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S. Daniel Jacob and Chester J. Koblinsky

Abstract

The effect of precipitation on the upper-ocean response during a tropical cyclone passage is investigated using a numerical model in this paper. For realistic wind forcing and empirical rain rates based on satellite climatology, numerical simulations are performed with and without precipitation forcing to delineate the effects of freshwater forcing on the upper-ocean heat and salt budgets. Additionally, the performance of five mixing parameterizations is also examined for the two forcing conditions to understand the sensitivity of simulated ocean response. Overall, results from 15 numerical experiments are analyzed to quantify the precipitation effects on the oceanic mixed layer and the upper ocean. Simulated fields for the same mixing scheme with and without precipitation indicate a decrease in the upper-ocean cooling of about 0.2°–0.5°C. This is mainly due to reduced mixing of colder water from below induced by the increased stability of the added freshwater. The cooler rainwater contributes a maximum of approximately 10% to the total surface heat loss from the ocean. The rate of freshening due to precipitation exceeds the rate of mixing of the more saline water from below, leading to a change in sign of the mixed layer salinity response. As seen in earlier studies, large uncertainty exists in the simulated upper-ocean response due to the choice of mixing parameterization. Although the nature of simulated response remains similar for all the mixing schemes, the magnitude of freshening and cooling varies by as much as 0.5 psu and 1°C between the schemes to the right of the storm track. While changes in the mixed layer and in the top 100 m of heat and salt budgets are strongly influenced by the choice of mixing scheme, integrated budgets in the top 200 m are seen to be affected more by advection and surface fluxes. However, since the estimated surface fluxes depend upon the simulated sea surface temperature, the choice of mixing scheme is crucial for realistic coupled predictive models.

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Robert S. Pickart, Daniel J. Torres, and Paula S. Fratantoni

Abstract

High-resolution hydrographic and velocity measurements across the East Greenland shelf break south of Denmark Strait have revealed an intense, narrow current banked against the upper continental slope. This is believed to be the result of dense water cascading over the shelf edge and entraining ambient water. The current has been named the East Greenland Spill Jet. It resides beneath the East Greenland/Irminger Current and transports roughly 2 Sverdrups of water equatorward. Strong vertical mixing occurs during the spilling, although the entrainment farther downstream is minimal. A vorticity analysis reveals that the increase in cyclonic relative vorticity within the jet is partly balanced by tilting vorticity, resulting in a sharp front in potential vorticity reminiscent of the Gulf Stream. The other components of the Irminger Sea boundary current system are described, including a presentation of absolute transports.

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Daniel J. Vimont, David S. Battisti, and Anthony C. Hirst

Abstract

An influence of midlatitude atmospheric variability on interannual ENSO and decadal ENSO-like variability is established and investigated in the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled general circulation models (CGCMs). The effect of midlatitude atmospheric variability is felt in the Tropics via the previously hypothesized “seasonal footprinting mechanism” (SFM), in which a tropical circulation is forced during spring and summer by tropical SST anomalies that are generated by midlatitude atmospheric variability during the previous winter. The tropical circulation includes equatorial zonal wind stress anomalies that act as a stochastic forcing for the CSIRO CGCM's damped ENSO mode.

Details of the SFM are investigated herein. A temporal analysis indicates that the SFM may explain 20%–40% of the model's interannual ENSO variability and nearly 70% of the model's decadal to interdecadal tropical variability. An analysis of the physical mechanisms that govern the SFM highlights the role of relaxed trade winds in producing tropical SST anomalies during winter, and identifies a weak positive coupled feedback between off-equatorial tropical SST anomalies and the atmospheric response to those anomalies.

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Daniel J. Vimont, David S. Battisti, and Anthony C. Hirst

Abstract

The structure and evolution of, and the mechanisms responsible for, interannual and decadal equatorial variability in a 1000-yr simulation of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) coupled general circulation model are examined. Principal component analysis is applied to the 0–270-m integrated heat content from the model to determine dominant modes of variability. The leading mode of unfiltered variability (annual) is best described by an AR2 null hypothesis with an implied periodic timescale of 6–10 yr. Spatial structures of the leading empirical modes of interannual (10-yr high-pass filtered) and decadal (9-yr low-pass filtered) variability closely resemble observations of interannual ENSO, and decadal ENSO-like variability. The amplitude of tropical SST anomalies is too small by a factor of 2–3 on interannual timescales, but is close to that observed for decadal timescales.

For interannual timescales, an equatorial heat budget analysis shows a positive feedback between zonal wind stress anomalies, thermocline depth, and SST anomalies along the equator: the so-called Bjerkness feedback. A delayed negative feedback is provided by westward-propagating signals (produced by the same tropical zonal wind stress anomalies) that reflect off the western boundary, and counter the local positive feedback. Combined with the results from the spectral analysis, the heat budget analysis shows that the processes responsible for the interannual variability are consistent with the delayed oscillator mechanism for ENSO; an emerging warm or cold ENSO event contains within it the seeds of its own destruction via the delayed, restoring ocean dynamics.

The decadal heat budget analysis shows the same positive feedback that exists on interannual timescales. However, unlike interannual timescales, for decadal timescales there is little evidence of a delayed negative feedback provided by signals reflecting off the western boundary. The lack of a negative feedback is partially explained by the different meridional structure of interannual and decadal zonal wind stress anomalies. Anomalies associated with the decadal variability decay by numerous processes, including strong surface heat fluxes, and advection by the equatorial undercurrent.

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Robert S. Pickart, Daniel J. Torres, and R. Allyn Clarke

Abstract

The hydrographic structure of the Labrador Sea during wintertime convection is described. The cruise, part of the Deep Convection Experiment, took place in February–March 1997 amidst an extended period of strong forcing in an otherwise moderate winter. Because the water column was preconditioned by previous strong winters, the limited forcing was enough to cause convection to approximately 1500 m. The change in heat storage along a transbasin section, relative to an occupation done the previous October, gives an average heat loss that is consistent with calibrated National Centers for Environmental Prediction surface heat fluxes over that time period (∼200 W m−2). Deep overturning was observed both seaward of the western continental slope (which was expected), as well as within the western boundary current itself—something that had not been directly observed previously. These two geographical regions, separated by roughly the 3000-m isobath, produce separate water mass products. The offshore water mass is the familiar cold/fresh/dense classical Labrador Sea Water (LSW). The boundary current water mass is a somewhat warmer, saltier, lighter vintage of classical LSW (though in the far field it would be difficult to distinguish these products). The offshore product was formed within the cyclonic recirculating gyre measured by Lavender et al. in a region that is limited to the north, most likely by an eddy flux of buoyant water from the eastern boundary current. The velocity measurements taken during the cruise provide a transport estimate of the boundary current “throughput” and offshore “recirculation.” Finally, the overall trends in stratification of the observed mixed layers are described.

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Daniel Vassallo, Raghavendra Krishnamurthy, Robert Menke, and Harindra J. S. Fernando

Abstract

This paper reports the findings of a comprehensive field investigation on flow through a mountain gap subject to a range of stably stratified environmental conditions. This study was embedded within the Perdigão field campaign, which was conducted in a region of parallel double-ridge topography with ridge-normal wind climatology. One of the ridges has a well-defined gap (col) at the top, and an array of in situ and remote sensors, including a novel triple Doppler lidar system, was deployed around it. The experimental design was mostly guided by previous numerical and theoretical studies conducted with an idealized configuration where a flow (with characteristic velocity U 0 and buoyancy frequency N) approaches normal to a mountain of height h with a gap at its crest, for which the governing parameters are the dimensionless mountain height G = Nh/U 0 and various gap aspect ratios. Modified forms of G were proposed to account for real-world atmospheric variability, and the results are discussed in terms of a gap-averaged value G c. The nature of gap flow was highly dependent on G c, wherein a nearly neutral flow regime (G c < 1), a transitional mountain wave regime [G c ~ O(1)], and a gap-jetting regime [G c > O(1)] were identified. The measurements were in broad agreement with previous numerical and theoretical studies on a single ridge with a gap or double-ridge topography, although details vary. This is the first-ever detailed field study reported on microscale [O(100) m] gap flows, and it provides useful data and insights for future theoretical and numerical studies.

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Peigen Lin, Robert S. Pickart, Daniel J. Torres, and Astrid Pacini

Abstract

Shipboard hydrographic and velocity measurements collected in summer 2014 are used to study the evolution of the freshwater coastal current in southern Greenland as it encounters Cape Farewell. The velocity structure reveals that the coastal current maintains its identity as it flows around the cape and bifurcates such that most of the flow is diverted to the outer west Greenland shelf, while a small portion remains on the inner shelf. Taking into account this inner branch, the volume transport of the coastal current is conserved, but the freshwater transport decreases on the west side of Cape Farewell. A significant amount of freshwater appears to be transported off the shelf where the outer branch flows adjacent to the shelfbreak circulation. It is argued that the offshore transposition of the coastal current is caused by the flow following the isobaths as they bend offshore because of the widening of the shelf on the west side of Cape Farewell. An analysis of the potential vorticity shows that the subsequent seaward flux of freshwater can be enhanced by instabilities of the current. This set of circumstances provides a pathway for the freshest water originating from the Arctic, as well as runoff from the Greenland ice sheet, to be fluxed into the interior Labrador Sea where it could influence convection in the basin.

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David S. Battisti, Daniel J. Vimont, and Benjamin P. Kirtman

Abstract

In situ observation networks and reanalyses products of the state of the atmosphere and upper ocean show well-defined, large-scale patterns of coupled climate variability on time scales ranging from seasons to several decades. We summarize these phenomena and their physics, which have been revealed by analysis of observations, by experimentation with uncoupled and coupled atmosphere and ocean models with a hierarchy of complexity, and by theoretical developments. We start with a discussion of the seasonal cycle in the equatorial tropical Pacific and Atlantic Oceans, which are clearly affected by coupling between the atmosphere and the ocean. We then discuss the tropical phenomena that only exist because of the coupling between the atmosphere and the ocean: the Pacific and Atlantic meridional modes, the El Niño–Southern Oscillation (ENSO) in the Pacific, and a phenomenon analogous to ENSO in the Atlantic. For ENSO, we further discuss the sources of irregularity and asymmetry between warm and cold phases of ENSO, and the response of ENSO to forcing. Fundamental to variability on all time scales in the midlatitudes of the Northern Hemisphere are preferred patterns of uncoupled atmospheric variability that exist independent of any changes in the state of the ocean, land, or distribution of sea ice. These patterns include the North Atlantic Oscillation (NAO), the North Pacific Oscillation (NPO), and the Pacific–North American (PNA) pattern; they are most active in wintertime, with a temporal spectrum that is nearly white. Stochastic variability in the NPO, PNA, and NAO force the ocean on days to interannual times scales by way of turbulent heat exchange and Ekman transport, and on decadal and longer time scales by way of wind stress forcing. The PNA is partially responsible for the Pacific decadal oscillation; the NAO is responsible for an analogous phenomenon in the North Atlantic subpolar gyre. In models, stochastic forcing by the NAO also gives rise to variability in the strength of the Atlantic meridional overturning circulation (AMOC) that is partially responsible for multidecadal anomalies in the North Atlantic climate known as the Atlantic multidecadal oscillation (AMO); observations do not yet exist to adequately determine the physics of the AMO. We review the progress that has been made in the past 50 years in understanding each of these phenomena and the implications for short-term (seasonal-to-interannual) climate forecasts. We end with a brief discussion of advances of things that are on the horizon, under the rug, and over the rainbow.

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Daniel J. Vimont, John M. Wallace, and David S. Battisti

Abstract

Midlatitude atmospheric variability is identified as a particularly effective component of the stochastic forcing of ENSO. This forcing is realized via a seasonal footprinting mechanism (SFM), in which the tropical atmosphere is forced during the spring and summer by SST anomalies generated by midlatitude atmospheric variability during the previous winter. The strong relationship between the SFM and ENSO may serve to enhance ENSO predictability and supports the view that ENSO is linearly stable in nature.

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Zachary S. Bruick, Kristen L. Rasmussen, and Daniel J. Cecil

Abstract

Hailstorms in subtropical South America are known to be some of the most frequent anywhere in the world, causing significant damage to the local agricultural economy every year. Convection in this region tends to be orographically forced, with moisture supplied from the Amazon rain forest by the South American low-level jet. Previous climatologies of hailstorms in this region have been limited to localized and sparse observational networks. Because of the lack of sufficient ground-based radar coverage, objective radar-derived hail climatologies have also not been produced for this region. As a result, this study uses a 16-yr dataset of TRMM Precipitation Radar and Microwave Imager observations to identify possible hailstorms remotely, using 37-GHz brightness temperature as a hail proxy. By combining satellite instruments and ERA-Interim reanalysis data, this study produces the first objective study of hailstorms in this region. Hailstorms in subtropical South America have an extended diurnal cycle, often occurring in the overnight hours. In addition, they tend to be multicellular in nature, rather than discrete. High-probability hailstorms (≥50% probability of containing hail) tend to be deeper by 1–2 km and horizontally larger by greater than 15 000 km2 than storms having a low probability of containing hail (<25% probability of containing hail). Hailstorms are supported synoptically by strong upper- and lower-level jets, anomalously warm and moist low levels, and enhanced instability. The findings of this study will support the forecasting of these severe storms and mitigation of their damage within this region.

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