SST Diurnal Cycle over the Western Hemisphere: Preliminary Results from the New High-Resolution MPM Analysis Wanqiu Wang, Pingping Xie, and Chenjie Huang Acknowledgments: This work is supported by CPO/CPPA

Motivation Diurnal coupling between the atmosphere and ocean is an important process in the air-sea interaction. An analysis of SST diurnal cycle based on observational data will help provide a basis for understanding the relationship in diurnal variability between the atmosphere and ocean, and for validating coupled atmosphere-ocean models.

Objectives To construct high-resolution SST analysis resolving the diurnal cycle over the western hemisphere for recent years through merging observations from in situ and space-based platforms (Multi-Platform Merged, MPM analysis); To document the diurnal cycle of sea surface temperatures (SSTs) around the North American Monsoon (NAM) region using the new MPM SST analysis; Diagnoses in this study include: Seasonality; mean and extreme amplitude; and variability of the diurnal cycle.

The MPM SST Analysis An analysis of three-hourly SST is constructed on a 0.25 o lat/lon grid over the western hemisphere [180 o -30 o W; 45 o S-60 o N] for a 6-year period from 2003 to 2008; The analysis is defined by merging SST observations from in situ measurements and retrievals from multiple satellite platforms using the algorithm of Wang and Xie (2005) –Quality control through comparisons with historical data and cross check among concurrent SST data –Bias correction to satellite data through matching the probability density function (PDF) against the in situ measurements –Optimal interpolation (OI) algorithm of Gandin (1965)

Comparisons with Other SST Analyses JJA Mean SST  Similar spatial patterns; MPM closer to RTG  NCDC colder than MPM / RTG over GOC and GOM NCDC RTG MPM NCDC-RTG NCDC-MPM RTG-MPM

Comparisons with Other SST Analyses Daily Mean SST Climatology over Selected regions  Similar patterns of temporal variations  Close agreement in magnitude with NCDC  NCDC is colder and presents smoother variations

Seasonality of the SST Diurnal Cycle Mean SST Diurnal Cycle Standard Deviation  Intensity of diurnal cycle for each day is defined as the standard deviation of eight 3-hourly values; Moves northward from northern winter to summer; The largest diurnal cycle observed in the Gulf of California, west cost of Mexico, and the coasts of the Gulf of Mexico during MAM and JJA.

Seasonality of the SST Diurnal Cycle Largest SST Diurnal Cycle Magnitude  The largest SST diurnal magnitude is defined as the 99% percentile of the daily standard deviations over the season during 2003 – 2008; The mean SST diurnal standard is usually less than 0.45 o K (last slide); The SST diurnal standard deviation could be as large as 1 o K, or 3 o K in difference between the maximum and minimum

JJA SST Diurnal Cycle Mean JJA 3-Hourly SST  Minimum SST observed in early morning  SST is the warmest in the afternoon 12Z-15Z 15Z-18Z 18Z-21Z 21Z-24Z 0Z-3Z 3Z-6Z 6Z-9Z 9Z-12Z

JJA SST Diurnal Cycle Mean JJA Diurnal Cycle  Arrows: Timing of maximum SST (N=00LST;E=06LST)  Shading:50% Percentile of SST Diurnal Standard Deviation  Variations in both the phase and magnitude over coastal regions

Variability of Diurnal Cycle over GOC Time - Latitude Section for Jun. 26 – Jul.5, 2006  Fine spatial and temporal structure of SST variations observed over GOC;  Strong diurnal cycle with range of up to 3.5 o K;

Variability of Diurnal Cycle over GOC Time Series of 3-hourly SST at [ o W; o N]  SST diurnal cycle presents variations of synoptic to intra-seasonal time scales

Variability of Diurnal Cycle over GOC Time – Latitude Section of SST Diurnal Standard Deviation  SST diurnal cycle presents inter-annual variations as well.

Summary  The SST diurnal cycle show great seasonal variations around the NAM region, with large diurnal amplitude in the Gulf of California, west cost of Mexico, and the coasts of the Gulf of Mexico during MAM and JJA.  The diurnal cycle varies at synoptic to interannual time scales. During periods of large diurnal cycle, SST diurnal standard deviation can be as large as 1 o K, corresponding to a diurnal range of about 3 o K.  The MPM SST analysis is helpful for studying SST diurnal cycle for its temporal and spatial variations. The data set can also be used for validating coupled model simulation and prediction, and for analyzing relationship in diurnal variability between the atmosphere and ocean.