Question: What ocean, intersected by the 30th Meridian, may also be an estuary of the Atlantic Ocean?
Context: The climate of the United States varies due to differences in latitude, and a range of geographic features, including mountains and deserts.  West of the 100th meridian, much of the US is semi-arid to desert in the far southwestern US, and Mediterranean along the California coast.  East of the 100th meridian, the climate is humid continental in the northern areas east through New England, to humid subtropical in the Gulf and South Atlantic regions.  Southern Florida is tropical, as is Hawaii and the US Virgin Islands.  Higher-elevation areas of the Rocky Mountains, the Wasatch, Sierra Nevada, and Cascade Range are alpine.  The West Coast areas in coastal Oregon and Washington are oceanic climate.  The state of Alaska, on the northwestern corner of the North American continent, is largely subarctic climate, but with a subpolar oceanic climate in the southeast (Alaska Panhandle), southwestern peninsula and Aleutian Islands. The 2010–11 Australian region cyclone season was a near average tropical cyclone season, with eleven tropical cyclones forming compared to an average of 12.  The season began on 1 November 2010 and ended on 30 April 2011, although the first tropical cyclone formed on 28 October.  The Australian region is defined as being to the south of the equator, between the 90th meridian east and 160th meridian east.  Tropical cyclones in this area are monitored by five Tropical Cyclone Warning Centres (TCWC's): Jakarta, Port Moresby, Perth, Darwin, and Brisbane, each of which have the power to name a tropical cyclone.  The TCWC's in Perth, Darwin, and Brisbane are run by the Bureau of Meteorology, who designate significant tropical lows with a number and the U suffix.  The Joint Typhoon Warning Center also issues unofficial warnings for the region, designating significant tropical cyclones with the "S" suffix when they form west of 135°E, and the "P" suffix when they form east of 135°E. The meridian 30° west of Greenwich is a line of longitude that extends from the North Pole across the Arctic Ocean, Greenland, the Atlantic Ocean, the Southern Ocean, and Antarctica to the South Pole. The Arctic Ocean is the smallest and shallowest of the world's five major oceans.  The International Hydrographic Organization (IHO) recognizes it as an ocean, although some oceanographers call it the Arctic Mediterranean Sea or simply the Arctic Sea, classifying it a mediterranean sea or an estuary of the Atlantic Ocean.  Alternatively, the Arctic Ocean can be seen as the northernmost part of the all-encompassing World Ocean. The border between Namibia and South Africa is 967 km long.  It runs along the Orange River from its mouth on the Atlantic Ocean to the 20th meridian east, and then northwards along that meridian to the tripoint with Botswana at the Nossob River. The 180th meridian or antimeridian is the meridian 180° east or west of the Prime Meridian, with which it forms a great circle dividing the earth into the Western and Eastern Hemispheres.  It is common to both east longitude and west longitude.  It is used as the basis for the International Date Line because it mostly passes through the open waters of the Pacific Ocean.  However, the meridian passes through Russia and Fiji as well as Antarctica. The National Hurricane Center (NHC) is the division of the United States' National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th parallel north in the northeast Pacific Ocean and the 31st parallel north in the northern Atlantic Ocean.  The agency, which is co-located with the Miami branch of the National Weather Service, is situated on the campus of Florida International University in University Park, Florida. The 2009–10 Australian region cyclone season was a below average tropical cyclone season, with eight tropical cyclones forming compared to an average of 12.  The season began on 1 November 2009 and ran through until it end on 30 April 2010.  The Australian region is defined as being to south of the equator, between the 90th meridian east and 160th meridian east.  Tropical cyclones in this area are monitored by five Tropical Cyclone Warning Centres (TCWC's): Jakarta, Port Moresby, Perth, Darwin, and Brisbane, each of which have the power to name a tropical cyclone.  The TCWC's in Perth, Darwin, and Brisbane are run by the Australian Bureau of Meteorology, who designate significant tropical lows with a number and the U suffix.  The Joint Typhoon Warning Center also issues unofficial warnings for the region, designating significant tropical cyclones with the "S" suffix when they form west of 135°E, and the "P" suffix when they form east of 135°E. Montague Aldous was a Dominion Topographical Surveyor from Charlottetown, Prince Edward Island.  He was a participant in foundation surveys of the Northwest Territories.  He was associated with A.L. Russell in 1877 in a survey west to the third meridian.  Taken on Special Survey in 1879, he surveyed the fourth meridian to a point north of Lloydminster, Alberta/Saskatchewan, then ran in the 14th base line west to Stony Plain where it intersected the 5th meridian.  His assistant, Charles Alexander Magrath, was later the first mayor of Lethbridge, Northwest Territories (now Alberta).  In 1880 Aldous surveyed the fifth meridian south to Waterton Lakes.  He spent the rest of the summer in southern Alberta's Porcupine Hills, then being taken up for ranching. As the Axis powers of Germany, Italy, and Japan cemented their military alliance by mutually declaring war against the United States by December 11, 1941, the Japanese proposed a clear territorial arrangement with the two main European Axis powers concerning the Asian continent.  On December 15 they presented the Germans with a drafted military convention that would delimit the continent of Asia into two separate "operational spheres" (zones of military responsibility) by a dividing line along the 70th meridian east longitude, going southwards through the Ob River's Arctic estuary, southwards to just east of Khost in Afghanistan and heading into the Indian Ocean just west of Rajkot in India, to split the "Lebensraum" land holdings of Germany and the similar "spazio vitale" areas of Italy to the west of it, and the Empire of Japan (and the Greater East Asia Co-Prosperity Sphere) to the east of it, after a complete defeat of the Soviet Union by the Third Reich.
Answer: Arctic Ocean