A32B-01 INVITED
Subseasonal SST variability in the tropical eastern north Pacific during boreal summer
Boreal summer intraseasonal (30-90 day timescale) SST variability in the east Pacific warm pool is examined using Tropical Rainfall Measuring Mission Microwave Imager sea surface temperatures (SSTs) during 1998- 2005. Intraseasonal SST variance maximizes at two locations in the warm pool: in the vicinity of 9°N, 92°W near the Costa Rica Dome, and near the northern edge of the warm pool in the vicinity of 19°N, 108°W. Both locations contain a significant spectral peak at 50-60 day periods, timescales characteristic of the Madden-Julian oscillation (MJO). Complex empirical orthogonal function (CEOF) and spectra coherence analyses are used to show that boreal summer intraseasonal SST anomalies are coherent with precipitation anomalies across the east Pacific warm pool. Spatial variations of phase are modest across the warm pool, although some evidence exists for northward progression of intraseasonal SST and precipitation anomalies. Intraseasonal SSTs at the north edge of the warm pool lag those in the vicinity of the Costa Rica Dome by about one week. The MJO explains 30-40% of the variance of intraseasonal SST anomalies in the east Pacific warm pool during boreal summer. Peak-to-peak SST variations of about 1.0°C occur during MJO events. SST is approximately in quadrature with MJO precipitation, with suppressed (enhanced) MJO precipitation anomalies leading positive (negative) SST anomalies by 7-10 days. Consistent with the CEOF and coherence analyses, MJO-related SST and precipitation anomalies near the Costa Rica Dome lead those at the northern edge of the warm pool by about a week. Equatorial SST anomalies during a composite MJO event are generally out-of-phase with those in the warm pool, although equatorial SST anomalies are generally patchy and less coherent than those in the warm pool.
A32B-02
Warm Eddy Structure Observed During EPIC in Eastern Pacific Ocean
During the NSF/NOAA sponsored Eastern Pacific Investigation of Climate (EPIC) field program in Sept. and Oct. 2001, oceanic current, temperature and salinity profiles were acquired by deploying expendable profilers from research aircraft flights above the warm pool grid centered on the TAO mooring at 10oN 95oW and the R/V Ron Brown, and along the 95oW transect from the NOAA WP-3D and the NCAR WC-130, respectively. Analyses of mooring, ship and aircraft observations suggest the propagation of a wind-forced, warm eddy in accord with remotely sensed fields from radar altimetry and TRMM microwave imager (TMI) measurements. This anti- cyclonically rotating warm eddy, consistent with Rossby wave dynamics, impacted both the oceanic and atmospheric mixed layer structure. To examine the evolving characteristics of this oceanic feature, SSTs, isotherm depths and oceanic heat content variations (relative to the 26oC isotherm depth referred to as OHC) were compared at the TAO buoy. Satellite- based OHC variations were estimated by inferring isotherm depths (20oC, 26oC) from blended and objectively mapped, altimeter-derived surface height anomaly (SHA) fields based on climatology and TMI-derived SSTs. Based on sequential maps of the SHA, the observed warm eddy had SHA elevation of 12 to 14 cm that indicated a propagation speed of 13 cm s-1 towards the southwest. Inferred isotherm depths and OHC variations agreed with those from the TAO mooring and profiler measurements. For example, the 26oC isotherm depth ranged from 35 to 40 m with OHC values of 40 kJ cm-2. Understanding the evolving 3-D structure of these features is central to assessing the upper ocean's role in hurricane intensity fluctuations in the Eastern Pacific Ocean. This approach is now being applied to several years of in situ and remotely sensed measurements in this regime to assess uncertainties in satellite retrievals to build climatology for use with hurricane intensity forecast models as in the Atlantic Ocean basin as part of the NOAA Joint Hurricane Testbed program.
A32B-03
On the Interannual Variability of the Eastern Pacific Warm Pool
The variability of the Eastern Pacific Warm Pool (EPWP) is studied analyzing observations from the Modular Ocean Data Assimilation System, results from the HYbrid Coordinate Ocean Model (HYCOM), and sea surface height anomaly data. Results indicate that the EPWP strengthens and weakens, but remains throughout the year. The monthly variability of the EPWP is primarily forced by the surface heat fluxes (SHF), which generate the EPWP's May (January) maximum (minimum) coverage of ~4,000,000 (~30,000) km2 and the July mid-summer minimum of ~2,300,000 km2. In addition to the SHF the interannual variability of the EPWP is influenced by the coastal-warm-water advected poleward by the Costa Rica Coastal Current and interannual coastally-trapped- waves (CTWs). During its poleward propagation the interannual CTWs generate westward propagating long Rossby waves that advect the coastal-warm-water offshore, contributing to the westward strengthening of the EPWP. Furthermore, it is shown that the dimensions of the EPWP increase (decrease) during El Niño (La Niña) years.
A32B-04
Upper Ocean Responses to Hurricane Frances in September 2004
Three new autonomous ocean velocity and density profilers were deployed ahead of Hurricane Frances as it passed north of Hispaniola in September 2004. These EM-APEX floats (velocity sensing versions of Webb Research Corp APEX floats) were launched from a C-130. The EM-APEX floats measured T, S and V over the upper 500 m starting about a day before the storm's arrival. One EM-APEX float was directly under the track of the storm's eye, another EM-APEX float went in about 55 km to the right of the track (where the surface winds are strongest) and the third float was about 110 km to the right. The EM-APEX floats profiled for 10 hours from the surface to 200 m then continued profiling between 30 and 200 m with excursions to 500 m every half inertial period. After 5 days, the EM-APEX floats surfaced and transmitted the accumulated processed observations, then the floats profiled to 500 m every half inertial period until recovered early in October aided by GPS and Iridium. The float array sampled in unprecedented detail the upper-ocean momentum, turbulence and salt and heat changes in response to the hurricane. Rapid acceleration of inertial currents in the surface mixing layer (SML) to over 1 m/s produced vertical mixing by shear instability at the SML base, as indicated by low Richardson numbers and SML deepening from about 40 m to 120 m under the strongest wind forcing. Surface cooling of about 2.2 C was primarily due to the SML deepening and entrainment of colder water, with a small contribution from surface heat flux. Intense inertial pumping was observed under the eye, with vertical excursions of 50 m or more. Comparison with a 3-D numerical model of the ocean response to Frances' winds simulates accurately SML deepening and surface cooling as well as significant differences in maximum currents and heat content changes. These differences highlight the sensitivity of the ocean's response to both the specification of the wind field and the parameterization of stress under high wind speeds. In particular, the momentum flux into the ocean supports a drag coefficient that does not increase with wind speed as proposed by recent GPS dropsonde and wind-wave tank studies.
A32B-05
2D optical array probe analysis of precipitating cumulonimbus clouds during EPIC 2001
During the 2001 East Pacific Investigation of Climate (EPIC) experiment, numerous measurements were made of the size distributions of raindrops in convective clouds that were developing over a region of the Mexican inter- tropical convergence zone (ITCZ). These measurements were made with optical array probes (PMS 2D-C and 2D-P) mounted on the National Science Foundation Hercules C-130, operated by the National Center for Atmospheric Research. In addition to capturing shadow images of individual drops between 25 μm and 6400 μm, these instruments also record the distance between each drop via a measurement of arrival times in the spectrometers lasers. The separation distance, along with the drop size, provides detailed information about the microstructure of precipitation. The 2D probe measurements have been analyzed as a function of altitude above cloud base, horizontal distance from cloud edges, cloud droplet size distributions (2-50 μm) and vertical wind velocities. The objective of the analysis is to evaluate the spatial distribution of precipitation events with respect to the microphysical and dynamical processes that are related to the development and evolution of rain in tropical convective clouds. In addition, the reflectivity is calculated from the size distributions and evaluated to assess how inhomogeneities in the precipitation might be observed by meteorological radars.
A32B-06
A theoretical study of the impact of anthropogenic CCN on deep convective clouds in the EPIC region
Several studies have explored the effect of anthropogenic emissions on the evolution and precipitation development of different types of clouds. However, there is still no consensus on the effect, particularly for the case of deep, mixed-phase clouds. In this study we have introduced changes in the parameterization of the autoconversion and accretion processes in the Advanced Regional Prediction System (ARPS). The cloud condensation nuclei (CCN) used in the model were obtained from in situ observations by the instrumented C-130 aircraft, during the East Pacific Investigations of Climate (EPIC) about 600- 800 km offshore in the Intertropical Convergence Zone. Two cases were selected, one of which showed evidence that the CCN were modified by anthropogenic emissions. The results indicate that precipitation development in the deep, mixed phase clouds simulated is sensitive to the input of large concentrations of anthropogenic CCN. Precipitation development is delayed and its amount is reduced in the simulations when more CCN are included. This results from the modified autoconversion scheme that in turn causes a delay in the formation of hail and modifies its spatial distribution. Melting of hail is the most important contributor towards precipitation and a reduction in the amount of hail falling below the Freezing Level produces an appreciable decrease in the precipitation at the surface in the polluted case. Changes in the initial concentration of CCN do not appear to influence the storm strength in terms of updrafts and cloudtop height, suggesting little sensitivity to cloud dynamics.
A32B-07
The Effect of Oceanic Eddies on Marine Stratocumulus in the Southeast Pacific
Air-sea interaction in the South East Pacific (SEP) plays an important role in the regional and global climate system. The combination of cold sea surface temperature (SST) and subsiding warm, dry air provide an ideal environment for the formation of marine stratocumulus clouds in the SEP. In this study, the modulation of clouds due to ocean meso-scale fluctuations has been examined using multi-year satellite data sets. Monthly averaged and 0.25 deg gridded sea surface temperature (SST), column water vapor (CWV), column liquid water (CLW), vector winds, and sea surface height (SSH) have been used to examine correlations among monthly anomalies of SST, SSH and cloud moisture parameters. Monthly anomalies, constructed by removing 6-year (1998-2003) monthly means, were averaged into 0.25C bins, over a 10x10 deg box centered at 20S, 85W. The analysis indicates that the anomaly of SST is highly correlated with anomalies of SSH and CWV. Wind divergence at height of 10-m shows a negative correlation with SST anomaly. A relationship of CLW anomaly with SST anomaly shows different phases of cloud formation. Preliminary interpretation suggests that high values of wind divergence are associated with low SST anomaly and relatively high CLW, consistent with descending air and relatively dense stratus clouds. For SST anomalies near zero and slightly positive, wind divergence is intermediate and CLW exhibits a broad minimum, consistent with weak subsidence and relatively less dense stratus. For SST anomalies in excess of 1C, there is evidence of the break-up of stratus into cumulus clouds driven by convection. Typically cold, negative SSH anomalies (cyclonic eddies) are regions of high biological productivity, which may lead to the generation of aerosols in the marine atmosphere. Therefore the clouds may be modulated by upper ocean biological productivity in addition to SST and wind.
A32B-08
Shipboard Investigation of Air-sea Interaction and Cloud Processes in the VOCALS Stratocumulus Region
NOAA ESRL and University of Miami have cooperated with the Woods Hole Oceanographic Institution (WHOI) for
series of research cruises to the stratocumulus region of Peru/Chile as part of NOAA's PACS/EPIC/VOCALS
program. Ship-based measurements have taken in October of 2001, 2005, and 2006, November 2003, and
December 2004, at the WHOI ocean reference buoy at 20 S 85 W during the annual cruise to service the buoy.
The goal of this work is to improve understanding of coupled air-sea processes in subtropical stratocumulus
regions and to gather statistics on flux, boundary layer, and cloud properties to promote the evaluation of models
and satellite data products. In this paper we will present a synthesis of results from all five cruises emphasizing
a consensus of the direct covariance flux data and an analysis of the diurnal cycle of cloud properties.
http:www.esrl.noaa.gov/psd/psd3/air-sea/