winter may exaggerate long-term ocean feedback. However, the role of ocean in the solar influence on the Earth's surface can be better seen. Although the momentum forcing differs from that of solar radiative forcing, the model results suggest that stratospheric changes can influence the troposphere, not only in the extratropics but also in the tropics through (i) a downward migration of wave–zonal mean flow interactions and (ii) changes in the stratospheric mean meridional circulation. These experiments support earlier evidence of an indirect solar influence from the stratosphere.

Hurricane interaction with the upper ocean in the Amazon-Orinoco plume region

The evolution of three successive hurricanes (Katia, Maria, and Ophelia) is investigated over the river plume area formed by the Amazon and Orinoco river outflows during September of 2011. The study focuses on hurricane impacts on the ocean structure and the ocean feedback influencing hurricane intensification. High-resolution (1/25° × 1/25° horizontal grid) numerical simulations of the circulation in the extended Atlantic Hurricane Region (Caribbean Sea, Gulf of Mexico, and Northwest Atlantic Ocean) were used to investigate the upper ocean response during the three hurricane-plume interaction cases. The three hurricanes revealed different evolution and intensification characteristics over an area covered by brackish surface waters. The upper ocean response to the hurricane passages over the plume affected region showed high variability due to the interaction of oceanic and atmospheric processes. The existence of a barrier layer (BL), formed by the offshore spreading of brackish waters, probably facilitated intensification of the first storm (Hurricane Katia) because the river-induced BL enhanced the resistance of the upper ocean to cooling. This effect was missing in the subsequent two hurricanes (Maria and Ophelia) as the eroded BL (due to Katia passage) allowed the upper ocean cooling to be increased. As a consequence, the amount of ocean thermal energy provided to these storms was greatly reduced, which acted to limit intensification. Numerical experiments and analyses, in tandem with observational support, lead to the conclusion that the presence of a river plume-induced BL is a strong factor in the ocean conditions influencing hurricane intensification.

North Atlantic Ocean OSSE system development: Nature Run evaluation and application to hurricane interaction with the Gulf Stream

A high resolution, free-running model has been developed for the hurricane region of the North Atlantic Ocean. The model is evaluated with a variety of observations to ensure that it adequately represents both the ocean climatology and variability over this region, with a focus on processes relevant to hurricane-ocean interactions. As such, it can be used as the “Nature Run” (NR) model within the framework of Observing System Simulation Experiments (OSSEs), designed specifically to improve the ocean component of coupled ocean-atmosphere hurricane forecast models. The OSSE methodology provides quantitative assessment of the impact of specific observations on the skill of forecast models and enables the comprehensive design of future observational platforms and the optimization of existing ones. Ocean OSSEs require a state-of-the-art, high-resolution free-running model simulation that represents the true ocean (the NR). This study concentrates on the development and data based evaluation of the NR model component, which leads to a reliable model simulation that has a dual purpose: (a) to provide the basis for future hurricane related OSSEs; (b) to explore process oriented studies of hurricane-ocean interactions. A specific example is presented, where the impact of Hurricane Bill (2009) on the eastward extension and transport of the Gulf Stream is analyzed. The hurricane induced cold wake is shown in both NR simulation and observations. Interaction of storm-forced currents with the Gulf Stream produced a temporary large reduction in eastward transport downstream from Cape Hatteras and had a marked influence on frontal displacement in the upper ocean. The kinetic energy due to ageostrophic currents showed a significant increase as the storm passed, and then decreased to pre-storm levels within 8 days after the hurricane advanced further north. This is a unique result of direct hurricane impact on a western boundary current, with possible implications on the ocean feedback on hurricane evolution.