The Setting - Climatology of the Hawaiian Archipelago
Link to Video of Maui Waves What caused this week’s weather ?
What caused this weekend’s weather ?
Today’s Objective:
Provide overview and description of the properties of the atmosphere and the ocean around Hawai‘i: marine climate water properties The Setting - Climatology of the Hawaiian archipelago
What drives surface ocean currents ?
What drives the wind patterns ? Uneven Heating Earth’s surface is heated unevenly by energy from the Sun
The
Sun
Tilt Angle:
23.4 degrees Winds Redistribute Uneven Heat
Cold Air Heating of the Earth’s surface Cool Air leads to seawater evaporation Tropic of Cancer 23.5 deg N Evaporation
Equator Warm Air
Tropic of Capricorn 23.5 deg. S
Cool Air Evaporation leads to lower sea-level atmospheric pressure
Cold Air Winds Redistribute Uneven Heat
Cool Dry Air Sinks
Tropic of Cancer Precipitation 23.5 deg N
Low Sea-Level Equator Atmospheric Pressure
Tropic of Capricorn Precipitation 23.5 deg. S
Cool Dry Air Sinks Surface Winds
90 N
60 N
30 N High Pressure
Equator Low Pressure Low Pressure
30 S High Pressure
60 S
90 S Remember: The Earth rotates to the East Wind Cells
High
Low
High
Remember: At the Earth’s surface, winds flow from areas of high to low sea-level atmospheric pressure Atmospheric Pressure - Terminology
Subpolar Low
Mid-lat Cell (Ferrell)
Subtropical High
Tropical Cell (Hadley)
Equatorial Trough
Tropical Cell (Hadley)
Subtropical High
Mid-lat Cell (Ferrell)
Subpolar Low Winds - Terminology
Westerlies
Northeasterly Trades
Equatorial Trough
Southeasterly Trades
Westerlies
Remember: Winds named by the direction they blow from Wind Maps
Winter:
Summer:
Scatterometer Climatology of Ocean Winds (SCOW) based on 8 years (Sept 1999 – Aug 2007) of QuikSCAT scatterometer data. Precipitation Cells
High
Low
High
Remember: At the Earth’s surface, winds flow from areas of high to low sea-level atmospheric pressure Precipitation Maps
30 N
Equator
30 S
30 N
Equator
30 S
Tropical Precipitation Band (Equator); Low Rainfall (15 - 45 degrees) Regional Salinity Patterns
Annual average surface salinity. Period: 1895-1993. Source: World Ocean Atlas, Ocean Climate Laboratory, NOAA.
Units: ppt (parts-per-thousand). Regional SST Patterns Summer Winter
Mean surface water temperatures for February to April ("winter", the coldest period) and August to October ("summer", the warmest period). Period: 1987 to 1991. Units: degrees Celsius.
Source: AVHRR weekly Multichannel Sea-Surface Temperature (MCSST) Project, Jet Propulsion Laboratory, NASA. Regional SST Patterns
SSTs have a strong north to south gradient, and a small annual cycle, being lowest around March 15, and highest around September 15.
The variations of temperature parallel the island chain, (i.e. surface waters are warmer to the west at a given latitude). Regional Atmospheric Patterns
Northeasterly trade winds (from the east) are the dominant atmospheric pattern over Hawaiian archipelago; westerlies (from the west) occur farther to the north.
This circulation follows the North Pacific anticyclone (subtropical high).
This High Pressure Cell shifts northward in summer, when trade winds intensify and reach on average 35 deg. N, and southward in winter, when westerlies extend as far south as 28 deg. N Regional Pressure Patterns
Summer Winter
Mean sea-level pressure in the Central Pacific in Summer (June to August) and Winter (December to February). Period: 1946-1993. Source: Comprehensive Ocean-Atmosphere Data Set, Environmental Research Laboratory, NOAA. Units: mbar. Regional Wind Patterns
Summer Winter
Mean surface winds in the Central Pacific for Summer (June to August) and Winter (December to February). Period: 1946-1993. Source: Comprehensive Ocean-Atmosphere Data Set, Environmental Research Laboratory, NOAA. Units: m/s (10 m/s = 22 mph). Large-Scale Wind Patterns
Annual average trade wind speed is 6 m/s (13.5 mph) at 20 N. Evaporation exceeds precipitation between 15 N and 36 N; further south and north, precipitation exceeds evaporation. There is a net cooling of the ocean by the atmosphere over the entire region. Large-scale Surface Currents
Regional Currents Patterns
Mean depth of the 10 C isotherm. Arrows indicate the direction of the flow. Period: 1895-1993. Source: World Ocean Atlas, Ocean Climate Laboratory, NOAA.
Units: meters.
Mean currents form a large basin-scale clockwise circulation, called gyre, centered at about 28 N. At the latitude of Hawai'i, the circulation is roughly from east to west and intensifies southward.
Local Current Patterns
Mean surface flow, based on 40,000 ship drift observations, 85,000 drifting buoy data points, and 8,000 current measurements.
Units: cm/s (25 cm/s = 0.5 knot).
North Equatorial Current (NEC) reaches a mean westward speed of 17 cm/s (0.35 knot) at 13 N. Between 18 - 22 N, currents strongly influenced by islands. A clockwise circulation at 19 N, merges with southern branch of NEC. Waves and Storms The significant wave height
(SWH or Hs) is the mean wave height (trough to crest) of the
highest third of the waves (H1/3)
Winter Summer
Mean significant wave height (H1/3). Period: 1992 to 1995. Source: TOPEX Altimeter mission, NASA and CNES. Units: meters. Waves and Storms
Strong seasonal cycle in wave height
Large monthly variability (storm events) Waves and Storms
Strong seasonal cycle in wave direction
NOTE: wave direction quantified by angle heading
Waves and Storms
Waves and Storms
Regional Swell Regimes Four dominant regimes responsible for swell in Hawai‘i: north Pacific swell trade wind swell south swell
Kona storms
Regions influenced by the four dominant swell regimes outlined by Moberly & Chamberlain (1964).
Wave rose depicting annual swell heights and directions added to original graphic (Vitousek & Fletcher 2008).
Regimes – North Pacific Swell
Hawai‘i receives large ocean swell from extra-tropical storms that track predominantly eastward from the northwest Pacific.
The north Pacific storminess reaches a peak in the boreal winter, as the Aleutian low intensifies and the north Pacific high moves southward. The strong winds associated with these storms produce large swell events, which can travel for thousands of miles until reaching the shores of Hawai‘i.
In summer, the north Pacific high moves northward and storms become infrequent (Flament et al. 1996). Regimes – North Pacific Swell
The mean winter wave heights in the north Pacific are ≥ 3 m while the mean summer wave heights are ≤ 2 m.
Seasonal cycle high in winter and low in summer, with mean daily wave height of 4 m.
However the system involves eastward storm wave heights of 5-10 m.
Satellite (JASON-1) derived average wave heights [m] in the summer and winter. Regimes – North Pacific Swell
Many north Pacific storm swells do not reach Hawai‘i. Storms that originate in high latitudes and those that track northeast send swells to Aleutians and Pacific Northwest.
Swells from lower latitude storms, and those that track to southeast, reach Hawai‘i with the largest wave heights.
Regimes – Trade Wind Swell Occur ~ 75% of the year, with an average speed of 15.7 +/- 5 (SD) mph and direction 73° +/- 23°(SD).
In winter, the north Pacific high weakens and moves south, decreasing trade wind persistence. Although the number of windy days in summer increases, the mean speed in summer and winter months is relatively similar.
Number of days per season when trade winds occur with a certain speed ( NOAA Buoy 51001)
Notice the persistence of typical trade winds (~ 16 mph) during summer. Regimes – Southern Swell Southern swell arriving in Hawai‘i is generated farther away than north Pacific swell. Generated by storms south of the equator: Australia, New Zealand and Southern Ocean.
South swells occur in summer months (Southern winter) and reach Hawai‘i with significant wave heights of 2.5 – 3.0 m.
The largest South Swell waves on record (June 1955) had faces over 6 m (20 ft) breaking in shallow water.
This first large Antarctic storm of the 2001 season produced significant wave heights of 35 - 45 ft (10 - 15 m)
Regimes – Southern Swell Peak storm surf reached Hawai‘i on 11 July 2001 (after 6 days)
Data from NOAA/NCEP.
Regimes – Kona Storms Giambelluca & Schroeder (1998) describe Kona storms as:
Low-pressure areas (cyclones) of subtropical origin that develop NW of Hawai‘i in winter and move slowly eastward.
They are accompanied by southerly winds (from whose direction the storm derives its name) and by rain•.
Kona storms generate wave heights of 3 - 4 m.
Regimes – Kona Storms While minor Kona storms occur practically every year in Hawai‘i, major Kona storms with strong winds and large wave heights occur every 5-10 years, during La Niña years and the negative phase of the PDO cycle.
El Niño years (warm phase) and the positive (warm) PDO phases seem to suppress Kona storms.
El Niño – Southern Oscillation Walker Cell
Low High Pressure Pressure
Southern Oscillation:
SL Pressure Tahiti - SL Pressure Darwin Regional ENSO Effects
El Niño La Niña Pressure
Local ENSO Effects
Weaker trades
Warm surface water flows east
Thermocline depth deepens
Stronger trades
Warm surface water flows west
Thermocline depth shoals Your Tasks After This Lecture
- Understand and use the terminology employed in this lecture
- Describe the oceanographic / climatological setting of Hawai’i
http://www.pacificstormsclimatology.org/index.php?page=glossary
References
Flament, P., Keenan, S., Lumpkin, R., Sawyer, M., Stroup, E.D., 1996. The Ocean Atlas of Hawai‘i. Department of Oceanography, School of Ocean and Earth Science and Technology, University of Hawai‘i (http://oos.soest.hawaii.edu/pacioos/outreach/oceanatlas/index.php)
Giambelluca, T.W., Schroeder, T.A., 1998. Climate. In: Juvik, S.P., and Juvik, J.O. (eds.), Atlas of Hawai‘i. Honolulu: University of Hawai‘i Press, pp. 49-59.
Moberly, R., Chamberlain, T. 1964. Hawaiian Beach Systems. Hawai‘i Institute of Geophysics Report 64-2, University of Hawai‘i.